Recent progress in anion exchange membranes (AEMs) in water electrolysis: synthesis, physio-chemical analysis, properties, and applications

Ganesan Sriram *^ab, Karmegam Dhanabalan ^ab, Kanalli V. Ajeya ^b, Kanakaraj Aruchamy ^a, Yern Chee Ching ^c, Tae Hwan Oh *^a, Ho-Young Jung *^bde and Mahaveer Kurkuri *^f
aSchool of Chemical Engineering, Yeungnam University, Gyeongsan 38541, Republic of Korea. E-mail: sriramyu@yu.ac.kr; taehwanoh@ynu.ac.kr
^bDepartment of Environment and Energy Engineering, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, Republic of Korea. E-mail: jungho@chonnam.ac.kr
^cDepartment of Chemical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia
^dCenter for Energy Storage System, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju, 61186, Republic of Korea
^eHybrid Power Pack Specialized Research Center, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju, 61186, Republic of Korea
^fCentre for Research in Functional Materials (CRFM), JAIN (Deemed-to-be University), Jain Global Campus, Bengaluru-562112, Karnataka, India. E-mail: mahaveer.kurkuri@jainuniversity.ac.in

First published on 2nd October 2023

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Ganesan Sriram

Ganesan Sriram received his PhD in Nanotechnology in 2020 from JAIN (Deemed-to-be University), India, under the supervision of Prof. Mahaveer Kurkuri. Later, in January 2022, he joined Chonnam National University, South Korea, as a postdoctoral researcher under the guidance of Prof. Ho-Young Jung to develop an anion-exchange membrane to apply in water electrolysis. Following his first postdoc, he moved to Yeungnam University, School of Chemical Engineering, in August 2022 to work on polymer processing and other sustainable materials research for diverse applications under the guidance of Prof. Tae Hwan Oh. In April 2023, he was appointed International Research Professor in the same department at Yeungnam University. His research interests include synthesizing polymers, membranes, and highly porous inorganic materials for environmental and energy applications.

Karmegam Dhanabalan

Karmegam Dhanabalan received his PhD in Nanoscience and technology from Alagappa University, India in 2016. After receiving his PhD, he worked as a postdoctoral researcher at Chonnam National University (CNU), South Korea, from 2016 to 2018. Subsequently, he moved to the University of Electronic Science and Technology, China, to conduct postdoctoral research on electrochemistry from 2019 to 2021. Later, he was appointed to CNU from February 2022 to March 2023 to work on water splitting and batteries. Presently, he is a research assistant professor at Yeungnam University, South Korea since April 2023. His research interests are polymer membranes for electrolysis and anode materials for electrochemical energy storage applications.

Yern Chee Ching

Dr Ching Yern Chee is an associate professor within the Department of Chemical Engineering at Universiti Malaya. She achieved her PhD in Engineering from Universiti Malaya in 2011. Dr Ching embarked on her academic journey within the Faculty of Engineering at Universiti Malaya as a Senior Lecturer in the same year. Dr Ching is actively engaging herself in research. Her main research area is polymer composites, nanocomposites, and nanomaterials. To date, she has secured >RM 5 million in research grants and completed 35 research projects as the principal investigator. She has published >138 ISI-cited publication, 5 invited book chapters, and 6 filed patents. Her current standing of h-index is 33 (Web of Science). In 2022, Dr Ching was selected as one of UM researchers among the World's top 2% Scientists by Elsevier.

Tae Hwan Oh

Prof. Tae Hwan Oh is currently a professor at the Yeungnam University of School of Chemical Engineering, Gyeongsan, Korea. Before joining Yeungnam University, he worked in Huvis Co. as a principal scientist. He was Vice-Dean of the Yeungnam University College of Engineering. In 2013, the Korea Federation of Textile Industries presented him with an award. Yeungnam University rewarded him for his teaching skills in 2018.

Ho-Young Jung

Ho-Young Jung is a professor in the Environment and Energy Engineering Department at Chonnam National University. In 2007, he obtained his PhD in Chemical and Biomolecular Engineering at KAIST, Korea. Prof. Jung's current work focuses on developing advanced materials for fuel cells, redox flow batteries, ultra-batteries, and water electrolyzer systems. The Environmental Energy Materials Laboratory (EEML) researches are focused on developing advanced materials for applying energy conversion devices. Specifically, cation/anion exchange membranes with low cost and high selectivity are intensively studied and produced by the pilot casting machine in the lab for commercialization.

Mahaveer Kurkuri

Prof. Mahaveer Kurkuri obtained his BSc degree from MM Arts and Science College, Sirsi, India, in 1996; he then moved to the University of Mysore, India, to obtain his MSc degree in Polymer Science in the year 1998. His interests in nature and basic science inspired him to pursue a PhD from Karnataka University, India, in Chemistry, which he completed in 2003. Though he could secure a job soon after his PhD in CIPET (under Ministry of Chemicals and Fertilizers, Govt. of India, India), his interest in science made him pursue postdoctoral experience in international laboratories; thus, he moved to KAIST, South Korea in the year 2004. In early 2006, he relocated to Australia and worked at Flinders University, Australia, University of South Australia, Australia and The University of Adelaide, Australia, in various capacities as a research staff until 2014. He joined Jain University, India, in 2014 as an associate professor and is currently heading the Centre for Research in Functional Materials, Jain University, India, as Associate Director. His research interests are functional high surface area materials for various applications.

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1. Introduction

1.1 Significance of hydrogen

A high level of energy consumption has been seen in the twenty-first century, which along with a rise in world population, has increased the industrialization levels in areas such as automobiles, textiles, food, pharmaceuticals, biotechnology, nano-engineering, alloys, petroleum and oils, constructions, mining, telecommunication, manufacturing, energy, electronics, and agriculture.^1–19 Due to population and high industrialization, there is increased usage of fuels and other sectors (power plants, mills, feedstocks, refining, etc.) are responsible for the high emission of carbon dioxide (CO₂) into the atmosphere, which reflects the global warming issue.^20–22 CO₂ emissions from 1975 to 2015 increased dramatically from 15.4 MTon to 32.2 MTon, respectively.²³ Based on the Global Carbon Project report, Fig. 1 shows the share of worldwide CO₂ emissions from fossil fuels in the year 2021, with the majority of emissions coming from China, the United States, India, Iran, and Russia.²⁴ On the other hand, energy stored in batteries can be suitable for short-term usage. Still, they have disadvantages such as hazardous waste production, volume expansion if the battery is overloaded, high discharge rate, low current density, and greenhouse gas emissions.²⁵ Researchers seek alternative renewable energy for a pollution-free environment and low-cost energy production technologies. Accordingly, hydrogen (H₂) is considered one of the most eco-friendly renewable energy options.²⁶ It is believed to be the best way to replace fossil fuels such as coal and petroleum.²⁷ Water and hydrocarbons contain hydrogen, which is considered one of the most abundant elements available on Earth. Hydrogen is a significant material used in various industries such as fertilizers, metallurgical, and oil processing.^28,29 Being an energy carrier, the energy content in hydrogen is higher than gasoline at 298 K and is in a state where it emits no greenhouse gases; therefore, hydrogen is environment friendly.^30–32 Hydrogen is a critical energy source, which can generate energy and be stored for future use, thus alleviating the energy problem from using non-renewable resources. Furthermore, the automobile sector has been criticized for generating pollution. Efforts are being made to develop electric vehicles, and at the same time, hydrogen is considered an emergent energy source. As a result, electric and hydrogen-based vehicles contribute to reducing pollution. These two sectors generate hydrogen economy, which is regarded as a critical solution to the challenge of providing energy to all end users.³³ There are many benefits of hydrogen, such as the reduced emissions of greenhouse gases, a suitable alternative energy source, releasing vast quantities of water and energy during combustion, and eliminating the production of greenhouse gases as opposed to hydrocarbon-based gases.³⁴ For hydrogen to be produced by green processes, renewable energy sources must be used, and the method must be carbon dioxide-free. However, hydrogen is primarily generated by steam-reforming fossil fuels such as gasoline, propane, methane, diesel, ethanol, methanol, coal, oil, and other natural gases.^35,36 Steam reforming yields low-grade hydrogen with toxic carbon monoxide (CO) as the by-product. Hydrogen is also produced using high energy consumption methods (reforming, biological process, thermal, and biomass) and electrolytic, photolytic, and water-based methods.^37–41Table 1 shows the various methods of hydrogen production. A long-term hydrogen energy source among them is water, which is abundant on Earth. Therefore, CO₂ emissions can be reduced using hydrogen from water instead of fossil fuels. Moreover, the productivity of water-based techniques will play an important role in renewable sources of conversion and energy-based systems in the next era.

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1.2 Water electrolysis (WE)

Water electrolysis is a promising technique that produces high-quality hydrogen. The process involves electrochemical conversion that converts water into hydrogen and oxygen (O₂) using electricity.^47,48 Water electrolysis could perform a significant role in producing hydrogen using renewable energy, and this fuel is applied for heating and energy storage applications. Moreover, it is highly possible to store excess energy as hydrogen using water electrolysis, and this stored hydrogen can further positively be utilized in fuel cells to produce electricity. With fuel cells, the process is reversed; they are suitable energy converters that electrochemically convert fuels (hydrogen) into electricity and, as a by-product, produce water that can be used in water electrolysis. Eqn (1) shows the overall process, and eqn (2)–(5) show the process of water electrolysis in an acidic and alkaline environment.^36,49,50 Based on eqn (3), the theoretical water electrolysis voltage is 1.23 V. At the same time, there is a need for other potential differences to solve the potential losses in the WE process due to the kinetic restrictions in the reaction of electrodes. However, we can overcome this issue using active electrocatalysts, which kinetically enhance the WE process.⁵¹ It is possible to run the WE process in different routes at different pH levels. To this degree, HER uses acidic electrolytes in which H₂ is produced when H⁺ is reduced.

Similarly, the pH conditions affect the oxygen evolution reaction (OER) route. In this reaction, O₂ is produced by oxidizing two molecules of H₂O in acidic conditions. The OER process involves the oxidation of four OH⁻ ions to H₂O and O₂ molecules in alkaline and neutral conditions.⁴⁹ Nicholson and Carlisle primarily explained this electrolysis process in the eighteenth century.⁵² There are several benefits to producing hydrogen using water electrolysis, such as an easy process, environmental friendliness, and guaranteed product quality. In rural areas with abundant solar and wind energy, poverty-stricken villages can utilize this electrolysis method to generate hydrogen to meet their energy needs for lighting and heating the households, telecommunication stations, and small-scale level industries.^53–55 The hydrogen generated by water electrolysis is considered as the best energy carrier to produce power for industries, domestic, etc.

Reaction in the process:

2H2O → 2H2 + O2, E0 = 1.229 V	(1)

In an acidic environment:

Cathode (negative): 2H+ + 2e− → H2, E0 = 0 V	(2)

Anode (positive): 2H2O → O2 + 4H+ + 4e−, E0 = 1.23 V	(3)

In an alkaline environment:

Cathode (negative): 2H2O + 2e− → H2 + 2OH−, E0 = −0.83 V	(4)

Anode (positive): 4OH− → O2 + 2H2O + 4e−, E0 = 0.40 V	(5)

Water electrolysis is favorable in both conditions, such as acid and alkaline, because of the availability of water molecules in the deprotonated form for OER in alkaline solutions and hydrogen evaluation reaction (HER) in acid solutions. The production of hydrogen by water requires a minimum free energy of 237.2 kJ mol⁻¹ for the overall process of water electrolysis.⁵⁶ Industrial water electrolysis began in the 19^th century. Since then, electrolysis has grown to 400 and above operation units.⁵⁷ Three main water electrolysis types can be considered, namely, alkaline water electrolysis (AWE) and proton exchange membrane electrolysis (PEMWE), that generate hydrogen at low temperatures (less than 90 °C), and solid oxide electrolysis (SOE), which is performed at high temperatures (700 to 950 °C) and is under development because a low voltage-based cell is required.^45,57 To this degree, alkaline electrolysis is a well-known technology for hydrogen production up to the megawatt scale, and it is the most employed electrolytic technology worldwide. The main challenges in this electrolysis is the corrosive electrolyte and minimal current densities because of the mild mobility of hydroxide (OH⁻). A few specialty applications of PEMWE can be found in the market with decent performance and stability. The disadvantages of this electrolysis are the requirements of specific materials to operate the corrosive acidic cell and expensive noble metal catalysts (Ru, Pt, and Ir). SOE has attracted special attention because it converts electrical energy into chemical energy, producing high-purity and efficient hydrogen. However, SOE needs high temperature and pressure to operate cells, has low stability, and leads to degradation of electrodes.⁵⁸ On the other hand, anion exchange membrane water electrolysis is a low-cost and ongoing efficient technology that can replace PEMWE, AWE, and SOE for producing hydrogen due to its ability to operate at high pressures and its reliance on anion exchange membranes (AEMs) and non-noble metal catalysts.

1.3 Anion exchange membrane water electrolysis (AEMWE)

AEMWE is an innovative hydrogen generation technology that is booming in electrolysis technology. Due to their many benefits, such as low cost and high performance, research on these processes is currently being conducted by various organizations and institutions. Because of this, while browsing scholars' sites, only a few articles have been found to be published on AEMs-based water electrolysis systems compared to alkaline and proton exchange membranes.^{23,27,29,59,60} AEMWE produces hydrogen via electro-catalytic water splitting (Fig. 2). It has several advantages compared to AWE's operation in alkaline conditions. In a corrosive environment (highly concentrated KOH) for hydrogen production, AEMWE can operate with less than 1 M concentration of KOH solutions, compact design, and ion conductor. The separator is easy to switch in AEMWE.⁶¹

The electrochemical cells in this water electrolysis utilize hydrocarbon AEMs and transition-type metal electrodes, and the electrolytes can be either water or a slightly less-concentrated alkaline solution. Moreover, its operation in an alkaline environment uses inexpensive metal catalysts for OER and HER at the anode and cathode, respectively.³² The catalytic activity of platinum (Pt) for HER and iridium (Ir) in OER is exceptional, but the metals are expensive. Also, low-cost and abundant transition metal electro-catalysts can be used instead of those noble metals. Therefore, these low-cost components of AEMWE significantly reduce the electrolyze system's production cost, making it more suitable for commercialization. Generally, AEM-based technology could be low-cost and incredibly stable for hydrogen generation. AEMWE's working performance must match or beat PEMWE's characteristics, such as hydrogen quality, lifetime, high current density, and clean water operation. AEMWE system is similar to PEMWE, constructed in a heap with membranes to offer higher energy density. It is compatible with several renewable energy sources, such as solar and wind.⁶² This electrolysis process is based on alkaline conditions with the functional groups immobilized on polymer chains as the interface of the membrane and, for the development of AEMWE systems, should be considered significant on account of membrane stability, ionic conductivity, and integration of catalysts into AEMWE systems. In recent years, the literature has focused on the progress of catalyst-based fabrications rather than anion exchange membranes. There are few research reports on AEMWE cell performance using water until 2021. However, researchers are working continuously to develop AEMWE for hydrogen production.

1.4 Anion exchange membranes (AEMs)

A wide range of materials for a variety of applications was developed, including silica, carbons, layered double hydroxide materials, perovskites, metal–organic frameworks, covalent organic frameworks, nanomaterials, zeolites, membranes, thin films, aerogels, xerogel, and clays, which were used for various applications such as energy, medicines, sensors, water treatment, solar cells, optoelectronics, and CO₂ conversion.^63–84 Among them, membranes are made of polymers, making them highly flexible, hydrophobic, high-performance, mass-produced, economical, and reliable for specific applications.^85–87 Accordingly, an anion exchange membrane (AEMs) is a semi-permeable membrane with ionic head groups attached to polymer substances.^88,89 The membrane's positively charged head groups repel cations, allowing anions to bind.⁹⁰ Furthermore, AEMs can be classed based on what types of anions pass by them; accordingly, AEMs give on non-alkaline forms of anions (such as PO₄³⁻, Cl⁻, and SO₄²⁻) and alkaline-AEMs pass on alkaline form of anions (such as OH, CO₃²⁻ and HCO₃⁻).⁸⁸ Nowadays, AEMs research is highly focused on developing alkaline anion exchange membranes for high-pH applications, anion exchange membrane fuel cells (AEMFC), anion exchange membrane water electrolysis (AEMWE) for high-temperature, and other important applications such as electro dialysis, acid recovery, redox flow battery, and dye removal.^{32,88,91–96}

AEMs are one of the most important components of AEMWE. Its main function is to transfer OH⁻ ions from the cathode to the anode chamber, limit gas movement, and transmit electrons during electrochemical reactions.⁹⁷ AEMs consist of a hydrocarbon polymer and an anion exchange functional group as the main and side chains. An AEM's main backbones are the polymeric materials normally used, such as polystyrene (PS) and polysulfone (PSF), to link divinylbenzene (DVB) and the relative ion-exchange groups that have ammonium or phosphonium groups.^27,98 For WE applications, the ion-exchange membrane must have good permselectivity, ionic conductivity, thermal and chemical stability, mechanical stability, etc.^99,100 It is still relatively challenging to fabricate AEM for useful applications to stabilize the tricky issues of decent mechanical stability and high ionic conductivity. Moreover, excess water may diminish the polymer's mechanical strength, while the polymer's chemical stability is not guaranteed, given the possibility of attack by hydroxide ions.^27,101,102 Besides, AEM's chemical stability is low at higher pH, a major concern.^29,103 There are two mechanisms, namely, Hofmann elimination, which involves degrading the anion-exchange groups under a basic environment through nucleophilic attack of OH⁻ ions on N-alkyl groups at high temperatures,¹⁰⁴ while the second degradation mechanism involves electrochemical oxidation.¹⁰⁵ Fluorocarbons replace hydrocarbon-based AEMs in WE applications to solve the above-mentioned problems. Incorporating fluorine atoms into the polymer's backbone is more desirable. Fluorocarbon-based polymers may exhibit exceptional properties, such as high stability in alkaline environments, high thermal stability, high hydrophobicity, high microphase separation, mechanical durability, low swelling, and high conductivity due to the strong C–F bond and low polarizability, which can be attributed to fluorine's smaller size and greater electronegativity.^106–108 More details on fluorocarbon-based membranes can be seen in section 2.2.

There have been recent developments in AEMs materials such as poly(arylene ether), polytetrafluoroethylene, poly(olefin), poly(phenylene), poly(arylene ether ketone), poly(vinyl alcohol), poly(ethylene-co-tetrafluoroethylene), poly(phenylene oxide), poly(aryl piperidinium), poly(ether imide), polysulfone, and cardo polyether ketone, and the conductivity of hydroxide ions is high in these polymers.^89,109–121 Despite this, most studies employ commercial membranes with membrane processes and applications.^122–127 Tokuyama A201, Fumasep FAA-3, NREL Gen 2, and Sustainion are all examples of commercially available membranes based on imidazolium or quaternary ammonium polymers; nevertheless, Sustainion was determined to have the highest performance for AEMWEs.⁶¹ However, due to the limitation of AEMs, there are important concerns for the long-term stability of AEMWE systems and the temperature. Accordingly, comprehensive research has been initiated to develop AEM materials with higher thermal and chemical stability in alkaline environments. Despite that, as novel AEMs materials are proposed and developed, they might improve hydrogen energy production. Scopus data shows that publications on AEMs in synthesis have steadily increased since 2010 (Fig. 3). But only a small amount of synthetic AEMs is used in water electrolysis. Since 2012, the number of publications has been slowly growing, and more studies may be done in the future. The AEMs' goal in water electrolysis is to replace commercial AEMs, make a low-cost process, speed up the movement of ions from the cathode to the anode, and get a high rate of electron transfer in the electrochemical process so that hydrogen can be made from water. Several AEMs have been developed, and their advantages and disadvantages are listed in Table 2.

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Recent reviews of an AEM focused primarily on fuel cell applications, an electrocatalyst for AEMWE, the technical evaluation of a water electrolysis cell, alkaline water electrolysis for H₂ generation, the economic analysis of H₂ production through electrolyzer technologies, the design of a catalyst and electrode for seawater electrolysis, and the challenges and future opportunities of a component in AEMWE.^{27,29,32,45,49,50,55,57,58,150–162} In addition, there are reviews on AEMs such as molecular design and microscopy analysis of membranes for fuel cell and water electrolysis, dealing with alkaline durability of imidazolium-based AEMs, synthesis of ion-exchange membranes for various applications, synthetic approaches for alkaline-stable AEMs, Friedel–Crafts reaction-based synthesis of AEMs for fuel cell and electrolysis applications, click chemistry in AEMs for energy-based applications, the significance of polymer ion exchange membrane structures and properties for hydrogen production by water electrolysis, synthesis of silica-AEMs composite, and anion exchange polyelectrolytes for the synthesis of AEMs and ionomers.^{50,88,145,160,163–173} However, no comprehensive review of AEMs from synthesis to application through physiochemical analysis and evaluation of properties has been identified in recent studies. This review is distinct from other contemporary works because it thoroughly reviews the synthesis of AEMs, such as hydrocarbon and fluorine, with choices of homogeneous (organic/organic) and heterogeneous membranes (organic/inorganic) to their chemical structure of backbones and functionalization of cationic groups and side chains. Furthermore, several analytical approaches were used to explore the structure of AEMs and their important attributes, such as ionic conductivity (IC), ion exchange capacity (IEC), water uptake and swelling ratio, mechanical and alkaline stabilities, and their application in water electrolysis. The synthesized AEMs and other commercial AEMs used in hydrogen production have been discussed and tabulated regarding cell voltage, current density, and durability. Further, an electrocatalyst is also an important component in water electrolysis for hydrogen production. Accordingly, the performance of various electrocatalysts in AEMWE is also discussed. The review aims to discuss the current reports to fill the research gaps of AEMs; hence, research areas in which more exploration and improvements are highly needed. Finally, we concisely conclude the topic and confer the challenges and prospects for the imminent development of AEM for WE application. The authors are hopeful that this review will be helpful to researchers who employ AEMs in their work on water electrolysis. Fig. 4 shows the overall review in a schematic diagram of the synthesis, properties, and application of AEMs for water electrolysis.

2. Synthesis of AEMs

AEMs pose many challenges in properties such as mechanical stability and ionic conductivity. It can be made more ionically conducting by adding different ion-exchange groups, resulting in too much water uptake. As a result of the high water uptake, the mechanical strength may decrease. Most AEMs are synthesized in the process involving chloromethylation, such as poly(styrene)/poly(sulfone), and benzyl chloride is quaternized with a tertiary amine to produce the cationic group quaternary ammoniums. Preparing AEMs by chloromethylation and quaternization is advantageous because the membrane is highly chemically and physically stable. The chloromethylation reaction of the polymer requires a chloromethylation agent and a Lewis acid catalyst. The generally used chloromethylation agents are chloromethyl methyl ether or bis-chloromethyl ether, which offer excellent conversions and yields. Quaternary ammoniums (QAs) have been studied widely for AEMs because they easily integrate into the membrane and produce gentle conductivity at low temperatures. In contrast, they are considered chloromethyl methyl ether (CMME (chloromethylating agent)) and trimethylamine (TMA (QAs agent)) carcinogens, and this kind of synthesis was well known for AEMs. New paths and materials for synthesizing AEMs are enabled, and accordingly, we have classified the membranes based on the types.

2.1 Hydrocarbon-based AEMs

Hydrocarbon-based polymers provide several benefits, including low-cost processes, synthetic flexibility, reduced gas-crossover fluxes, and eco-friendliness.¹⁷⁴ In most cases, chloromethylation, followed by quaternization, is the order of procedures that must be followed to prepare a hydrocarbon-based anion exchange membrane successfully. This approach has some advantages, not the least of which is that the membrane that results from using it has excellent physical and chemical stability. There is a possibility that large conversions and yields may be obtained by chloromethylation using chloromethylation agents such as chloromethyl methyl ether, chloromethyl octyl ethers, chloromethyl ether, and bis-chloromethyl ether.¹⁷⁵ On the other hand, they are considered hazardous to the environment. Therefore, researchers are putting more effort into finding alternatives that use non-toxic chloromethylation compounds or developing effective membranes that can be created without these agents. Examples of recent works that address developing effective membranes without utilizing these agents can be found in the following sections. In synthesizing AEMs, most membranes were prepared using techniques such as polymerization and cross-linking, phase-inversion, irradiation or grafting, and other chemical reactions to modify the polymers.^88,176 The polymer solutions are typically dispersed and dissolved in polar solvents, then cast onto the glass or Petri dish plate and dried in an oven to prepare the membrane. The subsequent section is based on the functionalization of various cations on AEMs, and it is followed by a subsection describing organic-inorganic and organic–organic composite-based hydrocarbon AEMs.

2.2 Fluorocarbon-based AEMs

Fluorine-based molecules are an essential class of aromatic complexes, and they have received significant interest for their distinctive physical and chemical properties. The substantial introduction of fluorine into the polymer backbone is considered a technique for structural modification that leads to considerable property improvement such as chemical stability, and high conductivity. For the past several decades, hydrocarbon AEMs have been developed; however, they are not very successful in terms of conductivity, durability, chemical stability, etc. Alternatively, fluorine is an aromatic component that is favored for use in the process of enhancing the properties of hydrocarbon membranes. They continue to be preferred by researchers due to their high chemical, high conductivity, and high mechanical stability. As a result, the following fluorocarbons are discussed.

2.3 Modification of commercial AEMs

Modifying cheap, commercially available membranes to synthesize ion-exchange membranes by grafting is an efficient, cost-saving method, and it has received much attention. Wang et al., therefore, applied a commercially available ethylene-tetrafluoroethylene (ETFE) membrane grafted with dimethylaminoethyl methacrylate (DMAEMA). Later, the grafted membrane was treated with γ-ray, and the treated membrane was further soaked with 1 mol L⁻¹ HCl to receive quaternary ammonium for further applications.³⁰⁸ Similarly, Garcia-Vasquez et al. used a commercial membrane (AMX-SB), and it was modified with the paste of poly(vinyl chloride) (PVC), divinylbenzene (DVB), and styrene (S) to produce stacked membrane.³⁰⁹ Further, the quaternary ammonium groups are introduced with chloromethylation, followed by an amination process. These typical AEMs could be a low-cost process. Another group studied commercially available A201 membrane without modification and tested it in a preliminary study for water electrolysis.³¹⁰ The membrane thickness is 28 μm and contains hydrocarbon and QAs functional groups. The study examined how the dimensions and conductivity of the membrane changed with changes in the temperature and water absorption.

Most conventional membrane synthesis protocols involve multi-step synthesis, which raises the overall cost. AEMs can also be fabricated from commercially available membranes using radiation grafting, an emerging method of synthesizing their functionalized forms. Accordingly, polyethylene with a thickness of 15 μm of commercially available AEMs was obtained from Goodfellow, UK, which was further irradiated and then grafted to achieve a sub-30 μm membrane by Wang et al.³¹¹ Specifically, the electron beam was generated at 4.5 MeV using a Dynamatron unit (Fig. 21a). This activates the membrane by producing peroxy groups covalently attached to the polymer chains. Dry ice was used after irradiation to store the membrane at −40 °C. The irradiated membranes were placed in glass vessels and further soaked in a mixture of vinylbenzyl chloride (VBC) and 1-octyl-2-pyrrolidone solutions, followed by N₂ purging for 2 h with the vessel sealed tightly. Afterward, the grafting procedure was performed by heating at 40 °C for 6 h. Then, amination was achieved by soaking the membrane in TMA solution for 24 h and washing it with high-purity water. In the end, chloride (Cl⁻) ions were formed by the membrane being soaked in NaCl solution, and finally, the final grafted membrane containing the chloride ions was stored in pure water until use. It was found that the modified membrane was highly robust. However, using high-end e-beam sources and after irradiation, the membrane stored at shallow temperatures (−40 °C) remains a costly and time-consuming process. Similarly, Mahmoud et al. studied a commercial ethylene-tetrafluoroethylene (ETFE) membrane irradiated using ⁶⁰Co radiation as a source.³¹² After irradiation, the membrane was treated with vinylbenzyl chloride under an argon atmosphere, soaked in TMA for 24 h, washed with water, and dried to obtain irradiated and grafted AEMs in the Cl⁻ ion form. As a result of using high-end radiation sources to irradiate the membrane, processed commercial membranes appear to be moderately expensive.

Similarly, Marino et al. explored the bromide form of bare membrane FAA-3 obtained from Fumatech GmbH. Anion exchange was accomplished by treating the membrane with 1 M Na-(Cl, F, OH) and NH₄I for 48 h.³¹⁴ Recently, Hossain et al. developed sulfonium-based AEMs from commercially available poly(ether sulfone) (PES) by initially introducing the chloromethyl groups, followed by functionalization with dimethyl sulfide.³¹³ The corresponding AEMs were obtained by anion exchange with 1 M KOH solution concentration, and the chloromethylation reaction was carried out with chloromethyl methyl ether and thionyl chloride (Fig. 21b). The catalyst ZnCl₂ was used as the Lewis acid. The impact of side-chain structures and spacer on membrane performance was studied recently by Li et al., who developed a series of PPO AEMs with different side-chain structures and spacer.¹¹³Fig. 21c depicts the preparation method of several side-chain structures on quaternized PPO AEM and the presence of a spacer group. In another work, alkaline doped polymers possess high hydroxide conductivity and chemical stability in an alkaline solution. Therefore, a commercially available polymer solution diluted with DMAc was recently cast on glasses and dried at different temperatures (90, 120, 170, and 190 °C).³¹⁵ The dried membranes were then doped with KOH to study the membrane properties further. This type of polymer solution obtained from the market to prepare AEMs could be cheaper than processing commercial AEMs with several steps.

2.4 Summary

For the quaternization process, the conventional method of dispensing polymers directly in QAs solution has significant limitations, including unpredictable reactions and undesired QA aggregates. Alternatively, cross-linking methods could be a better option to overcome these issues and improve the chemical and physical properties of AEMs. Most polymer synthesis procedures utilizing the toxic chloromethylation chemicals CMME, CMOE, CME, TMCS, and thionyl chloride are harmful to the environment. Therefore, researchers focus on using para-formaldehyde and trimethylsilyl chloride, the two non-toxic chloromethylation agents. Compared to conventional chemical crosslinking, the in situ crosslinking of a polymer solution via heat treatment may be a less expensive approach to create a self-crosslinking membrane. Compared to TMA-functionalized membranes, most imidazolium-functionalized membranes did not employ a chloromethylation agent during membrane manufacture, and imidazolium-based functionalization may be shown to use low-cost methods. Greener cations based on ionic liquids (ILs) may replace hazardous cations like TMA. Creating poly(aryl piperidinium)-based AEMs using polycondensation is straightforward, environmentally benign, and reasonably priced. It does not need any crosslinking, chloromethylation, or quaternization.

Most organic polymer membrane synthesis involving highly toxic organic solvents may harm the environment and humans. On the other hand, chitosan's naturally occurring hydroxyl and amine groups enable it to be quaternized without chloromethylation. In addition, poly(vinyl benzyl chloride) (PVBC) polymer contains naturally occurring chloromethyl classes and avoids the use of toxic chloromethyl ether agents. Therefore, future research focusing on these polymer types would be highly advantageous and cost-effective and reduce the use of hazardous organic solvents in membrane preparation. A hybrid method is typically used to produce composite membranes with effective ion channels. The uniform dispersion of inorganic particles may be difficult in organic/inorganic composite membranes, but this composite type would be advantageous for anti-fouling. In addition, future research should analyze the properties of inorganic particles composited with organic polymers for application purposes. Due to the membranes' physical, thermal, and chemical properties, fluorine incorporation in the hydrocarbon may be advantageous. However, the fluorination-based synthesis process is expensive due to the increased number of synthesis stages and the use of costly compounds. Compared to hydrocarbon membranes, fluorinated membranes are covered in fewer articles in the scientific literature. Therefore, future research on fluorinated polymers should focus on low-cost processes, and an effective fluorinated membrane is urgently required. Reinforced membrane fabrication is an excellent method for enhancing the membrane's structural stability; however, the reinforced membrane's thickness must be regulated, and the porous substrate must be coated uniformly. There are commercially available membranes, but they require additional processes such as surface modifications, quaternization, and ion exchange. Using commercial membranes increases the cost of this procedure. Ultimately, it was determined that numerous polymer synthesis processes involve multiple stages and could be time-consuming and costly. Therefore, it is recommended to synthesize a polymer with few straightforward steps as this is the most cost-effective method.

3. Characterization of AEMs

AEMs must be characterized to demonstrate that the structural data, such as organic functionalization, microstructure, morphology, hydrophilic–hydrophobic separation, and temperature stability, is applicable before application. Because AEMs cannot be recognized without analytical research, they must be characterized. In light of this, characterization methods such as small-angle X-ray scattering (SAXS), atomic force microscopy (AFM), proton nuclear magnetic resonance (¹H NMR), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA) were discussed. The physical, chemical, and thermal properties of AEMs were studied and analyzed in this section using a specific characterization technique. Based on the functionalization and preparation, it was also described how the structural, morphological, and other characteristics of AEMs changed. Therefore, the characterization section may be useful to researchers new to developing and studying AEM since it simplifies understanding its physiochemical features (Fig. 22).

3.1 Small-angle X-ray scattering (SAXS)

The microstructure of the AEMs has a significant impact that involves ion conduction in the membrane. Accordingly, manufacturing microphase-separated morphology (MSM) is essential for a high-conductivity AEM with MSM, and it is a practical approach to promote AEM's ionic conductivity. The hydrophobic backbone and the spacer linked with the hydrophilic cationic will likely form the membrane with MSM. The microphase separation of AEMs effectively influences their conductivity. Various techniques can be used to analyze the microphase separation of the AEMs. SAXS is one of the specific techniques for analyzing the structural properties of membranes. It is a tool used to study the morphological performance of AEMs, i.e., this technique was used to analyze the MSM of the membrane. The q value obtained from SAXS corresponds to the ion cluster size, and the diffraction peaks of SAXS reveal the membrane's MSM.¹¹³ Accordingly, SAXS was used to analyze the morphology of the p-quaterphenyl-having poly(aryl piperidinium) membrane (PQP-100).²³⁵Fig. 23a revealed that the MSM was confirmed via a broad peak formed at 3.21 nm⁻¹ (q value). This technique was also used to evaluate the membranes' ionic aggregation (formation of typical scattering peaks) and high ionic clusters (indicated by the highest Bragg's “d” spacing value).²³¹

Recently, Li et al. studied the PPO AEMs performance by adjusting the side-chain structures and the impacts of alkoxyl or alkyl extenders and spacers.¹¹³ They analyzed the various AEMs (DQEC, DQ, SDQ, and SDQEC) by SAXS that did not appear as apparent diffraction peaks due to not having a microphase separation. Among AEMs, SDQEO (alkoxyl extender and dual QAs were connected to the main chain along with a spacer) shows diffraction peaks due to having the presence of MSM (Fig. 23b). For this morphology, SDQEO could be a high ion conductivity. The piperidinium treated with aryl ether-free-based AEMs (PTP-75, 85, and 90) shows no significant ionic peaks due to the indiscriminate distribution of piperidinium groups on the main polymer backbones.²³⁴ Another study developed the microphase separation by the poly(biphenyl piperidinium) (PBP)-based AEMs (PBPCL-0.69, 1.29, 1.48 and PBPIL-55) structure containing the hydrophilic octopus-like side chain and the hydrophobic fluorinated moieties.²⁸⁸ The PBP containing a combination of side chains β-cyclodextrin (β-CD) and piperidinium ionic liquids (PBPCL) shows larger d-spacing and better MSM than PBP containing only piperidinium ionic liquids (PBPIL). According to this study, AEMs analyzed by SAXS have higher d-spacing, indicating a larger ion cluster and significant MSM (Fig. 23c).

Similarly, electrospinning and hot pressing methods obtained well-fabricated MSM structure for QAs-functionalized polyketone-based AEMs (QAFPF-1-n-E), and the calculated d-space values were obtained in the range of 9.1–10.8 nm.¹³³ However, in the case of QAs-functionalized polyketone (QAFPK-1-n-C) produced by casting, followed by the stripping method, low d-values (5.7–6.3 nm) were obtained, indicating that it could be a less microphase separated structure, and these higher and lower d-values specify that the ionic cluster of QAFPK-1-n-E is better than that of QAFPK-1-n-C (Fig. 23d). AEMs developed with aryl-ether-free backbones, long flexible alkoxy-containing bis-piperidinium side chains, and hydrophobic alkyl chains spacer (PISBr-NON) show characteristic diffraction peak at 1.6 nm⁻¹, indicating that this membrane has a good microphase separated structure.²²⁷

QAs-containing PPO-isopropyl AEMs showed a broad scattering peak (ionomer peak) at 1.9 nm⁻¹ due to the growth of ionomer aggregation.²³² No ionomer peak was observed for PPO containing n-propyl and cyclohexyl AEMs (Fig. 23e). The case of hydrophilic dual alkoxy side chains-grafted QAs-PPO shows more substantial diffraction peaks when compared to dual-alkyl side chains-grafted QAs-PPO and bare QAs-PPO.³¹⁶ Additionally, the d-spacing values of alkoxy side chains-grafted QAs-PPO are higher (11.9–13.8 nm) than others, indicating a well-long-scale ordered MSM (Fig. 23f). The reason could be that the grafting hydrophilic side chains (alkoxy) can stimulate MSM and the growth of ion conduction paths. Recently, PEG with molecular weight of 2000 with various percentages (1–5%) was composited with Q-PPO, showing that the broad diffraction peak was increased as the PEG weight percentage increased.²⁵⁵ These diffraction peaks indicate that all AEMs have MSM; thus, PEG 2000 could effectively induce the MSM in the membranes while compositing with PPO (Fig. 24a). The rigid backbone polymer structure and using a high dosage of guanidinium salt in the main polymer backbones do not give rise to significant ionomer peaks.²⁹³ Thus, the mechanical properties of the polymer backbone and the dosages of salt used for quaternization are essential in forming MSM.

Similarly, the ionomer peak (q = 0.18 nm⁻¹) was observed for the photoinitiator-mediated crosslinked (1,8-octanedithiol)1-vinylimidazole-functionalized PPO composite with PVA (crosslinked VImPPO-PVA), indicating that this AEM has moderate MSM.¹²⁸ Additionally, the d-spacing value found for these AEMs was 34.9 nm. However, these crosslinked AEMs do not show a long-level ordered MSM based on the obtained SAXS peak (Fig. 24b). In contrast, no more ionomer peak was found in the uncrosslinked VImPPO-PVA. Therefore, the 8-octanedithiol crosslinker could have the effect of producing an MSM in AEMs. Another study shows three types of crosslinked naphthalene-poly(vinylbenzyl chloride)-based block polymer AEMs with the ratio of 1:6, 1:8, and 1:10, named NAPEK-PVP-6-Q4, NAPEK-PVP-8-Q4, and NAPEK-PVP-10-Q4, respectively.²³⁹ Among AEMs, a more substantial scattering diffraction peak (q) at 0.52 nm⁻¹ with a d-spacing value of 12.08 nm was obtained for NAPEK-PVP-10-Q4 AEMs (Fig. 24c). It is revealed that the membrane containing naphthalene and a high ratio of hydrophilic polymer (1:10) backbone stimulates the ion clusters for membrane performance compared to low ratios of naphthalene and PVP (1:6 and 1:8) used to prepare AEM. The N-alkyl substitution on naphthalene-based PBI-QAs (e.g., NPBI-QAx-By) AEMs shows better microphase separation, confirmed through scattering ionomer peaks (2.7–2.9 nm⁻¹) with corresponding d spacing values (2.15–2.31 nm).¹⁸⁸ At the same time, no scattering peaks were observed when QAs separated from the naphthalene-based PBI polymer through an alkyl linker (e.g., NPBI-QAs). It could be the reason for the inter-chain hydrogen bond between the polymer backbones (NPBI) in the alkyl linker procedure to separate the QAs from the polymer, hindering the ionic aggregation (Fig. 24d). However, the N-alkyl substitution reaction greatly promotes the QAs on NPBI, and this procedure avoids the inter-chain hydrogen bond between the polymers. Thus, microphase separation was enhanced by the ionic aggregation through N-alkyl substitution reaction through QAs-functionalized NPBI AEMs.

Another study shows that C–NH₂ groups/linkages in the main polymer backbone generate hydrogen bonds in the AEMs, which could enhance the conductivity and mechanical strength. Accordingly, QAs-poly(arylene ether ketone) functionalized with C–NH₂ (QA-PE-NH₂) shows more substantial ionomer peaks (higher ion clusters) when compared to QA-PE-functionalized CO (QA-PE-CO).¹⁸⁹ The higher ion clusters in QA-PE-NH₂ AEMs were found to be hydrophilic networks (hydrogen bonds) that are spread throughout the membranes and could stimulate the growth of ion clusters to larger clusters (Fig. 24e). The effect of fluorine in SEBS membranes functionalized with hexyl QAs was studied for the MSM.¹³⁹ The fluorine content in the polymer backbone clearly showed high scattering peaks (high ionomer aggregation), confirming the long-scale ordered MSM in the membranes compared to hexyl QAs-functionalized SEBS. The increasing hydrophilic regions and d-spacing values with increased ionic group content were found in the membranes (3Q-PAP), attributed to the aggregate ionomer for the conductivity.²⁹⁵ Additionally, the d-spacing is minimal, and no scattering peak was detected for the low ionic content of AEMs. A highly enriched MSM/ion cluster was found in ionic liquid-crosslinked AEMs with fluorine-based polymer main backbones and long flexible hydrophilic multi-cation as side chains (e.g., TQ-PDBA-x).²⁸³ The obtained various scattering peaks “q” position (1.029–0.729 nm⁻¹) and “d” spacing values (6.1 to 8.6 nm) suggest that the invented AEMs have microphase separation and relative ion domain, which has a size that can help the formation of the ion cluster for the enhancement of AEMs performance for ion conductivity (Fig. 24f).

3.2 Atomic force microscopy (AFM)

AFM can identify the microphase-separated morphology (MSM) images of AEMs by the hydrophobic and hydrophilic areas, which are identified by the bright and dark regions, respectively. Hydrophobic and hydrophilic separation in membranes helps transport ions and dimensional strength. Highly developed hydrophilic/hydrophobic microphase separation in membranes can create better pathways for ion transport. Accordingly, the hydrophobic (bright) and hydrophilic (dark) areas found in the p-quaterphenyl have poly(aryl piperidinium) membranes (PQP-100).²³⁵ On the other hand, PBP-67 and PTP-83 show no such dark and bright regions that they do not have a microphase separation (Fig. 25a). The bulky isopropyl-grafted QAs containing PPO (PPO-iP) AEMs have enriched microphase separation, confirmed through the hydrophilic (dark) and hydrophobic region (bright).²³² However, there is no such dark, and AFM images do not observe bright areas for the bulky cyclohexyl (PPO-CH) and n-propyl (PPO-nP)-grafted QAs containing PPO AEMs (Fig. 25b). The spacer and lengthier alkoxyl coordination is helpful in microphase separation. Accordingly, PPO-based membranes observed the performances (microphase separation) based on their structural functionalization with different side-chain and the effects of alkoxyl or alkyl extenders and spacers (the connection between cation and polymer backbone).¹¹³ At this point, PPO-based membranes containing alkoxyl-extender-having dual QAs connected with the main chain and a spacer (SDQEO, SDQ, and SDQEC) show healthy microphase separation (hydrophilic (dark) and hydrophobic (white) regions) in the AFM images when compared to PPO-based membranes (DQEO, DQ, and DQEC) without the coordination of an alkyl extender and spacer (Fig. 25c).

Adjusting the degree of quaternization could affect obtaining a well-arranged microphase morphology. Accordingly, the poly(vinyl chloride) (PVC) membrane was quaternized with triethylenetetramine (TETA) with the degree of quaternization (4, 8, and 14 h) observed under moderate conditions.¹²⁹ At this point, a minimum of 8 h is needed for the quaternization of PVC to obtain a well-separated microphase morphology. Therefore, PVCs treated for 8 and 14 h quaternization time with TETA show better MSM than that treated for 1 h quaternization time, as confirmed through the AFM images. Another researcher observed that no MSM formation was found in the study of adjusting the viscosity of piperidinium-grafted aryl ether-free AEMs.²³⁴ It could be the reason for the unintentional distribution of the hydrophilic nature of piperidinium clusters on the backbones. Therefore, the adjusting viscosity does not benefit producing MSM in membranes.

Similarly, well-phase separated structured hydrophilic–hydrophobic morphology was observed for the linear (e.g., AEM-LCP) and star-shaped polymer (e.g., AEM-SCP) backbone polyisoprene-based AEMs.¹²⁷Fig. 26a shows various scale-resolution AFM images of the hydrophilic (black region) and hydrophobic (white region) separation of the AEM-SCP and AEM-LCP. Polyketone-based nanofiber AEMs (e.g., QAFPK) show good mobility and conductivity, enabling high interactions between cationic functional domains (hydrophilic) and hydrophobic polymer backbone that allow a healthy MSM structure. The authors claimed that the high conductivity polymer possesses well MSM.¹³³ Higher hydration could increase the smoothness on the surface of the PPO membrane, and the polymer chain (alkoxy) mobility tends to enhance the MSM.³¹⁶ At this point, the smaller hydrophilic clusters (dark regions) were observed for the membranes, e.g., QPPO17 and 16C25-10C25. On the other hand, alkoxy with extra side chains grafted on the polymer backbones enhanced the hydrophilic cluster formation for the 16C25-3O25 membrane. In contrast, the extender and other side chains led to a larger hydrophilic cluster observed in the 3O25-16C25 membrane (Fig. 26b). Therefore, alkoxy groups and extender with side chains could enhance the higher MSM. Well-preserved MSM structures are visible in hydrophilic (dark) and hydrophobic (bright) sections, such as the polymer backbone (VImPPO) and cross-linkers (1,8-octanedithiol), respectively, in a mechanically strengthened photoinitiator mediated on crosslinked VImPPO-PVA (e.g., sIPN-91).¹²⁸ However, the VImPPO and PVA composite membrane (e.g., n-64) did not exhibit any such active hydrophobic and hydrophilic areas in the absence of a crosslinker (Fig. 26c). Therefore, the crosslinker could be a crucial component in creating membrane microphase separation.

Recently, Zhou et al. studied MSM on polyfluorene piperidine (PBBP)-based AEMs. To this end, PBBP was crosslinked with DBMHDA, BAPTES, and BPPO in molar ratios of 0.1, 0.2, and 0.3 and then quaternized. PBBP was also studied without crosslinking for comparison sake.³⁰¹ Unlike BAPTES and BPPO crosslinking, effective MSM was found for a 0.3 molar ratio of DBMHDA-crosslinked PBBP quaternized membrane. AEM based solely on PBBP, however, did not exhibit any MSM. Hexyl QAs functionalized (HQAs) with mono and penta-based fluorobenzoyl modifications on SEBS AEMs recently yielded nicely organized MSM.¹³⁹ On HQAs-functionalized SEBS, however, MSM was not seen to perform any better. β-Cyclodextrin has a surface with many hydroxyl groups, making it ideal for roping side chains. The hydroxyl groups on the edge of the β-cyclodextrin may aid in causing the ionic groups to aggregate and provide an effective microphase separation (MPS). Therefore, the combination of ionic liquid and β-cyclodextrin-functionalized poly(arylene piperidinium) (PBP) AEM led to higher MSM as compared to only ionic liquid-functionalized PBP.²⁸⁸

It was discovered that adding a fluorinated moiety to the polymer enhanced the membranes' abilities to separate microphases, have minimal swelling, and maintain chemical stability, among other properties. The main chain uses a hydrophobic fluorine-contained polymer with unique functional sites. In the meantime, a multi-cation hydrophilic ionic liquid was used as the side chain to enhance the dimensional stability and ionic conductivity. Accordingly, diallyl bisphenol A poly(arylene ether) partially crosslinked with long flexible multi-cation ionic liquid-prepared AEMs shows high MSM.²⁸³ Friedel–Crafts polycondensation with coupling procedure may be used to produce aryl-ether-free QAs polyaromatics and stop the degradation of the polymer. Thus, ether-bond-free polyfluorene (PBF)-based AEMs encompassing stretchy alkyl pendant chains ended with piperidinium cations groups show high chemical stability, high ionic conductivity, and well-ordered MSM structure.²⁹¹ Self-assembly and microphase separation are caused in the membrane due to hydrophilic differences between the polymer backbone and the long stretchable ionic side chains. Additionally, the formed hydrophilic channels can improve ion conduction, thus adversely impacting the characteristics of AEMs. Therefore, side-chain-type poly(biphenyl-N-methyl piperidinium) (PBPip) AEMs containing bis-cation filaments with an extended stretchy methylene spacer shows high microphase separation, i.e., a separate phase separation morphology in the membrane may be created by including bendable side chains further into the polymer and increasing the concentration of ionic groups on the side chains.²⁹⁸

3.3 Proton nuclear magnetic resonance (¹H NMR)

¹H NMR can confirm the organic molecules' structures of the prepared membranes. Therefore, many organic-oriented researchers use this technique to determine the organic compounds' structure of AEMs. NMR analysis prefers deuterated (D) solvents, such as acetone, dimethyl sulfoxide, water, methanol, and chloroform, and solvents without hydrogen are also chosen, such as carbon disulfide and carbon tetrachloride. The quaternized naphthalene-based polybenzimidazole molecule structure (NPBI-QA25 and NPBI-QA25-B75) was confirmed by ¹H NMR.¹⁸⁸ This study used DMSO-d6 and trifluoroacetic acid to dissolve the membranes. The aromatic protons (7.5 and 8.7 ppm) in polymer backbones and methyl protons (3 ppm) in QAs were confirmed (Fig. 27a). Further, the NPBI polymer backbone was functionalized with n-butyl groups presented at 0.6 ppm. In another study, the DMSO-d6 soluble di-quaternized side-chains functionalized PPO (DQ-PPO) polymer was analyzed, and the peaks at 6.10–6.9 ppm were formed due to the phenyl rings in DQ-PPO.²³¹ For the solubility and protonation purpose, DMSO-d6-Trifluoroacetic acid (DMSO-d6/TFA), was used to confirm the methyl piperidine group on the PTP copolymer.²³⁴ PTP was functionalized with N-methyl-4-piperidone in the prepared membrane (e.g., PTPip-90). The structure was confirmed by the ¹H NMR peak formed at 9.77 ppm, which was due to proton–proton in the piperidine ring (Fig. 27b). After the quaternization of PTPip using methyl iodide, the piperidine peak disappeared because it was converted to piperidinium and correspondingly, the piperidinium rings of proton peaks formed at 3.39, 3.16, and 2.91 ppm. Additionally, due to quaternization, the PTPip polymer with methyl protons shown at 2.79 ppm was shifted to 3.16 ppm.

Organic functionalization was confirmed for the SEBS polymer, CMME-functionalized SEBS (SEBSC1), and lengthy side chain-functionalized SEBSC (SEBSC6) polymers using deuterated chloroform.²³³ Accordingly, –CH–, –CH₂– groups, and protons from benzene rings were credited to the SEBS polymer (Fig. 27c). Proton peaks from the chloromethyl groups were confirmed at 4.50 ppm, confirming the CMME functionalization with SEBS. Similarly, using deuterated chloroform, CMME functionalization on PSF (CMPSF) confirmed the peak form at 4.64 ppm because of the –CH₂– group in chloromethyl methyl from CMME.²⁰² Therefore, it can be assured that –CH₂– groups in chloromethyl methyl formed ¹H NMR peaks ranging from 4.50 to 4.64 ppm (Fig. 27d).

Further, the PDMB copolymers of –OCH₃, –COOCH₃, and methyl groups were confirmed by peaks at 3.3, 3.8, and 6.1 ppm, respectively, and in this study, deuterated chloroform was used for dissolving the polymer.¹⁴⁴ After chloromethylation (PDMB-Cl), the peak at 4.55 ppm was due to the methyl protons in benzyl chloride from CMME. Later, it was functionalized with piperidinium (PDMB-Pi), which confirms that the proton peaks at 1.4, 1.6, and 1.8 ppm belonged to piperidinium groups in the PDMB membrane. In another study, electrospun modified quaternized polyketone (Q-PK)-based AEM confirmed each step's organic functional process using ¹H NMR, and this study also used deuterated chloroform for dissolving the polymer.¹³³ After the Paal–Knorr reaction on PK (FPK), the peak at 5.4 and 4.99 ppm was due to the H₂ atoms of the pyrrole structure and the H₂ of the secondary carbon associated with pyrrole nitrogen. Later, quaternized FPK was confirmed by the peak at 2.03 ppm due to the methyl proton of QAs. The peak shift from 5.47 to 5.5 ppm was a proton moving from FPK to Q-FPK, which confirmed the QAs functionalization. Further, the new peak at 3.03 ppm was due to the amines of the side chains, and later, it disappeared due to quaternization.

Recently, quaternized poly(4-vinylbenzyl chloride-styrene) (QP(VBC-St)) AEM was assured through ¹H NMR, and deuterated DMSO was used for the polymer solubility.²⁶¹ As a result, the protons of the major backbone alkyl group confirmed a peak at about 2.2 ppm. Moreover, the protons of the chloromethyl group (CH₂Cl) were confirmed at 4.6 ppm due to the chloromethylation of the main backbone. Upon quaternization, the peak/chemical shift at 4.4 ppm showed that the methylene protons were linked to the QAs and the main backbone's benzene ring. Yang et al. developed 1-vinylimidazole (VIm)-functionalized BPPO (VImPPO) AEM, and their organic construction was confirmed using ¹H NMR. This study used deuterated chloroform for dissolving the BPPO and deuterated DMSO for dissolving the VImPPO.¹²⁸ Because of bromoethylation, the proton of the brominated –CH₂– group was detected at 4.3 ppm, while BPPO was responsible for the methyl proton signal at 2.0 ppm. The imidazolium ring was observed at 7.8, 9.37, and 7.7 ppm following VIm functionalization with BPPO, confirming the functionalization of VIm.

In another study, BPTMA-Br side chain-functionalized PBI (PBI-x%BTMA) membrane was studied, its organic functionalization was confirmed using ¹H NMR, and the membrane was dissolved using deuterated DMSO.¹⁸⁷ Accordingly, the protons of the benzimidazole and benzene ring of the main backbone were confirmed at 13.2 and 7.5–9.5 ppm, respectively, and the BPTMA-Br cation was identified at peaks ranging from 1.4 to 3.6 ppm (Fig. 28a). The peaks at 3.0 ppm and between 1.0 and 2.1 ppm after BPTMA functionalization with PBI were caused by the methyl and methylene groups in BPTMA. Further, Simari et al. identified the functional organic groups of chloromethylated polysulfone (CMPSU) and noted the chloromethyl and aromatic proton peaks at 4.64 and 7.5 ppm, respectively.¹³⁴Fig. 28b shows the ¹H NMR spectra of CM-PSU. Wang et al. used deuterated DMSO and chloroform to solubilize the synthesized polymers.²⁶⁵ Accordingly, benzyl hydrogen and the benzene ring were identified at 4.51 and 7.46 ppm, respectively, due to the chloromethylation of SEBS, where CH₃I reacted with PBP polymer to change the chemical shift in the tertiary and QAs nitrogen, observed at 2.7, 3.5, and 2.2 ppm.

A different study analyzed incorporating PEG into side chain-functionalized Q-PPO (20PDM-x%PEG2000) for the organic structure and used deuterated DMSO to dissolve the polymer.²⁵⁵ To this end, the MPi functionalization of piperidinium groups on BPPO was identified at 3.1 and 2.98 ppm (Fig. 28c). The incorporation of PEG into PPO was identified as a peak shift at 3.5 ppm due to the protons of –CH₂CH₂O– from PEG. Recently, Zn²⁺-incorporated imidazolium-functionalized polysulfone (PSF-ImCl/Zn-x) confirms their organic functionalization using ¹H NMR, and this study used deuterated DMSO.²⁵¹ The protons of the imidazolium ring were identified at 7.81 and 7.7 ppm, and these peaks were shifted to 7.65 and 7.56 ppm to incorporate Zn²⁺ ions into the polymer backbone. Additionally, evidence shows that the imidazolium ring's electron losses caused a higher chemical shift in the ¹H NMR spectra (Fig. 28d).

Compared to QAs cations, N-spirocyclic cations often have double-cyclic chemical structures and higher alkali stability. For improved results, polymer scientists concentrate on producing N-spirocyclic-based cations on membranes. As a result, an investigation was conducted on a newly developed hydrophilic N-spirocyclic-based cation and hydrophobic alkyl side chain-functionalized chloromethylated polysulfone.²⁴¹ The ¹H NMR signal at 4.64 ppm was owing to the protons of CH₂Cl, which served as a confirmation that polysulfone was cholomethylated. Later, at peaks 2.67, 0.86, and 2.48 ppm, hydrophobic alkyl side chains (n-octylamine and 1,6-hexamethylenediamine) were verified. Additionally, the proton peaks from 2.2 to 1.9 ppm were confirmed for hydrophilic N-spirocyclic cation-functionalized chloromethylated polysulfone, while 3.3 to 2.9 ppm corresponded to 8-hydroxyl-5-azonia-spiro[4.5]decane cations. After quaternization, the methylene protons of –NH₂ (2.67 ppm) were shifted to 2.70 ppm. Similarly, the bromomethyl-tethered copolymers (P(4PA-co-2PA)–Br) were analyzed, and the protons of aromatic phenylenes in the p-quaterphenylene of the main backbone were observed at 7.73 ppm.³⁰⁰ The formation of quaternization was evidenced by the fading of the Br–methyl group at 3.33 ppm and the advent of a new peak at 3.07 ppm, corresponding to the methyl groups from trimethylalkyl ammonium.

Lin et al. recently developed membranes made of poly(aryl piperidine) that had been quaternized with clustered cations (3QPAP), and deuterated DMSO was used to make the polymer more soluble.²⁹⁵ Before quaternization, PAP's primary backbone benzene rings were present from 7.0 to 8.0 ppm. The two signals at 2.7 and 3.1 ppm are attributed to methyl groups, while the signal at 8.4 ppm is attributed to the polymer's –NH groups. The aromatic protons signals remained constant after being quaternized with clustered cations (3QPAP), whereas the –NH peak vanished, and additional new peaks were seen due to the quaternization. To analyze their organic structural confirmation, bication cross-linked and quaternized poly(meta-terphenylene alkylene)s (m-PTPA) were utilized, and deuterated CHCl₃ was used to determine the polymer's solubility.²⁷⁹ After quaternization and the appearance of additional peaks at 2.9 and 3.0 ppm, which were attributed to the methyl protons of dialkyl and trimethyl alkyl QAs cation group, respectively, the bromomethylene proton of the brominated membrane (m-PTPA) at 3.33 ppm completely vanished. The indications of methylene and methyl protons in the spectrum further supported the conclusion that m-PTPABr was partly quaternized.

3.4 X-ray photoelectron spectroscopy (XPS)

Elemental analysis is widely performed using X-ray photoelectron spectroscopy (XPS). Various materials can be analyzed with it, providing valuable information about the chemical and quantitative properties of the material. XPS was used to determine the presence of several alkaline functional groups, including QAs and their crosslinking, tertiary amino, inorganic functionalization, and QAs. It may also be used to confirm the ion exchange of the membrane. Accordingly, the confirmation of ion exchange in NPBI-QA40 AEM was conducted in the bromide ions with hydroxide ions by dipping the prepared membrane in 1 M NaOH to produce ionic contact between N⁺ from the QAs and N⁻ from the benzimidazole rings. It was proved by the typical peak (N 1s) of N⁻ at 398.6 eV.¹⁸⁸ In addition, the authors asserted that the aqueous alkali medium was promising for the deprotonation of benzimidazole groups to develop an anionic species during the functionalization of PBI.

Several different weight ratios of the cation 4-dimethylaminopyridine were functionalized with a BPPO membrane, and this was done so that their elemental composition could be determined.²²⁴ As a result, the main peaks of Br 3d and N 1s, which represent BPPO and 4-dimethylaminopyridine, respectively, were seen, demonstrating the potential integration of these compounds into PPO (Fig. 29a). After quaternization, the intensity peak of the cation-functionalized BrPPO decreased because of the structural modifications made from benzyl bromide to QAs. The QAs of C₄N⁺ were discovered to be at 400.8 eV, confirming the successful quaternization of the nitrogen element and incorporation of 4-dimethylaminopyridine into the polymer backbone. In the same manner, procured BPPO was crosslinked with tertiary amino(dimethylamine) and quaternized with methyl iodide, and for comparison, uncrosslinked PPO was quaternized with trimethylamine (QPPO).¹⁹⁴ XPS showed that the N 1s binding energy differed for QAs in QPPO and QAs in dimethylamine (DMA)-crosslinked QPPO (Fig. 29b). It is evident that the binding energies at the N 1s peak are 401 and 403 eV, which correspond to the nitrogen in the tertiary amine and the nitrogen in crosslinked QAs, respectively. Additionally, the binding energy of DMA-crosslinked BPPO was measured following methyl iodide functionalization, and it was discovered that the N 1s peak at 399.5 eV was caused by tertiary amino, which was converted into QAs. TMA was used to quaternize PPO, whose N 1s peak appeared at 401.5 eV. Therefore, the crosslinking and quaternization of PPO were confirmed by such distinctive peaks at specific binding energies of functional groups.

To prove that an alkali was functionalized on PBI, the chemical composition of the alkaline-processed PBI polymer before and after was studied by Zeng et al., and the K peaks of alkali were observed at 376.8, 291.8, and 32.8 eV.³¹⁵ The single and double bond nitrogen (N) in the benzimidazole were represented as the N 1s peak; however, this N 1s was shifted after alkali treatment on PBI, and this peak shift indicated that the benzimidazole segment was deprotonation and obtained a negative charge as a result of delocalization with electrons (Fig. 29c). Similarly, Li et al. demonstrated the structural functionality of the poly(vinyl benzyl methylpyrrolidinium)-CMPSF (PVBC-MPy-CMPSF)-crosslinked dual-functional tertiary amines (diamine) AEMs through XPS.¹³⁵ N 1s peaks were seen at 400 eV as a result of crosslinkers as well as the insertion of chloromethyl groups in the polymer backbone (PVBC) and MPy-functionalized polysulfone. At 200 and 201.9 eV, respectively, the chloromethyl group, Cl 2p of chloromethylated polysulfone (PSF) and PVBC, was identified (Fig. 29d). Due to the functionalization of PVBC with MPy, the two binding energy peaks of PVBC were moved to a low energy state. After functionalization with MPy, it was found that PVBC's N 1s peak, which results from nitrogen in the pyrrolidinium, is at 402.1 eV. It was proven that diamine was functioning on the PVBC membrane due to the changes in the binding energy values from higher to lower and the appearance of the N 1s peak.

In a different study, Chen et al. investigated the chemical structure composition of Zn²⁺ ions-incorporated imidazolium-functionalized polysulfones.²⁵¹ Zn²⁺ ions were responsible for the peak at 1020 eV, whereas the peak (N 1s) at 401 eV resulted from nitrogen atoms in the imidazolium rings. Ion exchange between Ac⁻ and Cl⁻ ions is another possible reason for these peaks. AEM with dopamine functionalization, which is highly hydrophilic and has negative charges, was produced, and the structural composition of AEM confirms that it was quaternized since it reveals nitrogen elements in the bare AEM owing to QAs.²⁴⁶ The GO-modified AEM survey spectrum shows more oxygen content due to the high oxygen content of GO; however, the nitrogen content was lowered due to GO restricting the penetration depth of the rays falling on base QAs-containing AEM. Due to the presence of nitrogen in dopamine, the dopamine/GO-AEM (dopamine coated on a modified AEM) and the dopamine-AEM membrane exhibit significant nitrogen content. The functionalization of dopamine on the GO-modified AEM was therefore confirmed by increasing the nitrogen in the membrane, and a high nitrogen content in the membrane may be advantageous for high conductivity.

The surface area of cellulose nanomaterial crystals (CNCs) is adequate, lacking mechanical properties and hydroxyl groups. Therefore, they need additional surface modification to improve their mechanical and surface qualities. Accordingly, the structural composition was investigated for the quaternized CNCs (QCNCs), and it was discovered that these QCNCs had N 1s and Si 2p peaks at 399.1 and 101.4 eV due to the nitrogen and silica concentrations in QAs, respectively, while these peaks are absent in CNCs.²⁶⁷ Zeng et al. employed XPS to investigate rGO and PBI composite interaction.²⁶⁶ The N 1s peaks for rGO-modified PBI were identified at 398.0 and 399.9 eV due to double and single-bond nitrogen in the benzimidazole ring of PBI, respectively. The intensity of N 1s peaks was also diminished due to GO's adsorption on PBI. As a result, the presence of N 1s peaks in the PBI affected by rGO proves that GO externally modified the PBI.

Using CH₃I to quaternate the membrane, Wang et al. generated multi-cation 1,16-dibromo-5,11-(N,N-dimethylammonium)hexadecane (DBDMAH) ionic liquid-crosslinked poly(aryl piperidinium) AEMs.³¹⁷ They also prepared quaternized poly(aryl piperidinium) AEMs without cross-linking. The presence of the N 1s peak with a higher intensity than in uncross-linked AEM led researchers to conclude that the DBDMAH-crosslinked AEM had a high nitrogen content. Therefore, the functionalization of multi-cations may increase the nitrogen concentration and, as a result, have the potential to increase the membrane conductivity. Hao et al. developed an AEM based on poly(fluorenyl ether), and they quaternized the membrane using trimethylamine (e.g., QA-OMPFEK).³¹⁸ In this research, the author used homemade QAs (trimethylamine) for the quaternization of the membrane. The compositional structure of the generated AEM was discovered to be O 1s (531.6 eV), C 1s (286.6 eV), and N 1s (403.8 eV) owing to the polymer's main backbone and QAs, and Br (68.9 and 182.4 eV) appearance was caused by the bromide functionalization of the main backbone (Fig. 30a). The existence of such peaks confirmed the functionalization of the poly(fluorenyl ether) membrane by bromination and quaternization. The hydroxide ion can be increased by constructing composite inorganic–organic materials with graphitic-based nanomaterials and QAs-modified polymer backbones to boost the conductivity. As a result, quaternized polysulfone and graphitic carbon nitride (g-C₃N₄) were combined (g-C₃N₄-QPS).³¹⁹ The presence of N, C, and O elements demonstrate the surface properties of g-C₃N₄. At the same time, the author only confirms the elemental composition of bulk and exfoliated g-C₃N₄ using XPS (Fig. 30b). Additionally, functionalized g-C₃N₄ was found to have stronger and higher peaks than bulk g-C₃N₄ as a result of oxidation.

Porous silicon nanoparticles known as polyhedral oligomeric silsesquioxane (POSS) have a pore diameter of about 1.8 nm and many functional groups. POSS enhanced the electrolyte membranes' conductivity, mechanical, and dimensional stability. The poly(aryl piperidinum) (PAP100) membrane was therefore composited with imidazole-functionalized POSS (PAP100-POSSI), followed by quaternization using CH₃I, and ion exchange was investigated to increase the conductivity and other features.³²⁰ In this experiment, greater peaks in the composition of the composite membrane included O (548 eV), N (402 eV), C (285.1 eV), and Si (100 eV) and the appearance of N and C was attributed to the PAP membrane while Si was attributed to the POSS membrane (Fig. 30c). PAP, however, could have lower conductivity and fewer other properties because of the appearance of their weaker peaks (O, N, and C). The POSSI-modified PAP (PAP100-POSSI) polymer composite AEM was confirmed by such stronger peaks, which might indicate significant physical, chemical, and conductivity properties. Compared to bare PAP-100-dimethyl membrane, the N 1s peak for PAP100-POSSI remains stronger after the alkaline treatment. After alkaline treatment, the dimethyl-piperdinium group has a ring opening for the PAP100-dimethyl membrane, and the group was exhibited at 400.0 eV. However, following the alkaline treatment, the imidazole group in POSSI at 400.0 eV was unstable.

Furthermore, XPS was used to study the structural analysis of block copolymer-based PES and quaternized with high-density QAs (MPy) and crosslinked with a lengthy rigid side chain.³²¹ In this study, block copolymer PES that had been MPy treated showed that C–N and N 1s at 399.2 and 401.7 eV were obtained, respectively, by the polymer backbone and QAs (MPy). Only C–Br peaks at 70.9 eV were visible in the brominated PES polymer, while an additional peak at the Br ion at 67.1 eV was visible after functionalization with MPy. Specifically, Br ion and N 1s at certain binding energies on the block copolymer PES confirmed the bromination treatment and MPy quaternization. Similarly, the quaternized N-methylmorpholine (MM)-treated randomly organized copolymer polyether sulfone (rPES-MM) AEM showed nitrogen (401.7 eV) from QAs and C–N (399.2 eV) segment from the backbone.³²² Before quaternization, brominated rPES (Br-rPES) also displayed ionic bromine and C–Br at 70.9 and 67.1 eV, respectively, confirming that the bromination and quaternization processes were carried out on rPES.

Recently, PVA, which had been crosslinked with glutaraldehyde and further composited with crown ether-functionalized amine-SBA and the membrane, was then activated by KOH.³²³ Si, C, N, and O XPS peaks were obtained for the composite membrane, and following activation, a K peak could be seen. However, no significant structural changes were seen in the composite membrane before and after KOH activation. Similarly, the N 1s peaks were detected in the binding energy ranges from 403 to 398 eV for the combination of (2-bromoethyl)trimethylammonium bromide (BTAB) and hexylbromide (HB)-functionalized poly(biphenyl pyridine)-based AEM.³²⁴ As a result, the N 1s peak at 399.8 eV was caused by pyridine, and the pyridinium and trimethylammoniums of N 1s peak at 402.1 and 401.5 eV was caused by the main backbone and BTAB, respectively. In the XPS spectra, the bromination and quaternization functionalization was formally supported by the formation of the C–Br, ionic bromine, and nitrogen peaks, as well as the stronger and higher N 1s peak, which suggested that the functionalized cation's high nitrogen content might be beneficial for high membrane conductivity.

3.5 Fourier transform infrared spectroscopy (FT-IR)

The FT-IR technique is a cost-effective and consistent method of identifying inorganic, organic, and polymeric materials using infrared sources to analyze their bond properties. Therefore, FT-IR was used to determine the structural change of the membrane before and after ion exchange in an alkaline solution. The earlier report of the FT-IR study shows that the NPBIQA40-B60 membrane structure remained unaffected after treatment with 1 M NaOH and 1 M NaHCO₃, representing that the ion exchange process could not alter the structural change in the NPBI-QA40-B60 membrane.¹⁸⁸ In the case of NPBI-QA40, immersing this membrane in 1 M NaHCO₃ in the IR study shows no structural changes. However, structural changes were observed for NPBI-QA40 using 1 M NaOH, confirmed by the absorption peaks at 1120 and 1510 cm⁻¹, which is attributed to the bending and deformation of the ionic bond (N⁺N⁻), respectively. FT-IR confirms that the CuAAC reaction was used to produce the azidated PPO (APPO), which was then subjected to side chain tethering and di-quaternization (DQ-PPO).²³¹ The azide group (N₃) for APPO was visible in the IR spectrum at 2100 cm⁻¹. Still, it later totally vanished following di-quaternization, indicating that quaternization had taken place on the membrane (Fig. 31a). In a different study, three different QAs, such as isopropyl, n-propyl, and cyclohexyl-based QAs, were used to functionalize azide PPO.²³² All three cation functionalized PPOs displayed comparable functional groups without affecting their structural composition; however, following quaternization, the azide group vanished, proving that the PPOs achieved cation functionalization (Fig. 31b). On TETA-crosslinked PVC membrane (e.g., c-QPVC-1N, 2N, and 3N), the functional group changes with the degree of quaternization (4, 8, and 14 h) were investigated.¹²⁹ The stretching vibration of the C–Cl group was seen to be reduced following quaternization, and secondary N–H groups were found at 3370 cm⁻¹ in the IR spectra, which showed C–Cl groups in bare PVC at 685 cm⁻¹ (Fig. 31c). The N–H peak increased as the quaternization time increased, and the higher quaternization time results in a larger N–H peak, confirming that the higher duration enhances membrane quaternization.

Compared to quaternary nitrogen-based cations, phosphonium cations are more stable in alkali conditions. As a result, the pTAP membrane, a phosphonium-based polymer, was developed and mixed with polysulfone for electrochemical use.²³⁷ The composite form of the pTAP/polysulfone membrane included the functional group of pTAP and polysulfone in the IR spectra. The phosphorus compound of phosphonium was connected to the phenyl groups of polysulfone (1000, 1147, and 1441 cm⁻¹) present in the blended membrane. Due to membrane activation by KOH, the hydroxyl group (3421 cm⁻¹) in the blended membrane was detected. The cations such as Mpy, MIm, MPip, and 2-methyl-1-pyrroline (Mpyl) functionalized in the chloromethylated SEBS were investigated, and their functionalization was confirmed using infrared (IR) spectroscopy.²³³ As a result, the aliphatic groups of SEBSs were located at 2918 cm⁻¹ (–CH₃), 2849 cm⁻¹ (–CH₂–), and 1453 cm⁻¹ (–CH–), and the peak C–Cl (800 cm⁻¹) occurred as a result of chloromethylation. Finally, it was determined that the CN and C–N vibrations of Im or MPyl and Pip or MPy were the sources of the cations tethering on chloromethylated SEBS at 1503 and 1200 cm⁻¹.

Recently, TMA was used to quaternize polysulfone with crosslinked AEMs and their functionalization was examined using IR.²⁰² The peaks for the benzene ring and the sulfoxide group of the polymer backbone emerged at 1485 cm⁻¹ and 1073 cm⁻¹, respectively. Later, the C–Cl group emerged at 559 cm⁻¹ upon chloromethylation, confirming the chloromethylation of polysulfone. Following quaternization, the covalent interaction between the cations and the polymer backbone identified two additional peaks at 3363 (NR₄⁺) and 1631 cm⁻¹ (C–N). The presence of –OH groups from the crosslinker at 1731 cm⁻¹ confirmed the structure of TMPTA-crosslinked Q-polysulfone (Fig. 31d). Similarly, in another study, the absence of peaks at 1722 cm⁻¹ (–COOC) and 3400 cm⁻¹ (–OH), respectively, indicating the hydroxylation and chloromethylation processes of PDMB.¹⁴⁴ Later, a new peak caused by pyridinium appeared at 1627 cm⁻¹ (–C–N), confirming that the cation (pyridinium) was functionalized on hydroxylated and chloromethylated PDMB (Fig. 31e). Zhang et al. investigated the chemical structural variations of the synthesized quaternized bis-piperidinium side chains-based AEM using IR.²²⁷ To this end, the N–C bond and CH₃ functional groups were still present in the QAs group, as evidenced by the appearance of peaks at 1488 and 1465 cm⁻¹, respectively, in the alkaline resistance test before and after. These appearance of these peaks indicates that AEM has alkaline stable qualities under difficult circumstances. Recently, the fading of the C–Br (618 cm⁻¹) peak and the production of a new peak at 3335 cm⁻¹, which was caused by the hydroxyl group (–OH), both indicated that the bromomethyl group in the star-shaped copolymer had been transformed into a quaternary ammonium group.¹²⁷

Similarly, chloromethylated PAEK was aminated (CMPEAM) using TMA by a reductive mechanism.¹⁸⁹ The IR spectra of the compound show peaks at 3470 to 3350 cm⁻¹, indicating the presence of C–NH₂ in the polymer, while the peak CO of polymer backbone was lost at 1660 cm⁻¹ as a result of the conversion of the CO group to C–NH₂ in the polymer backbone. These results confirm that amination (quaternization) was done on chloromethylated PAEK (Fig. 32a). IR demonstrated absorption peaks at 2918 and 1103 cm⁻¹, respectively, confirming dual side chains-functionalized PPO, such as alkyl and alkoxy chains.³¹⁶ Similarly, the functional groups of brominated PPO (BPPO), which was then quaternized using DABDA, were examined.²³⁰ The peak C–Br was due to bromination and the bond C–C (1447 and 1596 cm⁻¹), whereas the peak C–O–C (1184 cm⁻¹) was due to the crosslinker DABDA, and the covalent link between DABDA and the polymer backbone appeared to be related to the highest C–N bond at 1017 cm⁻¹ (Fig. 32b). Thus, these absorption peaks supported the functionalization of PPO with crosslinkers and bromination.

Goel et al. studied aminopropylsilane-treated GO (AGO) that was functionalized with BPPO, and they used infrared spectroscopy to determine the chemical composition of the samples.²⁴² As a result, this study demonstrates that a C–Br peak formed at 590 cm⁻¹. When AGO functionalization was examined, the peak C–Br vanished, confirming the crosslinking of AGO with BPPO. Following quaternization, QAs were responsible for the peaks between 3200 and 3700 cm⁻¹, and a CN bond at 1568 cm⁻¹ may have resulted from a covalent interaction between QAs and AGO-PPO (Fig. 32c). The VImPPO and PVA polymers were made using a cross-linker and a light initiator.¹²⁸ Thus, IR was utilized to compare the crosslinking of composite polymers before and after. Weaker CC and C–H groups caused the peaks at 1640 and 940 cm⁻¹ compared to the uncrosslinked composite polymer (Fig. 32d). The functional groups of the lengthy side chain BTMA-Br functionalized on PBI were examined, and they revealed a peak at 1630 cm⁻¹ (CN group), which was related to the PBI (benzimidazole ring).¹⁸⁷ Possible causes of C–Br in BPTMA-Br include the typical peak at 740 cm⁻¹, which later faded following functionalization with PBI. The absence of C–Br showed that BPTMA-Br functionalized the PBI.

A composite PVBC polymer treated with MPy and chloromethylated polysulfone was produced, and its structural aspects have been examined.²⁶³ IR data indicates that the peak at 675 cm⁻¹ may represent a PVBC of C–Cl vibration. After the functionalization of MPy with PVBC, the peak C–Cl vanished (Fig. 32e). As a result of water molecules being absorbed on the membrane, hydroxyl groups are responsible for the hydrophilic MPy-treated PVBC, showing a peak at 3346 cm⁻¹. The peak indicates the composite membrane on which the cation was functionalized. The weight ratio of polysulfone in the composite membrane made with PVBC-MPy may be adjusted, but the authors found no change.

Recently, Zeng et al. studied GA-crosslinked PVA combined with QPVBC.²⁵⁷ The peak at 1488 cm⁻¹ in the IR spectrum was assigned to an aromatic ring (CC) by the polymer backbone, and the bond C–N at 890 cm⁻¹ was detected by QAs. These C–N and CC peaks in the composite membrane prove that QPVBC was constructed using cross-linked PVA. Furthermore, the composite PVA-QPVBC was formed, as evidenced by the peak at PVA's C–O–C bond in the composite membranes' IR spectra. Zhao et al. utilized quaternized chitosan/PVA composite AEM for the fuel cell application; however, they first confirmed the functional group of quaternized chitosan.²⁵⁰ As a result, it was revealed that hydroxyl and amine peaks were at 3432 cm⁻¹, which is due to water absorption and amine in chitosan, respectively (Fig. 32f). After quaternization, a couple of significant peaks (3018 and 1484 cm⁻¹) was because of the–CH bond of the QAs from the crosslinker (GTMAC). The epoxy group of GTMAC and the amine group of chitosan engaged in a ring-opening reaction, which resulted in the peak –NH at 1640 cm⁻¹ being decreased as well. However, this study did not do an IR analysis of the Q-chitosan/PVA combination, and it was confirmed that chitosan was quaternized when the –CH group formed and the –NH group decreased.

In a recent study, IR verified the presence of PEEK functionalized with quaternized piperidinium.²⁵⁴ A C–O–C bond was discovered at 1226 and 1151 cm⁻¹, which may be the polymer's backbone. The presence of a wide signal at 3400 cm⁻¹ brought on by hydrophilic piperidinium's water absorption on the membrane was subsequently utilized to prove that PEEK had been functionalized with piperidinium. Glutaraldehyde-crosslinked PVA-branched polyethyleneimine composite membrane was developed by Xiao et al. and used for fuel cell applications.²⁵⁸ They examined the crosslinked composite membrane's structural composition before the application. The polyethyleneimine's amine groups were shown by the wide peaks seen at 3367 cm⁻¹. The peaks at 1443 and 1605 cm⁻¹ were attributed to the C–O groups in PVA, while the interaction between the amine in polyethyleneimine and the –CHO in glutaraldehyde was thought to be caused by CN, respectively. Similarly, a peak developing at C–O–C was used to establish the interaction between the OH groups in PVA and the –CHO groups in glutaraldehyde (1100 cm⁻¹). Finally, the composite membrane's ion exchange (Cl⁻ to OH⁻) was proven by red shifting a wide peak from 3367 to 3282 cm⁻¹.

Kang et al. employed commercially available FAA-3 and Orion TM1™ AEM for water electrolysis application.³²⁵ Due to the hydrophilic nature of the membranes, both membranes exhibit a large peak at about 3600 cm⁻¹ due to OH. Both membranes' diphenyl ether component and aromatic bond (C–C) were also responsible for the peaks at 1302 cm⁻¹ and 1476 cm⁻¹. AEM was created by functionalizing poly(arylpiperidinium) (PAP) with the hydrophobic guanidinium cation.²⁹³ The PAP-based backbone underwent IR both before and after the guanidinium cation. Guanidinium salt caused the CN bond to appear to this extent at 1538 cm⁻¹. The appearance of such a CN peak proved that the PAP polymer was functionalized with cations.

Quaternized perfluorinated polymers such as Nafion-SO₂-F and Aquivion-SO₂-F were explored by Lee et al.²⁷⁶ They examined the functional groups of the polymers and found that sulfonyl fluoride peaked at 1467 cm⁻¹ and 796 cm⁻¹ (SO₂–F and S–F). Due to the presence of QAs, the sulfonyl fluoride peaks vanished, and a new peak (C–N) was found at 1020 cm⁻¹. The OSO peak was shifted following quaternization, and the sulfonamide (S–N) group was assigned at 1051 cm⁻¹. The perfluorinated and quaternization of Nafion and Aquivion membranes were validated by the peaks that appeared, including sulfonyl fluoride, C–N, and S–N. Similarly, using the 1,4-dimethylpiperazine (DMP) cation, the Nafion with SO₂-F form (Nafion-SO₂-F) was quaternized. As a result of DMB functionalization, peaks like –CH₃ and –CH₂ were seen at 2783 and 3040 cm⁻¹, respectively, whereas Nafion-SO₂-F missed such peaks.²⁷⁷ The authors also noticed that the –CH₃ and –CH₂ peaks were stronger as the quaternization duration increased (0 to 12 h). Time is another crucial factor in producing a more robust quaternization on the membrane.

Similarly, N,N-dimethyl-butylamine (DMBA)-functionalized PIM-block copolymer PSF membrane underwent an alkaline test in 1 M NaOH before and after.³⁰⁷ The C–N bond of QAs at 1296 cm⁻¹ was weaker after exposure to alkaline for 216 h; the nucleophilic substitution in the membrane may have caused this weakening peak. IR demonstrated that the functionalized ionic liquid membrane had greater alkaline stability than other cations (smaller side chains). Different cations, including DMIm, dimethylbutylamine, and dimethylhexylamine, were functionalized individually with perfluoroaklyl membrane.²⁸⁹ The spectra of the degraded qauternized perfluoro-based membrane were shown before and after the structural confirmations were checked using IR. Perfluoroaklyl membrane functionalized with quaternized dimethylhexylamine showed lower degradation than the membrane functionalized with other quaternized cations.

DMIm cation-functionalized PBI and PBI-Si hybrid confirmed their functional groups by IR.³⁰⁵ Accordingly, as a result of the cation (DIm), the imidazolium (NC–N) moiety peak appeared at 1536 cm⁻¹, and the peak shift from 1630 to 1620 cm⁻¹ was attributed to the benzimidazole (CN), which may have been the cause of the functionalization of DIm and Si. Due to the dimethylimidazolium in the PBI–DIm and PBI–DIm–Si, the C–H stretching vibration was recorded at 2861 cm⁻¹. When silica was discovered in a hybrid PBI–DIm–Si membrane, the Si–O–Si bond emerged at 1130 cm⁻¹. The produced AEMs of structures were verified using IR absorption analysis based on the functional groups that appeared at certain wavenumbers (cm⁻¹), such as –OH, –NH, –NH₂, C–Br, CO, CN, benzene ring, CC, –CF₂, and C–Cl, as well as red shifting peaks and membrane structural alterations following an alkaline test.

3.6 Transmission electron microscopy (TEM)

The hydrophilic and hydrophobic domains can cause microphase separation in the membrane and produce water paths for ion conduction. Hydrophobic and hydrophilic microphase separation in membranes helps simulate ion transport and control the hydrated materials' dimensional strength, and TEM can confirm these separations. Accordingly, Wu et al. developed microphase-separated morphologies in DQ-PPO membranes, i.e., hydrophilic and hydrophobic domains visible on TEM images as white (hydrophobic) and dark (hydrophilic) regions.²³¹ The TEM results revealed that the constructed di-quaternized side chains could efficiently promote microphase separation (Fig. 33a–f). This strategy might be helpful for linkages of alkyl ether and attaching a high volume of QAs in the side chains. The smooth morphology (dark hydrophilic region) observed for the PVC membrane was cross-linked with triethylenetetramine (TETA), followed by quaternization with an 8 h process in 0.1 M NaCl.¹²⁹ In contrast, the dark hydrophilic region was not observed clearly for the 14 h process due to ion suppression.

Interestingly, high ionic conductivity was achieved using the amphiphilic co-polymer to produce a hydrophilic and hydrophobic separation channel. Accordingly, different molar concentrations (72:28, 62:38, 53:47) of star-shaped copolymers were synthesized, and quaternized membranes showed well-ordered microphase separation (hydrophilic and hydrophobic).¹²⁷ The ionic domain sizes were increased (22–30 nm) with increasing copolymer molar concentrations, whereas linear-shaped copolymer-based AEM has a smaller ionic cluster size. Well-connected aggregated hydrophilic ion (darker region) and polymer backbone (hydrophobic) microphase separation were observed in the poly(bromoalkyl oxindolebiphenylylene)-based (PISBr) AEMs with cross-linked cation side-chain 1,2-bis(2-(2-methylpiperidine)ethoxy)-ethane (NON) (Fig. 33g).²²⁷ The side-chain (NON) may work well as a linker to improve microphase separation using a low-cost method. The hot pressing approach produced superior ionic channels for separating the hydrophilic and hydrophobic microphases during the manufacture of AEM (e.g., QAFPK) compared to the casting method.¹³³Fig. 33h and i shows the TEM images of QAFPK membranes produced by solution casting and electrospun-modified QAFPK membranes made by hot pressing. Zhang et al. recently developed a C–NH₂-linked QA-PAEK AEM.¹⁸⁹ They found that the consistent hydrogen bond channels in the invented AEM may be supplying active sites for hydroxide transport and encouraging the formation of ion clusters favorable for increasing ionic channels and OH diffusion, thereby boosting long-range hydroxide conduction. Similarly, highly ordered microphase separation was obtained for 8% 1,6-dibromohexane crosslinked with poly(aryl piperidinium) (PAP-8%)-based AEM compared to 0% crosslinked poly(aryl piperidinium) (PAP-0%) AEM.¹²¹ Although, the ionic cluster size of PAP-8% was higher than PAP-0% due to the self-assembly of polymer backbones and accumulation of hydrophilic phases therefore, the creation of larger ionic clusters in the AEMs was promoted by the 1,6-dibromohexane-based crosslinking agent.

Crosslinked PPO with various concentrations (40, 50, and 60%) of DABDA shows that well-ordered hydrophilic (dark) and hydrophobic (bright) microphase regions were formed, and the aggregation of hydrophilic ions with increasing concentrations of DABDA.²³⁰ DABDA is one of the excellent cross-linking agents for promoting microphase separation to aid in membranes with ionic conductivity; thus, it makes sense that it is one of them (Fig. 34a–c). In another work, crosslinked naphthalene-based AEMs (e.g., NAPEK-PVP-(6, 8, and 10)-Q4) of TEM images demonstrate that increasing the QAs groups in the polymer backbone improved the size of the ionic domain, which permits better microphase separation.²³⁹ Accordingly, Fig. 34d–f depicts a TEM image illustrating different ratios (6, 8, and 10%) of QAs-functionalized NAPEK-PVP, with the darker region representing the AEM's hydrophilic area and the lighter region representing the hydrophobic polymer backbone. High microphase separation was achieved by synthesizing varied weight ratios of molecular weight 2000 of hydrophilic PEG combined with crosslinked PPO.²⁵⁵ PEG's increasing weight ratio in modified PPO demonstrates increased ion sizes and microphase solid separation. However, as shown by TEM pictures, only modified PPO (20PDM) exhibits no microphase separation due to no hydrophilic regions in the membrane.

Intriguingly, imidazolium-functionalized PSF (Im-PSF) with the incorporation of different weight ratios (0 to 3.8%) of Zn²⁺ (Im-PSF/Zn²⁺) also outstandingly displays microphase separation and increasing ion cluster size with increasing the weight ratio of Zn²⁺.²⁵¹ In contrast, no microphase separation was observed for only Im-PSF. Zn²⁺ ion in the Im-PSF, causing an expanding ionic cluster, which electrostatically combines with imidazolium (Fig. 34g–j). Recently, high levels of microphase separation were found in the hydrophilic N-spirocyclic (8-(4-(chloromethyl)benzyloxy)-5-azonia-spiro[4.5]decane) cation, and the hydrophobic n-octylamine functionalized with PSF AEM shows high levels of hydrophilic and hydrophobic microphase separation and the ionic cluster size was also increased with increasing hydrophobic contents.²⁴¹ Similarly, a type of N-spirocyclic (6-azonia-spiro[5.5]undecane) cation was functionalized with GO (ASU-GO) in another study, and it was later further functionalized with QPPO. The functionalized PPO AEM produced ordered hydrophilic (cation-functionalized GO) and hydrophobic (PPO) microphase separation morphology.²⁴³N-Spirocyclic-based cations are key in generating microphase separation and can enhance the ionic conductivity in polymer membranes.

Well-ordered microphase separation was observed in quaternized poly(terphenyl) (m-TPN) functionalized with CH₃I (Pi) compared to quaternized m-TPN-functionalized N,N-diisopropylethylamine (Py) and α,α′-dibromo-o-xylene (Be), respectively.²⁴⁰ The reason could be the higher degree of microphase (hydrophilic and hydrophobic) separation and increasing cations clouds in Q-m-TPN-Pi compared to others. Accordingly, the larger ionic cluster size was found to be Q-m-TPN-Pi. Interestingly, the hexyl QAs (HQAs)-functionalized SEBS AEM exhibit smaller and greater degrees of microphase separation, respectively. The pentafluorobenzoyl-functionalized HQAs-SEBS shows larger ionic clusters than monofluorobenzoyl-functionalized HQAs-SEBS.¹³⁹ Without fluorobenzoyl-functionalized SEBS, however, no hydrophilic and hydrophobic separation morphology was observed. Lin et al. produced a series of poly(terphenylene) AEMs modified with piperidinium (cation).²⁹⁵ To this extent, multi-cation-functionalized poly(terphenylene) displays a higher microphase separated structure, and the ionic cluster size was increased compared to single cation functionalized poly(terphenylene) due to cation accumulation in multi-cation-functionalized polymer compared to single cation functionalization. As a result, cation side-chain (hydrophilic) functionalization is crucial for developing microphase separation structures during the fabrication of AEMs. According to the amount of functional or cations groups, the size of the ionic cluster increased.

3.7 Scanning electron microscopy (SEM)

For use in water electrolysis and other applications, it is crucial to investigate the microstructure and morphology of AEM and cross-linked AEMs. Therefore, AEM's surface morphology and microstructure can be analyzed by SEM. Triethylenetetramine (TETA) cross-linked PVC AEM of the cross-sectional image shows smooth and dense morphology.¹²⁹ However, after cross-linking, quaternized PVC shows high smoothness compared to bare PVC membranes. Tetraaryl-phosphonium ionomers (pTAP)-blended polysulfone (PSF) AEM was discovered to have a homogeneous structure and extremely closely-packed particles, as well as no surface flaws and phase separation membrane shape.²³⁷ Recently, diverse cations (Im, Mpyl, Mpy, and Pip) embedded in SEBS-based AEM display uniform structures and are very transparent. These membranes have non-porous, highly dense structures that can be used for fuel cell application.²³³ Zhou et al. investigated the surface morphology of an AEM (QAFPK-1-x-E) made from polyketone nanofibers that were produced utilizing hot pressing and casting techniques.¹³³ They noticed that the smooth morphology of the membrane nanofiber produced by the cast approach (Fig. 35a–c) contrasts with the blocked pores and reduced fiber diameter made by the hot-pressing method (Fig. 35d–f). As a result, the casting process works well for creating nanofiber-based AEM. Comparably, the cross-sectional view of the BPPO membrane and the cation side-chain (DABDA)-functionalized PPO membrane revealed homogeneous and dense structural morphology.²³⁰ Similarly, the N,N,N,N-tetramethyl-1,6-hexanediamine (THMDA) cross-linked SEBS membrane (e.g., C6-PQASEBS-62.3%) exhibited a smooth, homogeneous, and dense structure.²⁰³ Additionally, phase separation is seen in the SEM images, and the cross-sectional image demonstrates thick AEM (Fig. 35g and h).

The membrane's smooth and dense structure aided in preventing fuel from leaking into energy-based devices, and this type of structure offers structural stability and mechanical qualities for energy-related applications. Accordingly, Zhang et al. observed a smooth, neat surface and dense (cross-sectional view) in bis-imidazolium-treated block-copolymer-based polynorbornene (e.g., rP(NB-MGE-b-[NB-O-BisIm⁺] [OH⁻]₂)-20) AEM.²²⁶Fig. 35i and j shows morphology and cross-sectional focus of rP(NB-MGE-b-[NB-O-BisIm⁺][OH⁻]₂)-20 membrane. The surface morphology of cross-linked PPO with aminopropylsilane-treated GO (PPO/AGO-8) and uncross-linked PPO AEMs (PPO/AGO-0) were similarly observed by Goel et al.,²⁴² in contrast to cross-linked PPO (Fig. 36a and b), which produced a rough and non-uniform structured morphology. At the same time, uncross-linked PPO produced a clean surface (uniform), free of pores and densely organized (Fig. 36c and d). In contrast to non-cross-linked PPO, the authors asserted that great flexibility was discovered in cross-linked PPO AEM.

Recently, poly[2-20-(mphenylene)-5-50-bibenzimidazole] (ABPBI) was composited with PVA to fabricate electrospun nanofibers, considering that the incorporation of polymer nanofibers allows obtaining it due to ABPBI providing thermal resistance, mechanical strength, low permeability, and their durability toward the PVA membranes. Accordingly, its composition (ABPBI-PVA nanofiber) was prepared with a cross-linked (glutaraldehyde (GA)) structure, and it was analyzed through SEM.¹³¹ Composite fiber (PVA-ABPBI) morphology was unsmoothly compared to PVA fiber due to the incorporation of ABPBI, and the cross-linked composition (C-PVAf-ABPBI) morphology shows a lamellar structure (cross-sectional view) and homogeneous (Fig. 36e–h). In the case of VBC-functionalized LDPE-based AEM, a rough surface morphology and dense structure were obtained. The cross-sectional image shows the two-layer (VBC + LDPE) formation.¹³² Highly smooth, no separation phase, homogeneous, dense, and neat morphology was obtained for the three piperdinium cations-functionalized poly(terphenylene)-based AEM (3QPAP-x) compared to a single piperdinium-functionalized poly(terphenylene) membrane (1QPAP).²⁹⁵ Therefore, this study reveals that the smoothness of AEM depends on the amount of piperidinium functionalization (Fig. 36i).

It's interesting to note that surfaces made of (vinylbenzyl)trimethylammonium (VT) and N-vinylimidazole (NV)-functionalized poly(vinylidene fluoride-co-hexafluoropropylene)-based AEM were very clean, smooth, dense, uniform, and free of any other potentially dangerous substances.³²⁶ For energy-related applications, the functionalization of VT and NV are highly compatible with the copolymer to manufacture smooth AEM. Recently, dimethylimidazolium-functionalized PBI (PBI–DIm) and its hybrid with silica (Si) (PBI–DIm–Si) AEM obtained highly transparent, dense, smooth, and homogeneous surfaces.³⁰⁵ However, the Si on the PBI surfaces causes the hybrid structured membrane to lose its smoothness.

The organic–organic hybrid polymer composite membrane with a smooth, dense, homogeneous, and neat surface was made using PVBC crosslinked with poly(vinyl acetate).²⁶⁴ Compared to an organic–inorganic membrane, a composite organic–organic polymer could perform better because of its compatibility with polymer–polymer interactions. Additionally, the procedure is low-cost because poly(vinyl acetate) acts as a cross-linker agent, so no additional costly organic cross-linking agent is required. Wu et al. noticed that the polymer backbone and cross-linkers are critical in membrane morphology, which is crucial to highlight.²⁷¹ As a result, when the polymer's 3 g BPPO backbone was functionalized with a medium concentration of DVB and VBC combination, it produced a tidy and uniform shape as opposed to a low (2%) concentration. However, using more than 3 g BBPO at the same medium concentration (6%) DVB shows that superior morphology was not achieved. Therefore, having the right amount of cross-linker and polymer backbone is crucial. As a result, the smooth and dense shape of the membrane relies on the weight proportion of the cross-linker and polymer backbone and the selection of composites of organic–organic and organic-inorganic matters.

3.8 Thermogravimetric analysis (TGA)

An additional crucial issue must be the membrane's thermal stability in the energy-based devices under optimum working temperatures (50 to 150 °C). Accordingly, thermogravimetric analysis (TGA) was used to determine the thermal stability of the membrane, and the kind of polymer backbones also impacted this thermal stability. Accordingly, the piperidinium and aromatic (p-tetraphenyl segment) segment-containing polymer backbones have been reported to be stable up to 320 °C in poly(aryl piperidinium)-based AEM (PQP) in the form of iodide.²³⁵ While p-biphenyl and p-terphenyl aromatics were employed to construct poly(aryl piperidinium)-based AEMs (PBP and PTP) in the form of iodide, they did not exhibit more excellent thermal stability (293 and 287 °C) than p-tetraphenyl (320 °C). Fig. 37a shows the TGA spectra of PBP, PTP, and PQP-based membranes. The n-butyl side chains-tethered QAs-functionalized naphthalene-based PBI membranes exhibit moderate thermal breakdown; three weight loss temperatures were shown at 200, 450, and 543 °C, suggesting QAs degradation, side-chain, and polymer backbone (NPBI) decomposition, respectively.¹⁸⁸ At the same time, bare NPBI do not continue to function thermally over 500 °C. Aliphatic side chains and QAs contributed to the NPBI membrane's thermal stability up to 600 °C. However, Wu et al. found that side chains with di-quaternization-functionalized PPO-based AEM displayed low thermal stability, i.e., the weight loss temperatures were low, such as 160, 220, and 400 °C, correlating to di-quaternization degradation, side chain degradation, and PPO decomposition.²³¹ As a result, specially made side chains incorporating di-quaternization did not make the membranes more thermally stable.

Another research utilized the cross-linker triethylenetetramine (TETA) on PVC-based AEM. To determine the degree of quaternization, PVC was immersed in TETA solution for periods of 4, 8, and 14 h and the thermal stability of the membrane was accordingly studied.¹²⁹ TGA curves indicate that weight loss occurred in stages, with the first occurring between 85 and 160 °C due to water evaporation and the second occurring at 220 °C due to the loss of TETA. The last stage occurred at 460 °C with the breakdown of the backbones (Fig. 37b). At a temperature of 430 °C, bare PVC began to degrade due to poor thermal performance. As a result, increasing the degree of quaternization via TETA over time improved the thermal stability of the PVC-based AEM. Only two stages of weight loss were discovered to be caused by aryl-ether-free PTP-based AEMs, showing that the first stage occurred between 250 and 350 °C, indicating that broken-down polymer backbones caused the breakdown of cations and weight loss at 400 °C.²³⁴ Thus, aryl-ether-free PTP-based AEM has decent thermal stability and may be employed for water electrolyzers and other applications (Fig. 37c).

Terminal alkyne (TA)-containing QA such as n-propyl (TA-nP), isopropyl (TA-iP), and cyclohexyl (TA-CH)-functionalized QPPO membrane in the iodide form demonstrate that weight loss temperature was initiated at 180 °C due to QAs degradation, and side chain decomposition was found to be in the range of 236 to 239 °C.²³² The polymer backbone (PPO) was then decomposed at 400 °C. As a result, the thermal stability at high temperatures (400 °C) was improved by coupling massive QAs via an alkyl spacer (Fig. 37d). PPO was functionalized with alkyl-extender-containing dual-QAs devoted to the main chain through a spacer (PPO-SDQEC) showing low thermal stability, i.e., the side chains and cations were degraded at 150 °C, and the backbone (PPO) was decomposed in between 350 and 500 °C.¹¹³ The specific SDQEC-functionalized PPO did not exhibit significantly enhanced thermal stability.

The thermal stability of increasing amounts of cross-linking agent on quaternized polymer backbones was investigated, and it was found that when compared to quaternized polysulfone that had not been cross-linked, the rate of thermal degradation was slower for TMPTA cross-linked and TMA was used to quaternize (Q) polysulfone.²⁰² All the prepared AEMs exhibit thermal stability up to a temperature of 370 °C; however, increasing the amount of TMPTA cross-linking on Q-polysulfone causes slower degradation than Q-polysulfone AEMs (Fig. 38a). Therefore, one of the factors affecting the membrane's thermal stability is the cross-linking agent with a higher degree that is chosen. Recently, quaternized PPO was cross-linked with aminopropylsilane-functionalized GO (AGO), demonstrating stability up to 450 °C.²⁴² However, because AGO has a silica component, it was shown to have a slower degradation rate than Q-PPO when crosslinked with AGO. Therefore, adding silica components may improve the membrane's thermal stability.

The weight ratio of the two polymer solutions was adjusted for membrane preparation and thermal stability. As a result, 1,8-octanedithiol was used to cross-link VImPPO, which was then mixed with a photoinitiator.¹²⁸ Composite membranes were created by adjusting the cross-linked VImPPO and PVA in various ratios (8:2, 7:3, and 6:4). Compared to other ratios, the 8:2 composite membrane achieved remarkable thermal stability (Fig. 38b). Also tested was a VImPPO and PVA composite without cross-linking, which exhibits poor thermal stability when compared to cross-linked VImPPO and PVA composite membranes. However, the membranes showed initial stability up to 180 °C and afterward weight loss, from 190 to 450 °C due to the decomposition of the cross-linker, polymer backbones including PPO and PVA. As a result, 1,8-octanedithiol utilized to cross-link membranes may improve their thermal stability.

Similarly, in a different study, TMHDA was used to cross-link poly(vinyl benzyl chloride) (PVBC), followed by its quaternization (Q) using MPy.²⁶³ Later, the cross-linked Q-PVBC was composited with polysulfone. The thermal stability of the composite membrane showed that the loss of water molecules caused weight loss at 130 °C; the degradation of MPy and TMHDA caused weight loss at 200 °C (Fig. 38c). Then, PVBC and polysulfone polymer backbones were finally broken down at 480 °C. However, compared to cross-linked Q-PVBC-(10 and 30%) polysulfone composites, only the PVBC membrane exhibits low thermal stability since it decomposed at 400 °C. MPy and TMHDA may thus improve the thermal stability of membranes. In another study, ionic liquid-crosslinked naphthalene and its composite with Q-PVP were found to be the weight loss temperatures at 210 and 370 °C, and this weight loss was attributed to the degradation of the crosslinkers and the polymer backbones, respectively.²³⁹ The type of composite membrane, however, did not significantly increase the thermal stability (Fig. 38d).

The cation's chemical structure greatly influences the thermal stability, which was observed by Wang et al.²⁴⁰ Accordingly, meta-poly(terphenylene) (m-TPN) AEMs were synthesized by quaternization with various cation structures such as CH₃I, 1,4-dibromopentane, and α,α′-dibromo-o-xylene. To this end, m-TPN functionalized with CH₃I shows low thermal stability than m-TPN functionalized with the spiro-cyclic structure cation of 1,4-dibromopentane and α,α′-dibromo-o-xylene membranes. Recently, the thermal stability of the chitosan membrane was investigated using TGA, showing stability up to 500 °C; however, chitosan following quaternization did not exhibit any significant stability (480 °C).²⁵⁰ Therefore, quaternization alone could not increase the membrane's thermal stability. Similarly, it was observed that the hexyl QAs-functionalized SEBS and the mono- and penta-fluoro benzoyl-modified hexyl QAs-SEBS had no appreciable impact on the thermal stability.¹³⁹ Fluoro-functionalization does not influence hexyl QAS-SEBS membranes since all membranes have a comparable thermal effect (maximum stability 450 °C).

Another research compared the commercially available membrane FAA-3 with Orion TM1™, which has a backbone made of polyphenylene and functional groups QAs.³²⁵ The membranes exhibit a two-step weight loss, while the maximum stability was determined to be 450 °C for FAA-3 membranes and 500 °C for Orion TM1™ membranes. Jiang et al. investigated the thermal stability of bications cross-linked and uncross-linked poly(meta-terphenylene alkylene).²⁷⁹ However, no heat impact was seen for all of the membranes. The membranes' stable temperature was up to 360 °C, while QAs and cross-linker degradation caused weight loss at 210 °C. As a result, the thermal stability of membranes made of poly(meta-terphenylene alkylene) did not significantly change due to bication functionalization. Guanidinium salt is one component that substantially improves the membrane's thermal stability. Gao et al. developed a long multi-cation-based side chain (NQQN) cross-linked with fluorine-based PDBA AEMs with varying weight percentages (30 to 70%).²⁸³ All membranes exhibit three phases of weight loss at temperatures of 200 °C, 200 to 555 °C, and above 555 °C, and all membrane types have more robust thermal stability (555 °C) than the most current AEMs. To improve the thermal stability of membranes for applications, multi-cation (NQQN) functionalization is the optimum method. The functionalization of membranes, such as the addition of single or multiple cations, composites with other polymers, or inorganic materials, affects the thermal stability of such membranes. However, producing membranes with excellent thermal stability ought to be simple and practical for usage over an extended period.

3.9. Summary

The MSM of membranes such as PBP, PISBr-NON, and (PBP)-c-SEBS-based AEMs was examined using SAXS, and a high MSM level was found due to the formation of diffraction ionic peaks. However, no discernible ionic peaks are seen in piperidinium treated with aryl ether-free-based AEMs due to no MSM structure. Thus, the MSM in the membranes is necessary to form diffraction peaks. AFM can identify the MSM of membranes by the hydrophilic and hydrophobic regions (bright and dark, respectively), which can be observed in membranes such as PQ-100 and PPO-iP. For instance, PBP-67 and PTP-83, PPO-CH, and PPO-nP do not have such dark and bright areas; thus, they do not have a microphase separation. MSM structures may be achieved by increasing the amount of (QAs groups, degree of quaternization, degree of hydration, alkoxy and extender with side chains, and spacer and alkyl longer coordination). Membranes made of fluorine exhibited greater MSM. The membranes' function to separate the microphase was significantly improved by adding a fluorinated component to the polymer backbone. ¹H NMR may confirm the structures of the organic molecules in the produced membranes. For global organic-based researchers, it will be helpful to identify different organic compounds in the polymer structures. However, the organic polymer should completely dissolve in the deuterated organic solvent for the best result. At the same time, the chemical composition of different inorganic materials and organic polymer structures was analyzed using XPS. The organic matter or elements on the surface of the materials are particularly sensitive to XPS. More research should also be done with XPS on the organic functionalization, organic–organic polymers, inorganic–organic polymers, and fluorinated polymer structures.

FT-IR is one of the most effective methods for analyzing the functional groups of diverse organic and inorganic materials. The ability to recognize the functional groups would be beneficial to new researchers who are working with polymers. For example, it confirms that the hydroxyl group, the bending and deformation of ionic bonds, the aliphatic groups of polymers, the benzene ring and the sulfoxide group, bromomethyl group into QAs changes, alkyl methacrylate, CF₂ group, CO group to C–NH₂ due to QAs functionalization on chloromethylated polymer were found. Moreover, for the imidazolium (NC–N) group, the changes in the structure before and after crosslinking, and the addition of inorganic particles such as silica and GO to the polymer were all confirmed. Improving the MSM by increasing the weight ratio of inorganic elements in the polymer backbone and the concentration of piperidinium in the polymer backbone is possible. These changes were attributed to larger ionic domain sizes, which was confirmed by TEM. The size of the ionic cluster may expand depending on the number of cation groups present in the membrane. Di-quaternized side chains have the potential to enhance microphase separation effectively.

SEM confirmed the smoothness, cross-sectional, and thick structure of the AEM, and it was found that the smoothness of the membrane can be lost when it is in composite form. Compared to a single polymer membrane and an inorganic–organic composite structure, a composite made from AEMs is very dense. Aromatic segment polymer structure, AEM based on piperidinium and p-tetraphenyl in the polymer is stable up to 350 °C. Though aliphatic side chains and thick QAs-linked AEMs showed that the maximum was stable up to 600 °C, this membrane type can be used for high-temperature applications. Cations such as MPi, MPy, and TMHDA made the membrane more stable at high temperatures. Unlike membranes made of a single material, composite membranes can make the membrane more stable in high temperatures. But, the inorganic–organic composite membrane is more stable at high temperatures than organic–organic polymer composites. The fluorination of the hydrocarbon polymer can also improve the temperature stability of the membrane very well. Up to 550 °C, fluorinated membranes with metal ions and metal-cation-based side chains were very stable.

4. Properties of AEMs

AEMs should undergo physiochemical analysis in pure water to determine the impact of polymer backbones since the amount of absorbed water is a crucial factor in maintaining swift ion conduction and solid mechanical properties in hydrated AEMs. Their long-term alkaline stability is among the most vital and challenging criteria for using AEMs in water electrolysis. For AEMs, knowing the behavior of the absorbed water is crucial since too much water results in mechanical weakness, dimensional instability, and the production of charge carriers.²³⁴ Among the essential characteristics of AEMs is ion transport or conductivity, which directly impacts the yield performance of AEMs used in electrochemical-based devices. Ion exchange capacity (IEC) can increase a membrane's hydroxide conductivity but at the risk of a high swelling ratio (SR) and reduced mechanical stability. Before employing the membrane in an application, it is crucial to measure the conductivity of the membrane to get good, balanced high conductivity and appropriate stability. However, the hydroxyl ion (OH⁻) conductivity is linked with the membrane's IEC. In polymer matrices, conductivity depends on stable anionic sites and networks that connect them for ion transit across membranes. The concentration of QAs (cations), which is essential for ions to move over AEMs, affects the properties of the membrane, including its mechanical, dimensional, and swelling stability. In an energy-producing device, the membranes operate in a high pH atmosphere, making them vulnerable to OH⁻ ion assault. As a result, the membrane has to be stable and robust enough to withstand OH⁻ ion attack. During the operation of the electrolysis cell, the interfacial stability, electrode assembly, and long-term stability of the AEMs depend on the material's mechanical properties. Accordingly, the following discussion will center on the primary qualities of the AEM, which are based on ion exchange, conductivity, water absorption, swelling ratios, durability, and membranes' physical stabilities (Fig. 39).

4.1 Ion exchange capacity (IEC)

In an ideal situation, ion exchange capacity (IEC) is proportional to the AEM's conductivity, meaning that membrane conductivity rises as IEC rises and is independent of the polymer structure.¹⁵⁹ Even though IEC and membrane water content are connected, high membrane IEC might result in high water absorption. Following this, swelling causes the ionic conductivity of the membrane to diminish. IEC may be distinguished by one mol of cation present on the AEM per unit mass of dry membrane, and if there are only monovalent functional groups, the IEC is expressed as mmol g⁻¹ or meq g⁻¹.^260,327 A mmol quantity of cationic ions contained in one gram of dry membrane is another definition of IEC. It represents the number of ionic segments per dry QA-tethered membrane, an important factor controlling the polymeric membranes' conductivity and water absorption. Techniques including titration, potentiometric, and spectroscopy were applied to find the IEC. Accordingly, the titration of AEM with AgNO₃ is used to measure the exchanged Cl⁻ and NO₃⁻ ions. In contrast, the potentiometric method measures the change in pH caused by the increased concentration of hydroxide (OH⁻) ions in the solution. The spectroscopic detection technique is based on the NO₃⁻ ions exchanged with Cl⁻ ions in the AEM.³²⁸ In the IEC experiment, the Cl⁻ form of AEMs was checked the IEC using the Mohr titration technique, and before this experiment, the membrane was dried at 100 °C for 24–48 h under a vacuum. Subsequently, the membrane was soaked in 1 M Na₂SO₄ to discharge the Cl⁻ ion at 75–80 °C. Further, the Cl⁻ ions in the solution were titrated by 0.01 M AgNO₃, and K₂CrO₄ was used as an indicator. The IEC was calculated by following eqn (6).

	(6)

V_AgNO3 and C_AgNO3 refer to the volume and concentration of spent AgNO₃ solution, and m_dry is the dry membrane's weight in Cl⁻ form.

(Or)

Eqn (7) may also be used to compute the IEC of the AEMs. This equation titrates the membrane with a sodium hydroxide solution and uses phenolphthalein as an indicator after converting it to the OH⁻ form using an HCl solution.²³⁴

	(7)

V_HCl denotes the HCl solution volume, and C_HCl indicates the HCl concentration. The volume and concentration of the NaOH solution, respectively, are V_NaOH and C_NaOH, while the dry weight of AEM is W_dry.

(Or)

	(8)

IEC calculation may also be assessed using eqn (8). The dry membrane (W_dry) was submerged in hydrochloric acid (HCl) at 60 °C for two days to neutralize the membrane's hydroxide ion. A KOH solution (C_KOH) was used to titrate the remaining HCl molecules, and the consumed volume (V_x) was measured. V₀ is the amount of KOH solution used to neutralize an HCl solution. To determine the amount of ion exchange in membranes, the reported papers provide a choice of IEC equations.^{128,134,187,239,240,257,287,301}

Recently, PQP-100, PBP-67, and PTP-83 AEMs were produced by aromatic monomers p-quaterphenyl, p-biphenyl, and p-terphenyl and functionalizing with N-methyl-4-piperidone, respectively.²³⁵ There was no difference between these AEMs because they all had identical IEC values of 2.30 mmol g⁻¹ at 20 °C. In another study, compared to the PEGDA (0.18 mmol g⁻¹) membrane, the PS-DVB (1.5 mmol g⁻¹) membrane had a higher IEC in the presence of Cl⁻, whereas PEGDA (0.190 mmol g⁻¹) showed higher IEC in the HCO₃⁻ form compared to Cl⁻.²⁶⁰ An adjusted ratio of QAs (hydrophilic) and n-butyl groups (hydrophobic) was used to produce a naphthalene-based PBI membrane (NPBI-QA55-B45) to test its IEC.¹⁸⁸ As a result, increased QA groups in the NPBI resulted in a higher IEC (0.92 to 1.80 mmol g⁻¹) because more hydrophilic materials absorb more water (Fig. 40a). Additionally, IEC was decreased when n-butyl was functionalized with QAs-NPBI (0.83 to 1.56 mmol g⁻¹). However, no IEC measurement was found after a certain quantity of QAs comprised NPBI-n-butyl since high WU causes dimensional change and membrane breakage. As a result, the optimization of increasing number of QAs should be considered on the AEM. Using Mohr titration, the IEC of dual QAs-functionalized PPO (DQ-PPOx-OH) could be determined with varying azidation degrees (12–26%).²³¹ It was discovered that the IEC increased with increasing azidation degrees, from 1.35 to 2.03 mmol g⁻¹. In order to improve the IEC of the membrane, the degree of azidation may therefore have an impact.

Recently, IEC testing was done on N-cyclohexyl-N-hexyl-N-methylcyclohexanaminium (CH)-functionalized PPO (PPO-CH), N-methyl-N,N-dipropylhexan-1-aminium (nP)-functionalized PPO (PPO-nP), and N,N-diisopropyl-N-methylhexan-1-aminium (iP)-functionalized PPO (PPO-iP) with varying degrees of azidation (between 31 and 44%).²³² The results of this study indicated that higher azidation levels led to higher IEC (Fig. 40b). As a result, PPO-iP and PPO-nP with higher azidation (44%) groups both demonstrated higher IECs of 1.90 and 1.90 mmol g⁻¹, respectively, when compared to PPO-iP (1.56 mmol g⁻¹) and PPO-nP (1.56 mmol g⁻¹) with 31% azidation. Besides, a lower IEC of 1.39 and 1.65 mmol g⁻¹ was observed in PPO-CH functionalized with azidation levels of 31 and 44%, respectively. To this degree, the azidation degree of 44% of PPO-iP obtained higher IEC than cations-functionalized PPO with azidation degrees of 44 and 31%. Thus, the IEC depends on the chosen cation, and the degree of azidation is crucial, as revealed by this study's findings.

Intriguingly, PPO-based AEM with hydrophilic alkoxyl extender side chain and hydrophobic alkyl spacer (e.g., SDQEO) was received on the high WU, and minimal dimension change resulted in high IC.¹¹³ The dual-function PPO has the benefit of accumulating ions in the spacer between the primary and side chains to improve the conductivity. Even then, the IEC (1.13 mmol g⁻¹) for this created AEM was not high (Fig. 40c). The N-cyclic piperidinium functionalized AEM with an alkyl ether-free bond (e.g., PTP-90) produced high IEC (2.52 mmol g⁻¹), while the same membrane showed high WU.²³⁴ It is then proven that the membrane has a high IEC and can absorb more water when compared to PTP-75 and PTP-85-based membranes (Fig. 40d). An IEC of 1.5 mmol g⁻¹ was received by poly(biphenyl piperidinium) AEMs based on ionic liquid and containing higher concentrations of piperidinium cation (e.g., PBPCL1.48).²⁸⁸ AEM's low cation concentration also exhibited a lower IEC (1.29 mmol g⁻¹). As a result, the IECs of membranes rely on the concentration of cations within the membranes. In another study, the polymer composite polysulfone/tetraaryl-phosphonium (PS/pTAP) with the adjusted ratios 40:60, 50:50, and 60:40 showed IEC of 1.2, 1.11, and 0.95 mmol g⁻¹, respectively.²³⁷ The increasing ionomer pTAP amount in the PS polymer can increase the IEC due to its high hydrophilic nature to promote ion transport.

IEC was tested for the SEBS membrane grafted with shorter (CMME) and extended side chain (6-bromohexanoyl chloride) groups, after which they were quaternized with different nitrogen heterocyclic complexes.²³³ It was discovered that quaternized CMME-SEBS (e.g., SEBS-C1-MPy) displayed higher IEC (0.99–1.13 mmol g⁻¹) than quaternized extended side chain-functionalized SEBS (0.44–0.48 mmol g⁻¹). Therefore, the longer side chain could not benefit the membrane by enhancing the IEC. Goel et al. recently combined PPO with GO, which had been treated with aminopropylsilane (PPO/AGO).²⁴² PPO was a composite with different weight percentages of AGO, which were changed by 0–8% for the efficiency tests. The IEC rose as the weight percentage of AGO in PPO increased. IEC often grows with WU. As a result, IEC (1.45–2.25 mmol g⁻¹) and WU were raised in this study by increasing the weight percentage of AGO (2–8%) in PPO. Consequently, the hydrophilic qualities of AGO in PPO improved the IEC of the composite membrane.

In another study, when compared to bare (1.42 mmol g⁻¹) and single antioxidant-doped (1.28 mmol g⁻¹) membranes, the double antioxidant membrane (e.g., QP(VBC-St)-MB/EA_0.3) doping (1.02 mmol g⁻¹) did not aid in enhancing the IEC of the membrane.²⁶¹ It could be because a single antioxidant-doped membrane produces better microseparation morphologies than a doped membrane with two antioxidants, which results in superior IEC. Therefore, adding two antioxidants to the membranes may not help get improved IEC. In the recent testing for IEC, several radical inhibitor structures doped with quaternized poly(4-vinylbenzyl chloride-styrene) (QP(VBC-St)) were evaluated.²⁶² In comparison to bare QP(VBC-St) (1.72 mmol g⁻¹), doped-QP(VBC-St) (1.44–1.38 mmol g⁻¹) had lower IEC values. When compared to another radical inhibitor-doped AEMs (1.42–1.38 mmol g⁻¹), the 4-tertbutylphenol-based radical inhibitor-doped QP(VBC-St) (e.g., QP(VBC-St)-4-TBP_0.7) showed a higher IEC (1.44 mmol g⁻¹). In contrast to radical inhibitor-doped AEMs, where the separation morphology influences the IEC of the membrane, QP(VBC-St) has a greater IEC because it has a well-ordered microseparation morphology.

The impact of varying the VImPPO-PVA (sIPN) semi-interpenetration polymer network ratio on IEC was recently investigated.¹²⁸ The IEC values decreased from 1.46 to 0.78 mmol g⁻¹ due to the lowering and increasing proportions of VImPPO and PVA, respectively. IEC was more significant in the higher and lower ratios (9:1) of the VImPPO-PVA membrane (sIPN-91) than in other ratios (8:2, 7:3, and 6:4). As a result, VImPPO may be an excellent composite for PVA to improve the IEC. A higher PVA ratio in the composite membrane did not impact the IEC. The long-side chain structured QAs will improve the ion exchange of the AEMs. Tang et al. investigated several ratios (between 27 and 48%) of long-side chain-structured BPTMA that tethered PBI (BPTMA-PBI) evaluated for IEC.¹⁸⁷ Finding out that the IEC (1.1–1.8 mmol g⁻¹) rose as the proportion of BPTMA grew (27–48%) was quite interesting. Investigating the same thing using a different long-side chain structured QAs attached to AEM to evaluate the IEC would be preferable. Layered double hydroxides (LDHs), which are positively charged materials with high mechanical strength and a large concentration of hydroxyl groups, may aid in permitting better mechanical stability and hydroxide ion conduction when composited with AEMs. Due to the benefits of LDHs, Simari et al. recently explored several Mg–Al LDH compositions (1 and 5%) with quaternized polysulfone (QPSU) for IEC testing.¹³⁴ Findings revealed that LDH-qPSU (e.g., qPSU-LDH₅) displayed greater IEC than pristine QPSU. Interestingly, the QPSU's IEC (0.85–0.88 mmol g⁻¹) values rise as the weight percentage of LDH (1–5%) increases, in contrast to the pristine QPSU's IEC of 0.81 mmol g⁻¹. However, LDHs may not be the best choice to raise the IEC when combined with the membrane.

Poly(vinyl benzyl methyl pyrrolidinium) (PVBMPy), an ionomer, was recently crosslinked with varying amounts (10–30%) of TMHDA before being composited with polysulfone (e.g., PVBMPy-CL-PSF) to evaluate the IEC.²⁶³ With an increasing proportion of crosslinkers (0–30%), the IEC (3.6–1.6 mmol g⁻¹) dropped in this study (Fig. 41a). The IEC values, however, were unaffected by the addition of polysulfone. As a result, adding polysulfone did not affect ion exchange. PVBMPy-PSF AEM demonstrated 2.1 mmol g⁻¹ of the 15% optimized crosslinker (TMHDA). Increased IEC, on the other hand, resulted in increased water absorption and swelling ratio, which harmed the AEM. Therefore, the authors optimized a 15% crosslinked composite membrane for this research. In a different study, higher levels of crosslinking (5–15%) result in lower membrane IEC (2.99–2.74 mmol g⁻¹).²⁶⁵ The ions are not exchanged due to the increasing crosslinking between membranes (e.g., 10%-PBP-c-SEBS). This investigation thus shows that improving the IEC of the membranes by increasing the crosslinking is not possible. However, since WR and IEC are strongly correlated, lowering the IEC would be beneficial for enhancing the mechanical stability of the membrane.

Recent attempts to enhance the IEC by introducing hydrophilic oligomers in the polymer backbones have been unsuccessful. Increasing percentages (0.5–5%) of PEGs were used to produce composites with functionalized PPO (e.g., 20PDM-(0.5–5%)PEG2000).²⁵⁵ Consequently, the increasing proportion of PEGs in the functionalized PPO lowered the IEC from 2.27 to 2.16 mmol g⁻¹ compared to bare functionalized PPO. The PEG's lack of ions in the structure precluded ion exchanges; thus, the IEC was declining. Thus, the ions in the polymer serve as the foundation for the IEC in membrane improvement. The mass ratio of two polymers can be changed to increase the IEC; however, changing the mass ratio of the crosslinker and polymer results in a drop in the IEC for membranes. Accordingly, Zeng et al. studied IEC on composite membrane structures by modifying the mass ratio of two polymers (PVA:QVBC) and adjusting the crosslinker and polymer (DVB:QVBC) to generate PVA_0.6–0.9QVBC_30–50%.²⁵⁷ IEC increases and reduces with the mass ratio of PVA with QPVBC (0.6–0.9%) and crosslinker (DVB) with QVBC (30–50%), respectively, rises. Therefore, ion exchange is feasible between the two polymer chains, not between the crosslinker and polymer. Thus, increasing the weight percentage of two polymers may be a better strategy to improve the IEC.

When a polymer is exposed to ionizing radiation, radiation-induced pathways cause significant changes in the polymer's intrinsic characteristics. As a result, several doses of electron beam irradiation (20, 40, and 100 kGy) on VBC-grafted ETFE-based AEM were investigated in an environment of air or N₂ at either RT or below zero degrees.²⁹⁹ IEC (2.62 mmol g⁻¹) was discovered to be higher for a high degree of grafting and 100 kGy radiation exposed to air atmosphere at low temperature (less than −10 °C) (e.g., 100-air-LT/RT) compared to other doses of exposure to air or N₂ atmosphere at particular temperatures with a low degree of grafting. Compared to AEM made chemically, the IEC of the membrane may be improved by the low-temperature irradiation method. The method might minimize the need for hazardous chemicals and a labor-intensive manufacturing process for creating membranes. Researchers have been interested in N-cyclic-based cations because of their alkaline stability. Therefore, Wang et al. investigated various poly(terphenyl) (m-TPN)-tethered N-cyclic cations to produce membranes m-TPNPiQA, m-TPNPyQA, and m-TPNBeQA.²⁴⁰ It was found that m-TPNPiQA (2.54 mmol g⁻¹) had a higher IEC than TPNPyQA (2.32 mmol g⁻¹) and m-TPNBeQA (1.99 mmol g⁻¹). Smaller cationic groups or microstructure configurations that result in significant ion exchanges might cause high IEC by m-TPNPiQA (Fig. 41b).

Zhou et al. investigated the impact of crosslinking on AEMs for IEC. The produced PBBP membrane was crosslinked with DBMHDA (AEM 1 to 3), BAPTES (AEM 4 to 6), and BPPO (AEM-7) to test for IEC.³⁰¹ As a result, PBBP functionalized with DBMHDA (PBBP-DBMHDA) displayed a higher IEC (2.65 mmol g⁻¹) than BAPTES (1.71 mmol g⁻¹) and BPPO (1.55 mmol g⁻¹). It could cause a much higher QAs occupancy rate in DBMHDA compared to BAPTES and BPPO (Fig. 41c). One of the causes may be that the silica groups in BAPTES block the hydroxyl ion exchange to the backbone. For BPPO, a smaller functional group mass per unit mass resulted in a lower IEC. Besides, the uncrosslinked PBBP AEM obtained higher IEC (1.9 mmol g⁻¹) than BAPTES (1.71 mmol g⁻¹) and BPPO (1.55 mmol g⁻¹)-crosslinked PBBP. Therefore, DBMHDA could lead to better crosslinking to enhance the IEC of the PBBP compared to BAPTES and BPPO. The molecular weight ratio (0.24 to 0.63 mol%)-based copolymer (quaterphenylene alkylene and biphenylene alkylene) membranes (e.g., P(4PA-co-2PA)-24) has recently been shown to lower IEC (2.17–1.87 mmol g⁻¹) by increasing the molecular weight ratio.³⁰⁰ The IEC was lower (1.85 mmol g⁻¹) for the membrane-made biphenylene alkylene solely (Fig. 41d). Because of this, the membrane benefited from increased IEC in the copolymer form.

More cations in the AEMs mean more ion exchange between the cations and the backbone since IEC is directly proportional to the number of ion-conducting groups in the membrane. Thus, the impact of adding a fluorobenzoyl group on quaternized AEMs on their IEC was investigated. Accordingly, Munsur et al. investigated quaternized SEBS functionalized with monofluorobenzoyl (F1) and pentafluorobenzoyl (F5) to ascertain the IEC.¹³⁹ Thus far, increasing fluorine substitution in the quaternized membrane did not improve the IEC. As an alternative, the IEC of quaternized SEBS (1.51 mmol g⁻¹) was found to be greater than that of quaternized SEBS with fluorine (F1 (HQA-F1-SEBS) and F5 (HQA-F5-SEBS)) substitutes (1.49 and 1.45 mmol g⁻¹), respectively. The restriction of ion exchange in QAs and SEBS by fluorine moieties is probably responsible for the decline in IEC. The mechanical stability caused by water uptake concerning IEC may be enhanced using a membrane of this form, reducing water absorption and swellings. Increases in IEC (1.62–2.26 mmol g⁻¹) are shown when the ratio of poly(aryl piperidinium) with clustered cations-functionalized AEM (e.g., 3QPAP) is changed (0.3–0.5%) as compared to poly(aryl piperidinium) membrane with a single cation functionalized, e.g., 1QPAP (2.01 mmol g⁻¹).²⁹⁵ Increased backbone ratio may result from improved ion exchange between clustered cations and the backbone, which raises the IEC.

Various monomers were employed in synthesizing different molarities (30–60%) of poly(ether sulfone)-based AEM, which was then tested for its impact on IEC.²⁸⁷ An increase in the molar ratio of monomer and the degree of bromination (e.g., 60MePh-2.07) caused the increase in IEC. The ion exchanges have undergone a comprehensive transformation because of the increased bromination in the membranes. Unlike conventional AEMs, block-copolymers AEMs do not perform well in the IEC. As a result, AEMs based on polystyrene and poly(phenylene) copolymers (e.g., HTMA-DAPP) had IECs of 1.7 and 1.5 mmol g⁻¹, respectively.³²⁹ Incomplete or limited ion exchanges between the cation and backbones may be due to the block copolymer-based architectures of AEMs. This way, the IEC values of AEM structures based on block copolymers are lower than those found on conventional AEM structures. Recently, an AEM based on poly(arylene piperidinium) (PAPQ83) was synthesized and compared to commercially available AEM (Aemion™) in terms of IEC.²⁹⁴ PAPQ83's IEC value (2.29 mmol g⁻¹) was similar to Aemion™ (2.1–2.5 mmol g⁻¹). Thus, PAPQ83 may work on a commercial scale.

The IEC values of a single cation structured organic molecule-functionalized AEM with quaternization are lower. The functionalization of bication-structured organic compounds in AEMs has recently enhanced IEC. As a result, poly(meta-terphenylene alkylene) (m-PTPA) polymer was functionalized with bication structured compound (m-XPTPA-2N) and quaternized with TMHDA in different ratios (0–40%).²⁷⁹ The IEC of the bication functionalized m-PTPA was more significant (2.99 mmol g⁻¹). However, following quaternization with TMHDA, its ratio (10–40%) is raised to reduce the IEC values (2.82–2.49 mmol g⁻¹). As a result, the increased content of TMHDA could not exchange ions highly with the m-PTPA-2N membrane. Compared to a quaternized high number of QAs on a carbazole pendant cation-functionalized fluorinated membrane, the ion exchange was much higher in a fluorinated membrane that tethered a long alkyl chain-based carbazole pendant cation (e.g., PAEK-HQACz-0.7).¹¹⁰ The increasing ratio of alkyl chain-based carbazole cation increased the density of QAs in the hydrophobic fluorinated polymer to increase ion exchange and transport. In contrast, increasing the concentration without long alkyl chain-based carbazole cation with increased QAs in fluorinated polymer did not provide significantly better results in IEC. Recently, various ratios (4–10%) of (vinylbenzyl)trimethylammonium chloride were functionalized with the fluorinated polymer (PVIB), revealing the increase in the IEC (1.43–1.82 mmol g⁻¹).³²⁶ Increasing the number of (vinylbenzyl)trimethylammonium chloride in the membrane could enhance the ion exchanges between the polymer backbone and QAs. Another research found that the IECs decreased from 2.84 to 1.55 mmol g⁻¹ when the amount of extended alkyl chain-based bis-piperidinium cation groups in the quaternized membrane (e.g., QPBPip-Ac) was increased (from 33% to 67%).²⁹⁸ For the membrane to function correctly, it would need a different kind of bis-piperidinium cation group than one based on a lengthy alkyl chain.

IEC's influence on the fluorine-contained membrane with end-cross-linked structure (e.g., PHFB-VBC-DQ) was examined to determine the degree of multi-cation (30–80%).²⁸⁴ IEC was shown to rise (1.59–2.24 mmol g⁻¹) when the percentage of cations (30–80%) in the crosslinked fluorinated membrane increased. The QAs tethered to the multi-cation structure may cause the fluorinated polymer backbone's enhanced ion exchange. Dimethylimidazolium-functionalized PBI composites with silicon (PBI–DIm–Si hybrid) exhibited an IEC of about 1.87 mmol g⁻¹, as measured by Jheng et al.³⁰⁵ In contrast, those without an Si membrane showed 1.85 mmol g⁻¹ hybrid membranes containing silicon increased the ion exchange. Quaternized poly(p-phenylene) and poly(arylene ether) copolymers were produced and evaluated for IEC by altering the molar ratio (2:1 and 4:1) of monomer and trimer.³⁰⁶ It was shown that a more significant amount of monomer (e.g., QCPPAE-4/1) was used in the copolymer to get a greater IEC (1.75 mmol g⁻¹) than that achieved with a smaller amount of monomer (e.g., QCPPAE-2/1) used in the membrane (1.08 mmol g⁻¹). For this reason, increasing the monomer quantity is crucial for better IEC. In another study, different ratios (50 and 75%) of fluorinated polymer were recently created with cross-linked and uncross-linked THMDA for IEC.²⁷⁸ TMHDA cross-linked 75% fluorinated polymer (e.g., L-FPAEO-75-MIM) exhibited a greater IEC (1.8 mmol g⁻¹) than cross-linked (1.36 mmol g⁻¹) and uncross-linked 50% fluorinated polymer (1.32 mmol g⁻¹). This work showed that IEC depends on cross-linking (quaternization) and polymer quantity.

Perfluorinated polymers have well-ordered structures and morphologies that assist water movement and minimal water absorption owing to their hydrophobic nature. Nafion and Aquivion, two perfluorinated polymers, were recently quaternized via TMA and evaluated for IEC.²⁷⁶ It was shown that the Aquivion-TMA (1.06 mmol g⁻¹) had higher ionic conducting groups because it received a higher IEC than the TMA-treated Nafion (0.88 mmol g⁻¹). Miyanishi et al. investigated AEM in the form of Cl⁻ and OH⁻ based on high molecular weight polyfluorene for IEC.¹⁰⁰ The hydroxide form of polyfluorene-based AEM (e.g., PFOTFPh-C6-TMA (OH)) obtained high IEC, which led to the discovery that AEM-containing OH⁻ had more ionic conducting groups than Cl⁻, a form of polyfluorene AEMs. Recently, it was shown that increasing the ratio (30–70%) of fluorene-based polymer precursors in multi-cation ionic liquid cross-linked fluorene-based AEMs (TQ-PDBA) boosted the IEC (1.09–2.16 mmol g⁻¹).²⁸³ They increased the membrane's fluorine precursors to increase the number of ionic conducting groups per area of the polymer and increased ion exchanges. In their latest work, Li et al. found that the IEC (1.76 mmol g⁻¹) of fluorine-tethered PPO followed by the tri-QAs membrane (e.g., PPO-22-3QA8F) was greater than that of tri-QAs-tethered PPO (1.71 mmol g⁻¹) and single QAs-tethered PPO (1.72 mmol g⁻¹).²⁸² Higher hydrophobic and lesser hydrophilic properties of AEMs were found to have higher ion conducting groups than lesser hydrophobic and more inferior hydrophilic properties of AEMs because higher hydrophobic and lesser hydrophilic properties of fluorocarbon-tethered PPO with side chain tri-QAs received higher IEC than the lesser hydrophobic and lesser hydrophilic property one.

4.2 Water uptake (WU) and swelling ratio (SR)

Membranes need water to function normally because maintaining optimal hydration is necessary for excellent performance. Water uptake (WU) in the membrane depends on the operation temperature, number of ionic groups, dissociation constant, counter ions, polymer elasticity, hydrophobicity of the main polymer backbone, membrane pretreatment, etc. When ion exchange cluster density rises, membrane conductivity increases, thus increasing the IEC. The coordination interaction of water molecules surrounding the ion exchange clusters caused membrane swelling and was attributed to an increased WU. With increasing temperature, WU climbed considerably. This is because high temperatures speed up the movement of water molecules. The membrane's main chain moves faster and faster simultaneously. More water molecules were absorbed due to increased free space between molecules, raising the WU and SR. As a result, high and low water uptake and high and low swelling ratios were obtained by hydrophilic and hydrophobic AEMs, respectively. The linked hydrophilic segments of a thick membrane are primarily responsible for OH⁻ conduction; hence, a minimal amount of water is required for OH⁻ conductivity to prevent the membrane from swelling excessively.

Recently, an attempt showed decreased IEC and hydroxide conductivity caused by the poor WU of hydrophobic crosslinked membranes.³³⁰ Higher WU is attributed to membrane swelling, mainly affecting the mechanical degradation and chemical properties. At first, the required size of the membrane was modified to the form of OH⁻ ions according to the WU measurements. After that, the length was measured, dried, and weighed. The membrane was then sealed firmly and submerged in water for 24 h at the specified temperatures. Before degassing, WU was computed in water. After soaking, the membrane was immediately cleaned to eliminate any extra water accumulated on its surfaces, and it was then weighed and calculated. The collected measures were applied to measure the WU and SR. The membranes with OH⁻ counter ions were soaked in water at a particular temperature, and the changes in mass and length of the membrane were observed. WU and SR were evaluated by following eqn (9) and (10), respectively.

	(9)

	(10)

W_wet and L_wet are the weight and length of the hydrated (water uptake) membrane, and W_dry and L_dry are the weight and length of the membrane after drying under vacuum at a particular temperature for 12 h, respectively.

Additionally, the amount of water particles absorbed per QAs group is known as the “hydration number (λ),” and it may be determined using the following eqn (11) or (12).^234,239,295

	(11)

(or)

	(12)

The WU and SR were tested to be for the PQP-100, PBP-67, and PTP-83 AEMs, revealing that the p-biphenyl (PB) used to synthesize PBP-67 was claimed high WU (53.5%) and high SR (24.3%) compared to PQP-100 (20% and 20.6%) and PTP-83 (18.5% and 13.9%) at 20 °C was due to the less rigidity and high hydrophilicity of PB.²³⁵ The high rigidity and increased hydrophobicity of the p-quaterphenyl monomer in PQP-100 limit the high-water absorption and dimensional change and, thus, make it a viable AEM for water electrolyzer application (Fig. 42a). Moreover, the hydration number (λ) of PQP-100, PBP-67, and PTP-83 was 4.8, 12.9, and 4.5, respectively, and these numbers suggest that the PQP-100 received low hydration, which means less water absorption compared to others. Therefore, this AEM (PQP-100) results in reduced water absorption, and a less dimensional change will benefit the application. In another study, two different kinds of AEMs, based on poly(ethylene glycol) diacrylate (PEGDA) and polystyrene-divinylbenzene (PS-DVB), were developed by Kim et al.²⁶⁰ In the Cl⁻ form, the PEGDA membrane (72%) displays higher WU than PS-DVB (18%). Later, it was found that the WU in the HCO₃⁻ form of both the membranes was greater than the Cl⁻ form. The increased uptake of the PEGDA and PS-DVB membranes was caused by their fabrication using hydrophilic polymer (polyethylene glycol) and hydrophobic polymer (polystyrene)-based backbones, resulting in high and low WU. Furthermore, based on the findings of hydration number, it was confirmed that HCO₃⁻ membrane absorbed more water than the Cl⁻ form of the membrane.

Increasing the QA amount in the primary NPBI very attentively revealed that the WU (7 to 35%) grew.¹⁸⁸ However, after functionalization and adjusting the n-butyl (hydrophobic) group, the number of QAs-functionalized NPBI (e.g., NPBI-QA55-B45) showed an increment in WU (3.3 to 39.0%). As a result, this work demonstrates that WU only depends on the hydrophilic groups of QAs and is unaffected by the interference of other hydrophobic segments in the primary backbone (Fig. 42b). Additionally, the swelling ratio (SR) increased (5.4 to 11.8%) with the number of QAs in NPBI. Still, this impact later diminished (2.9 to 10.0%) after the QAs in NPBI were functionalized with hydrophobic n-butyl groups. As a result, adding hydrophobic material to QAs-functionalized membrane may lessen the swelling impact. Furthermore, the fact that the hydration number of n-butyl-functionalized QAs-NPBI (2 to 12.1 λ) was larger than that of QAs-NPBI (3.6 to 10.1λ) suggested that the functionalization of n-butyl increased the water absorption. The CuAAC click reaction results in triazole formation, which increases the number of active sites on the membrane and causes a rise in the WU and SR. Accordingly, dual QAs-tethered azidated PPO demonstrated increased WU (23.0 to 170.0%) and SR (4.6 to 70.4%) concerning the raised azidation (12–36%) of PPO and increasing ionic cluster size.²³¹ The greater WU and high SR might not be helpful for the application. As a result, the membrane may be damaged under extremely high WU conditions that cause excessive membrane swelling. However, authors optimized the azidated manufactured membrane, i.e., DQ-PPO-17-OH, showing that the WU and SR were 60.0% and 12.7%, respectively. As a result, increasing the azidation of the polymer backbone may not be helpful due to the high WU and high SR damage to the membrane. Additionally, it was stated in this study that the WU and SR are influenced by the temperature, meaning that these values rise as the temperature increases.

At an azidation degree of 44%, the high hygroscopic nature of isopropyl-based cations-functionalized PPO (e.g., PPO-iP-44) exhibits higher WU (192.0%) than n-propyl (141.0%) and cyclohexyl (101.0%)-based cations-functionalized PPO.²³² The PPO functionalized with identical cations and a 31% azidation level revealed lower WU values of 47.8, 38.8, and 22.6%. The hygroscopic isopropyl-based cation absorbs more water than other cations, which is what caused the higher WU to be measured. A higher azidation degree (44%) is another characteristic that helps the membrane absorb more water. Therefore, hygroscopic cations and azidation degrees are crucial in the WU. Moreover, these same investigations verified that higher WU and lower WU resulted in higher and lower SR, respectively. The hydration number of the PPO-iP membrane (56.1λ) was then found to be higher than that of the PPO-nP membrane (41.4λ) and the PPO-CH membrane (29.5λ) at a degree of azidation of 44%, and even PPO-iP (17.0λ) was found to have higher hydration than the other membranes (13.8 and 9.0λ) at a degree of azidation of 31%. The PPO-iP's higher hydration result demonstrated that it had a higher WU.

Due to the hydrogen bonding between the extender with the side chain and the polymer backbone when employing a hydrophilic-based alkoxyl extender, the WU and SR can be enhanced. However, very little WU and SR were seen when using a hydrophobic-based alkyl extender. Alkyl spacers are attached to the side chain and improve the WU and SR since consuming large amounts of water might encourage ion accumulation. Accordingly, Li et al. developed a hydrophilic alkoxyl extender into the side chain, and they linked it to the PPO main chain via a hydrophobic alkyl spacer (e.g., SDQEO) to the extent they did.¹¹³ At 25 °C, the alkoxy extender and alkyl spacer in the PPO were both high WU (120.1%) and showed a low SR (33.2%). Although the experiment (WU (130.0%) and SR (33.7%)) was carried out at 80 °C, there is still room for improvement. To assist the membrane and have high conductivity, the hydrophilic extender and hydrophobic alkoxyl might reduce the SR and increase the WU. N-Cyclic piperidiniums-functionalized PTP-based AEMs (e.g., PTP-90) strongly responded to an increase in WU (40–86.9%) and low SR (7.5–15.2%) with the increasing temperature (20–80 °C).²³⁴ It was proven that the low SR of PTP-based AEMs was acquired with an intense hydration number (8.7λ). For example, a low hydration number causes a low WU, and a low SR is related to high dimensional stability. Furthermore, the authors noted that the significant dimensional stability for PTP-based AEM was demonstrated at higher temperatures (80 °C).

A high SR and low OH⁻ ion concentration in the membrane caused by an excessive WU will reduce the mechanical strength and IC of the AEMs. Therefore, a minimum quantity of water absorption would benefit the AEM's functioning. Designing an AEM with a hydrophobic moiety and long alkyl chains will reduce excessive water absorption. Accordingly, hydrophobic-based poly(biphenyl piperidinium) AEM was prepared with a higher concentration of cations β-cyclodextrin and piperidinium-based ionic liquid (PBPCL1.48), which was shown to have high WU (46.4%) and high SR (20.8%) in comparison to lower cation concentrations of similarly designed PBPCL1.29 (32.6% and 13.3%) and PBPCL0.79 (9.3% and 3.8%).²⁸⁸ Thus, one of the factors affecting the WU is the cation concentration in the primary backbone. Increased WU (5.4%, 5.6%, and 8.6%) was seen in the hydrophobic polysulfone with a rising hydrophilic ionomer (PS/pTAP) ratio (60:40, 50:50, 40:60), and the external temperature (at 60 °C) also had an impact on the membranes' WU (7.8–10.3%).²³⁷ The increased WU was caused by the increase in the pTAP ionomer and higher temperatures (60 °C) because of the ionomer's hydrophilic nature and the enhanced mobility of polymer chain moieties and the production of free volume, respectively. However, this type of PS/pTAP AEM did not absorb much water, as evidenced by the finding that the WU at ambient temperature was only 8.6%.

Yu et al. investigated WU on several cations grafted onto shorter side chains (CMME) and longer side chains (6-bromohexanoyl chloride) SEBS membranes, respectively.²³³ At 30 °C, the WU (25–31%) of the longer side chain-grafted SEBS (e.g., SEBS-C6-Pip/MPy) was more significant than the shorter side chain-grafted SEBS (15–24%) such as SEBS-C1-Pip/MPy. Additionally, the hydration number at 80 °C showed that the longer side chain structured membrane (55–59λ) was superior to the shorter side chain-structured membrane (10–25λ). Longer side chain-tethered SEBS and shorter side chain-tethered SEBS exhibit high and low hydration numbers, 55 and 25, respectively, in the quaternization. WU was also increased as the temperature rose (30–80 °C). Therefore, the factors contributing to the membrane absorbing more water are the cation and the length of the side chains. Also, the SR of the piperidinium-functionalized shorter and longer side chain-tethered SEBS membranes was greater than that of the other cation-functionalized membranes due to the higher WU. As the temperature rises, the membranes' area and volume expand, i.e., the SR of the membrane is influenced by temperature.

As the wt% of 3-aminopropyl trimethoxysilane-functionalized GO (AGO) increased from 2–8%, the WU (24–37%) as well as SR (6.3–9.8%) were enhanced after the composite with PPO (PPO/AGO).²⁴² But according to PPO, WU and SR were just 20.3% and 4.7%, respectively (Fig. 43a). Thus, the AGO's hydrophilic capabilities might improve the membrane's intrinsic features. In a recent study, Peng et al. found that double antioxidant doping of quaternized poly(4-vinylbenzyl chloride-styrene) (e.g., QP(VBC-St)-MB/EA_0.3) significantly reduced the amount of WU (33.8%) compared to undoped (44.1%) and single antioxidant doped (59.7%) versions.²⁶¹ It could cause two antioxidants to be added to the membrane to increase its hydrophobic property. Similar results were found in the SR of quaternized poly(4-vinylbenzyl chloride-styrene) doped with two antioxidants (39.6%) compared to membranes that were left bare (60.7%) and membranes that were doped with a single antioxidant (45.5%). Antioxidant doping into membranes can also be used to regulate WU.

Modifying membranes using radical inhibitors can increase the WU. As a result, the changed QP(VBC-St) AEM with radical inhibitors (e.g., QP(VBC-St)-4-TBP_0.7) showed greater WU (39–47%) than unmodified QP(VBC-St) AEM (37%).²⁶² However, compared to other radical inhibitor-modified AEMs (39–45%), the benzophenone-based modified membrane (47%) had a higher WU. It could cause benzophenone's more hydrophilic structure compared to other radical inhibitors. The SR of the radical inhibitors-doped membranes (26–39%) showed a similar pattern, which was greater than that of bare QP(VBC-St) (27%). Coppola et al. improved the WU of an ABPBI with a PVA.¹³¹ As a result, the membrane ABPBI only exhibited 48.0% and was increased by 67.0% after compositing with PVA. Due to the PVA's hydrophilic properties, which allow it to absorb more water into the membrane, its WU was higher. The structure of the membranes is crucial for controlling the SR; for example, a membrane in the shape of a fiber (e.g., C-PVAf-ABPBI) had a lower SR (27%) than a membrane in the form of a sheet (84%) analyzed under 15% KOH solution up to 7 days. Compared to sheet-shaped membranes made by casting, the fiber (by electrospinning method) form's lower SR can result from less free space or volume to absorb less water. In a different investigation, higher WU, higher SR, and higher hydration numbers were shown to correlate with higher IEC values achieved by VImPPO-PVA (sIPN) composites.¹²⁸ However, when the IEC levels fall, so do the WU, SR, and hydration values (Fig. 43b). The declining WU, SR, and hydration numbers of composite membranes may result from functional group aggregation at higher concentrations, resulting in worse performances. In comparison to other ratios of VImPPO–PVA (8:2, 7:3, and 6:4), the composite membrane ratio 9:1's (sIPN-91) higher IEC values revealed greater WU (48.68%), SR (20.61%), and hydration numbers (16.41λ). While composite membranes with higher hydration numbers had higher WU, SR, and IEC, those with lower hydration numbers had lower WU, SR, and IEC. As a result, the hydration levels also affect the membrane's characteristics.

Improving the long-side chain-structured QAs-tethered membranes are expected to enhance the WU and SR. Accordingly, Tang et al. explored long-side chain structured BPTMA (27 and 48%)-tethered PBI (48%BPTMA-PBI) AEMs for testing the WU and SR.¹⁸⁷ To this extent, the WU (37–57%) and SR (25–30%) rose along with the percentage of BPTMA (27–48%) in the PBI. Due to its hydrophobicity, however, bare PBI had lower WU (16%) and SR (7%). BPTMA may be the superior QAs to increase the membrane's hydrophilicity and improve the WU and SR. According to Simari et al., Mg–Al LDHs-modified QPSU membrane (e.g., qPSU-LDH₅) demonstrated more water absorption (60%) than pristine QPSU (58%).¹³⁴ Additionally, increasing the LDH wt% from 1 to 5 results in a decrease (from 60% to 58%) rather than an increase in water absorption. It could cause the blocking of the composite membrane pore structures at higher concentrations. The LDHs-modified QPSU WU findings, however, showed that there is not much water absorption by the LDHs.

According to a recent study, PVBMPy AEM's high hydrophilic content with an IEC of 3.6 mmol g⁻¹ can lead to greater WU and SR.²⁶³ However, these values are useless for application usage because of the membrane's decreased mechanical stability brought on by greater WU and SR. As a result, PVBMPy was crosslinked with TMHDA (10–30%) before being combined with PSF and having verified WU and SR (Fig. 43c). The WU (49.4–16.3%) and SR (34.6–17.6%) decreased due to the rising crosslinker concentration (10–30%) in PVBMPy-PSF. PSF's hydrophobic nature is also one reason to lower the WU and SR of the PVBMPy-THMDA% at this stage. Thus, TMHDA and PSF were added to improve the PVBMPy performance by reducing WU, which in turn reduced the SR and can enhance the membrane health.

Similarly, increasing the crosslinking percentages (5–15%) of the PBP membrane into (SEBS-PBP-c-SEBS) revealed that decreasing the IEC resulted in lower WU (131–34.3%) and SR (13.2–91%) values.²⁶⁵ However, the WU and SR of the crosslinked PBP-SEBS membranes increased with rising temperature, indicating significant responsiveness to temperatures. To sum up, increased PBP crosslinking might lessen the WU, which is what causes lower SR to be attained. It could be the hydrophobic properties of PBP that are impeding the WU. Water molecules stimulate hydroxide ion conduction by the Grotthuss mechanism, which is why water absorption is directly proportional to the IC of the membranes. WU is further dependent on the membrane's hydrophilicity. Therefore, PEG, a hydrophilic oligomer, was investigated for WU and SR in the composite form with hydrophobic PPO (20PDM-PEG2000) at varying amounts (0.5–5%) of PEG.²⁵⁵ As the PEG percentages grew up to this point, the WU (73.4–89.7%) and SR (69.3–77.5%) both increased compared to that of pristine functionalized PPO (WU (68.2%) and SR (66.3%)). PEG can, therefore, improve the WU and SR properties of the membrane. Increased PEG, however, increased the likelihood that more water would be consumed, and swelling harmed the membrane's structural stability.

On the other hand, it was shown that the WU and SR decreased when the IEC values rose. As a result, the WU and SR were controlled by increasing the mass ratios (0.6–0.9%) of QVBC with PVA (PVA-PQVBC) and increasing the mass ratio (30–50%) of divinylbenzene (DVB) with QVBC in the composite membrane of PVA_0.6–0.9PQVBC_30–50% (cross-linked Q-PVBC/cross-linked PVA).²⁵⁷ Since the WU and SR are controlled when both QVBC with PVA and DVB with QVBC increase, the resulting crosslinked composite membrane may help improve the membrane's mechanical stability. Recently, different doses of electron beam irradiation at various atmospheres with varying temperatures have been investigated for WU and hydration numbers on VBC-grafted aminated ETFE AEMs.²⁹⁹ In comparison to a low degree, the smaller dosage of the beam, air, and RT, it was found that higher radiation degree-grafted ETFE had higher WU (256%) and hydration number (54λ) when exposed to a high dose (100 kGy) electron beam radiation under air environment and low temperature (e.g., 100-air-LT/RT). Possible causes include high-degree VBC grafting, high-dose electron beam treatment, and low-temperature enhancement of the high amination, which increased the WU and hydration number.

AEMs (m-TPN) were functionalized with N-cyclic-based cations to synthesize m-TPNPiQA, m-TPNPyQA, and m-TPNBeQA for evaluating the WU and SR.²⁴⁰ The WU and SR were found to be higher for m-TPNPiQA (52.28 and 21.14%) than m-TPNPyQA (45.12 and 16.15%) and m-TPNBeQA (30.5 and 12.5%). Due to the stiff structure, the AEMs had lower WU and SR than m-TPNPiQA and had lower IEC. Due to its high hydrophilicity, m-TPNPiQA has a greater WU, resulting in a higher SR and IEC. Additionally, it was verified that m-TPNPiQA (10.92λ) had a higher amount of hydration than m-TPNPyQA (10.36λ) and m-TPNBeQA (8.19λ) AEMs. As a result, high and low WU and SR are produced by hydrophilic and hydrophobic-based cations, respectively. Crosslinking's impact on fluorine-based PBBP membranes was observed on WU and SR by Zhou et al.³⁰¹ The BPPO-crosslinked PBBP had lower WU (28.1%) and SR (5.9%) at this point than DBMHDA-crosslinked PBBP (PBBP-DBMHDA) WU (53.5%) and SR (14.3%), BAPTES-crosslinked PBBP (BAPTESDBMHDA (WU (40.3%) and SR (11.2%)), and bare PBBP (WU (90.8%) and SR (28.2%)) at 80 °C. The shorter side chains between them, which result in less volume to take in water and fewer swellings and this crosslinked membrane may have high hydrophobicity, may cause a lower value of BPPO-PBBP. In this instance, more hydrophilic cations in the DBMHDA caused a significant increase in water absorption and swellings. The OH⁻ groups produced by the silica structure in BAPTES inhibit water absorption and result in low swelling values.

Intriguingly, WU and SR can be controlled by preparing copolymer-based AEMs. Thus, by raising the molar ratio (0.24–0.63 mol%) of their total units, the WU (18.8–9.0%) and SR (3.6–0.6%) were kept under control by the synthesized alkylene-based copolymer (quaterphenylene and biphenylene) (e.g., P(4PA-co-2PA)-24).³⁰⁰ In contrast, the WU (28.7%) and SR (6.1%) were enhanced in the quaterphenylene alkylene-free copolymer. Because of its hydrophobic nature, water absorption and swelling may decrease by increasing the copolymer unit's quaterphenylene alkylene molar ratio. It was also shown that the rigid/hydrophobic structure of quaterphenylene alkylene causes the hydration number of the copolymer (4.5–2.5λ) to decrease with an increase in the total molar ratio (0.24–0.63 mol%) of the copolymer unit, i.e., the water absorption of the copolymer unit was reduced with increase in the molecular weight of the total unit. On the other hand, when comparing the copolymer structure forms, the quaterphenylene alkylene-free membrane revealed a higher hydration number (7.8λ), indicating a greater capacity for water absorption.

The quantity of hydrophilic groups in the membrane determines how much water may be absorbed. Low water uptake indicates that hydrophobic membrane groups are much more abundant than hydrophilic ones. Because of this, when the number of hydrophobic fluorobenzoyl groups (from mono to penta) in the quaternized SEBS (HQA-F1-SEBS to HQA-F5-SEBS) membranes increased, the WU (150.9–136.0%) and SR (39.0–36.8%) declined at 80 °C.¹³⁹ More water WU (165.9%) and SR (47.9%) were seen with the fluorobenzoyl-free hydrophilic quaternized SEBS. Therefore, the increased water and swelling in the membrane may be reduced by the hydrophobic groups of fluorobenzoyl. When compared to single cations (less concentrated cations)-functionalized backbone (21.8% and 10.9%), clustered cations-functionalized backbone (e.g., 3QPAP-0.5) exhibits an increase in WU (8.3–26.0%) and SR (2.5–9.2%).²⁹⁵ In contrast to cluster cation-functionalized backbone, it is uncommon to observe that the SR was better for single cation-functionalized backbone. As a result, compared to a single cation-functionalized membrane, cations in more excellent concentration cluster forms stimulate hydrophilicity to absorb more water. It was also demonstrated that clustered cation-functionalized membranes (6.4λ) had more significant hydration numbers than single cation-functionalized membranes (6.0λ), which produced higher and lowered WU, respectively. In another investigation, raising the ratio of monomers and degree of brominations (e.g., 60MePh-2.07) led to improved cationic groups, attributed to increasing the WU and SR.²⁸⁷ The membrane's conductivities might benefit from increased cationic group sites, thus boosting water uptake.

It is well known that a higher membrane IEC results in a higher WU. It was determined that the IEC (1.7 mmol g⁻¹) and WU (144%) of polystyrene copolymer-based AEM (e.g., HTMA-DAPP) were superior to those of poly(phenylene) copolymer-based AEM (1.5 mmol g⁻¹ and 98%).³²⁹ As a result, the hydrophilic nature of the polystyrene copolymer-based AEM structure may cause it to absorb a more significant proportion of water than the poly(phenylene)-based copolymer. Another investigation found that because of the PAPQ83 AEM's highly hydrophilic structure, its effective water absorption resulted in a 100% WU.²⁹⁴ The membrane may include hydrophobic structures, leading to a 37% decrease in WU compared to commercially available Aemion™. As a result, it is possible that WU is dependent on the hydrophilic structure of the AEM and that WU plays a significant role in ion conduction across the membrane. Water molecules move more freely at higher temperatures, allowing membranes to absorb more water. TMHDA and its ratio increase quaternization to reduce the WU and SR. Therefore, bication-functionalized AEM (e.g., m-XPTPA20-2N) without quaternization displays greater WU and SR at higher temperatures.²⁷⁹ Increased hydrophobic characteristics in the membrane may result from decreased WU and SR caused by quaternization using THMDA on bication-tethered AEM. It was also shown that the hydration number (15.6–10.0λ) of the bication-functionalized AEMs decreases with an increase in the cross-linking ratio (TMHDA (10–40%)). This work indicates that the water inside the membrane decreases proportionately to the cross-linking present.

Quaternized PAEK with a long alkyl chain-based carbazole pendant cation (e.g., PAEK-HQACz-0.7) demonstrated higher WU and better SW control than quaternized PAEK with a carbazole-based cation functional group but no long alkyl chain.¹¹⁰ When the concentration of cations increased with the increase in the number of QAs, the WU and SR rose accordingly (Fig. 44a). However, the entanglement effect was thought to be responsible for limiting the size alterations of the fluorinated membranes when a long alkyl chain was added to the fluorinated membrane. IEC results are linked to membrane SR. To that end, the SR of a (vinylbenzyl)trimethylammonium chloride-tethered fluorinate membrane (e.g., PVIB-10) rises from 13.2 to 20.5% as the IECs (1.43 to 1.82 mmol g⁻¹) are raised.³²⁶ It may be because (vinylbenzyl)trimethylammonium chloride raises the membrane's hydrophilicity, thus increasing its SR. According to another study, when the percentage of long alkyl chain-based bis-piperidinium cation groups in the quaternized membrane (e.g., QPBPip33Ac) was raised from 33 to 67%, the WU was reduced by 53.3% to 8.7%, and the SR was decreased from 23.50% to 23.75%.²⁹⁸ The potential causes of this rise in hydrophobicity include a decrease in the membrane's water utilization efficiency and swelling. With increasing levels of bis-piperidinium cation functionalization, it was shown that the hydration number of the membrane dropped from 10.4 to 3.1λ, indicating that less water was absorbed.

The WU (19.6–50.7%) and SR (16.7–26.1%) at 80 °C have recently been shown to increase with increasing ratios of multi-cation (30–80%) structured compound in the quaternized fluorinated cross-linked membrane (PHFB-VBC-DQ).²⁸⁴ Increased water absorption and some swelling may result from the hydrophilic nature of multi-cation. Higher cation ratios resulted in more water absorption by the AEM, as seen by a rise in hydration (6.8–12.5λ). It is well known that WU is proportional to IEC. However, Jheng et al. found that WU drops as the IEC values climb.³⁰⁵ Dimethylimidazolium-tethered PBI AEMs demonstrated 62.7% WU and 26.7% SR. After being hybridized with silicon (Si) (PBI–DIm–Si hybrid), the WU (30.9%) and SR (10.0%) were both reduced, and the hydration number revealed that the bare membrane had a more effective water absorption rate (18.8λ) than the hybrid form (9.2λ). Accordingly, the WU and SR could be controlled if the membrane was composited with Si. Similarly, membranes composed of block co-polymer structures comprising quaternized hydrophobic fluorinated polymer and hydrophilic polymer (e.g., DMBA-PIM-b-PSF-0.91) may exert control on the WU and SR.³⁰⁷ The increases in WU and SR were also seen in quaternized hydrophilic polymer membranes that lacked a block-copolymer structure-based membrane (Fig. 44b). The copolymer structure that contains fluorinated hydrophobic polymer restricts the WU.

Intriguingly, the copolymers (poly(p-phenylene) and poly(arylene ether))-based AEMs (QCPPAE) were studied for their effects on WU and SR when the monomer-to-trimer ratio was altered from 2:1 ((QCPPAE)-2/1) to 4:1 ((QCPPAE)-4/1).³⁰⁶ When the copolymers' monomer content is raised, the WU (24–40%) and SR (11–23%) are raised. Since a more significant monomer concentration results in more water absorption through the membrane, utilizing a minimum monomer concentration to control the membrane's dimensions is preferable. By constructing the membrane with a TMHDA cross-linking structure (e.g., L-FPAEO-75-MIM), it can exert control over both the WU and the SR.²⁷⁸ WU has recently been evaluated for quaternized perfluorinated polymers such as TMA-Nafion and Aquivion-TMA.²⁷⁶ The WU of the two membranes was 21% and 23%, respectively. As a result, the SR of the two membranes was only 5% comparable. It is possible that the hydrophobic character of these membranes limits water absorption, resulting in reduced swelling, and that they may be utilized for long-term applications in fuel cells, water electrolysis, and so on. Polyfluorene membranes in the OH⁻ form (e.g., PFOTFPh-C6-TMA(OH)) were shown to have improved WU and SR over those in the Cl⁻ form, as corroborated by Miyanishi et al.¹⁰⁰ The results of this research make it clear that the OH⁻ form AEMs are more hydrophilic than the Cl⁻ form AEMs, which suggests that OH⁻ form polyfluorene AEMs may have higher WU and SR.

Very little water WU (4.1–10.8%) and SR (1.7–3.23%) were discovered in the multi-cation ionic liquid-functionalized fluorene-based (e.g., TQ-PDBA-70%) AEMs.²⁸³ It could be because the long and flexible multi-cation ionic liquid side-chain acts as a crosslinker between the leading backbone chains, providing adequate space to store water. However, the crosslinked cations structure somewhat restricts the top chains' transport and the membranes' bulging. As a result, the multi-cation functionalized fluorene-based AEM reduced WU and swelling, which might be helpful in long-term applications. Recently, WU and SR were examined using single and tri-QAs tethered PPO and fluorine-tethered PPO with tri-QAs.²⁸² Tri-QAs-tethered PPO (e.g., PPO-22-3QA8F) was found to have greater WU and SR than single QAs-tethered PPO and fluorine-tethered PPO-tri-QAs. Because tri-QAs-tethered PPO is more hydrophilic than other AEM, it has a greater WU and SR. Other AEMs may have weaker hydrophilic properties, reducing the WU and swellings.

4.3 Ionic conductivity (IC)

The ionic conductivity (IC) of AEM is a crucial component in water electrolysis; it depends on the quantity and mobility of hydroxide ions (anions) within the membrane and is also influenced by the transport characteristics of the membrane. A higher anion conductivity in the membrane results in lower power losses, which boosts the power density. The IC of a membrane is one of the most important factors in determining the current density of an AEM. This is because a high hydroxyl ion conductivity makes it possible for the membrane to reach large current densities.³² IC measurements often account for steps such as AEM pretreatment, AEM impedance measurement, and IC calculation from the data obtained via impedance. It is assumed that pre-treating membranes for ion exchange involves converting them to hydroxide (OH⁻) or chloride (Cl⁻) ions. Most researchers carried out the following procedure for the IC measurement. The membranes are soaked in NaOH solutions of varying concentrations and lengths of time. After ion exchange, AEMs are carefully washed with clean water to remove any remaining OH⁻ or Cl⁻ form ions. Impedance spectroscopy is the usual test method for determining the ion conductivity and is applied to the AEM following pre-treatment. Generally, the conductivity (σ) of the membrane was tested on an electrochemical workstation. The 1 cm × 4 cm membrane was fixed in a Teflon mould and dipped in pure water. Under hydrated conditions, ohmic resistance (R) in kiloohm (kω) was measured by electrochemical impedance spectroscopy (EIS) (50 mHz–100 kHz), and the conductivity of the membrane was calculated by eqn (13).²³⁵

	(13)

Eqn (13) shows the membrane's cross-sectional area and the distance between reference electrodes A and L.

According to research by Liu et al., p-quaterphenyl, an aromatic monomer utilized to create the AEM PQP-100 (56.7 mS cm⁻¹) has a higher IC value than p-biphenyl (PBP) and p-terphenyl (PTP), which were used to develop the AEM PBP-67 (39.2 mS cm⁻¹) and PTP-83 (47.2 mS cm⁻¹), respectively.²³⁵ Compared to PBP-67 and PTP-83, whose conductivities were low, PQP-100's enhanced conductivity was caused by the well-ordered microphase hydrophilic (p-quaterphenyl)-hydrophobic (polymer backbone) separation effect. Additionally, the ionic conductivity of AEMs increased with respect to the temperature. AEMs such as the PEGDA and PS-DVB of conductivities in the form of Cl⁻ (1.2 and 7.0 mS cm⁻¹) and HCO₃⁻ (0.8 and 4.1 mS cm⁻¹) were found to be the membrane in the form of HCO₃⁻ and had lower conductivity than the conductivities in the form of Cl⁻.²⁶⁰ Low water absorption-based membranes may achieve high IC in the Cl⁻ form because the state of AEM hydration in Cl⁻ was lower than in the HCO₃⁻ form. In another investigation, the IC value was measured at 25 and 80 °C with and without an n-butyl-functionalized QAs-NPBI (NPBI-QA55-B45) membrane.¹⁸⁸ After n-butyl was functionalized, IC increased along with the number of QAs in NPBI (8.9 to 31.7 mS cm⁻¹ at 25 °C). But adding more n-butyl did not significantly improve the IC. Therefore, more hydrophobic n-butyl content might lower the IC of QAs-NPBI, and the membrane at 80 °C had a better IC than at low temperature (25 °C). Increasing the number of QAs into NPBI without n-butyl showed significantly lesser IC (2.8 to 2.96 mS cm⁻¹ at 25 °C). Therefore, to improve conductivity in the membrane, n-butyl-based groups may be helpful.

Dual QAs side chains are promising for membrane ionic conduction, resulting in improved hydroxide conductivity. Accordingly, dual QAs-tethered azidated PPO in the form of OH⁻ membrane (e.g., DQ-PPO-17-OH) showed high IC (1.35 to 1.70 mS cm⁻¹), although the IC depends on the temperature.²³¹ On the other hand, the ionic conductivity of the membrane increases with increasing temperature, such as 20 °C (19.1 to 40.5 mS cm⁻¹) and 80 °C (35.0 to 63.9 mS cm⁻¹) (Fig. 45a). Therefore, dual QAs, azidation degrees, and temperature are all three factors that influence the IC conductivity in the membranes. In a different study, the stronger WU character of PPO-iP (e.g., PPO-iP-44) yielded higher IC (64.8 mS cm⁻¹).²³² The higher IC might cause the membrane's higher WU and good microphase separation. Consequently, the low IC values of 60.4 and 59.6 mS cm⁻¹ were produced using the low WU features of the membranes PPO-nP and PPO-CH.

The IC's dependence on the membranes' capacity for water absorption has been proven. Li et al. reported that the hydrophilic extender built into the side chain was coupled to the PPO via a hydrophobic alkyl spacer (e.g., SDQEO) and had a high IC (16.9–40.1 mS cm⁻¹) at 25 °C.¹¹³ However, at 80 °C, this AEM had a high IC (87.3 mS cm⁻¹). It might be the case that alkyl spacers encourage increasing ions to high WU, leading to increased conductivity without dimensional change. Therefore, the membrane's dual function as a hydrophilic extender and hydrophobic spacer might improve its conductivity for energy generation. Due to the membrane constructed without an alkyl ether bond by Hu et al., the IC of the N-cyclic piperidiniums-functionalized PTP-based AEMs (e.g., PTP-90) was investigated as a function of both temperature and IEC.²³⁴ The membrane's conductivity (64.4–129 mS cm⁻¹) rose when the temperature (from 20–80 °C) and IEC (from 2.0–2.52 mmol g⁻¹) increased (Fig. 45b). The rapid water movement in the membranes caused the conductivity to improve as a function of temperature.

Poly(biphenyl piperidinium)-based AEM (PBPCL1.48) functionalized with high cation groups demonstrated better conductivity (64.2 mS cm⁻¹) at 30 °C than PBPCL0.79 (31.4 mS cm⁻¹) and PBPCL1.29 (51.8 mS cm⁻¹), which have lower cation concentrations.²⁸⁸ It can be the case that AEMs with high cation concentrations will improve their hydrophilicity and support ion transport through self-aggregation (Fig. 45c). In contrast, it was found that the IC of the polymer composite PS/pTAP AEM with a 40:60 (PS/pTAP (40:60)) ratio rose with rising temperature. However, at 40 °C, this kind of polymer composite did not show a significant IC (4.2 mS cm⁻¹).²³⁷ Using a specific cation and extending the side chain with membranes, the IC of the membrane could not be enhanced. As a result, MPip-functionalized longer side chain-grafted SEBS membranes (e.g., SEBS-C1-MPy) exhibit little higher IC than those functionalized with other cations (MIm, MPy, and 2-methyl-1-pyrroline) for shorter side chains.²³³ SEBS-C1-MPy, however, had a lower IC (23 mS cm⁻¹). It's low WU brought on the membrane's lower IC, and when the temperatures increased, the membrane's IC did not change as much (Fig. 45d).

The combination of aminopropylsilane-treated GO and QPPO (QPPO/AGO) exhibits good ion transport and conductivity.²⁴² Additionally, the hydrophilic pathways that facilitate ion transport were exposed by the membrane containing aminopropylsilane-treated GO forms (Fig. 45e). As a result, the conductivity of pure Q-PPO (35.2 mS cm⁻¹) was lower than that of the Q-PPO-AGO composite (83.8 mS cm⁻¹). The AGO's hydrophilic characteristics might cause the composite membrane's improved conductivity. Due to the dispersion of water molecules and ion transport at higher temperatures in the membrane, IC generally increased with temperature. Similarly, IC examined a single antioxidant or two antioxidants (e.g., QP(VBC-St)-MB/EA0.3) with a quaternized membrane for functionalization.²⁶¹ However, functionalizing one or two antioxidants with the quaternized membranes did not affect the membrane's IC (Fig. 45f). As a result, antioxidants did not aid in increasing the IC of the membranes. Recently, Ye et al. found that the IC of modified QP(VBC-St) membranes with various radical inhibitors (RI) (QP(VBC-St)-4-RI_0.7) were lower than the IC (40.8–56.1 mS cm⁻¹) of bare QP(VBC-St) membranes (68.7 mS cm⁻¹) at 80 °C.²⁶² The lower IC of the radically modified membranes was caused by the membranes' poorly organized microseparation morphology or the lack of hydroxide groups in the radical inhibitors or not having functional groups for OH⁻ conduction. In contrast to radical-modified AEMs (40.8–53.7 mS cm⁻¹), tempamine-based radical inhibitor-modified membrane (QP(VBC-St)-TPA_0.7) and 4-tert-butylphenol-based radical inhibitor-modified membrane (QP(VBC-St)-4-TBP_0.7) had a higher IC 56.1 and 53.7 mS cm⁻¹, respectively.

By combining the membrane with organic structured polymers, the IC of the membrane might be increased. Recently, PVA was crosslinked with ABPBI (e.g., C-PVAf-ABPBI) polymer using glutaraldehyde (43 mS cm⁻¹).¹³¹ This composite material performed better in IC than bare PVA crosslinked with glutaraldehyde (28 mS cm⁻¹) and PVA crosslinked with ABPBI fiber (36 mS cm⁻¹) at 60 °C. The higher IC of the composite membrane may be attributable to the separation of hydrophilic and hydrophobic molecules in a well-ordered manner to increase the conductivity of the membrane. VImPPO was combined with PVA in different ratios (9:1 to 6:4) to prepare AEM (sIPN).¹²⁸ In comparison to PVA, VImPPO has higher hydrophilicity. Thus, the conductivity of polymer networks with an adjusted ratio of VImPPO:PVA (sIPN-91 to sIPN-64) from 9:1 to 6:4 was evaluated. It was discovered that the composite IC (28.8–16.4 mS cm⁻¹) dropped for the membrane's ratios from 9:1 to 6:4. It could be because VImPPO in more significant concentrations has more hydrophilicity, which allows for more water absorption and better conductivity. Additionally, reduced water absorption and conductivity were linked to a reduction in hydrophilicity when VImPPO concentration was lower. As a result, this investigation amply demonstrated that IC was dependent on the hydrophilicity of the membrane's ratio.

Due to the membrane's strong ion-conducting functional groups, membranes with higher IEC exhibited higher IC. The longer side chain BPTMA-tethered PBI (e.g., 48%BPTMA-PBI) had higher ion conduction groups up to this point, which resulted in higher IEC.¹⁸⁷ Due to the increasing IEC data, the IC should also increase. As discussed and proved in the IEC section, these membranes have a high IEC value. It was concluded that increasing the percentage (27–48%) of BPTMA in the PBI membranes at 80 °C will result in a higher IC (8.3–29.8 mS cm⁻¹).¹⁸⁷ Additionally, the longer side chain 48% BPTMA of the QAs-tethered PBI's IC had a strong temperature response, increasing the IC values from 9.9 to 29.8 mS cm⁻¹ with a temperature rise (30 to 80 °C). The 27% BPTMA-tethered PBI also exhibits a similar pattern. As a result, longer side chain-organized QAs may help to improve the IC for membranes. Interestingly, raising the temperature from 30 to 60 °C results in a corresponding rise in the IC of the LDH-modified quaternized polysulfone (e.g., qPSU-LDH₅) from 19 to 60 mS cm⁻¹.¹³⁴ The membrane's IEC determines its IC. As a result, the crosslinking degrees of the PVBMPy composite (2.4–1.6 mmol g⁻¹) with PSF decreased as they increased (10–30%).²⁶³ To a similar extent, crosslinking degrees (10–30%) reduced the IC (40–17.5 mS cm⁻¹) of PVBMPy-PSF AEMs (PVBMPy-CL-PSF). It could cause a narrow constant ion conduction path and hamper the ionic migration caused by the PSF polymer chain.

Wang et al. demonstrated that the IC of the membranes is independent of the amount of crosslinking.²⁶⁵ The conductivity was raised to 10% of the crosslinked composite membranes (e.g., 10%-PBP-c-SEBS) and then dropped at 15% crosslinking, according to the IC analysis of increasing the crosslinking percentage (5–15%) of the PBP polymer to SEBS membranes. It may have contributed to the membranes' WU quickly dropping to 15% for crosslinked composite membranes. As a result, the IC of the membranes may depend on the WU. Similarly, at 80 °C, the IC of a functionalized PPO membrane with varying PEG composite membrane (e.g., 20PDM-PEG2000) percentages (0.5–5%) was investigated.²⁵⁵ At this point, increasing the PEG percentages up to 2% (e.g., 20PDM-2%PEG2000) raised the IC (85.7–97.2 mS cm⁻¹), and then IC was lowered (Fig. 46a). The conductivity may be limited due to ion buildup at greater concentrations. The obtained IC for functionalized bare PPO was 73.2 mmol g⁻¹. However, the inclusion of PEGs can improve the PPO of IC.

Water uptake in the membrane and IC are linked, meaning that IC rises with the water uptake. It is rarely seen that WU decreases while IC increases. The strategy was applied to the structure of glutaraldehyde-crosslinked PVA_0.6–0.9QVBC_30–50%.²⁵⁷ The IC values were increased and decreased in this composite structure by increasing the mass (0.6–0.9%) ratios of the two polymers (PVA and QVBC) as well as increasing the mass (30–50%) ratio of the DVB and the polymer (QVBC). Therefore, it was demonstrated that WU reduces when the mass ratio of polymers is increased as well as when crosslinkers (glutaraldehyde) are used with polymers (Fig. 46b). It has been shown that increasing the mass ratio of QVBC and PVA increases IC, but increasing the mass ratio of DVB and QVBC reduces IC. Similarly, Biancolli et al. observed higher WU, and the higher IEC of high-degree electron beam-treated VBC-grafted ETFE (e.g., 100-air-LT/RT) AEMs demonstrated low conductivity (113.5 mS cm⁻¹), low WU, and low IEC of low-degree electron beam-treated VBC-grafted membranes (146 mS cm⁻¹).²⁹⁹ Recent research has demonstrated that the high hydrophilic (WU) property of m-TPNPiQA (22.1 mS cm⁻¹) had a higher IC than the lower WU properties of m-TPNPyQA (14.9 mS cm⁻¹) and m-TPNBeQA (12.1 mS cm⁻¹).²⁴⁰ m-TPNPiQA with a well-arranged microseparation morphology made of the hydrophilic CH₃I cation and hydrophobic polymer backbone (m-TPN) to create a channel for the ions to conduct may have contributed to the higher IC.

The fluorinated segment in the polymer backbone is anticipated to increase the polymer's hydrophobicity, ultimately resulting in a higher degree of MSM. This configuration should be advantageous for developing an MSM and a highly efficient ion-conducting channel. Recently, cations such as DBMHDA, BAPTES, and the polymer BPPO crosslinked the fluorine-based fluorinated PBBP membrane.³⁰¹ The DBMHDA-crosslinked PBBP (PBBP-DBMHDA) showed higher IC (74.7 mS cm⁻¹) than BAPTES (60.0 mS cm⁻¹), BPPO-crosslinked PBBP (36.0 mS cm⁻¹), and bare PBBP (34.8 mS cm⁻¹) because of its well-ordered microphase separation morphology, which provided more cations (Fig. 46c). Additionally, BAPTES-PBBP showed a morphology with well-separated hydrophilic and hydrophobic regions; nevertheless, no new cations were generated, and BAPTES silica structures blocked OH⁻ from being transported, resulting in low WU and decreased IC. The example of BPPO-functionalized PBBP revealed that extremely low IC might be caused by lower cation generation or microphase separation morphology. As the molar ratio (0.24–0.63 mol%) of the whole unit increases, the IC (35–22 mS cm⁻¹) of the copolymer-based AEMs (e.g., P(4PA-co-2PA)-24) decreases, which is an intriguing phenomenon to consider.³⁰⁰ Generally, excellent IC is accomplished by the microphase separation morphology of the membrane, i.e., the well-ordered hydrophilic and hydrophobic architecture supports the conductivity. However, as the concentration or molar ratio of the copolymer's unit rises, the conductivity falls because an increasing number of ions accumulate and block the ionic pathway, thus limiting ion conduction in the membranes (Fig. 46d).

The conductivity of the membrane might be improved in the presence of ions with high electronegativity and ionic clusters. Elevating the electronegativity and ionic clusters of fluorobenzoyl groups in the quaternized SEBS improved the membrane's IC at 8 °C.¹³⁹ The quaternized SEBS with a mono- or penta-fluorobenzoyl group (e.g., HQA-F1-SEBS and HQA-F5-SEBS) added to it has a larger IC (71.2 and 87.0 mS cm⁻¹) than when it is fluorobenzoyl-free (63.6 mS cm⁻¹). Elevating the number of fluorobenzoyl groups from one to five resulted in a similar rise in electronegativity, and ionic clusters were responsible for the subsequent increase in IC. Due to the solid microphase separation, clustered cations functionalization with increased weight ratios (0.3–0.5%) of the backbones (e.g., 3QPAP-0.5) had higher conductivities (21.4–45.0 mS cm⁻¹) than single cation-functionalized backbones (27.7 mS cm⁻¹).²⁹⁵ The higher order of microseparation by clustered cations (hydrophilic) and polymer backbone (hydrophobic) structures may cause the rising conductivity. Increases in the monomer ratio and bromination degree (e.g., 60MePh-2.07) have recently been discovered to boost the membrane conductivity.²⁸⁷ The strong hydroxide conductivity may be attributed to the cationic group sites created with the increased monomer/bromide molar ratio.

In most cases, higher water intake corresponds to higher conductivity. However, the microphase separation in the membrane structures is crucial to the conductivity. Motz et al. agreed that the AEM's conductivity is not directly connected to its ability to absorb water but may result from microscopic phase separation structures.³²⁹ Polystyrene copolymer-based AEM has low IC values (50–61 mS cm⁻¹) and high WU values (100–144%). Poly(phenylene) copolymer AEM (e.g., HTMA-DAPP), on the other hand, has a low WU (98%) and a high IC (90 mS cm⁻¹). Compared to polystyrene copolymer-based AEM, the higher IC produced by poly(phenylene) copolymer-based AEM structure may be due to high microphase separation (hydrophilic–hydrophobic) morphologies. When the membrane is very hydrophilic, it has a high conductivity. Because of the high microphase separation and WU of this membrane structure, the PAPQ83 AEM exhibited a high conductivity (148.0 mS cm⁻¹).²⁹⁴

Another research found that when the cross-linking ratio of TMHDA in a bication-functionalized membrane (e.g., m-XPTPA20-2N) increased from 10% to 40%, the conductivity reduced from 54.5 to 50.0 mS cm⁻¹ at 80 °C.²⁷⁹ It may be due to lessening WU and a lack of order in the microphase separation that would occur with a higher cross-linking ratio. Developing a lengthy alkyl chain-containing hydrophilic cation-tethered fluorinated hydrophobic backbone promotes a well-ordered microphase separation structure (MSM) and characteristics that lead to high conductivity. Altering the cation ratio from low to high respectively acquired low to high dense amination group following quaternization, which allowed for very thick QAs. Increasing the concentration of QAs is essential for improving the AEM's conductivity. Accordingly, high MSM and conductivity (98.1 mS cm⁻¹) were found in PAEK with a long alkyl carbazole side chain, followed by a tethering high density of QAs (e.g., PAEK-HQACz-0.7).¹¹⁰

Similarly, membrane IECs findings are linked to WU, SR, and IC. Accordingly, increases in the weight ratio of (vinylbenzyl) trimethylammonium chloride's (4–10%) increases the fluorinated (vinylbenzyl)trimethylammonium membrane (PVIB), which increases the IC (41.2–48.4 mS cm⁻¹), leading to increases in both the IECs (1.43–1.82 mmol g⁻¹) and the SR (13.2–20.5%).³²⁶ A higher SR and conductivity with IECs may be attributed to (vinylbenzyl)trimethylammonium chloride's hydrophilic character. Thus, the membrane's hydrophilicity is crucial to its conductivity. The membrane's microphase separation morphology (MSM) determined its conductivity. Because of this, increasing the long alkyl bis-piperidinium cation functionalization of the membrane did not result in a good MSM, meaning that not enough hydrophilic–hydrophobic segregation was produced.²⁹⁸ Better IC (27.7 mS cm⁻¹) was achieved with a lower concentration of bis-cation-functionalized membrane (e.g., QPBPip33Ac) compared to the AEM with a greater concentration of bis-piperidinium cations (19.8–7.5 mS cm⁻¹). It was discovered that the membrane has a structure of multi-cations and cross-linking, which accounts for its high conductivity.²⁸⁴ A higher ratio (30–80%) of multi-cations in the fluorinated cross-linked membrane (e.g., PHFB-VBC-DQ-80%) increased the conductivity in this study from 55.1 to 135.8% mS cm⁻¹. Higher conductivity indicates that the membrane has a well-structured MSM.

In most cases, the hybrid membrane's hydroxide conductivity exceeded that of the bare membrane because of the orderly separation of hydrophilic and hydrophobic phases. Dimethylimidazolium (DIm)-functionalized fluorinated PBI bare membranes had a hydroxide conductivity of 50.2 mS cm⁻¹. In contrast, Dim-functionalized fluorinated PBI-silicon hybrid membranes (PBI–DIm–Si hybrid) had a conductivity of 22.2 mS cm⁻¹, as measured by Jheng et al.³⁰⁵ The poor conductivity was obtained because the microphase separation in the hybrid membrane did not develop correctly compared to the bare membrane. According to another research, membranes having a quaternized mixed block co-polymer structure (e.g., DMBA-PIM-b-PSF-0.91) had more conductivity than those without such a structure.³⁰⁷ They obtained a well-ordered MSM structure in the hybrid block-copolymer structure, consisting of fluorinated hydrophobic and hydrophilic polymers with increased conductivity. The conductivity of the copolymer-based AEM (e.g., QCPPAE-4/1) produced by Cha et al. was found to be improved by increasing the quantity of monomer (2–4%) while keeping the amount of trimer constant (1%).³⁰⁶ Increasing the monomer content has been shown to promote hydrophobic–hydrophilic separation for ion movement to boost the membrane's ability to conduct ions. The membrane's conductivity may be increased by developing it with cross-linking and increasing the quantity of the polymer ratio in the membrane's composition. Accordingly, an N-methyl imidazole-functionalized 50% fluorinated polymer membrane exhibited 5.4 mS cm⁻¹, and following cross-linking (quaternization) by TMHDA, this value rose to 6.5 mS cm⁻¹.²⁷⁸ When the fluorinated polymer ratio was increased to 75% (e.g., L-FPAEO-75-MIM), either with or without crosslinking, the results were 16.9 and 17.0 mS cm⁻¹, respectively.

Perfluorinated polymers are designed with well-ordered microphase separation morphologies (MSM) to improve the conductivity. However, MSM's conductivity varies depending on the polymers from which it is made. Compared to quaternized Nafion (27 mS cm⁻¹), quaternized Aquivion (e.g., Aquivion-TMA) was shown to have more excellent conductivity (38 mS cm⁻¹).²⁷⁶ Aquivion AEM suggests that increased conductivity may be due to a shorter side chain, high IEC, and higher MSM. At 80 °C, the OH⁻ form of polyfluorene-based AEMs (e.g., PFOTFPh-C6-TMA(OH)) was shown to have a higher conductivity (101–156 mS cm⁻¹) than the Cl⁻ form (52–133 mS cm⁻¹), perhaps because the hydroxide form of the membrane had a higher hydrophilic–hydrophobic separation and higher water absorption.¹⁰⁰

Higher IEC findings resulted in higher water uptakes related to high IC. As a result, an adjusting ratio (30–70%) of PDBA in the fabrication of multi-cation (NQQN)-functionalized fluorene-based AEMs (e.g., TQ-PDBA-30–70%) was attributed to increased IEC (1.09–2.1 mmol g⁻¹) and WU (4.1–10.8%).²⁸³ At this stage, the membrane had a higher IC (78.8–136.2 mS cm⁻¹). As a result, multi-cation-functionalized fluorene-based AEM may have good hydrophilic-hydrophobic separation, which would lead to good ion conduction across the membrane. In general, a higher WU membrane led to a higher IC. However, a recent study has demonstrated that IC depends not on WU but on microphase separation morphologies.²⁸² As a result, the low WU property of single QAs-tethered PPO (35.2 mS cm⁻¹) and fluorine-tethered PPO with tri-QAs (e.g., PPO-22-3QA8F) exhibited more excellent IC (75.5 mS cm⁻¹) than the higher WU property of tri-QAs tethered PPO (23.5 mS cm⁻¹). Thus, fluorine-tethered PPO-tri-QAs with high IC could achieve high microphase separation.

4.4 Alkaline stability (AS)

The alkaline resistance (AS) of the AEMs was considered by seeing the changes in the hydroxide conductivity at various times. Generally, membranes were immersed in 1 mol L⁻¹ KOH or NaOH for varying times at 80 °C, and the OH⁻ conductivity was then measured. When constructing membrane electrode assemblies (MEA), AEMs must be stable and resistant to oxidizing and reducing species. The chemical stability of AEMs has to be enhanced as there are several reports of their instability in alkaline media, particularly at high temperatures. Therefore, the membrane's chemical stability (CS) must be boosted. The swelling index may be used to study the CS of AEMs and ought to be high, especially at alkaline pH.³³¹ QAs are present in most AEMs and are generally stable in an alkaline solution. Hydroxide anions, which are excellent nucleophiles, replace the QAs in AEMs, which causes them to be unstable in alkaline environments.¹⁰³ The destabilization of cations and main polymer backbones with relevant aryl ether linkers in hostile alkaline conditions is one of the hurdles for AEMs. When subjected to high-pH conditions, AEM conductivities vary over time, and the alkaline stability (AS) measures this change. The basic idea is to immerse the AEM stability for longer durations in a high-pH environment (1 to 10 M KOH) at a specific temperature (such as RT or a higher temperature) while occasionally evaluating the membrane conductivity to observe how it varies over time.⁸⁸ To this degree, the alkaline stability of AEMs (PQP-100, PBP-67, and PTP-83) was studied by dipping the membrane in 1 M NaOH solution at 80 °C with time variation.²³⁵ Accordingly, the IC value of PQP-100 slowly decreased with increasing time, showing that it is high alkali resistance (70–75% conductivity remaining after 1344 h); however, still, it has a higher IC value compared to PBP-67 and PTP-83 as it was a low hydrated membrane (contains high hydrophobic aromatic monomer) that is safe from OH⁻ ions attack.

Wang et al. observed that QAs-NPBI with n-butyl functionalization (e.g., NPBI-QA55-B45) demonstrated 48.2% conductivity, which is still present after 1200 h in 1 M NaOH at 80 °C.¹⁸⁸ In contrast, QAs-NPBI without functionalization showed an abrupt conductivity decrease after 50 h owing to OH⁻ attack (Fig. 47a). The membrane's stability may benefit from n-butyl functionalization in an alkaline environment. The developed dual QAs side-chains were effective in improving the AS. As the amount of time decreased, the conductivity of the dual QAs-functionalized azidated PPO (e.g., DQ-PPO-17-OH) AEMs steadily reduced.²³¹ The maximum initial conductivity loss (7.4%) was found for the optimized membrane (DQPPO-17-OH), indicating that the membrane had a high AS (Fig. 47b). The high ion content (dual QAs side-chains) may have contributed to the high AS. Liu et al. observed high alkaline stability for low WU characteristics of the PPO-CH membrane at 1 and 2 M NaOH at 60 °C with retained conductivity of 99.2% and 98.8%, respectively, for 216 h.²³² Besides, the high WU characteristics of PPO-iP (e.g., PPO-iP-44) and PPO-nP showed high conductivity loss of 65% and 74%, respectively, at 2 M NaOH. The high AS of PPO-CH was due to high cyclohexyl substitution on QAs, which restrict the OH⁻ ion attack. AEMs with an alkyl spacer (e.g., SDQEO) demonstrated more excellent chemical stability (77% conductivity was retained after eight days) than AEMs without one in 1 M KOH and at 60 °C due to the deteriorating effect of the electron-retreating backbone on the cations.¹¹³ Additionally, membranes are extremely susceptible to nucleophilic (OH⁻) attack without spacers because cations are close to the benzene ring, which raises the risk of OH⁻ ion attack (Fig. 47c). At 80 °C under 1 M NaOH, N-cyclic piperidinium-functionalized PTP-based AEMs (e.g., PTP-90) had remarkable chemical stability, and after 934 h, 73% IC remained.²³⁴ The authors also noted good AS in a comparable PTP-based AEM with low WU and low IEC. The designed poly(biphenyl piperidinium)-based AEMs (PBPCL1.29) show high AS under 1 M NaOH at 80 °C, i.e., the membrane conductivity retained almost 87% after 480 h.²⁸⁸ This higher chemical stability of the AEM was due to the distinctive octopus-shaped piperidinium side chain assembly, which can ease the microphase separation, and the hydrophobic main backbone can diminish the hydroxide attack.

In the form of the hydrophilic ionomer and hydrophobic polymer (e.g., PS/pTAP (40:60)), AEM obtained higher AS, i.e., 98% conductivity was retained after 300 h in 1 M NaOH at ambient temperature.²³⁷ Due to its role in maintaining the membrane's stability in alkaline circumstances, the functionalizing cation with the nonpolarizable ring-based structure in the membrane for the quaternization is crucial. Thus, at 1 M KOH at 80 °C, higher AS is demonstrated by MPy and Pip cations, which were utilized to quaternize longer and shorter side chains connected to the SEBS membrane (e.g., SEBS-C1-MPy), respectively.²³³ The maintained conductivity was about 87% and 84% after 600 h (Fig. 47d). Compared to other cations with functionalized side chains attached to SEBS, the heterocyclic cations (MPy and Pip) have good chemical stability and can protect the membrane from OH⁻ attack. In another study, high chemical stability was observed in the composite PPO/aminopropyl silane-treated GO (PPO/AGO) under the environmental conditions of 5 M NaOH solution.²⁴²

Poly(4-vinylbenzyl chloride-styrene) membranes were doped with double antioxidants for the AS for the first time at 60 °C and 2 M KOH.²⁶¹ As a result, diphosphite-based antioxidants-doped membranes (e.g., QP(VBC-St)-MB/EA_0.3) showed significant AS (69.6% conductivity was retained after 1300 h) compared to bare AEM (42.5% conductivity was retained after 1300 h). In contrast, the doped membranes showed less stability (47.9% conductivity was kept after 1300 h) in 1 M KOH at 80 °C, while bare quaternized membranes revealed that just 21.2% conductivity was retained after 1000 h. Compared to a pure poly(4-vinylbenzyl chloride-styrene) membrane, these antioxidant-doped membranes exhibit more excellent stability, even at higher alkaline concentrations (2 M KOH). It could be the effect of a diphosphite-based antioxidant decomposing the intermediate byproduct and preventing the generation of free radicals. Radical inhibitor-modified QP(VBC-St) showed higher AS than bare QP(VBC-St) in 8 M KOH solution at 80 °C for 24 h.²⁶² It was found to be 4-tertbutylphenol-based radical inhibitor-modified QP(VBC-St) AEM (e.g., QP(VBC-St)-4-TBP_1.4) that showed higher AS (66%) than QP(VBC-St)-4-TBP_0.7 (45%) and QP(VBC-St)-4-TBP_0.1 (16%) AEMs (Fig. 47e). Therefore, the radical inhibitor could protect the QAs from the OH radicals. The authors also examined how more radical (OH⁻) ions were consumed when the concentration of 4-tertbutylphenol in the QP(VBS-St) membrane increased, thus significantly the protecting QAs. Coppola et al. investigated AS on the fiber form of a PVA-crosslinked ABPBI membrane (C-PVAf-ABPBI) in a 15% KOH solution at 30 °C and found that the membrane was more stable than bare PVA fiber or glutaraldehyde crosslinked PVA for up to 7 days.¹³¹ The ABPBI membrane may thereby improve the membrane's stability in an alkaline setting.

Yang et al. investigated AS for VImPPO:PVA semi-interpenetrated polymer networks (e.g., sIPN-91) in 1 M NaOH at 80 °C.¹²⁸ The membrane's IC rose for the first two days before gradually declining as the free imidazolium ions in the NaOH solution degraded at higher temperatures (Fig. 47f). After 1000 h of soaking, the composite membrane maintained its IC of 66.5%. Similar to this, Tang et al. found that the AS dropped (100–65.3%) with an extended soaking period (0–240 h) of 48% BPTMA-tethered PBI (48%BPTMA-PBI) under 1 M KOH at 80 °C.¹⁸⁷ It could be the cause of the nucleophilic assault that caused PBI's core to degrade. Thus, the longer side chain-tethered PBI membrane does not show much alkaline stability under high pH conditions because it declined the conductivity to 65% in a shorter time (240 h). However, by setting up a large QAs group, it may be possible to fend off OH⁻ attack on the membrane. Interestingly, quaternized polysulfone deteriorated entirely in 360 h due to nucleophilic attack on the QAs groups in the backbone.¹³⁴ It did not exhibit much AS at 1 M KOH and 80 °C. However, the positive charges of LDH electrostatically attracted the OH⁻ ion to keep the backbone from being attacked by nucleophiles. They thus kept it stable for up to 400 h after adding the LDH composite (e.g., qPSU-LDH₅). It may be best to combine positively charged elements to protect the polymer backbone from nucleophilic attack.

A considerable increase in alkaline stability may be achieved by comparing crosslinked AEMs to a pristine QAs-functionalized membrane. Most reports indicate that higher temperatures can hasten the disintegration of AEMs; the AS of the membranes at high temperatures is crucial. As a consequence, the alkaline stability of the membrane reduces as the temperature (60 and 80 °C) increases.²⁶³ Additionally, it lowered the AS of the membrane discovered in the TMHDA-crosslinked PVBMPy-PSF AEM and raised the cross-linking degree (15, 20, and 25%). It was shown that the membrane conductivity at 60 °C (27.5, 23.5, and 17.5 mS cm⁻¹) and 80 °C (23.5, 19.8, and 15.0 mS cm⁻¹) declines after 576 h. The hydrophilicity of the membranes may have decreased, and their QAs groups may have deteriorated with time, which would account for the AS decline. As a reason, both the temperature and degree of crosslinking affect the AS of the membrane. However, after 576 h, this crosslinked composite membrane (e.g., PVBMPy-CL-15%PS) showed stronger AS. Recently, it was found that the high AS of the PBP membrane only degraded by 9.1% at 1 M NaOH after 800 h.²⁶⁵ The membrane showed even higher AS and only degraded by 6.0, 4.7, and 3.2% after 800 h about 5, 10, and 15% PBP-crosslinked SEBS membranes such as 5%-PBP-c-SEBS, 10%-PBP-c-SEBS, and 15%-PBP-c-SEBS, respectively. Therefore, in highly alkaline environments, the crosslinking degrees improve the membrane AS.

Interestingly, functionalized PPO was combined with different molecular weights (400–4000) of PEG and tested for AS under 1 M KOH at 80 °C.²⁵⁵ With the increase in the AS, i.e., the conductivity of the composite membrane was maintained (61–68%) after 300 h by increasing the molecular weight (400–2000) of PEG (e.g., 20PDM-2%PEG2000). The 4000PEG composite with PPO was observed to have lower maintained conductivity (52%). The undeveloped microstructure (microseparation morphology) patterns may be caused by the diminishing retained conductivity of PEG at increasing molecular weights. Thus, this work demonstrates that AS depends on the structure of the membranes' microseparation. In another study, in 1 M or 6 M NaOH at 80 °C, PVA-incorporated crosslinked QVBC composite was evaluated for the AS. After 216 h, the initial conductivity decreased from 45 to 17 mS cm⁻¹.²⁵⁷ Later, if the period was extended to 1584 h, the initial conductivity (17 mS cm⁻¹) remained unchanged. After 96 h in 6 M NaOH, a second examination of the composite membrane revealed that the conductivity had dropped to 18 mS cm⁻¹. The crosslinked QVBC combined with a PVA membrane (e.g., PVA-0.9PQVBC30%) exhibits improved AS at a mole concentration of 1 M NaOH but cannot withstand a mole concentration of 6 M NaOH.

AEM with VBC grafts recently subjected to electron beam irradiation at low or RT with a dosage of 100 kGy (e.g., 100-air-LT/RT) did not display chemical stability (high conductivity loss) when subjected to 1 M KOH at 80 °C.²⁹⁹ However, on irradiation with an electron beam at a dosage of 40 kGy, while the procedure was carried out at ambient temperature, demonstrated remarkable chemical stability (significantly less conductivity loss). The better chemical stability provided by a dosage of 40 kGy of AEM has considerable flexibility compared to a high-dose beam (100 kGy) of AEM, resulting in a high chain scission reaction. This flexibility may protect the backbone against attacks by hydroxide ions. AEMs treated with an electron beam have the same level of chemical stability as AEMs that have been treated chemically.

The problem of developing AEMs for energy storage and conversion devices concerns the long-term durability of the membranes in alkaline environments. The breakdown of AEMs occurs in an alkaline environment because the cationic groups and backbones are susceptible to OH⁻ ions at high temperatures. As a result, it is crucial to develop AEMs functionalized with very stable cations. As a result, N-cyclic-based cations are very stable at high temperatures and in very alkaline environments. Wang et al. constructed m-TPN AEMs and functionalized them with various N-cyclic cations to create m-TPNPiQA, m-TPNPyQA, and m-TPNBeQA AEMs, which were then used to test an AS at 80 °C with 1, 2, and 5 M NaOH.²⁴⁰ In contrast to m-TPNBeQA, m-TPNPiQA and m-TPNPyQA were shown to have higher AS. As the molarity of NaOH increases, the initial IEC values of m-TPNPiQA and m-TPNPyQA are slightly reduced. When the alkaline concentration was increased from 1 to 5 M NaOH, m-TPNBeQA diminished the IEC values rapidly. The AS of m-TPNBeQA was decreasing, which may have been caused by the cation and backbone being attacked by nucleophiles, causing it to degrade.

Comparing crosslinked AEMs to bare AEMs, increased AS was seen. Compared to BAPTES and BPPO-crosslinked PBBP and bare BPPO under 2 M NaOH at 25 °C, DBMHDA-crosslinked PBBP (PBBP-DBMHDA) demonstrated more excellent AS.³⁰¹ However, the conductivity of the crosslinked membranes rapidly diminished over time as the cations in the crosslinking (DBMHDA) were increasingly prone to attack by hydroxide ions. BAPTES, on the other hand, has silica (Si–O–Si) built into its structure by hydrolysis, which surrounds the functional group and shields it from OH⁻ attack, resulting in shallow conductivity loss. However, a high IC was found in crosslinked membranes after 720 h in 2 M NaOH, compared to bare and BPPO crosslinked PBBP. The high AS of the copolymer-based membranes (e.g., P(4PA-co-2PA)-24) under 1 M NaOH at 80 °C was found by Jiang et al., with no change in the conductivity after 720 h.³⁰⁰ However, after being exposed to 5 M NaOH for 720 h, these copolymers-based AEMs were shown to be degraded (Fig. 48a). The n-alkyltrimethylammonium (QAs)-functionalized copolymers degrade in a 5 M NaOH environment. Still, they are stable in 1 M NaOH conditions. Additionally, ¹H NMR results exhibit vinylic peaks, which signals that Hofmann elimination occurring in the pentyl spacer was used to demonstrate the stability of the membrane following 5 M NaOH treatment.

It is important to note that at 1 M KOH and 80 °C, the AS of fluorine-substituted (mono and penta-fluorobenzoyl)-quaternized SEBS membrane was higher than that of fluorine-free quaternized membrane.¹³⁹ After 500 h, the conductivity of the mono and pentafluorine-substituted (e.g., HQA-F1-SEBS and HQA-F5-SEBS) membranes were still 93.8 and 96.6%, respectively, far higher than that of the fluorine-free quaternized membrane (88.1%). These findings suggest that the fluorobenzoyl group has high alkaline stability and protects the polymer backbone from nucleophilic attack (Fig. 48b). Therefore, membranes attached to fluorine moieties would be helpful for their increased chemical stability over the long term. Poly(aryl piperidinium) functionalized with piperidinium-based clustered cations (e.g., 3QPAP-0.5) was found to have high chemical stability under 1 M NaOH at 80 °C. Minor conductivity loss was observed from 0 to 144 h (45.0–40.1 mS cm⁻¹); later, it was stable up to 720 h (40.1 mS cm⁻¹).²⁹⁵ Due to clustered cations in the membrane protecting the backbone from OH⁻ assault, excellent stability of the membrane was achieved. However, poly(aryl piperidinium) functionalized with a single cation similarly experienced a loss in conductivity. As a result, piperidinium-based cations, whether in a cluster or single form, improve the chemical stability in an alkaline environment.

Low WU resulting in low IEC may protect membranes from the attack of OH⁻ ions in an extreme environment, which may explain why membranes with low IEC values have shown good alkaline stability. Cross-linking and adapting to low IEC values might help to improve the alkaline stability (AS). However, with 1 M NaOH at 80 °C, Chen et al. found that AEMs (e.g., 60MePh-2.07) with higher and lower IEC values displayed higher and lowered AS; therefore, the conductivity test in an alkaline media shows a slight decrease in these membranes.²⁸⁷ Compared to poly(phenylene) copolymer-based AEM, which showed modest conductivity variations (119 to 88 mS cm⁻¹), the polystyrene copolymer-based AEM (e.g., HTMA-DAPP) exhibited very high AS with the initial conductivity (63 to 64 mS cm⁻¹) not changing even after 500 h under 1 M NaOH.³²⁹ Unlike other AEMs, the AS values for these two copolymer-based AEMs were much higher (Table 6).

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Similarly, after 2000 h in 1 M KOH at 100 °C, the AS of poly(arylene piperidinium)-based AEM (PAPQ83) was high.²⁹⁴ The piperidinium-based cation may defend the skeleton from the hydroxide attack. Recently, the bication-functionalized poly(meta-terphenylene alkylene) (m-PTPA-2N) quaternized by varied ratios (10–40%) of TMHDA was found to have high AS.²⁷⁹ It was found that the AS of TMHDA cross-linked m-PTPA-2N with a higher ratio of 40% (e.g., m-XPTPA40-2N) was greater than that of uncrosslinked m-PTPA-2N and cross-linked with a lower percentage (10–30%) membranes such as m-PTPA10-2N, m-PTPA20-2N, and m-PTPA30-2N (Fig. 48c and its inset picture). After 300 h, only slight variations were discovered in the membrane's conductivity, and it was still able to maintain a high level of stability even after 1000 h. The quaternary ammonium groups and the polymer backbone could be protected from OH⁻ assault by the bication structure.

Polymer backbones and QAs may be protected from nucleophilic attack by fabricating a membrane with a high cloud long alkyl chain-containing cations. Liu et al. found that the fluorinated membrane (PAEK) was prepared by the quaternization of a polymer composed of long alkyl chains-containing carbazole-based cations.¹¹⁰ Even after 7 days in 4 M NaOH at 80 °C, the produced AEM (e.g., PAEK-HQACz-0.7) exhibited good alkaline stability (AS). The chemical stability and membrane protection afforded by a long alkyl chain enclosed inside a cation structure may be enhanced throughout extended use. The chemical stability of the bis-cation-functionalized membrane (e.g., QPBPip33Ac) was investigated by Liu et al. for 3 days in 2 M HCl.²⁹⁸ In an acidic environment, its effectiveness is minimal. A decrease in the membrane mass was linked to a rise in bis-piperidinium cation concentration. Since the membrane contains the piperidinium cation, which shields the QAs from the attack of the hydroxide ions, it may be very stable in NaOH or KOH environments. Intriguingly, the multi-cation-based compound-functionalized fluorinated cross-linked membrane (e.g., PHFB-VBC-DQ-80%) was discovered to have a high AS.²⁸⁴ After 500 h in 2 M NaOH, the membrane retains 92.9% conductivity. The membrane's multi-cation structure may be what protects the polymer backbone and QAs from nucleophilic assault (Fig. 48d).

Under 2 M NaOH at 80 °C for 120 h, the membranes PBI–DIm and hybrid PBI–DIm–Si were discovered to have a high AS.³⁰⁵ Both membranes demonstrated 100% loss-free conductivity. It is possible that the decreased nucleophilic assault of OH⁻ ions to DIm, especially for the membrane PBI–DIm, is due to sufficient water in the region of cationic side groups that dilute the hydroxide ions. Therefore, significant AS was also seen in silicon AEM hybridized with PBI–DIm (PBI–DIm–Si hybrid). However, if the authors investigated the AS for at least 500 h, the results may have been more impressive when the membranes were ready. The conductivity with time was investigated for the block copolymer-structured quaternized fluorinated PSF hybrid with hydrophilic PSF (e.g., Q-PIM-b-PSF-0.91) under 1 M NaOH at 60 °C for 216 h.³⁰⁷ As the time passed, the conductivity decreased; after 216 h, the conductivity stayed at 80.0%. The membrane's reduced conductivity might be due to OH⁻ attack on polymer backbones and cations that cause the membrane to deteriorate progressively. Using an optimized ratio (2:1 and 4:1) of monomer to trimer, Cha et al. found that quaternized copolymer-based AEMs made in this way exhibited moderate chemical stability.³⁰⁶ The conductivity (48–35 mS cm⁻¹) decreased steadily with an increase in the days (1–7) for the 4:1 ratio of AEM (e.g., QCPPAE-4/1), indicating that the produced AEMs were not stable under 1 M NaOH at 60 °C. Alternatively, a 2:1 AEM ratio leads to 24.5–18.2 mS cm⁻¹ conductivity for 1–7 days. The QAs groups may be degraded by nucleophilic assault, which may account for the lowering of conductivity. However, the copolymer-based AEM with a greater monomer concentration exhibited a higher AS than the one with a lower monomer level.

AS testing was conducted with 0.5 and 1 M NaOH at 25 °C on perfluorinated polymers such as quaternized Nafion (Nafion-TMA) and Aquivion (Aquivion-TMA).²⁷⁶ Intense hydroxide ion assault causes quaternized perfluorinated polymers to deteriorate rapidly, leading to a significant drop in IECs. These membranes might be functionalized with a solid cation to shield them against hydroxide ion attack. High molecular weight polyfluorene-based AEM (e.g., PFOTFPh-C6-TMA(OH)) was resistant to 8 M NaOH solution at 80 °C for 168 h.¹⁰⁰ With the progression of time, the conductivity loss slowed down. However, 88% of the original conductivity remained there after 168 h. High resistance to hydroxide attack that led to high AS in hostile conditions suggest that polyfluorene-type AEM might be viable membranes. Accordingly, 1000 h of testing with 1–10 M NaOH at 80 °C revealed that the membrane, multi-cation ionic liquid-functionalized fluorene-based (e.g., TQ-PDBA-70%) AEM exhibited remarkable chemical stability.²⁸³ A steady decrease in conductivity (from 99% to 59%) was seen over 1000 h when the NaOH concentration was increased from 1 to 10 M. It is possible that the fluorine-based main chain's strong hydrophobicity, which can prevent the assault on the main backbone from OH⁻, contributes to the improved chemical stability of these membranes. Similarly, the membrane in the form of fluorine-tethered PPO crosslinked with tri-QAs demonstrated improved alkaline stability.²⁸² For 7 days, the AEM showed good strength in 1 M NaOH at 80 °C. Even after 7 days, the conductivity remained at 99%. In contrast, without fluorine in the PPO-based AEMs, conductivity was preserved at 55% and 60% after OH⁻ assault on the polymer backbone. However, fluorine-functionalized PPO-tri QAs (e.g., PPO-22-3QA8F) demonstrated that solid chemical stability and fluorine could protect the backbone against OH⁻ assault. The characteristics of several AEMs are shown in Table 6, along with an overview of how well they performed in the IEC, WU, SR, IC, and AS and their hydration numbers (λ).

4.5 Mechanical stability (MS)

For electrochemical devices to operate for a long time, the mechanical qualities of membranes are crucial, and the membrane's mechanical strength depends on the polymer's primary backbone.¹⁵⁹ Increased water uptake-induced membrane swelling or dehydration-induced membrane hardness are blamed for the excess concentration of ion exchange clusters, which is believed to have decreased mechanical stability. Therefore, it is crucial to establish a firm limit on the number of ion exchange clusters and adhere to it. A typical method for enhancing the membrane tensile strength is a chemical process called crosslinking that raises the Young's modulus and high strength. However, it might lower the ion concentration and produce membrane embrittlement. It has demonstrated that mechanical characteristics can be enhanced by increased the crystallinity. It may be done by changing the chemistry of the polymer, for example, by shortening the side chain. An inert, granular, alkaline stable polymer coating can also strengthen the membranes, thus increasing the longevity. For the determination of the mechanical characteristics of AEM, a standard analytical technique can be utilized, such as Young's modulus (YM), tensile strength (TS), and elongation at break (EB). Membranes are often dried after being soaked in clean water as part of a mechanical test. Dynamic thermomechanical analysis or an electronic tensile tester can be used to get measurements.³³²

The YM results (2134, 2105, and 2127 MPa), EB (15.8, 17.3, and 14.6%), and TS (83.9, 81.4, and 82.5 MPa) demonstrated the superior performance of the aromatic monomer used to prepare 30 μm thickness of PQP-100, PBP-67, and PTP-83 AEMs, and these findings show the rigid backbones. The high molecular weight of the PQP-100 membrane obtained better mechanical properties at 30 °C.²³⁵ The membranes of the hydrophobic n-butyl group-tethered quaternized naphthalene-based PBI (e.g., NPBI-QA55-B45) were studied for TS and EB.¹⁸⁸ The n-butyl group concentration in NPBI-Q increased from 20 to 75%, and TS (19.1–47.3 MPa) and EB (5.4–17.6%) increased. In addition, without n-butyl, Q-NPBI displayed lower TS and EB values, even though the number of QAs (25–100%) were increased due to the accumulation of highly hydrophilic (QAs), which leads to a defect in the mechanical strength of the membrane; there is no improvement in the TS (40.2–14.5 MPa) and EB (12.7–2.3%). Therefore, increasing or adding n-butyl's hydrophobic characteristics might improve the membranes' mechanical qualities. In a different investigation, completely hydrated dual QAs-treated azidated PPO-OH AEMs (e.g., DQ-PPO-17-OH) were found to have low TS (7.2 to 10.9 MPa) and high EB (19.1 to 31.8%).²³¹ Given that the membrane was tethered with dual QAs, the obtained low TS may cause the membrane's high WU and high SR (high hydrophilic). High levels of WU and SR may cause the polymer backbone chains to get entangled, thus reducing the membrane's stiffness (tensile strength).

Liu et al. investigated the mechanical characteristics of QAs-functionalized PPO (e.g., PPO-iP-44) that included high and low cations.²³² In contrast to less cation-containing QAs-functionalized PPO such as PPO-iP-44 (1599.2 MPa (YM), 34.9 MPa (TS), 2.4% (EB)) and PPO-nP-44 (1437.8 MPa (YM), 27.6 MPa (TS), and 2.0% (EB)), the highly bulky QAs-functionalized PPO (PPO-CH-44) demonstrated low YM (1212.5 MPa), TS (8.6 MPa), and EB (1%). As a result, bulky cations containing QAs caused the membrane's mechanical characteristics to decline. In contrast, fewer cations containing QAs could benefit the membrane's mechanical properties and could be the reason for less water uptake. The PPO-based AEM prepared with a hydrophilic alkoxyl extender side chain and a hydrophobic alkyl spacer (e.g., SDQEO) was found to have a low TS (10.5 MPa) and a high EB (27.3%) due to the longer side chain and the spacer's higher free volume, which reduced the inter-chain forces between the main backbone and side-chain and resulted in lower values being obtained.¹¹³ In addition, the absence of a spacer and a smaller side chain synthesized by PPO-based AEM demonstrated stronger TS. It lowered the EB because the smaller free volumes strengthen the main backbone and side-chain inter-chain forces. Due to the high viscosity of N-cyclic piperidinium, functionalized PTP-based AEM (e.g., PTP-90) showed moderate TS (29.2 MPa), low EB (6.8%), and noticeable YM (466.7).²³⁴ The TS and EB of the poly(biphenyl piperidinium)-based AEMs (e.g., PBPCL1.29) decreased and increased as the cation concentration increased in the membrane.²⁸⁸ It is possible to assert that the mechanical stability reduction contributed to the membranes' high IEC tendencies to increase the WU.

The mechanical stability of composite membranes with polysulfone/pTAP ratios of 60%:40% (PS/pTAP (60:40)) and 40%:60% (e.g., PS/pTAP (40:60)) was examined by Arunachalam et al.²³⁷ The TS was reduced (22.5 to 12.5 MPa), and the EB was raised (1.8 to 2.5%) due to the rising ratio of ionomer pTAP (Fig. 49a). It could be because the hydrophilicity of the ionomer content increased the water absorption, causing the TS to drop and the EB to rise. As a result, an excess of the ionomer (pTAP) can degrade the mechanical characteristics of the PS/pTAP AEM. Higher WU and high SR are related to the membrane's mechanical strength deterioration. Accordingly, Yu et al. found that more extended side-chain-functionalized membranes (e.g., SEBS C1-x and C6-x) with higher WU and SR had lower mechanical strength.²³³ In contrast, shorter side-chain-functionalized membranes with lower WU and SR had better mechanical strength (Fig. 49b). According to Peng et al., doping one or two antioxidants in the AEMs (e.g., QP(VBC-St)-MB/EA_0.3) improved the mechanical characteristics of the membranes.²⁶¹ Accordingly, increasing one or more doping antioxidants led to intermediate results for the quaternized P(VBC-St) of mechanical strength, such as TS (8.2–10.3 MPa), EB (10.8–4.7%), and YM (374–773 MPa). The rising TS and YM and falling EB demonstrated the stiffness characteristics of antioxidants. When compared to the bare membrane QP(VBC-St) (10.7 MPa), the QP(VBC-St) membranes' tensile strength (6.6 to 4.6 MPa) reduced with the addition of radical inhibitors (e.g., QP(VBC-St)-4-TBP_0.7).²⁶² At the same time, the membrane's flexibility (EB) increased with the addition of the radical inhibitors (8.1 to 19.0%) compared to the bare (QP(VBC-St)) membrane (6.4%). Therefore, a radical inhibitor might increase the membrane's flexibility for applications.

It is interesting to note that structural matter can affect the mechanical stability of an AEM. As a result, PVA was crosslinked with the ABPBI polymer by glutaraldehyde to create nanofiber (e.g., C-PVAf-ABPBI) and nanosheet forms utilizing the electrospinning and immersion processes, respectively.¹³¹ In comparison to the nanosheet form (TS (2.8) and YM (64.0 MPa)), the fiber form had more excellent TS (5.0 MPa) and YM (103 MPa) values. It was discovered that the fiber form of the membrane had more stiffness than the sheet form. Additionally, it was revealed that the fiber membrane's flexibility (EB) was lower than that of the sheet kind of membrane (16.0%), at just 6.0%. Therefore, the membrane's structure firmly approached its stiffness and flexibility. Interestingly, PPO and PVA's stiff and flexible properties were combined in a composite (VImPPO:PVA) with a range of ratios, from 9:1 to 6:4, to create an AEM (sIPN 91 to sIPN 64).¹²⁸ By decreasing the PPO and increasing the PVA ratios, the membrane's TS (30.8–20.9 MPa) was reduced, and the flexibility (EB) (20.5–60.2%) was raised (Fig. 49c). Because the stiffness characteristics of PPO decreases with an increase in the flexible features of PVA in the composite form, the membrane had a low stiffness and high flexibility property. The VImPPO-PVA-mixed type demonstrated moderate mechanical stability. Therefore, PPO may increase the membrane's stiffness, while PVA was utilized to increase the membrane's flexibility.

If the mechanical characteristics are inadequate, the AEM will rupture, and the application will be unsuccessful. As a result, the AEM should need mechanical strength. The mechanical strength of various percentages (27 and 48%) of longer side chains-structured BPTMA-tethered PBI was recently investigated and compared to bare PBI membranes.¹⁸⁷ Due to the longer side chain's-grafted membrane that interrupts the hydrogen bonding connection and subsequent increase in the macromolecular chain's distance from the backbone, the mechanical strength of the longer side chain-grafted PBI (e.g., PBI-27%BPTMA) was lower than that of bare PBI. However, the BPTMA percentage increase to 48% was attributed to lower YS, TS, and EB. The PBI membrane's stiffness and flexibility decreased as the grafting degree percentage increased. In contrast to grafted membranes, PBI, which has a complex structure, exhibits excellent stiffness and flexibility. Crosslinking and a stiff polymer backbone structure may improve the membrane's MS.

Interestingly, AEMs with a composite rigid and flexible polymer structure might improve the MS of the membranes. As a result, flexible polymer pristine membrane (SEBS) has low TS and greater EB, whereas stiff pristine polymer (PBP) exhibits high TS and lower EB.²⁶⁵ The benefits of these two membranes allowed for the formation of composites (e.g., 10%-PBP-c-SEBS) with a 5–15% degree of crosslinking. They were tested for MS. PBP's degree of crosslinking was increased from 5 to 15% when composited with SEBS membranes, demonstrating how the amount of PBP makes the composite membranes stiffer and causes the TS to increase while the EB decreases. Due to PBP's rigid polymer structure, membranes may be stiffer and crosslinked. Recent research has demonstrated that improving the MS may be accomplished by combining an increasing hydrophilic content with a hydrophobic polymer backbone (e.g., 20PDM-2%PEG2000).²⁵⁵ As a result, different percentages (0.5–50%) of PEG2000 composited with PPO were examined for the MS. It was demonstrated that as the PEG percentages were raised, the EB rose as well. However, the TS and YM values decreased as the PEG concentration increased because high WU caused the TS and YM to drop and the EB to increase. PEG can increase the flexibility of membranes by adding to the polymer backbone. Crosslinked PVBMPy-PSF composite membrane thus shows higher MS than that of bare PVBMPy.²⁶³ Additionally, increasing the crosslinking (10–30%) strengthens the membrane's MS (1.3–22.6 MPa). The growing MS of the membrane may lead to increased crosslinking, and the addition of PSF reduced the WU and SR (Fig. 49d).

Recently, the TS of the membrane reduced while the mass ratio of the two polymers, PVA and QVBC, grew (e.g., PVA-0.6PQVBC_40%).²⁵⁷ However, an increase in the TS is shown by increasing the mass ratio of the crosslinker (DVB) with QVBC (Fig. 50a). The increase in the hydrophilic nature of the PVA ratio that led to the membrane's weakening might be caused by the declining TS. Increasing hydrophobic QVBC with the crosslinker DVB was the cause of the rising TS. As a result, the membrane's mechanical strength may be improved by the polymer's hydrophobic properties. The mechanical stability of a PBBP membrane based on fluorene (e.g., BPPO-PBBP) was investigated by Zhou et al.³⁰¹ Comparatively, the TS (18.2 MPa) and EB (10.3%) of BPPO-crosslinked PBBP were higher than those of BAPTES (14.3 MPa and 7.7%) and DBMHDA (11.9 MPa and 4.9%)-crosslinked PBBP and bare PBBP (1.53 MPa and 0.9%). This crosslinked membrane's high TS and EB may be attributed to BPPO and PBBP being very hydrophobic and hydrophilic. In addition, the low TS and EB values obtained with DBMHDA can be attributed to the presence of hydrophilic cations in the structure. In contrast, the presence of silica in the BAPTES's structure could contribute to greater mechanical strength, and the presence of hydroxyl groups makes the membrane more adaptable.

The rigidity of quaterphenylene alkylene may be advantageous in the process of improving the membrane's MS. Recent research has shown that biphenylene alkylene-based AEM (e.g., P(4PA-co-2PA)-63) has a high mechanical strength of TS (35.4 MPa) and EB (17.9%).³⁰⁰ However, after the incorporation of quaterphenylene alkylene, the TS (37.3–42.5 MPa) of the copolymer increased with an increase in the amount of quaterphenylene, while the EB (11.7–10.4%) values decreased; this indicates that the membrane's stiffness and flexibility increased and decreased, respectively. It was conceivable to improve the stiffness of quaterphenylene despite the material's lack of flexibility because of the quaterphenylene's inherently inflexible character. Similarly, fluorobenzoyl group-grafted quaternized SEBS (e.g., HQA-F5-SEBS) had a higher MS compared to quaternized SEBS without a fluorobenzoyl group.¹³⁹ Since fluorobenzoyl is hydrophobic, it does not absorb water and reduces swellings. To avoid swelling or shrinking and to make the membranes more rigid, the membranes must absorb relatively little water. In this way, the fluorobenzoyl group-incorporated SEBS (HQA-Fx-SEBS) showed low WU and SR, which led to high mechanical strength, compared to the fluorobenzoyl-free quaternized SEBS (HQA-SEBS), which showed greater WU and SR (Fig. 50b). Thus, AEMs-containing fluorine had a higher MS than those without fluorine substitution. Recently, it was discovered that the clustered cations-tethered poly(aryl piperidinium) (e.g., 3QPAP-0.3) had high mechanical strength and low flexibility (38.7 MPa and 20.4%).²⁹⁵ In contrast, the functionalized PAP with single piperidinium cation (1QPAP) had low mechanical strength and increased flexibility (31.8 MPa and 26.9%). However, the mechanical stability of poly(aryl piperidinium) decreased (38.7–28.0 MPa), and the flexibility increased (20.4–34.7%) with an increase in the weight ratio (0.3–0.5%) of PAP (Fig. 50c). Therefore, it was found in this study that the polymer backbone, rather than the side chain cations, is what determines a polymer's mechanical stability and flexibility.

Generally, the backbone's hydrophobic character and the cations' hydrophilic nature are responsible for the material's mechanical stability and flexibility. The mechanical strength with high and low degrees of electron beam-grafted ETFE AEM were recently investigated by subjecting it to varying amounts of electron beam irradiation in N₂ atmospheres.²⁹⁹ When comparing the 40 kGy of electron irradiation in an N₂ environment at RT (e.g., 40-N₂-RT), it was found that the mechanical stability of the membrane with a high degree of grafting was not improved by electron irradiation at 100 kGy. The material's increased stiffness due to the higher degree of grafting made it very rigid. Therefore, the mechanical stability of the membranes greatly depends on the degree of grafting.

Increasing the molar ratio and degree of bromination increases the number of cation group sites, raising the IEC and WU (e.g., 40MePh-1.72).²⁸⁷ IEC and WU growth contributes to a less rigid membrane by reducing the mechanical stability. Therefore, increasing the WU and IEC did not aid in creating membranes with ideal flexibility and stiffness. It was found that medium IEC and WU in the membrane exhibited necessary mechanical stability (YM (480 MPa), TS (18 MPa), and EB (83%)). Compared to regular AEM made via cross-linking or grafting (SES-TMA and HTMA-DAPP), reinforced membranes often exhibited high strength (Fig. 50d). In this light, reinforced PTFE AEM (e.g., Durion) demonstrated excellent mechanical stability and lower flexibility than copolymer structure-based AEMs.³²⁹ Because of this, reinforcing increases the material's rigidity, which might be advantageous for long-term energy cell-oriented applications.

The membrane's mechanical stability (MS) rises as the number of cross-linked bication structures (e.g., m-XPTPA40-2N) rises.²⁷⁹ The cross-linking method supports the tangles of macromolecular chains in the membrane to improve the MS. On the other hand, the high flexibility of the membrane is due to its high molecular weight and moderate water absorption. However, this study showed that the increase in MS caused the crosslinked bication-functionalized membrane to become less flexible. The cross-linked AEM is less flexible because its extensions and chains cannot move as freely as they used to. Thus, crosslinking molecules with a bication structure also dramatically affects the materials' stiffness. Compared to fluorinated PAEK with merely the carbazole pendants connected, the stiffness and flexibility of the membrane improved when a long alkyl chain-based carbazole pendant cation was introduced into the PAEK backbone (e.g., PAEKHQACz-0.7).¹¹⁰ Because of the entanglement created by the long alkyl chain, the polymer's flexibility is increased (increase the elongation break). In contrast, membranes devoid of lengthy alkyl chains in their cation tethers were brittle and less flexible (decreasing the elongation break). As the amount of long alkyl chain cations in the membrane increases, its stiffness decreases, and the flexibility rises. Therefore, long alkyl chain-based cations in the polymer membrane may be appropriate for increasing the membrane flexibility in energy cells in the long term.

The membrane's mechanical stability is typically strong in its hybrid state. Compared to bare membrane PBI–DIm, PBI–DIm–Si hybrid AEM demonstrated superior mechanical strength (MS) and flexibility.³⁰⁵ In another investigation, the membrane's MS and flexibility were high and low in the form of a hybrid block co-polymer structure (e.g., DMBA-PIM-b-PSF-0.91).³⁰⁷ In contrast, the same membrane without the block co-polymer structure demonstrated low MS and high flexibility. The block copolymer structure with fluorinated hydrophobic polymer might improve the membrane's mechanical strength, whereas the hydrophilic polymer in the membrane without the block copolymer structure demonstrated excellent flexibility. Thus, fluorinated hydrophobic polymers are essential for enhancing the mechanical stability, whereas the hydrophilic polymer structures in the membrane are necessary for improving the flexibility. Cha et al. produced a copolymer-based AEM (e.g., QPPAE-4/1), and they evaluated it for MS after altering the monomer-to-trimer ratio (2:1 and 4:1).³⁰⁶ It was discovered that a more extensive monomer content in the membrane resulted in higher mechanical strength and less flexibility than a smaller monomer quantity employed to construct a quaternized copolymer-based membrane (Fig. 50e). The membrane's mechanical strength increases with more monomer content, and the membrane remains remarkably flexible with less monomer content.

High molecular weight polyfluorene-based AEMs in the form of OH⁻ and Cl⁻ were recently investigated for MS.¹⁰⁰ Polyfluorene-based membranes often have stiff backbones. However, the mechanical strength of membranes containing OH⁻ and Cl⁻ might vary. It was determined that the Cl⁻ form of polyfluorene AEM (PFOTFPh-C6-TMA (Cl)) had more mechanical strength than the OH⁻ form of the membrane. The explanation might be that the OH⁻ form of polyfluorene can absorb more water, decreasing the mechanical stability. In comparison, the Cl⁻ form of polyfluorene can resist absorbing more water, resulting in high mechanical strength. The multi-cation-functionalized fluorine-based AEM (TQ-PDBA-70%) has remarkable mechanical stability with a TS and EB of 33.4 MPa and 14.4%, respectively.²⁸³ Generally, the mechanical strength and flexibility of fluorine-based AEMs are high and stiff, respectively (Fig. 50f).

Recent research has shown that fluorine-containing block copolymers with a bend-twisted structure (NCBP-Im) exhibit higher flexibility (24.7%) and lower stiffness (28.5 MPa).²⁸⁵ As a result, the membrane structure of the bending and twisting type could have a high degree of flexibility. Similarly, another research found that a claw-type head with a partially fluorinated (PAEFPAE-3B-1.0-PD) membrane had a high rigidity (27.9 MPa) but low flexibility (9.6%).²⁹⁰ Thus, mechanical stability is determined by the structure of fluorinated membranes. In another study, according to Wang et al., the cross-linking process, also known as quaternization, increased the MS of the fluorinated membrane while simultaneously lowering its flexibility.²⁷⁸ Furthermore, enhancing the fluorinated polymer ratio in the membrane led to reduced MS and boosted flexibility. As a result, an overabundance of the fluorinated membrane may lower the mechanical strength of the membrane. Thus, the cross-linking (quaternization) technique does not affect the MS. Therefore, they observed that the fluorinated poly(aryl ether oxadiazole) with a TMHDA cross-linked structure (C-FPAEO-50-MIM) exhibited improved stiffness properties (28.0 MPa) and low flexibility (13.1%), whereas the same membranes lacking cross-linking structure revealed reduced stiffness (17–26 MPa) and increased flexibility (17–22%).

By increasing the quantity of bis-piperidinium-based cationic side chain in the poly(bromoalkyl oxindolebiphenylylene) (PISBr–NON-1), Zhang et al. found that the membrane stiffness (13.0–7.7 MPa) decreased and flexibility (15.7–49.2%) improved.²²⁷ As a result, cation side chains based on bis-piperidinium may enhance membrane flexibility owing to the hydrophilic feature of these chains. Similarly, increasing the proportion of piperidinium in the polymer PBMB (PDMB-Pi-0.5) reduces the membrane's stiffness and increased flexibility.¹⁴⁴ Even though the backbone of the membranes is made of triphenylmethane, which is stiff and has high hydrophobic properties, the membranes are not very rigid. However, the high hydrophilic property of piperdinium contributes to the increased flexibility of the membrane.

The tensile strength (TS) is reduced and flexibility (elongation at break (EB)) is enhanced on increasing the crosslinking molarity in the AEM. Accordingly, with an increase in the molarity (0.5–1.0 M) of the crosslinker ether group containing DACEE in the polysulfone AEM, the TS (17.7–12.9 MPa) and EB (14.5–23.8%) were lowered and raised, respectively (e.g., PSf₈₆-DACEE_0.5).²²⁸ It may be justified by increasing the number of crosslinkers-containing ether groups, which may raise the membrane's hydrophilicity and lower its rigidity. However, compared to the ether group-containing DACEE, crosslinker DABDH had polysulfone (e.g., PSf₈₆-DADBH_0.75) and exhibited moderate TS (15.8 MPa) and low EB (17.4%). Therefore, the ether group-containing crosslinker may also contribute significantly to the mechanical stability of membranes. In another research, several crosslinkers (TMEDA, TMBDA, and TMHDA) with increased crosslinking (7.31–65.49%, 9.3–80.2%, and 8.75–90.03%) in the SEBS membrane were examined for mechanical stability (MS).²⁰³ TMHDA-crosslinked SEBS (e.g., C6-CQASEBS) had better TS (11.9–18.4 MPa) and EB (231.4–101.8%) than TMEDA (TS (8.2–13.7 MPa) and EB (191.2–88.7 MPa)) and TMBDA (TS (6.1–15.5 MPa) and (137–49%)) crosslinkers. Additionally, increasing the cross-linking degree might raise the TS while decreasing the EB of all membranes (flexibility). The TS was raised because the selected crosslinkers (TMHDA) may have a stiffer structure than other crosslinkers.

The crosslinker (DABDA) and increasing its grafting ratio (40–60%) in PPO (PPO/DABDA) caused a reduction in the tensile strength (38.1–14.2 MPa) and Young's modulus (1170–350 MPa) but caused an increase in the elongation at break (2.4–4.0%).²³⁰ The crosslinker DABDA could have hydrophilic qualities, which would boost the elastic properties of the PPO/DABDA-based AEM while decreasing its stiff properties. Therefore, as compared to higher crosslinking degrees (50–60%) of PPO, lower grafting degree (40%) of DABDA with PPO (PPO/DABDA-40) demonstrated better TS (38.1 MPa), higher Young's modulus (1170.0 MPa), and reduced elongation at break (2.4%). Researchers Qaisrani et al. found that the degree of chloromethylation in the polysulfone-Cl-DABDA (PSF-Cl-DABDA) crosslinked structure increased from 58 to 84%, and the tensile strength decreased from 25.0 to 17.5 MPa. In comparison, the elongation at break increased from 17.4 to 21.5%.²²⁰ These results were found when the chloromethylation degree was increased. Increasing the chloromethylation level lowers the tensile strength and can raise the membrane's flexibility. Elevating the chloromethylation level can raise the membrane's hydrophilic characteristics.

Consequently, selecting an appropriate crosslinker ensures the membrane's continued mechanical integrity. The lengthening of the side chains in AEMs affects the membrane's mechanical stability. Accordingly, increasing the side chain length in the quaternized polysulfone (e.g., acS₁₀QAPSF) increased the TS, decreased the EB, increased the membrane's rigidity, and reduced the flexibility.²⁰¹ Additionally, compared to sidechain-functionalized quaternized polysulfone, AEM with no side chains had extremely low TS and EB. Thus, the sidechain also affects the membrane's mechanical stability, and adding more sidechains will only improve the membrane's tensile strength. On increasing the imidazolium concentration in the PEEK (DI-PEEK) AEM, it was found that the tensile strength gradually decreased.²¹⁹ It was discovered due to increased membrane plasticity caused by water absorption. Therefore, increasing the amount of imidazolium did not significantly benefit the membrane's mechanical stability.

Adjusting the monomer ratio in the synthesized PES (PES-BTP) AEM from 20 to 30% was examined for its mechanical stability, and the results revealed that increasing the monomer content in the membrane tends to lower the tensile strength while simultaneously increasing the elongation at break.¹⁸⁴ The monomer is to blame for the membrane's increased flexibility rather than its rigidity. It might be because the monomer makes the membrane more hydrophilic, allowing it to take in more water. The ratio that is the lowest possible functionalization of 20% of the monomer PES (PES-BTP-20%) was shown to have greater flexibility (46.8%). Still, lower tensile strength (21.7 MPa), while increasing the monomer ratio, decreased the tensile strength and increased the flexibility. As a result, the monomer plays a significant part in improving the membrane's flexibility. The dried QPPO membranes and their recycled structure dried disulfide bond cross-linked RC-QPPO AEMs were evaluated for mechanical stability, and the results indicated that the recycled membrane revealed stronger tensile strength and higher flexibility than the bare dried QPPO membrane.¹⁹⁶ For technical understanding, the mechanical properties of various AEMs have been represented in Table 7.

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4.6 Summary

5. Application

Hydrogen produced by water electrolysis is of very high purity, making it a popular subject of study amongst experts in the field of energy. Water electrolyzers are classified into alkaline, proton-exchange-membrane (PEM), and AEM. However, alkaline and PEM have drawbacks, such as the traditional alkaline electrolyzer's low operating current density and lack of pressurization.⁵⁵ On the other hand, PEM was required; the highly acidic environment necessitated using costly platinum-group metal (PGM) catalysts.⁴⁸ Therefore, AEM-based electrolyzers have recently been explored because of their great performance and stability in continuous operation using non-PGM catalysts. The capacity to employ low electrocatalysts in alkaline environments and the purity of hydrogen produced are also benefits of AEM electrolyzers.³³³ A fundamental challenge for implementing water electrolysis in a commercial setting is the production of anion exchange membranes (AEMs) that have high conductivity and are physically stable. While AEM-based water electrolyzers show promise, widespread adoption is yet to occur because of problems with membrane stability and component polymers, which are further aggravated at high temperatures necessary for successful electrocatalysis. Boosting membrane electrolyzer performance may be possible by increasing the membrane IECs or using a concentrated alkaline solution to lessen the electrolyzer's ohmic resistance.⁹⁷

The water uptake of AEM is essential if one wishes to achieve excellent performance in water electrolysis and long-term durability in this activity.³³⁴ This is because high water uptake led to high conductivity, which enabled one to achieve high performance in water electrolysis. An advantage of AEM-based water electrolysis over traditional electrolysis is that either water or a low-concentration alkaline medium could be employed as the electrolyte.³² The cell voltage and current density are two of the most important operational parameters in electrolyzing water. In most cases, the amount of energy a cell needs to function properly is determined by its voltage. On the other hand, the creation of H₂ is directly related to the current density. Accordingly, a high current density results from a fast electrochemical reaction occurring in the cell, and an AEM water electrolyzer may be operated in the current density range between 100 and 500 mA cm⁻².³² However, one of the difficulties associated with the cell is that when it is working at a greater current density, bubbles are formed on the surface of the electrode. To this degree, this results in an increase in the amount of overpotential that is required. As a result, there is a need for more studies to be conducted to undertake an analysis of the optimal range of current density for the development of hydrogen. While the AEM's potential in fuel cell applications has been extensively studied, its use in water electrolysis has received comparatively less attention. Therefore, future studies should focus more on synthesizing efficient AEM for the benefit of water electrolysis for hydrogen generation. Table 8 presents the results of recent studies on using AEMs in water electrolysis for hydrogen production, and the following objectives were discussed: cell voltages employed, attained current density, temperatures, electrolytes, and durability.

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5.1 AEMs in water electrolysis

Recently, a composite with a larger weight% of inorganic material and a polymer membrane performed better when exposed to lower voltage.³³⁵ Accordingly, the electrolysis study of three types of membranes, such as varying percentages (1 and 3%) of Ni–Fe LDH composite with QAPPT, was performed at 60 °C at a cell voltage of 2 V and current density of 1 A cm⁻². QAPPT/3%Ni–Fe LDH demonstrated the best electrolysis (1 M KOH) performance, with electrolytic voltages of 1.82 V compared to 1%LDH-QAPPT (1.97 V) and bare QAPPT (1.87 V) owing to better conductivity (178 mS cm⁻¹) and tensile strength (TS) of 23.31 MPa. Furthermore, the prepared AEM (QAPPT/3% Ni–Fe LDH) demonstrated remarkable electrolytic stability throughout the 87 h trial, with the voltage (1.8 V) remaining almost constant. At a cell voltage of 2.0 V, the PQP-100 AEM (40 μm) with high conductivity and high TS demonstrated a higher current density (J) of 1.5 A cm⁻² than the PBP-67 and PTP-83 membranes for alkaline water electrolysis.²³⁵ Due to the material's greater conductivity, increased electrolytic performance may be attributed to PQP-100's lower ohmic resistance. Further testing in a comparable setting using a thinner membrane, PQP-100 (10 μm), demonstrates a reduction in the current density from 1.5 A cm⁻² to 1.3 A cm⁻² (Fig. 51a). Because of its poor conductivity and high swelling, a thin membrane may have high internal resistance. Therefore, the efficiency of water electrolysis also relies on the membrane thickness. Based on performance, PQP-100 was also evaluated for durability under 1 M NaOH at 60 °C. It shows that after 400 h, the cell voltage rose from 1.75 V to 1.89 V. The increasing voltage after 400 h was caused by membrane electrode assembly deterioration; hence, the PQP-100 membrane could maintain stability for 400 h.

On the polydiallylammonium-incorporated cationic network sIPN AEM (e.g., BD₃/50EVOH), a similar current density (1.57 A cm⁻²) observation may be seen.³³⁶ It was possible to achieve a higher current density because of the membrane's strong conductivity (161 mS cm⁻¹), good mechanical stability, and elevated alkaline stability. However, increasing the quantity of poly(vinyl alcohol-co-ethylene) (EVOH) in the sIPN membranes causes a reduction in the current density since the sIPN membranes' ionic conductivity is also reduced (161–62.8 mS cm⁻¹). The ionic conductivity of the membrane will undoubtedly have an impact on this investigation of current density. Additionally, the high alkaline stability of the membrane may have contributed to the great durability of the sIPN AEM, which has been shown to exhibit no significant voltage fluctuations up to 300 min at 1.6 V at 70 °C.

Interestingly, water electrolysis was performed by commercial AEMs TM1-MEA and FAA-MEA under various mixtures of solutions such as KOH/KOH, KOH/water, water/KOH, and water/water.³²⁵ To this degree, TM1-MEA showed high current density under all conditions of solutions compared to FAA-MEA due to high conductivity. Additionally, TM1-MEA showed maximum current density (2.75 A cm⁻²) in KOH/KOH than other conditions at 1.9 V. Besides, TM1-MEA-conducted electrolysis study in water/water conditions exhibited very low performance, and the obtained current density was 0.175 A cm⁻². Therefore, higher current density was obtained by TM1-MEA under KOH/KOH conditions compared to FAA-MEA. It could be the polymer structure of TM1 AEM and suggested possible conditions for water electrolysis with at least a mixture of KOH rather than only water. This membrane's durability was evaluated for 50 h, and it was discovered that after 200 cycles, the current density of the TM1-MEA was only reduced by 32%, compared to 85% for the FAA-MEA, indicating that the TM1-MEA had greater stability. As the cell's temperature rose from 50 to 80 °C, so did the breakdown rate of the TM1-MEA membrane (from 40 to 321 mV h⁻¹). Therefore, raising the temperature of the cell affects its membrane.

In another investigation, the performance of poly(biphenyl piperidinium)-based AEMs (e.g., PBP) at varying thicknesses (15–60 μm) was evaluated.³³⁷ It was found that the thickness of the AEMs had a significant impact on the current density. Accordingly, the current density (2.8–1.54 A cm⁻²) decreased as the thickness (15–60 μm) of AEMs rose, and this might be because lower and higher thickness membranes had higher and lower series resistance, respectively. High-resistance membranes may have poor conductivity and low current density, while low-resistance membranes have high conductivity and high current density. As a result, PBP (PBP-15) with a 15 m thickness showed excellent performance at 2.8 A cm⁻² at 2.2 V in 1 M KOH electrolyte at 60 °C. The higher current density by the membrane was likely caused by its higher conductivity (185 mS cm⁻¹) and thinner thickness (15 μm). When dual-methylpiperidinium AEMs (e.g., QBNTP-MP11) were used to crosslink poly(binaphthyl-co-terphenyl piperidinium) at electrolyzer settings of 2.0 V, 1 M KOH, and 80 °C, significant current density (2.75 A cm⁻²) was found on the polymer.³³⁸ Due to the cross-linking of dual-methylpiperidinium, the high current density of the membrane may cause the membrane's high conductivity (181.2 mS cm⁻¹). Conversely, cross-linker-free AEM (QBNTP) had poorer current density (1.9 A cm⁻²) at the same voltage, which may have been caused by lower ionic conductivity (140 mS cm⁻¹). Compared to QBNTP at 1 M KOH, 80 °C, and an applied current density of 0.2 A cm⁻², the QBNTP-MP11's durability was superior for up to 100 h. The QBNTP-MP11's exceptional durability was made possible by its strong ionic transport, high mechanical strength, and great chemical stability. On the other hand, QBNTP showed poor performance after 40 h, which might result from membrane breakdown because of their poor mechanical and alkaline stability.

The current density of the membrane (e.g., C-PVAf-ABPBI) was measured at cell voltage 2.0 V and different temperatures, such as 50 and 70 °C, in the presence of 15 wt% KOH.¹³¹ Because of the strong ion mobility at high temperatures, the membrane's current density rose from 0.23 to 0.27 A cm⁻² as the temperature increased from 50 to 70 °C (Fig. 51b). As a result, the temperature affects the current density in the membranes for water electrolysis. Despite this, it has been shown that the current density of the C-PVAf-ABPBI is rather low. The AEM durability test was completed in 60 min, revealing that the starting cell voltage (1.98 V) did not vary significantly. Conducting a durability test on the membrane for at least 50 h would be preferable. As a result of ionic conductivity, the AEMs' current density may vary. By analyzing PTP-based AEMs, Hu et al. verified that PTP-75 achieved 0.767 A cm⁻² at 2.2 V. In contrast, a higher current density of 0.910 A cm⁻² was detected in this report for alkaline water electrolysis.²³⁴ When the temperature is raised from 55 °C to 75 °C, the current density of the AEMs rises from 0.910 A cm⁻² to 1.0 A cm⁻²; this rise in current density could be caused by the AEMs' enhanced conductivity (Fig. 51c). The authors carried out a durability study for the PTP-85 AEM. It was assessed at a current density of 0.4 A cm⁻² at 55 °C. Cell voltages are seen to rise (to a maximum of 2.8 V) with time (to a maximum of 120 h), which may result from a slight deterioration of the membrane electrode assembly.

The membrane with a lower hydroxide conductivity requires a higher voltage to work well in water electrolysis, whereas the membrane with a greater conductivity performs well at low voltages. For water electrolysis, commercially available AEMs such as Sustainion, Aemion, and PiperION were tested, and the findings showed that they produced 2.25, 2.24, and 2.05 V, respectively, at current densities of 1.0 A cm⁻² in water. Because of their low hydroxide conductivities, Sustainion and Aemion could operate at higher voltages; however, piperION, which had a higher conductivity, could function at a lower voltage. PiperION showed a voltage deterioration rate (VDR) of 0.67 mV h⁻¹ at a current density of 0.5 A cm⁻² at 55 °C for 175 h. In contrast, other commercial membranes failed to withstand and shut down above 2.9 V during the same test.¹²² Since the hydroxide conductivity improves with temperature, the operating voltages of the PiperION show a drop as the temperature rises, resulting in improved performance. Remarkably, the PiperION has a durability of 175 h when compared to membranes such as Sustainion and Aemion. Curiously, at 1.9 V, high water uptake-AEM (HWU-AEM) has good performance under pure water input to obtain a current density of 0.254 A cm⁻² at 60 °C in contrast to utilizing 1 M KOH, which exhibits a current density of 0.911 A cm⁻² at 30 °C.³³⁹ On the other hand, it was discovered that the HWU-AEM membrane has very high durability, demonstrating a modest increase in cell voltage from 1.97 to 2.0 V after being tested for 150 h. In another study, under the circumstances of 1 wt% K₂CO₃, AEM–water electrolysis with a poly(phenylene)-based membrane and a binder poly(fluorene) ionomeric gave the best result, which was 1 A cm⁻² at 2 V.³²⁹ For the durability test, at a current density of 750 mA cm⁻², it was discovered that the VDR was 0.27–0.55 mV h⁻¹ after being measured for more than 400 h.

Aemion+® is a poly(bis)imidazolium ionenes-based commercial membrane with strong ionic conductivity, high mechanical strength, and great chemical stability. As a result, polyolefin-reinforced Aemion+® (e.g., AF2-HWP8-75-X) AEM is utilized for water electrolyzer testing.³⁴⁰ Although this reinforced membrane has a high ionic conductivity (112 mS cm⁻¹), it exhibits low current density (0.2 A cm⁻²) at the applied voltage of 2.0 V when tested with 1 M KOH at 70 °C. Therefore, the membrane's current density could not rely on ionic conductivity but rather on the structure and backbone of the polymer. However, this reinforced membrane may demonstrate high durability up to 5000 h and twelve months with virtually unaltered conductivity operation. Although this membrane could have a low current density, its continued usage might be useful in water electrolyzers for hydrogen production. In an electrolysis system, the efficiency of the cells may depend critically on the concentration of KOH.

Recently, MEA (catalysts (Ni/Fe and Mo) and binder (SEBS)) with PBI/mTPN-50.120 AEM demonstrates that the current density (from 0.01–0.172 A cm⁻²) increases when the KOH concentration rises from 0.01 to 1 M.²⁷² This membrane was put through its durability test at a current density of 0.25 A cm⁻². Almost all cell voltages (1.98 V) remained stable for over 200 h. The high chemical stability membrane of crosslinked piperidinium-functionalized norbornene-introduced alicyclic PBI membrane (e.g., PcPBI-Nb-C2) may not perform well since the current density of the membrane was determined to be 0.368 A cm⁻² at 2.1 V.³⁴¹ The membrane could not achieve a significant current density despite having a high OH⁻ conductivity (105.5 mS cm⁻¹). The designed membrane's durability was also tested for a short time (8.3 h) at 60 °C with a current density of 0.15 A cm⁻²; however, it passed with excellent performance since the voltage was maintained between 1.49 and 1.51 V. N-Spirocyclic-quaternized copolymer PBI AEMs with flexible alicyclic and stiff benzene incorporation (e.g., cPBI-0.4p-0.6s) show strong ionic conductivity, mechanical stability, and alkaline stability.³⁴² But this type of structured membrane (long-side chain-based piperidinium and N-spirocyclic QAs) could only deliver modest performance in a water electrolyzer. For example, the current density by the cPBI-0.4p-0.6s-based AEMWE was found to be 0.553 A cm⁻² at 2.1 V, 1 M KOH, and 80 °C. Due to the high conductivity (110.3 mS cm⁻¹), the prepared membrane achieved a modest performance. Also, the membrane's durability was average, and there were only small changes in the voltage (1.78–1.85 V) after 10 h of testing. It might be better if the study lasted at least 100 h.

The crosslinkers and electrolyte content affect the membrane's current density as well. Accordingly, 1,6-hexanedithiol cross-linked polynorbornene-based AEMs (V-1.5-H-1) and 3,6-dioxa-1,8-octanedithiol cross-linked polynorbornene-based AEMs (V-1.5-O-1) were put to the test in a water electrolyzer with 1 and 0.1 M KOH and an applied voltage of 2.0 V at 60 °C.³⁴³ Better current density (0.86 A cm⁻²) was achieved with 1 M KOH and 3,6-dioxa-1,8-octanedithiol crosslinked AEMs system than 1 M KOH with 1,6-hexanedithiol-crosslinked AEMs system (0.513 A cm⁻²). However, the current density was reduced when using 0.1 M KOH for the membranes V-1.5-O-1 (0.60 A cm⁻²) and V-1.5-H-1 (0.21 A cm⁻²). Therefore, the concentration of electrolytes has a significant impact on the membrane current density. Although, at 40 °C, the V-1.5-H-1's ionic conductivity (112 mS cm⁻¹) was slightly higher than that of V-1.5-O-1 (108 mS cm⁻¹). However, a lower current density is seen in V-1.5-H-1 than V-1.5-O-1. The fact that V-1.5-O-1 had a superior current density might be attributed to the effects of the crosslinker (3,6-dioxa-1,8-octanedithiol) and the molarity of the electrolytes (1 M KOH). Additionally, the V-1.5-O-1's durability test was superior to the V-1.5-H-1's at 0.1 M KOH, 60 °C, and at a current density of 0.2 A cm⁻², perhaps owing to the membrane's high alkaline stability. There were no appreciable changes in voltages over time up to 70 h. Vinylbenzyl chloride/styrene-tethered copolymer-based membranes were shown to have low durability and moderate current density (0.8 A cm⁻²) at 2.0 V in the 1 wt% KOH solution (e.g., g-VBC-5-co-Sty-16-Q).³⁴⁴ This structured copolymer membrane had poor ionic conductivity (5.7 mS cm⁻²) and maybe low alkaline stability, resulting in a moderate current density and short durability (stable for only 3 h). Therefore, an additional study on membrane modification is required to increase the conductivity and alkaline stability of the membranes in light of AEMWE performances.

Imidazolium-functionalized membranes are difficult to test in alkaline electrolysis because OH⁻ attacks the C2 position and breaks the membrane. It is possible that adding C2-methyl to the membrane could make it more stable under alkaline conditions. The chemical stability of the AEM could also be improved by adding N3-butyl. Thus, C2-methyl and N3-butyl-incorporated imidazolium-functionalized PAEK membrane (e.g., PAEK-APMBI) was studied for water electrolysis performance at 2 A cm⁻² in 10 wt% KOH solution at 60 °C, and they were compared with commercial AEM (FAA-3).¹⁴⁷ The high conductivity of imidazolium-functionalized PAEK obtained better results (2.53 V at 2.0 A cm⁻²). It had better electrochemical performance than the FAA-3 membrane (2.63 V at 2.0 A cm⁻²) as it was made. The benefit of the C2-methyl substitute in the membrane is that it improves the alkaline stability and keeps the imidazolium from being attacked by hydroxide.

The longer side chain-crosslinked piperidinium-functionalized membrane (e.g., SEBS-P206) was put through tests for water electrolysis using clean water and 0.1 M KOH, and the results showed that its current density was 0.275 A cm⁻² and 0.680 A cm⁻², respectively.³⁴⁵ Thus, the KOH solution that uses the AEMWE system can potentially attain higher results. Similarly, longer side chain-crosslinked pyrrolidinium-functionalized membranes, such as SEBS-Py206, demonstrate greater current densities (0.5 A cm⁻²), which were achieved in the presence of 0.1 M KOH, compared to lower current densities (0.17 A cm⁻²) when using pure water and a cell voltage of 2 V.³⁴⁶ Nevertheless, the current density of the SEBS-Py206 was lower than SEBS-P206 AEMs in this study's testing of water electrolysis using a PGM-free catalyst or conductivity issues. Triazole may be utilized to crosslink the two polymers to enhance the AEMWE performances and mechanical qualities. Accordingly, PPO-SEBS with triazole crosslinking (such as TriPPO-50SEBS) was found to have a high current density (0.71 A cm⁻²) compared to PPO-SEBS without crosslinking (such as PPO-50SEBS), which had a low current density of 0.42 A cm⁻².³⁴⁷ Ionic conductivities of high (77.3 mS cm⁻²) and low (66.0 mS cm⁻²) values may be caused by larger and lower current densities by the membranes. Triazole may, thus, increase the membrane conductivity and hence increase the current density. The crosslinked PPO-SEBS's durability was extremely high and lasted for 3.8 h as opposed to the triazole-free PPO-SEBS, which degraded after 0.06 h at a current density of 0.2 A cm⁻².

High levels of current density (>0.5 cm⁻²) are often required for hydrogen production on an industrial scale when electrochemical techniques are used. Therefore, electrolysis was investigated by Wan et al. using a commercially available AEM (FAA-3-50) in conjunction with a manufactured MEA (Pt/C and IrO₂ catalyst and Nafion ionomer).²⁵³ At 2.15 V, the membrane attained a current density of 1 A cm⁻². Despite this, the commercial AEM's performance was inferior to that of the developed PTFE/LDH composite membrane (1 A cm⁻² and 1.88 V). It was discovered that additional catalyst usage (LDH) with mass transfer and the improved OER (oxygen evolution reaction) performance of LDH were the reasons for the composite membrane's greater performance (Fig. 52a). The cause may also be that the interfacial resistance between the catalyst and FAA-3-50 is greater than that between the catalyst and PTFE/LDH. At a current density of 1 A cm⁻², the researchers found that an increase in temperature from 20 to 60 °C resulted in a drop in the voltage from 2.1 to 1.88 V. Because an increase in temperature facilitates the passage of hydroxide ions, the electrolyzer's ohmic resistance will noticeably decrease as the temperature rises. This might be the cause of the falling voltage. In addition, the transfer of charge that occurs at the interface between the catalyst layer and the membrane in the MEA is caused by the mobility of OH⁻ ions. Therefore, in the MEA with a composite membrane, the number of OH⁻ ions that flow between the electrode and membrane increases, ultimately resulting in a higher degree of reaction but a lower cell voltage. When evaluated for durability, the two membranes, PTFE/LDH and FAA-3-50, showed strong and poor stability for more than 180 h at a current density of 0.5 A cm⁻², respectively (Fig. 52b). Although the catalysts were washed out of the electrodes, the voltage was raised by 8 h for FAA-3-50, demonstrating that the catalyst and flat AEM interface did not retain the catalyst particles during continuous operation, which resulted in instability. At the same time, the three-dimensional structure of the PTFE/LDH composite was used to hold the catalyst particles that increase the stability during continuous operation.

According to recent research, utilizing the alkali metal media available in the electrolysis system may aid in designing and optimizing AEMWE.³⁴⁸ The polarization of AEMWE showed that an increase in the electrolyte (KOH) concentration resulted in an improvement in the operating performance of the electrolyzer while simultaneously leading to a reduction in the cell's high-frequency resistances. Research demonstrates that additional hydroxide ions play a crucial function, including acting in the AEM, the catalyst layer's ohmic resistance, and working in cell performance reaction rates. In addition, it was discovered that the cell, including the electrolyte 1 M KOH, performed better than the cell containing only water. Therefore, it was determined that an electrolyte solution with a high pH in AEMWEs is essential to accomplish a high level of cell electrolyze performance. It was discovered that ionomers with strong ion exchange capacity and a greater interface with electrocatalysts might increase the AEMWE performance. In addition, the significance of conductivity in the electrode and electrolyte supplied in the electrode for the performance of the AEMWE was shown by the high level of performance achieved when the 1 M KOH solution was circulated.

Mechanically reinforced and high ion conduction diamine cross-linked PSF and PVBC-MPy-based AEMs have recently been hooked up to improve water electrolysis performance.¹³⁵ Cell performance was proven to be dependent on membrane conductivity. It was found that the AEM (e.g., M-6#) lasts in the cell for up to 48 h, after which the cell voltage stays the same (Fig. 53a). Better results for OER were achieved using polysulfone-based AEMs (e.g., PSF-TMA) in alkaline water electrolyzers with the inexpensive catalyst Pb-ruthenate-pyrochlore (Pb-R-P).³⁴⁹ This research aimed to quantify electrolyzers' stability and long-term performance and identify the root causes of performance degradation. Results from the study demonstrated that water electrolysis at 50 °C produced a current density measurement of 0.4 A cm⁻² at 1.8 V. These composite-structured membranes may perform poorly for water electrolysis in hydrogen production due to their low current density.

The significant current density (3.7 A cm⁻²) for composite-structured quaternized polysulfone-TMA-functionalized silica AEMs (e.g., qPSU/NIM-N⁺) at 2.2 V under 1 M KOH at 80 °C.³⁵⁰ This composite membrane may be particularly useful for producing hydrogen in real-world applications because of its high current density and high membrane conductivity (109 mS cm⁻¹). Long-term behavior is crucial for AEMs; hence, consideration should be given to their resilience over time when developing AEM electrolysis. As a result, the test was run at 2 V for 15 h while submerging in a 0.5 M KOH solution, and it was found that the AEM remained stable for up to 14 h. Interestingly, the straightforward organic method to create quaternized polysulfone (qPSU) AEM demonstrated a much greater current density (4.2 A cm⁻²) at 2.2 V under 1 M KOH at 90 °C.³⁵¹ It was shown that the current density might not depend on ionic conductivity, although the qPSU's ionic conductivity (77 mS cm⁻¹) feature allowed for achieving a very high current density. However, the membrane's increased current density at higher temperatures is the sole issue with the reported study.

Recently, higher amounts of imidazolium-functionalized blended AEMs (e.g., PISPVA46) with high conductivity and high hydration characteristics have shown superior electrochemical performance than smaller amounts of imidazolium-functionalized blended AEMs such as PISPVA37 and PISPVA 28.¹⁰¹ Since PISPVA46's charge transfer, ohmic, and mass transport resistance was lower than those of PISPVA37 and PISPVA28, thus, PISPVA46 had good electrochemical performance. The PISPVA46 membrane's stability was discovered to be somewhat less stable, dropping from 0.35 A cm⁻² to 0.22 A cm⁻² from 30 min to 80 h, respectively (Fig. 53b). Long side chain (LSC) and short side chain (SC) QAs, connected to AEM-containing piperidinium (Pi) groups, respectively, demonstrated electrolysis of pure water at 50 °C.³⁵² The research found that LSCPi and SCPi AEMs had a current density of 0.3 and 0.2 A cm⁻² at 1.8 V, respectively. Comparing LSCPi AEM to SCPi AEM, the improved performance of the potential cell with LSCPi AEM was attributed to their lower ohmic resistance or greater membrane conductivity (Fig. 53c). On the LSCPi AEMs, the possibility of delamination between the membrane and catalyst was attributed to the degradation of MEAs, resulting in moderate durability. It was found that there was a rapid increase in cell voltage after 35 h.

Another research examined the effects of changing the catalyst loading (10–40 mg cm⁻²), ionomer quantity (9–33%), electrolytes (K₂CO₃, KHCO₃, KOH, and water), and temperature (40–80 °C) on AEMWE performance.³⁵³ Catalyst loading between 30 and 40 mg cm⁻² was optimal, with 10 mg cm⁻² being the bare minimum due to the increased overpotential. It refers to the extra energy required to start the water-splitting process above and above what is anticipated on the principle of thermodynamics. It was shown that just 9% of the ionomer was enough to prevent cracking in the catalyst layer. Nonetheless, a nonlinear connection existed between temperature and voltages, with higher temperatures resulting in lower onset voltages. Better performance was seen with a 1% K₂CO₃ electrolyte compared to 1 M KOH, 1% KHCO₃, and water owing to the electrolyte's stronger stability and lower overpotential (Fig. 53d). Finally, the 1% K₂CO₃ electrolyte at 60 °C showed the greatest performance, with 0.5 A cm⁻² for 1.95 V. In addition, a durability test over 200 h demonstrated the same promising results for hydrogen production, with the performance remaining almost constant at 0.5 A cm⁻² at 2.08 V. LDHs generally have high ionic conductivity, can transport more OH⁻ ions, and are quite stable over the long term. To investigate water electrolysis in an alkaline environment containing 0.1 M NaOH, Zeng et al. developed an inorganic-based AEM (cold pressed Mg–Al LDH membrane).³⁵⁴ At a temperature of 70 °C and a cell voltage of 2.2 V, the integrated inorganic membrane-containing MEA demonstrates an electrolysis performance that produces a current density of 0.208 A cm⁻². The findings of the cell durability test showed that the voltage of the cell grew slightly during the whole of the study, exhibiting strong stability when using either 0.1 M NaOH (voltage increased from 1.88 to 1.95 V) or 0.1 M Na₂CO₃ (voltage increased from 2.03 V to 2.09 V).

Under various conditions, including the electrolyte concentration and temperature, the electrolysis performance of dense side chain-functionalized partially-fluorinated PAES membranes was evaluated.³⁵⁵ It was observed that the electrolyte concentration (0.5–2.0 M NaOH) and temperature (20.0–80.0 °C) affected the amount of current that was flowing. This indicates that the resistance dropped as the electrolyte content and temperature rose. The current density that could be attained in this investigation was 1.2 A cm⁻² under 2 M NaOH and at a temperature of 80 °C. The membrane's durability (e.g., PAES-TMI-0.25) was determined by exposing it to 2 M NaOH at 0.5 A cm⁻² at 30 °C for 480 h. It was discovered that there were slight variations in the voltage over time and that the voltage decreased as the concentration of NaOH increased. This was considered an indicator of the durability of the battery's performance. A study evaluated a partially-fluorinated PAES membrane (e.g., PAES-MQA-0.18) with extremely flexible side chains and numerous QAs for water electrolysis at two different NaOH molarities (1 and 2 M).³⁵⁶ The AEMs of such types exhibit higher performance because of their strong ionic conductivity (125.5 mS cm⁻²). As a result, the membrane produced a high current density (1.12 A cm⁻²) at a voltage of 2.0 V while operating in a 2 M NaOH electrolyte at 30 °C. When the electrolyte concentration was 1 M NaOH, the same membrane could not demonstrate a superior current density (0.50 A cm⁻²). Therefore, the electrolyte solution's concentration could improve the AEMWE's current density. At 0.5 A cm⁻², 2 M NaOH, and 30 °C conditions, the durability test was high up to 480 h, and a very low degradation rate was discovered. It could be due to partial fluorination, numerous QAs, and flexible side chains that helped to increase the membrane's alkaline stability.

Recently, it was shown that morpholinium-tethered polyketone (e.g., PK_0.5 step 3) with low conductivity (0.029 mS cm⁻¹) had extremely poor water electrolyzer performance, only showing a current density of 0.15 A cm⁻² at 2.0 V in pure water at 80 °C.³⁵⁷ The AEM's poor ionic conductivity may have caused the low current density. Therefore, mopholinium may not be the best modification to enhance the polyketone membrane's characteristics. To guarantee a secure and productive operation of electrolyze, one of the most significant tasks is the measurement of hydrogen crossover. Accordingly, Bensmann and colleagues researched an in situ approach for determining the hydrogen gas crossover that occurs during the water electrolysis of polymer electrolyte membranes.³⁵⁸ The electrolysis measurement is based on the electrochemical output of the flux, which can transform the mass flux into an electric current. The method that has been suggested has a simple setup and a high degree of measurement precision; these qualities make it suitable for use in electrolysis applications at an industrial level.

In another research, increasing the concentration of an ionomer (e.g., TMA) with MEA resulted in higher current density when compared to bare MEA under pure water-electrolyte conditions, and the influence of phenyl groups-loaded ionomer was shown to be identical.³⁶⁰ Nevertheless, the current density varied depending on the electrolyte solution, with pure water obtaining a greater current density than the NaOH solution owing to an acidic phenol from the phenyl group being neutralized by the NaOH solution. Moreover, the same research discovered that the durability of higher amounts of ionomer MEA was poorer than smaller amounts of ionomer MEA because more elevated amounts of MEA do not retain the catalyst particles throughout the operation. Similarly, in another study, MEA was equipped with various commercial AEMs such as Sustainion, AEMION, and A-201, which showed that Sustainion obtained better electrochemical performance (Fig. 53e), i.e., the resistance was less than that of others under low concentration KOH solution at a current density of 0.5 A cm⁻² and cell voltage of 2.1 V.³⁵⁹ Additionally, the cell voltage of Sustainion-MEA was decreased with increased temperature due to increased membrane resistance. Therefore, the temperature does not play a greater role in the commercial AEM for cell efficiencies.

AEM has high conductivity, high mol. weight, and high mechanical strength, which might be advantageous for water electrolysis. Accordingly, Miyanishi et al. promised that the polyfluorene-based AEM (such as PFOTFPH-C₆-TMA) could be highly useful in water electrolysis because they had high conductivity (100 mS cm⁻¹), high mol. weight (170–240 kDa), low swelling, high mechanical strength (25–42 MPa), and high chemical stability (under 8 M NaOH).¹⁰⁰ Moreover, they possessed minimal swelling characteristics, which may have been an advantage of the AEM. Recently, high conductivity (123 mS cm⁻¹), durability, and efficiency of 7-bromo-1,1,1-trifluoroheptan-2-one-incorporated poly(biphenyl alkylene) (e.g., PBPA) AEM were discovered to have excellent performance in water electrolyzer, which obtained a very high current density 4.0 A cm⁻² at an applied voltage of 2.0 V, using 1 M KOH at 80 °C.³⁶¹ Furthermore, the PBPA membrane-based AEMWE responded effectively to increased KOH concentration from 0.1 to 1 M, corresponding to an increase in the current density from 1.3 to 2.8 A cm⁻². The excellent durability of the PBPA membrane was discovered to be very high, with no significant changes in the voltages up to 3500 h at the conditions of 1 A cm⁻², 1 M KOH, and 60 °C, which could be attributed to the incorporation of fluorine moiety in the membrane, which is attributed to the membrane's high alkaline stability. Because of its great durability (membrane degradation rate less than 6.5 μV h⁻¹) and high current density (4.0 A cm⁻²), PBPA might be a viable choice for real-time hydrogen synthesis utilizing a water electrolyzer. Due to the hydroxyl groups present in its structure, the crosslinker tris(dimethyl aminomethyl) phenol (TDMAP) may form a hydrogen bond with H₂O, increasing the conductivity as it does so. Accordingly, TDMAP-crosslinked bromohexyl pentafluorobenzyl SEBS membrane (e.g., TDMAP-50%-SEBS) exhibits excellent conductivity (109.9 mS cm⁻¹), yet this crosslinked SEBS membrane may demonstrate current density of 1.19 A cm⁻² at 2.0 V under the 1 M KOH at 70 °C.³⁶² Crosslinked SEBS performed better in water electrolysis (1.19 A cm⁻²) compared to the FAA-3-50 membrane (0.96 A cm⁻²), which may be related to FAA-3-50's lower conductivity (80.4 mS cm⁻²). However, no study on durability was found for this investigation.

The stability of the functionalized methacrylate monomers with the fluorinated alkane side chain (2,2,2,-trifluoroethyl methacrylate (TFEMA)) to the quaternized poly(DMAEMA-co-TFEMA-co-BMA) membrane (e.g., QPDTB) was found to be the current density, which remained steady for 300 min.³⁶³ However, the water-electrolyte used in the AEMWE with QPDTB achieved a low current density (0.1 A cm⁻²) at a cell voltage of 1.9 V when the temperature was kept constant. Perfluorinated commercial membrane Aquivion® (e.g., AQ720A4-Q) integrated QAs achieved excellent current density (0.9 A cm⁻²) at a voltage of 2.2 V and functioned at high temperature (90 °C). No durability testing was carried out in this study.³⁶⁴ According to Chen et al., the characteristics of QAs-tethered poly(fluorenyl ether) (PFEQA) membranes make them highly applicable in water electrolysis.³⁶⁵ These characteristics include high hydroxide conductivity, mechanical strength, and relatively moderate chemical and thermal stability. The developed AEMs showed features capable of use in water electrolyzers, which can function in pure water without electrolytes. TMA-functionalized poly-(fluorene-alt-tetrafluorophenylene) (e.g., PFT-C10-TMA (MEA-1)) has high durability and is very effective despite its 56 m thickness. Under 1 M KOH at 80 °C, AEM was measured at 1.8 V, showing 1 A cm⁻² and at 0.1 V, showing 0.0 A cm⁻².³⁶⁶ As a result, applying voltages is crucial for achieving a current density. Up to 1000 start–stop cycles, the voltage of the MEA-1 was seen to remain unchanged, demonstrating its outstanding durability. However, the PFT-C10-TMA membrane (MEA-2) was tested after being adjusted to a thickness of 35 μm. Current density (1.0 A cm⁻²) and durability (1000 start–stop cycles) were determined to remain unchanged at the same voltage. Due to their very durable structure, the generated fluorocarbon AEMs (MEA-1 and MEA-2) may be a suitable candidate for real-time use in hydrogen generation.

Suzuki cross-coupling was employed to produce aryl ether-free and propyl spacers in a polyfluorene-polymer membrane (e.g., PFPB-QA) with pendent ammonium in the side chains, which was tested for water electrolysis.³⁶⁷ The polymer's propyl spacer may have the advantage of increasing flexibility and having the capacity to contain a large ionic cluster. In this work, water electrolysis was tested using AEMs and ionomers. Accordingly, PFPB-QA membranes employing ionomers such as PFPB-QA and QA-PPO compared these results to commercial membrane FAA-3-50 using ionomers such as FAA-3, PFBQA, and QA-PPO. It showed that PFPB-QA produced current densities of 1.53 and 1.19 A cm⁻², respectively, using the ionomers PFPB-QA and QA-PPO. Contrarily, the ionomers (FAA-3, PFBQA, and QA-PPO) utilized in FAA-3-50 exhibit lower current densities (0.942, 0.946, and 1.05 A cm⁻²). As a result, it was investigated using a synthetic polymer membrane known as an ionomer that worked well in the electrolysis system. It showed a current density of 1.53 A cm⁻² at a voltage of 2.0 V in 1 M KOH electrolyte at a temperature of 70 °C. This may be because the PFPB-QA membrane had a greater conductivity (122 mS cm⁻¹), but the FAA-3-50 membrane had a lower conductivity (76.9 mS cm⁻²). Additionally, a durability test over a very short time (0.4 h) was performed, and the results showed that PFPB-QA was almost stable up to 0.4 h, whereas FAA-3.50 was unable to do so because PFPB-QA has an aryl-ether-free structure and FAA-3.50 has an unstable PEEK structure. Table 8 shows various AEMs used in water electrolysis, and their corresponding parameters, including current density and durability, have been summarized.

5.2 Performance of electrocatalyst in AEMWE

The HER (cathode) and OER (anode) half-cell reactions are essential to water electrolysis. Noble metals, including Ir, Pt, Rh, and Pd, are believed to be the best electrocatalyst materials for HER. OER catalysts, on the other hand, are often made of noble metals, including platinum, palladium, rhodium, iridium, gold, and their alloys.³⁶⁸ However, OER is quite sluggish in terms of kinetics and thermodynamics. Thus, it is essential to use a catalyst not based on a noble metal to reduce the total cost of AEM electrolysis. For example, in recent years, Ni, IrO₂, graphene, and Ni–Fe have been used as catalysts for OER, while Pt, Ni–Mo, CuCoO, Ni, and graphene have been utilized as catalysts for HER. To improve the electrocatalytic activity, scientists are working on producing efficient OER catalysts.⁵⁶

To make large quantities of hydrogen at an industrial scale, researchers need to find low-cost, highly stable electrocatalysts with identical catalytic performance to catalysts for water splitting at greater current densities. Any acidic, basic, or neutral environment suits HER and OER. HER works well in acidic electrolytes because there are enough protons (H⁺) to adsorb on the electrode surface and react. Still, utilizing alkaline and neutral electrolytes is not easy because water dissociation weakens the kinetic process.³⁶⁹ In contrast, basic electrolytes are highly favored as OER catalysts for water splitting because of their superior activity and great stability. At the same time, acidic solutions cause the catalyst to disintegrate and quickly inactive the catalyst. HER or OER catalysts do not perform significantly in a neutral medium. Suppose the neutral-based water-splitting process could be satisfied by the catalyst. In that case, the real-time seawater splitting system may be conceivable without the desalination matter for pH settings. This would produce a greater quantity of hydrogen in a more environment-friendly manner. Metal oxides, hydroxides, perovskites, and carbon-based electrocatalysts have been used for water-splitting. Carbon-based catalysts are particularly effective in producing OER catalysts because of their large surface area, high conductivity, and excellent corrosion resistance. Despite this, they have a high cost of synthesis and are a notoriously difficult technique. On the other hand, scientific communities are trying very hard to synthesize an effective OER catalyst with a low price and a straightforward procedure for employing it in AEMWE. Therefore, a discussion will be made of low-cost-based effective electrocatalysts for the catalytic process, leading to hydrogen generation in an AEMWE system in the following sentences.

For hydrogen to be produced via water electrolysis, purified water is required. On the other hand, if only clean water were utilized to generate hydrogen everywhere, then significant problems with water supply may emerge. Therefore, the electrolysis of sea salt water, which is one of the most plentiful characteristics in the world and is regarded as a component bestowed by nature in the generation of hydrogen and seawater desalination, without concerns about the use of clean water, is viable options. Accordingly, the AEM-water electrolyzer (AEMWE) that Park et al. developed with a composed Ni–FeOOH (AEMWE-NF) catalyst was evaluated for its electrolysis capability using 1 M potassium hydroxide and seawater.³⁷⁰ The constructed configuration revealed that a current density of 0.729 A cm⁻² could be attained, demonstrating a cell efficiency of 76.3%. Additionally, they examined the electrolysis performance of AEMWE-IrO₂, which revealed a lower performance (73.2%). These results demonstrated that performing AEMWE utilizing seawater while maintaining high efficiency is feasible. The endurance of these catalyst-composed AEMWE was examined, and the results showed that AEMWE-NF was superior to IrO₂-composed AEMWE (high voltage loss) over 15 h at 0.5 A cm⁻². Due to the superhydrophilic surfaces of the Ni–FeOOH catalyst, the AEMWE-NF had a greater efficiency than the AEMWE-IrO₂, which was the case. Therefore, the Ni–FeOOH surface possesses the superior property that generated oxygen gas is swiftly removed from the surface of the electrode, which led to cell voltage being proficiently used due to the mass transference resistance in the electrolyzer being efficiently minimized. This resulted in the cell voltage being used to its full potential.

The OER is an important process in various energy storage applications, such as water electrolysis; however, its sluggish reaction and large overpotentials make it difficult to employ in these applications. For optimal oxygen evolution reactions (OER), an electrocatalyst has to have a lot of surface area and strong electrical conductivity. Many investigators have exerted great effort for seeking ideal catalysts to enhance the reaction speed and stability. In light of this, Busacca et al. developed Ni–Mn and Ni–Co-based catalysts on carbon fiber for use in OER in AEMWE.³⁷¹ The electrochemical test was carried out in water and 6 M KOH solution. The results showed that the Ni–Mn-based catalyst performed better due to its high electrical conductivity. Additionally, the OER activity of both the catalysts improved with increasing temperature when tested in the alkaline solution, in contrast to when tested in water. Recently, the active bifunctional electrocatalyst graphitic nitride-carbon fiber (e.g., g-CN-CNF-800) was designed by Park et al. for AEMWE testing, and the electrochemical performance test was studied by modifying the loading of the catalyst.³⁷² The investigation discovered that loading a thicker catalyst layer might reduce the electrolysis performance. The researchers then refined the catalyst loading, and they found that 6 mg cm⁻² led to achieving the optimum current density (0.734 A cm⁻²) when the voltage was set to 1.9 V (Fig. 54a). In addition, g-CN-CNF-800 shows an excellent electrolysis performance compared to IrO₂ catalyst, i.e., the performance of the carbonaceous (g-CN-CNF-800) catalyst was 53% greater than IrO₂ at a current density of 1.9 V. Due to their outstanding electrochemical activity and stability for energy conversion in cell operations, the functionalization of NiFeOOH on nitrogen-doped carbon-based materials (e.g., a-NiFeOOH/N-CFP) is now a desirable electrode.³⁷³ Additionally, doping a high electronegative nitrogen element into a carbon substrate may provide the carbon atoms with an unequal supply of electrical charge, which enhances the oxygen electrochemistry on nitrogen-doped carbon materials. Additionally, plasma doping nitrogen enhances the electrode's surface wettability, which enhances the integration of electrodes with the carbon substrate. The amorphous phase and nitrogen-doped carbon support were the only compensations to achieve a current density.

Proton exchange membrane water electrolyzers (PEMWEs) and alkaline water electrolysis (AWE) are well-established industrial technologies for producing H₂. Despite this, AWE has some demerits, such as forming toxic carbonates after OH radicals react with CO₂ in the atmosphere to reduce the IEC and the system's overall effectiveness.^125,374 As for PEMWEs, they use expensive platinum (Pt) metal electrocatalysts, which makes them a high-priced technology, with the possibility of degradation when long-term stability testing is conducted. It is also not durable when combining a PEMWE with renewable technologies.^59,374,375 In addition to these two approaches, AEMWEs, which combine the benefits of PEM and alkaline electrolyzers, are now the subject of intense study. AEMWEs have a compact design, superior carbonate formation prevention, and employ non-precious metal catalysts such as Ni, Fe, and Co. In a while, an environment with a high alkaline concentration and high temperatures may cause the membrane to degrade. Therefore, Niaz et al. recently looked into the effects of two distinct operating voltage conditions on the deterioration behavior of AEMWE over 1000 h and carried out an experiment to find a major degradation factor concerning the operation modes, such as short and long periodic operating modes.³⁷⁴ They observed that the long periodic operating mode resulted in superior stability, with the ohmic and non-ohmic electrical resistance staying practically constant throughout the test. To develop novel catalyst materials with high catalytic activity, high conductivity, and electrocatalytically excellent catalyst supports for electrolysis, Chanda et al. constructed NiFeS–Ti₃C₂ MXene electrocatalysts on Ni foam framework for AEMWE.³⁷⁶ Compared to electrocatalysts such as Pt/C–RuO₂ cells in AEMWE, it achieved cell efficiency of 67.6% and attained a current density of 0.4 A cm⁻² at a cell voltage of 1.85 V. The study's durability was found to be quite good, allowing continuous operation of the cell for up to 750 h with little fluctuations in current density (Fig. 54b). Because of the low running expenses of this sort of system, the proposed electrolyzer is among the best-performing AEMWE systems in recent years.

Another research discovered that the current density was dependent on the electrocatalysts; for example, at a voltage of 1.85 V, NiCoO–NCo/C (0.504 A cm⁻²) exhibited a larger current density than NiCo/C (0.071 A cm⁻²), which showed a lower current density in AEMWE in the single-cell setup.³⁷⁷ For this reason, the form of NiCoO–NCo/C electrocatalysts that is used is a crucial consideration for use in water electrolysis. In addition, it was observed that the durability of the electrocatalyst was very steady for 10 h, but the NiCo/C electrocatalyst showed a rapid increase after just 3 h of use. Moreover, operating a 5-cell setup with electrodes such as NiCoO–NiCo/C (cathode) and CuCoO (anode) obtained a high current density of 0.74 A cm⁻² at 9.25 V_stack (Fig. 55a–e). The 5-cell stack AEMWE demonstrated superior results than the single-cell AEMWE, and the current density grew quickly while utilizing this method. The 5-cell configuration's endurance was discovered to be degrading at a rate of 2.0 mV h⁻¹. Furthermore, it was found that the cell efficiency was initially 69% (0 h), but it later revealed an efficiency of 59%. High-purity H₂ (99.95%) may be produced using the newly invented 5-cell system, equivalent to commercial grade H₂ (99.999%).

Recently, the electrocatalyst with chemically etched CuCo-based AEMWE has shown excellent performance in oxygen evolution reaction (OER).³⁷⁸ This system also has a higher current density (1.39 A cm⁻²) owing to the electrocatalyst's high electrical conductivity. The durability of the CuCo-based AEMWE was tested for 64 h and found to be good. The cell voltage slightly increased from 1.66 to 1.74 V (Fig. 56a). In contrast, the IrO₂ electrocatalyst-based AEMWE system showed lower performance in OER, and the obtained current density was only 0.9 A cm⁻² due to the low conductivity of the electrocatalyst. Similarly, for AEMWE, an effective electrocatalyst (such as H–Co_0.9Fe_0.1-CNF) was used.³⁷⁹ It was discovered that the cell voltage was almost stable and only increased slightly with the current density up to 1.7 V with a high current density of 0.794 A cm⁻², which may have been caused by the electrocatalyst's high electrical conductivity (Fig. 56b). The H–Co_0.9Fe_0.1-CNF-based electrocatalyst was found to have good performance in terms of stability, with no noticeable variations in cell voltage up to 290 h. Recently, high AEMWE electrochemical performance was achieved to attain current densities of 1.5 and 1.15 A. cm⁻² at 1.9 and 1.8 V, respectively, by adjusting the MEA manufacturing process, MEA characteristics, architectures, ionomer-membrane selection, and operating conditions.³⁸⁰ The parameters of low ohmic, high charge transfer, and high mass transport resistance maximized the cell performance.

Researchers have been drawn to transition metal sulfides because of their low cost, rapid charge-transfer kinetics, high conductivity, high active sites, high mechanical strength, stability, and high storage. Nevertheless, nickel sulfides in crystalline form have strong OER energy resistance. As a result, by building a heterojunction interface, the performance of OER might be improved by tuning the electrical structure. A catalytic arrangement based on heterojunctions might reduce the energy consumption, enabling a low-cost hydrogen economy. As a result, Wang et al. developed a high-conductivity heterojunction-structured Ni₂P/Ni₇S₆ catalyst for OER performance.³⁸¹ Compared to a single catalyst system (Ni₂P, Ni₇S₆ and Ni), they discovered that the new heterojunction-structured catalyst needs overpotentials of 330 mV to attain 1.0 A cm⁻² and is durable at 0.5 A cm⁻² for 700 h. Compared to lower temperatures, the heterostructure catalyst on Pt/C at 75 °C revealed the best cell performance, with a current density of 1.0 A cm⁻² at 1.88 V. The developed catalyst-supported AEMWE exhibits 1.0 A cm⁻² of current density for 140 h at 1.88 V and 75 °C in a 1 M KOH environment. Substantial water splitting for industrial use might be accomplished using the advanced heterostructure electrocatalyst since it is very active and stable. Pt is a very active catalyst for HER; however, its sluggish kinetics results in a high cost due to the large loading of Pt quantities needed. Intermetallic structured catalysts showed better HER activity in AEMWE than Pt/C catalysts, suggesting a potential solution. As a result, very stable intermetallic MoO₂/Ni cathode and Ni foams were prepared for AEMWE HER testing.³⁸² A current density of 0.55 A cm⁻² was obtained at 2 V at 60 °C and was found to be stable for more than 300 h at 1 M KOH at a current density setting of 0.5 A cm⁻². The highly porous structure of MoO₂/Ni in this work enables high active areas for water electrolysis, and the study was built up to make MoO₂/Ni with an extent of 78.5 cm², which was evaluated in a three-cell AEMWE stack and might be beneficial for industrial use.

To achieve optimal cell efficiencies, it is essential first to develop OER catalysts that are both effective and economical. As a result, one of the low-cost-based binaries or ternary electrocatalysts might play a significant role in the OER process for improved water electrolysis applications. Accordingly, the researchers observed that ternary electrocatalysts exhibited greater electrochemical performance than binary electrocatalysts in the AEMWE system.³⁸³ Moreover, when placed in an alkaline solution, the ternary electrocatalysts offered a very low OER overpotential. It was discovered that the ternary electrocatalyst had a durability of not changing at a current density of 0.5 A cm⁻² for up to 3.5 h in a highly alkaline medium at 60 °C. The electrocatalyst Co–Fe ratio was recently modified and tested for electrochemical performance in AEMWE.³⁸⁴ The results showed that the onset potential decreased as the catalyst Co content increased, while the OER activity increased as the catalyst Co content increased. Examining the stability of AEMWE with Co–Fe and Pt/C catalysts at 1.8 V reveals that the current density did not change after 16 h. Recently, a porous transport layer on the anode material considerably impacted the OER's AEMWE.³⁸⁵ Higher porosity benefited numerous transport capabilities, such as water and oxygen, while decreasing the adherence of the catalyst layer to the porous transport layer. Furthermore, the Ni-based porous transport layer had a lower operation voltage than the stainless steel-based porous layer. Nevertheless, by sintering the materials, the operation voltage may be increased. The thickness of the porous layer (1.2 mm) has a negligible influence on the electrolyzer process. In this study, AEMWE with enhanced porous transport layer material achieved an electrolyzer voltage of 1.64 V at 1.0 A cm⁻² and at a cell voltage of 2.0 V, it acquired a current density of 3.5 A cm⁻².

Perovskite oxides are extensively used as OER electrocatalysts due to their low cost, high durability, cation deficiency, flexible electronic structure, high conductivity, high catalytic material, and eco-friendliness. Perovskite oxides also have high conductivity and high catalytic material. Hence, Chen et al. developed a hybrid structured perovskite catalyst to boost the oxygen diffusion rate, ultimately increasing the OER.³⁸⁶ The hybrid perovskite has superior OER activity, as demonstrated by the measurement of 0.1 A cm⁻² achieved at 0.36 V in an environment containing 1.0 M KOH. Moreover, the hybrid-structured perovskite catalyst was applied as the anode of WEs, and this application was ascribed to a current density of greater than 1.0 A cm⁻² at a voltage of 1.8 V. The durability of the AEMWE catalyzed by a hybrid catalyst was evaluated for 40 h at a current density of 0.5 A cm⁻²; despite this, the AEMWE that was formed exhibited a degradation rate of 3 mV h⁻¹, indicating low durability.

Similarly, the commercial membranes Fumasep and Sustainion were employed as AEMs to investigate the electrolysis of water utilizing Pt/C as an electrocatalyst.³⁸⁷ A schematic diagram of the AEMWE setup is provided in Fig. 57a. At a voltage of 2 V and a temperature of 60 °C, the authors discovered that the electrolysis performance of Sustainion, which attained a current density of 2.74 A cm⁻², was superior to that of Fumasep (1.40 A cm⁻²). The catalyst (Pt/C)-coated carbon paper and carbon cloth testing for electrolysis performance revealed that catalyst-coated carbon cloth had high electrochemical activity. Based on its superior performance, only Sustainion was tested for stability using Pt/C-coated carbon paper and Pt/C-coated carbon fabric for 205 h at 60 °C. Compared to employing Pt/C-coated carbon paper, the Pt/C-coated carbon cloth demonstrated great stability without any changes throughout the test.

Hydrogen may be stored under pressure in a cell or stacked on a particular design of an AEM water electrolyzer. The manufacture of compressed hydrogen would facilitate the storage and consumption of hydrogen by allowing for direct storage and enhancing the energy output for impending energy conservation practices. Therefore, Ito et al. established a hydrogen pressure of 8.5 bar in the cell of the AEM water electrolyzer and examined the effect of hydrogen pressure on the electrolysis performance of water, including current density, cell voltage, anode gas, and the quantity of water in the produced hydrogen.³⁸⁸ The investigation findings demonstrated that a pressurized AEM-based water electrolysis method enabled the synthesis of hydrogen with low relative humidity. Schematic depiction of home-made AEMWE cell outlines and inlet gases arrangement for hydrogen pump tests for anode catalysts such as Pt/C (upside) and CuCoO_x (downside) are shown in Fig. 57b. The configured cell was utilized for AEM water electrolysis as well as used for the hydrogen-pump test. To measure current in the hydrogen pump tests, the voltage applied to the AEMWE cell by the DC supply is 0.50 V (downside) and 1.80 V (upside). The gas transfer for the anode measurement causes the current to fluctuate over time in these hydrogen pump experiments.

The impact of electrolytes and their various concentrations on the performance of AEMWE was examined.³⁸⁹ In the electrochemical performance, electrolytes such as K₂CO₃, KOH, and water were utilized, and it was determined that the electrolysis system functioned more efficiently in the favorable environment of KOH or K₂CO₃ than in water, owing to the membrane's high resistance in the water. In addition, the K₂CO₃ (around pH of 12) electrolyte performed better than KOH for electrolysis due to its lower resistance, and increasing the concentration (0.1–10 wt%) of K₂CO₃ resulted in an even greater improvement in electrolysis performance. Researchers have found catalysts that can replace expensive noble metal catalysts, such as metal oxides, spinel ferrites, organic matter, and perovskites, after searching for non-noble-based OER catalysts for several years. Among them, spinel ferrites are drawing interest because of their cheap cost, great durability, and excellent catalytic characteristics, which make them useful in catalysis. But spinel ferrite cannot be used as a catalyst in the electrolysis process for industrial applications because it has low conductivity. However, Pandiarajan et al. recently designed a spinel ferrite (Ce_0.2MnFe_1.8O₄) catalyst as the anode material.¹²⁴ In brief, Ni was coated on the cathode, and the Ce_0.2MnFe_1.8O₄ catalyst was used as the anode material, and both were set up to provide MEA and AEM (FAA-3-PK-130). In the electrolysis test, the applied cell potential was found to rise slowly, which caused the current density to rise to 0.3 A cm⁻² at 1.8 V. In the stability test, the current density (0.39 A cm⁻²) was almost the same after 100 h at 2 V. Thus, a spinel ferrite-based catalyst could improve the electrolysis performance and keep the OER stable over time. Also, the Pt–Ti meshes were used to make the electrolyzers last longer by reducing anode corrosion.

On the other hand, due to its naturally high availability and large supply, salt water is an excellent source for water electrolysis. However, seawater electrolysis is harder than water electrolysis because salt (chloride anions) produces a chlorine evolution reaction (CER), which increases the resistance of the AEM and could cause all the metals in the cell to corrode. At higher overpotentials, the CER can race with the OER, which causes all metal-based catalysts to corrode. Also, ions in the seawater, such as magnesium and calcium, can settle on the surface of the electrode during the electrolysis process and cover up the active catalytic spaces. Research into the fundamentals of electrolysis in seawater has been extended to prevent these issues. Accordingly, Xing et al. developed active, low-cost, and durable (1000 h) seawater-splitting electrocatalysts (NiFe-LDH/Pt/PTL (anode) and RANEY®-Nickel/Ni (cathode)) for the AEMWEs.³⁹⁰ The performance of the constructed electrolyzer cells, when operated with seawater, shows improvement, and after approximately 1000 h of processing time, the same current density (0.3 A cm⁻²) was attained.

Transition metal oxides (TMO) or a combination of various transition metals show an excellent performance toward OER catalysis due to the possibility of tunable stoichiometry of the lattice point.³⁹¹ This could be because transition metal oxides can be made with different transition metals. TMOs are not good conductors of electricity, and the isotherm process in the OER is caused by the oxygen vacancies created by electrocatalytic reduction.³⁹² But making TMO through orderly hierarchical growth in two-dimensional or three-dimensional structures gives the OER high electrochemical stability. Thus, recently, a hydrothermal method was used to make NiFe₂O₄ in a hierarchical structure to improve the catalytic activity of OER in AEMWE.³⁹³ When the hierarchical catalysts were made, it was found that they had a high catalytic activity toward OER. The study (NiFe₂O₄ catalyst) had a current density of 2.7 A cm⁻² at 2.2 V, more than that recorded with an IrO₂ catalyst at the anode for AEMWE based on FAA3-50 AEM. However, the 72 h durability test showed that it needed to be fixed to work longer. AEMWE is a new technology that uses platinum-free catalysts and pure water as the electrolyte to make it cheap to make hydrogen, and this kind of electrolyzer also has the capacity to create compact big cell stacks. Thus, considering the problems with PEMWE, an AEMWE has a great chance of supplanting the technologies. For the industrial uses of AEMWEs, it is important to find stable catalysts for OER that don't use platinum metal. To solve this problem, Osmieri et al. made a La–Sr–Co catalyst for OER and studied the effect of ionomer-to-catalyst and binder-to-ionomer on AEMWEs on water and how long they last.³⁹⁴ Both 1% K₂CO₃ and 0.1 M KOH, which are used in AEMWE operations, reveal that the electrolyzer's performance is far lower than that in water. Table 9 shows various electrocatalysts used in AEMWE, and their corresponding parameters have been summarized.

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6. Challenges

The fundamental challenge to the widespread use of membrane-based electrochemical devices is their poor chemical stability. Further substantial trial and error on the manufacturing processes should be carried out to create good AEMs. Instead of homemade membranes, commercial membranes have been used extensively in AEMWE to produce hydrogen. This choice may increase the overall cost of the system. Therefore, the usage of AEM that has been synthesized in a laboratory would be recommended to lower the overall cost of the AEMWE system and enhance the high performance of the system. The problem of developing AEMs for energy storage and conversion devices concerns the long-term durability of the membranes in alkaline environments. The breakdown of AEMs occurs in an alkaline environment because the cationic groups and backbones are susceptible to OH⁻ ions at high temperatures. As a result, it is crucial to develop AEMs functionalized with very stable cations. The significant swelling is another problem that needs fixing as is the necessity for vigilance against volatile oxygen species. Finding active functional groups and super cationic head materials with excellent stability are potential solutions to the degradation difficulties in an alkaline environment. The strength of cationic chemical compounds attached to the polymer backbone may determine the membrane's alkaline stability. The nucleophilic attack of strong cationic groups induced backbone degradation; hence, AEMs generated via guanidinium, imidazolium, phosphonium, and benzimidazolium functionalization may resist degradation. Electrocatalysts made of transition metals perform less than those made of noble metals, and the utilization rate of the catalysts is poor. A perfect operating state to promote the activity and longevity of the AEMWE cell must be further explored from the literature's perspective. Further study of the evolution of large-scale techniques from the perspective of low-cost procedures must supply the world's energy demand. The primary difficulties that may reduce the WE performance are the very poor alkaline durability and extremely low IC of AEMs. The best membrane for WE applications may be developed by ongoing research into AEM produced with varying thicknesses, forms, and homogeneous and heterogeneous reinforced structures. The AEMs are still in the developmental stage of realizing the AEM with high performance. The endurance of AEMWE is another issue that must be resolved before it can be used extensively, and this constraint requires more study.

7. Future scope

Cost-effectiveness, high IC, great alkaline stability, good thermal and mechanical stability, low swelling ratio, etc., are all desirable outcomes of future AEMs research. Using imidazolium-based cations to increase features such as IEC and IC improved the membrane. A potential approach to enhance the alkali stability of membranes is using flexible polymer backbones free of ether linkages. Hydrogen production using AEM-based WE has the potential to become standard because of its many benefits over the conventional method, including its ease of use, efficiency, and throughput. Consideration should be given to future research on AEMs with large-scale production for water electrolysis. Due to their high conductivity, inorganic/organic composites can produce high current densities at low voltage. AEM-water electrolysis technology is in its earliest stages of development. The kinetically-poor OER process is a primary concern in developing AEMWE. Consequently, further action of efficient, structurally robust, cost-effective electrocatalysts for OER is required. In alkaline environments, perovskites, transition metal oxides, and hydroxide electrocatalysts outperform PGM-based catalysts in activity and cost. Future research on a catalyst for AEMWE should consider the following factors. These factors include a high surface area, synergistic effect of the diverse oxide components, and quick electron transfer due to the bifunctional structure. From a materials science standpoint, highly efficient and long-lasting non-PGM-free electrocatalysts should be investigated.

Studies should concentrate on the stability of electrocatalysts and the maintenance of their performance in neutral environments. Additionally, in water electrolysis systems based on a single-cell stack, there have been relatively few reports of the performance of the water electrolysis equipment deteriorating. Low alkaline durability and low conductivity of membranes are the major problems that decrease the ohmic drop, which is attributed to the energy loss in the AEMWE. A complete understanding of catalysts in electrocatalytic mechanisms should be further investigated. Many electrocatalysts have been analyzed considering performances for water electrolysis, and their catalytic mechanism in the cell is not yet fully explored. The practical use of the developed non-PGM-based water electrolysis catalysts could be a potential direction in industrial applications. Using alkaline stable and low-cost AEMs with non-PGM catalysts will be appropriate for reducing the total cost of H₂ production and the operation of AEMWE. The start-up expense of the whole system may be reduced with the use of noble metal-free catalysts in AEM water electrolysis. Ni–Fe and other metal compositions make up the vast majority of OER catalysts; this might be the reason for their relatively cheap cost and average efficiency. In the future, the research and development of PGM-free, low-cost catalysts that can achieve high catalytic activity reactions will be an important part of the water electrolysis process. The performance of hydrogen-generating systems operating at laboratory and industrial scales must be compared, and laboratory AEMs must be scaled to industrial cell stack systems.

8. Conclusion

This study focused on current developments and trends in the synthesis, characterization, and various membrane variables and the performance of AEMs for water electrolysis applications. With novel approaches for synthesizing AEMs and establishing a unique tool for analyzing their physiochemical structures and chemical stability properties, the polymer science community has pushed for developing a new polymeric backbone material. The physical and chemical characteristics of AEMs may be enhanced through cross-linking techniques. Compared to TMA-functionalized membranes, imidazolium-based functionalization uses less chloromethylation agent and is cheaper. Natural polymers containing hydroxyl and amine groups may be quaternized without chloromethylation. The inorganic particle composite with organic polymers could be more effective than the organic/organic membrane composite. As a result of improved membrane physical, thermal, and chemical characteristics, fluorine incorporation into hydrocarbon polymers may be advantageous. Additionally, the fabrication of reinforced membranes is an excellent method for improving the structural stability of the membrane. Several techniques have shown promise in synthesizing ion exchange polymers, such as polymerization, polycondensation, pore filling, sol–gel, solvothermal, electrodeposition, and blending. It is important to consider AEMs, such as ensuring that the membrane preparation process is straightforward, environment-friendly, effective, and economical.

Microphase separation morphology (MSM) is a crucial key for the AEMs for ionic conductivity, and SAXS, AFM, TEM, and SEM can confirm these morphologies of AEMs. It was found that MSM is founded on a variety of hydrophobic and hydrophilic configurations in the membrane. Degree of quaternization, a higher level of hydration, alkoxy groups and extender with side chains, and increasing the number of quaternary amines groups are all necessary to achieve good MSM. Interestingly, the ability of the membranes to segregate microphases was significantly improved by adding a fluorinated component to the polymer backbone. New structural insight into the possibility of a high conductivity AEM is obtained by maintaining consistency between hydrophilic and hydrophobic phase modification during microphase separation. ¹H NMR may confirm the organic structures of AEMs, although well-soluble polymers in deuterated solvents provide the best results. FT-IR is a low-cost analytical method for studying organic and inorganic compounds and their composite structures. It might benefit researchers globally due to its accessibility and affordability. XPS detects surface elements of modified and unmodified organic/inorganic composites. Due to its high cost, rural researchers may find it difficult to employ this technique for polymers and their organic/inorganic composites. Aliphatic side chains and densely structured QAs-tethered AEMs showed high thermal stability. Unlike single polymer-constructed membranes, composite membranes have better thermal stability. The fluorination of hydrocarbon polymers results in increased membrane thermal stability.

Conductivity and water uptake determine membrane IECs. High IEC may cause high conductivity and water absorption. Therefore, membranes with high conductivity increase the IEC. Due to their strong conductivity, fluorinated membranes have higher IEC than hydrocarbon membranes. Membrane hydrophilicity affects water intake and swelling ratio. Higher hydrophilic may increase water absorption and swelling ratios, thus weakening membrane dimensional stability. Organic/organic composite hydrocarbon membranes increased the WU compared to single and inorganic/organic polymer membranes. Hydrophobic copolymer-structured AEMs with hydrophobic extenders to the hydrophilic backbone have lower water absorption and swelling ratio. Reinforced membranes decrease water swelling and improve the membrane stability. Fluorinated membranes are hydrophobic and have limited water absorption compared to hydrocarbon membranes. Water absorption and membrane MSM influence IC. Due to water absorption, hydrophilic membranes may have higher IC than hydrophobic ones. The organic/organic composite's IC relies on the membrane's hydrophilic groups. However, the IC of these composite membranes could be lower that of the inorganic/organic composite. Fluorinated membranes have higher IC than hydrocarbon membranes because fluorine's electronegativity facilitates ion separation and movement. The very low IC in AEM suggests that it may be chemically unstable due to OH⁻ assault on the immobile ion. To a large extent, the conductivity of an AEM may be hampered by the Hoffmann elimination process and the degradation of QAs groups by nucleophilic substitution. The IC is crucial to the AEM's functionality since greater OH⁻ conductivity enables significantly larger current densities.

The membrane's alkaline stability (AS) relies on its functionalized cations such as cyclohexyl substituted QAs, heterocyclic cations, long side-chain quaternary ammonium groups, N-spirocyclic, and N-cyclic piperidinium as well as the high density of cations. Due to their aromatic nature and reduced nucleophilic attack, copolymer-structured AEMs have high AS. High AS membranes with low WU and IEC protect backbones from nucleophilic attack. Pore-filled membrane method can improve the mechanical properties using porous substrates. Inorganic/organic composites (e.g., PBI–DIm–Si) preserved 100% conductivity in alkaline conditions. Fluorine-substituted quaternized AEMs were more stable in alkaline solution than fluorine-free AEMs, and the membrane preserved the conductivity better than hydrocarbon-based membranes. Due to low water absorption, cross-linking improves the membrane mechanical stabilities (MS). TMHDA and DABDA, which are hydrophilic and decrease membrane stiffness, may not improve MS. Due to limited water absorption, inorganic particles strengthen inorganic–organic composite membranes, endowing them with higher MS. Organic or inorganic rigidity may boost membrane MS. Hydrocarbon-based membranes are mechanically weak. Fluorinated quaternized membranes have higher MS than bare ones.

In terms of electrochemical performance, blended AEMs functionalized with imidazolium have excelled. Interestingly, longer side-chain-functionalized AEMs may be more advantageous than shorter side-chain-functionalized AEMs in improving the water electrolysis performance. The electrolyte conditions can affect the performance of AEM in water electrolysis. Therefore, KOH conditions demonstrated higher performance than water. As the temperature rises, the strong ions become more mobile, allowing for a higher current density in the membrane. An increase in current density may not be beneficial if a higher cell voltage is the cause of a higher temperature. Water electrolysis for hydrogen generation may be carried out with lower voltages using membranes with higher conductivities. The membrane's current density depends on the electrolytes because the AEM achieved a lower current density in water than in KOH. In an electrolysis system, the efficacy of the cells may be highly dependent on the electrolyte concentration, i.e., the current density increases as the KOH concentration rises. Compared to commercially available AEMs, AEMs synthesized in the lab exhibited a higher current density. The best results using AEMs to electrolyze water and using electrocatalysts for AEMWE are as follows: the best water electrolysis is shown in 1 M KOH at 80–90 °C with the following components: AEM: qPSU/NIM-N⁺, catalyst: Ni (anode)-Pt/C (cathode), voltage: 2.2 V. AEM: qPSU, catalyst: Ni (anode)-Pt/C (cathode), voltage: 2.2 V. AEM: PBPA, catalyst: IrO₂ (anode)-Pt/C (cathode), voltage: 2.0. These AEMs work effectively to achieve greater current densities of 3.7, 4.2, and 4.0 A cm⁻², respectively. AEMWE's best performance so far is 2.74 A cm⁻² at 2.0 V for the following configuration: Sustainion X37-50 Grade RT (AEM), stainless steel (anode), and Pt/C (cathode) in 1 M KOH at 60 °C.

Author contributions

Ganesan Sriram: writing – original draft, conceptualization, methodology, investigation. Karmegam Dhanabalan: formal analysis, validation, writing – review & editing. Kanalli V. Ajeya: writing – review & editing, resources, visualization. Kanakaraj Aruchamy: resources, software, visualization, formal analysis. Yern Chee Ching: writing – review & editing, visualization. Tae Hwan Oh: investigation, writing – review & editing, supervision, funding acquisition. Ho-Young Jung: methodology, formal analysis, writing – review & editing, supervision, funding acquisition. Mahaveer Kurkuri: formal analysis, writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We sincerely acknowledge the funding support from the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. 2022R1A2C1004283), and the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20213030040590). We also thank Karnataka Govt., India (K-FIST L1/GRD/1053).

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