Recent advances in two-dimensional polymers: synthesis, assembly and energy-related applications

Yumei Ren ^ab and Yuxi Xu *^a
aSchool of Engineering, Westlake University, Hangzhou 310024, Zhejiang Province, China. E-mail: xuyuxi@westlake.edu.cn
^bSchool of Materials Science and Engineering, Zhengzhou University of Aeronautics, Zhengzhou 450046, China

First published on 9th January 2024

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Yumei Ren

Yumei Ren received her PhD degree from Zhengzhou University in 2018. Now she is working at the Zhengzhou University of Aeronautics and is currently a visiting scholar in Xu's group. Her current research interests focus on the functionalization of 2D materials and their applications in energy conversion.

Yuxi Xu

Yuxi Xu received his B.S. degree from Wuhan University (2007) and PhD degree from Tsinghua University (2011). He then worked as a postdoctoral fellow at the University of California, Los Angeles. He joined the faculty of Fudan University in 2015 and moved to Westlake University in April of 2019. His research interests include chemically modified graphene, 2D polymers, and self-assembled functional materials.

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1. Introduction

In the context of carbon neutrality, the research on green, low-carbon, and renewable energy has become a hot spot for governments and scientists around the world. Two-dimensional (2D) materials have broad application prospects in the field of energy conversion and storage because of their unique structure and adjustable characteristics.^1–3 As the first 2D material to represent a major scientific discovery and technological advance, the innovative and potential properties of graphene have inspired great interest in the research of other novel 2D materials.^4–7 Since then, a series of inorganic (e.g., transition metal dichalcogenides (TMDs), MXenes, layered double hydroxides (LDHs), etc.)^8–11 and organic (e.g., organic 2D supramolecular polymers and organic 2D covalent polymers)^12–17 2D materials have been successfully prepared. Compared with 2D inorganic materials, 2D organic materials have the advantages of lightweight, good flexibility, good designability, and large-scale production, so their preparation and research are of great significance.

Graphene is a naturally occurring 2D polymer (2DP) whose unique atomic-level thickness and 2D conjugated structure give rise to many extraordinary properties, such as large specific surface area, ultra-high carrier mobility, and excellent mechanical flexibility.^18,19 Attracted by these fascinating properties, the rational design and synthesis of novel 2DPs with graphene-like structures at the atomic or molecular level has aroused wide interest among researchers.

Based on the concept of polymers proposed by Staudinger, Schlüter et al. put forward a clear definition of 2DPs, that is, 2DPs are a class of topological planar polymers with one monomer unit thick and periodicity connected by structural units through covalent bonds.^20–22 They pointed out that 2DPs should be able to independently support their structure at monolayer thickness. However, 2DPs usually exist in the form of single-, few-, or multilayers due to the presence of π–π packing and weak intermolecular forces such as van der Waals forces between polymer lamellae. In particular, 2DPs uniquely combine the inherent properties of 2D materials and polymers, exhibiting many interesting properties.^23–27 More importantly, 2DP materials can be accurately functionalized at the molecular level through the selection of monomers, the regulation of reaction conditions, and the design of reaction routes.^28–30 The study of thin-layer 2DPs not only broadens the traditional concepts of polymers and low-dimensional materials, but also introduces many special properties to polymers. On the basis of these attractive advantages, 2DPs have been rapidly developed in the field of synthetic preparation and energy-related applications in recent years.

The overall purpose of this review is to describe the design and synthesis of 2DPs and their applications in energy-related fields. We will first introduce the historical development and advantages of 2DPs, focusing on the unique 2D characteristics and diverse topologies of 2DPs, which will pave the way for the synthesis and preparation of 2DPs below. Then, various typical “top-down” and “bottom-up” methods for preparing 2DPs are discussed. Next, we explore the assembly and recombination of 2DPs, revealing the importance of their processability. We then introduce the applications of 2DPs in energy storage and conversion (e.g., batteries, supercapacitors (SCs), electrocatalysis and photocatalysis). Finally, the further research on 2DPs is summarized and prospected. Here we make it clear that the 2DPs mentioned in this review are thin networks formed by periodic arrangement of monomer units connected by covalent bonds, and to a certain extent, the exfoliated single- or few-layer 2D covalent organic frameworks (COFs) and covalent triazine frameworks (CTFs) are regarded as 2DPs. We consider 2DPs linked by coordination bonds or other non-covalent interactions to be outside the scope of this review.^22,23 In addition, graphene, carbon nitride, and hexagonal boron nitride, while meeting the criteria for the definition of 2DPs, are not discussed since they are not synthesized from monomers whose structures remain within 2D macromolecular sheets and they have been extensively reviewed.^23,31–36

2. General overview on two-dimensional polymers

2.1. Historical development of 2DPs

About a century ago (1920), Staudinger first proposed the concept of polymers (or macromolecules).³⁷ After a hundred years of continuous research and development, polymer science has made great progress, and polymer materials have touched almost every aspect of modern life. Unlike traditional linear (one-dimensional) polymers, 2DPs are a new kind of material that realize structure changes from line to surface, resulting in revolutionary changes in properties. Therefore, 2DPs have received more and more attention from researchers, and it is expected that the unique planar structure of 2DPs will bring new physicochemical properties and a wide range of applications. As for the relevant research on 2DPs, as early as 1935, Gee et al. reported that the polymerization reaction based on β-elaeostearin at the air/water interface could form an independent monolayer polymer flake, which was the earliest exploration of 2DPs by researchers, and also an important first step in this field.³⁸ Shortly, Talmud and coworkers introduced the term “2D polymerization” when describing interfacial crosslinking between stearaldehyde and multifunctional amines, and speculated that reactions at the interface would produce “2DPs” (1941).³⁹ Subsequently, in 1958, Blumstein et al. proposed that “sheetlike” macromolecules could be prepared by cross-linking the monomers adsorbed between the lamellar layers of montmorillonite clays, and 2D space-confined cross-linking polymerization was achieved for the first time utilizing a hard template.⁴⁰ With further expansion of the research, Beredjick et al. reported 2D polymerization of monolayers of octadecyl methacrylate and divinyl ester at the air/water interface under ultraviolet radiation (1970).⁴¹ In 1993, Stupp et al. constructed independent polymer nanosheets with a monodisperse thickness of 50.2 angstroms by a bulk reaction.⁴² Despite early interest in 2D polymerization, researchers did not obtain structured 2DPs, and the limited synthesis and analysis tools at the time hindered their further development.

Until 2004, the successful exfoliation of single-layer graphene ushered in the dawn of the graphene era and the rise of 2D materials.⁴³ Thanks to the similar structure to graphene and the rich structural and functional variations, 2DPs have once again become the focus of attention and an emerging direction in the field of polymer research. In 2005, Yaghi and coworkers for the first time used the reversible dynamic covalent chemistry principle to prepare and characterize 2D COFs formed by the polymerization of boronic acid and catechol, which could be regarded as potential 2DPs, but whether their 2D structure could be isolated remained to be confirmed.⁴⁴ Subsequently, in 2008, Thomas et al. for the first time prepared crystalline CTFs with sheetlike structures by a high-temperature ionothermal method.⁴⁵ Professor A. D. Schlüter, a well-known polymer scholar, is an active promoter of 2DPs and has done a lot of pioneering work in this field. After his group proposed the clear definition of 2DPs in 2009, it was not until 2012 that they used organic synthesis methods for the first time to synthesize independent monolayer 2DPs through solid-phase photo-polymerization and liquid-phase exfoliation processes.⁴⁶ This was the first 2DP to meet the precise definition, and the discovery marked a major breakthrough in the synthesis of 2DPs. With the deepening of the research on 2DPs, people's understanding of the structure and properties of 2DPs is also improving. It is very important to define the crystal structure, bond length, bond angle, and specific atom–atom connection for the further development of 2DPs. In 2014, Kissel and Kory et al. successively reported two 2DPs synthesized from single-crystal-to-single-crystal (SCSC), and the single-crystal XRD analysis established clear structural proof of the synthesized 2DPs for the first time, confirming the feasibility of solid phase polymerization to construct 2DPs.^47,48 In the past 10 years, researchers have continuously expanded the types and application range of 2DPs by adopting various new synthesis techniques and structural regulation methods, and have synthesized hundreds of 2DPs with different chemical structures. The mentioned key developments in 2DPs are shown in the timeline of Fig. 1. The research results show that 2DPs possess many unique and interesting properties, and have been explored for various applications.

Although some scientists have made important explorations into 2DPs, research in this field has only just begun, and 2DPs are full of opportunities and challenges in both basic and applied research. Some important key scientific problems, such as efficient synthesis, unique physical and chemical properties, ordered and controllable construction, novel functional materials based on 2DPs, and their unique applications, are yet to be resolved. This article reviews the unique advantages, typical preparation methods, assembly processes, and applications in energy storage and conversion of 2DPs. It is believed that with the progress of science and technology, the preparation and application of 2DPs will be more deeply developed and applied in continuous practice.

2.2. Advantages of 2DPs

As “new star” materials in the field of polymers, 2DPs integrate the advantages of 2D materials and polymers. Due to their unique structural characteristics, 2DPs exhibit many novel properties compared with 1D linear and 3D cross-linked polymers, which have attracted wide attention in the fields of energy, catalysis, sensing, gas separation, and adsorption. In particular, ultrathin 2DPs with atomic thickness have a broader application prospect because of their high surface area, abundant active sites, unique electronic structures, and excellent processability.

3. Approaches to two-dimensional polymers

As a new material form, 2DPs have broad prospects in the fields of physics, chemistry, materials science, etc. The research on them can not only explore the characteristics and advantages of new materials, but also have high academic value and practical significance. In recent years, researchers have developed a variety of reliable preparation methods to obtain high-quality 2DP materials. In general, it can be summarized as top-down and bottom-up methods (Table 3).^{46–48,95–121,130–143} In this section, we will introduce some of the more mature preparation methods, and discuss their advantages and disadvantages.

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3.1. Top-down methods

The preparation of 2DPs by a top-down method is to weaken or destroy the weak interaction force between the layered polymers by various physical or chemical means, so as to obtain ultrathin 2DPs. At present, many commonly used preparation methods have been developed, including mechanical exfoliation, liquid phase exfoliation, self-exfoliation, and crystal methods.

3.2. Bottom-up methods

Unlike the top-down method, the bottom-up method directly synthesizes ultrathin 2DPs through linking monomer molecules or structural units via covalent bond interactions. Such a method prevents the structure from damage via exfoliation, and can effectively regulate the size and thickness of the 2DPs, which has been extensively explored recently. At present, the top-down method mainly uses interface polymerization and solution polymerization.

4. Assembly and processing of two-dimensional polymers

Suitable assembly and processing determine the usefulness of the material, so the processability of 2DPs is critical to fully utilize their unique properties, but integrating them into devices remains challenging.^52,173 In this section, we describe some of the current materials forms based on the assembly and processing of 2DPs, such as thin films, membranes, aerogels, heterostructures and hybrids, and discuss some typical examples as well as the corresponding processing methods.

4.1. Thin film

4.2. Membranes

The ultrathin thickness of 2DPs, coupled with their ability to rationally chemically design and adjust pore size by selecting the building units, made it a promising candidate for building highly permeable and highly selective membranes for applications in water purification, gas separation, and ion exchange/transport in fuel cells and lithium–sulfur batteries.^184,185 Due to the diversity of 2DP monomers and the characteristics of covalent bonding, they usually exhibited better stability and membrane-forming ability.

With highly ordered in-plane pores, interlayered channels, and excellent stability, CTF nanosheets are promising materials for preparing fast and selective membranes. Wang et al. successfully fabricated single-layer CTF nanosheets through interfacial synthesis, mild oxidation, and ultrasonic-assisted exfoliation processes (Fig. 15a).¹⁸⁶ Then, thin CTF-nanosheet membranes could be constructed via vacuum filtration of CTF nanosheets onto the porous AAO substrate (Fig. 15b), and the thickness of membranes could be flexibly adjusted by changing the deposition density, which showed high water permeability and high retention for target molecules. The intrinsic proton-conducting COF (IPC-COF) membrane and Tp-AD-50 membrane could also be prepared using the same vacuum filtration method.^187,188

The LB method is also one of the effective methods for preparing membranes. By using this synthesis method, Lai and co-workers successfully prepared 2D crystalline TFP-DHF 2D COF membranes for the first time, whose thickness could be precisely adjusted layer by layer (Fig. 15c and d).¹⁸⁹ The membrane was constructed with 1,3,5-triformylphloroglucinol (TFP) and 9,9-dihexylfluorene-2,7-diamine (DHF) as precursors through β-ketoamine linkage at the water–air interface. The as-obtained membranes had good porous structure and displayed excellent thermal stability and high permeability. In another example, Zhao et al. performed the layer-by-layer assembly method to horizontally transfer two thin iCONs with different pore sizes and opposite charges on porous α-Al₂O₃ support using Langmuir–Schaefer (LS) method (Fig. 15e and f).¹⁹⁰ Driven by electrostatic attractive interaction, ultrathin 2D membranes were prepared with reduced aperture size, compact structure and long- term hydrothermal stability, which could be used for highly efficient molecular separations.

4.3. Aerogels

The preparation of macroscopic objects can greatly expand their practical application range. In particular, 3D porous aerogels have been widely studied in the field of energy storage and conversion due to their advantages of ultra-low density, extremely high porosity and specific surface area. Our group successfully prepared ultralight aerogels with different densities (5–15 mg cm⁻³) by simply freeze-drying the as-obtained non-covalently functionalized highly water-dispersible single-layer/few-layer 2D triazine polymer nanosheets, and for the first time realized the construction of porous 2DP macro-assembly (Fig. 16a and b).¹³⁰ The resultant aerogels had a 3D network structure and abundant porosity and exhibited excellent thermal stability and mechanical strength (Fig. 16c–e). Such a study had a positive effect on the development of 2D functional polymers. It is well known that aerogels have unique structural and performance advantages, so the development and research of aerogels based on 2DPs has potential advantages.

4.4. Heterostructures and hybrids

Composite materials are composed of two or more substances with different physical and chemical properties, which can give full play to the advantages of various materials, overcome the defects of a single material, and expand the application range of materials. In recent years, 2DP composites have also received extensive attention from researchers. Typically, Xu et al. reported the construction of van der Waals heterostructures as a Z-type photocatalytic system based on ultrathin 2DP nanosheets.¹⁹¹ The authors first prepared 2D aza-CMP and C₂N polymer nanosheets with a thickness of ∼4 nm by solution polymerization and liquid phase ultrasonic exfoliation, respectively. By further ultrasonic mixing and annealing of the two nanosheets, aza-CMP/C₂N van der Waals heterostructures could be obtained (Fig. 17a and b). X-ray absorption near-edge structure (XANES) spectroscopy demonstrated that there were strong interlayer interactions between aza-CMP and C₂N (Fig. 17c).

Graphene is one of the most promising components in heterostructures, and combining 2DPs with graphene can synergistically take advantage of their respective advantages.¹⁹² Zheng et al. used graphene/SiO₂/Si as substrate and deposited 2DP1 by horizontal transfer at the air–water interface to prepare the 2DP1-graphene heterostructures, which showed excellent mechanical strength and high flexibility, as well as high chemical and thermal stability (Fig. 17d–g).¹⁹³ The electrical conductivity was increased by about 2.5 times compared to pure graphene (Fig. 17h). Such work opened the door to the rational design of the properties of 2DP heterostructures for specific applications. In another work, Feng and co-workers first synthesized 2DPI through the LB method, and then achieved the scalable synthesis of 2DPI-graphene (2DPI-G) van der Waals heterostructures at the water interface by face-to-face co-assembling the 2DPI with exfoliated graphene (EG) (Fig. 17i).¹⁹⁴ AFM image demonstrated that the as-prepared 2DPI film had a single-layer feature (Fig. 17j). TEM image of 2DPI-G after twice deposition showed that EG flakes are distributed in 2DPI-G without obvious aggregation (Fig. 17k). Other reports showed that large-area 2D CMP/rGO heterostructures could be performed by direct synthesis of 2D conjugated microporous polymers (2D CMPs) on a rGO substrate through a low-cost full-solution process.^195,196 The preparation of aerogel composites based on 2DPs is of great significance for making full use of their chemical function and porosity. Thomas's group prepared COF/rGO ultralight aerogels using a simple hydrothermal method (Fig. 18a and b).¹⁹⁷ The COFs grew in situ along the surface of the graphene sheets, showing a 3D layered porous sponge-like structure, which could be compressed and expanded several times without breaking (Fig. 18c–e). The resulting COF/rGO aerogels showed good mechanical strength, excellent adsorption, and electrochemical properties. Additionally, Kleitz et al. have also successfully fabricated an ultralight anthraquinone-based COF/graphene aerogel (CGA) via a hydrothermal approach, in which ultrathin 2D COFs were uniformly grown on the surface of the graphene template.¹⁹⁸

In addition to graphene, other materials such as MoS₂, MXene, CNTs, etc., have also been used to compound with 2DPs.^97,132,199 For instance, Wang et al. reported the rational design and optimal synthesis of the electronically coupled semiconducting 2DP/MoS₂ heterostructures, successfully achieving the direct exfoliation of high crystallinity TIIP 2DP films condensed by pyrene and thienoisoindigo moieties down to a few nanometers (Fig. 18f–i).⁹⁷ The first thickness-dependent study of 2DP/MoS₂ heterostructures demonstrated that the photoluminescence (PL) quantum efficiency of TIIP films with a few nanometers thick could be improved by more than 2 orders of magnitude (Fig. 18j).

The processability of materials is a fundamental aspect of commercializing 2DPs as useful materials for different purposes, however, relevant research is still in the development stage. The reasonable design of different assembly forms and composite assemblies, as well as the search for simple and economically promising processing methods remains the key to achieving the widespread application of 2DPs.

5. Characterisation of 2DPs

As an important part of materials science, material characterization techniques play an important role in the process of realizing high-precision, high-efficiency and high-reliability material preparation. In recent years, the development level of material characterization material characterization techniques has made great progress, and new analytical means and tools continue to emerge, making the research and application of material characterization more in-depth and extensive. In this section, we describe the commonly used techniques for characterizing the structure, morphology, mechanical and other properties of 2DPs, which have contributed to the continuous progress and development of polymer science.

5.1. Structure characterization

It is important to study and determine the structure and composition of 2DPs to elucidate the relationship between their microstructure and properties. The most traditional and classical method is XRD, which can be combined with FTIR, Raman spectroscopy, XPS, NMR spectroscopy and other techniques to obtain more accurate crystal structures.

XRD is the main method to study the phase and crystal structure of matter. For 2DPs prepared by single-crystal-to-single-crystal photopolymerization, the use of single-crystal XRD can establish accurate structural evidence for such synthesis of 2DPs.^47,48 For example, King's group characterized the layered structure of 2DPs with atomic precision using single-crystal XRD analysis, confirming that the individual layers were composed of monomers with quasi-hexagonal packing motif, and their photoreactive blades were stacked face to face (Fig. 19a and b).⁴⁷ In addition, single crystal XRD analysis of poly(fantrip) crystal exposed to dry in the dark for one month demonstrated that their crystal lattice is completely solvent-free (Fig. 19c and d). For other crystalline 2DPs, the crystal structure is often analyzed by PXRD.^{95,104,134,138} However, determining the structure of 2DPs from the PXRD pattern usually requires computational simulation with the help of certain software. The 2D CTF-DCB synthesized with assistance of microwave was characterized by PXRD (Fig. 19e).¹³³ It can be clearly seen that the experimental PXRD pattern of 2D CTF-DCB exhibited very narrow and strong peaks, which was consistent with the simulated one. Such result indicated that it had good crystallinity. Using PXRD, Loh's group confirmed that the introduction of macrocycles to the as-prepared 2D COFs did not destroy the in-plane or out-of-plane crystallization in the absence of threading units.¹⁰³ The experimental result was in good agreement with the Pawley-refined PXRD pattern and the simulated PXRD pattern of the antiparallel form, suggesting that the preferred structure of the 2D COFs was antiparallel stacking. The grazing incidence diffraction (GID) technique can characterize the microstructure of thin films more truly and comprehensively. Using this technique, Bein et al. provided strong evidence for the oriented growth of COF films on the substrates.²⁰⁰ It was clear from the GID patterns that the intense reflection generated by the COF layer parallel to the substrate at q(y) = 0 indicated the orientation of the COF in the c direction (Fig. 19f). In addition, wide-angle XRD (WAXD) could also be used to characterize the structure of visible floating films.¹³⁷

Molecular spectroscopy (such as FTIR, Raman spectroscopy) is a common means to characterize the chemical structure of polymer materials.^136,148 FTIR has significant fingerprint characteristics, which can identify the functional group structure and chemical composition of 2DPs. Raman spectroscopy uses the principle of light scattering to characterize the vibration of molecular skeleton bonds, which can be mutually verified and supplemented with the functional group vibration information of infrared spectrum, so as to reflect the structure of 2DPs more accurately. By FTIR characterization of the as-synthesized 2DPI, we found that the –NH₂ of the used melamine (MA) monomer was almost completely converted to C–N–C of 2DPI, indicating that the 2D polymerization reaction was quite efficient (Fig. 20a).¹²⁰ Raman spectra further showed that the 2DPI had a sharp G peak similar to that of graphene compared to conventional PI, which verified that the 2DPI had a 2D honeycomb structure and high crystallinity (Fig. 20b). In addition, XPS studies the surface composition, chemical state and electronic structure of 2DPs by measuring photoelectron spectroscopy. By comparing the XPS spectra of the exfoliated f-CTF-1 and pristine CTF-1, Zhu et al. found that abundant –NO₂ groups were formed in f-CTF-1, which was due to the oxidation of nitrogen components derived from hydrolysis of triazine ring in the system in the H₂SO₄/HNO₃ mixture (Fig. 20c).¹³⁶ As is known, NMR spectroscopy is an effective means to analyze the chemical structure, conformation and relaxation of polymers.^99,100,130 The result was confirmed by further characterization of solid-state ¹³C NMR spectra (Fig. 20d). Compared to pristine CTF-1, the peak of the C atoms corresponding to the triazine units in f-CTF-1became wider and shifted to ∼167 ppm, and the peak at ∼138 ppm attributed to the C atom directly connected to the triazine unit.

In addition to the characterization techniques mentioned above, there are many other techniques that can be used to distinguish molecular structures and provide supplementary information for 2DPs, such as XANES spectra, grazing incidence wide-angle X-ray scattering (GIWAXS) and small-angle X-ray scattering (SAXS).^{97,100,135,168}

5.2. Morphology characterization

Unlike traditional 1D polymers, 2DPs are molecule sheets connected by covalent bonds and have a periodically arranged structure on a 2D plane. The analysis of 2DPs morphology (surface morphology, interface morphology, defect morphology, etc.) is of great significance for optimizing their performance, improving their preparation technology and developing new 2DPs materials. With the development of materials science and technology, the methods of material topography analysis have also been continuously developed, and new research methods and technologies are emerging constantly. So far, some advanced characterization methods such as optical microscopy (OM), SEM, AFM, TEM, STM, etc. have been widely used to study 2DPs.^23,90,126

In general, OM uses visible light to observe the sample and can provide information about the surface morphology and structure of the sample (such as shape, size and thickness). In the analysis of triazine-based 2DP synthesized from the solution, OM image showed a uniform color contrast, confirming that the thickness of the thin sheet was relatively uniform.¹¹³ Feng's group observed 2D BECOF film suspended on a copper grid through OM and could see cracks in the film (Fig. 21a).¹⁶⁷ In addition, the characterization result also reflected the large size and excellent mechanical stability of the film. However, due to the large wavelength of visible light, OM cannot resolve structures with smaller sizes or higher resolutions. SEM and TEM are commonly used high-resolution microscopy techniques. SEM can provide surface morphology and surface structure information of 2DPs (see Sections 3 and 4). While TEM uses high-energy electron beams to illuminate materials, which can give higher resolution cross-section and atomic-level structural information. Combined with TEM accessories (selected area electron diffraction (SAED), energy-dispersive spectrometry (EDS), etc.), more detailed and accurate morphology, crystal structure and composition information of 2DPs can be obtained. We could clearly see the lattice fringes of the prepared 2D-TPs assigning to (100) plane with lattice constant of 1.12 nm through the HRTEM image (Fig. 21b).⁹⁹ The SAED resented a sharp hexagonal symmetrical pattern, indicating a high crystallinity of 2D-TPs (Fig. 21c). Moreover, high angle angular dark field-scanning TEM image and the EDS mapping showed that carbon and nitrogen elements were uniformly distributed on the surface of 2D-TPs (Fig. 21d). Notably, using AFM characterization, surface topography and surface roughness information at nanoscale resolution can be obtained by scanning a 2DPs surface with a probe (see Sections 3 and 4).

STM is a very common means of 2DPs characterization, which uses quantum tunneling effect to achieve atomic resolution imaging of material surface. The height and topology of the sample surface are measured by varying the voltage applied between the probe and the sample, which has very high resolution and sensitivity for observing the lattice structure and defects of 2DPs. To understand the process of preparing single-layer 2D-CAP via Au surface-mediated polymerization, Loh's group performed STM characterization of the resulting monolayer film.¹⁰⁷ It could be clearly seen that the initial individual 2,7,13,18-tetrabromodibenzo[a,c]dibenzo[5,6:7,8]-quinoxalino-[2,3-i]phenazine (2-TBQP) molecules had no obvious adsorption order on Au(111) (Fig. 21e). While at high temperature, the precursor molecules were polymerized into 2D-CAP, whose STM images appeared as a rectangular-grid network (Fig. 21f). The STM height profiles showed that the pore sizes was ∼13.0 Å along the long axis and ∼7.4 Å along the short axis, which was in good agreement with the theoretical pore size of the monolayered 2D-CAP (Fig. 21g). In order to avoid the rotational misorientation, Au(110) was used as the growth substrate to obtain better orientation of the grid network (Fig. 21h). Besides, there are other techniques that are used to characterize 2DP morphology, such as brewster angle microscopy (BAM), polarized light microscopy (PLM), etc.^134,161,167

5.3. Mechanical characterization

2DPs has excellent mechanical properties and processability, so it is particularly important to characterize their mechanical properties in the material development stage. The AFM nanoindentation method can directly obtain mechanical properties of thin film materials without separating the film and the substrate.^177,182 Wang et al. transferred the as-prepared ultrathin 2D COF films onto the patterned Si wafer, and calculated the 2D elastic modulus of the 4.7 nm COF thin film as 119.1 ± 2.9 N m⁻¹ through AFM indentation experiment, and the corresponding Young's modulus value was 25.9 ± 0.6 GPa, indicating that the COF film had good mechanical property (Fig. 21i–l).¹⁷⁷ Loh et al. reported a quantitative in situ tensile study of ultrathin COFs films.¹⁸² The tensile mechanical properties of COFs films were measured by in situ SEM nanomechanical testing platform. The tensile modulus E and fracture strength σ_f were 10.38 ± 3.42 GPa and 0.75 ± 0.34 GPa, respectively (Fig. 21m–o). The fracture toughness of the COFs films was also measured by the same method, and the defect insensitive fracture behavior could be observed. In addition, typical tensile and compression testing were usually used to study the mechanical response of 2DPs.^174,187,197

In order to fully understand and master the structure and properties of 2DPs, apart from the above characterization of structure, morphology and mechanical properties, other characterization techniques such as UV-Vis absorption spectroscopy, UV-Vis diffuse reflectance spectroscopy, PL and PL excitation (PLE) spectroscopy can be used to characterize the optical properties of 2DPs.^136,139,195 The N₂ adsorption and desorption isotherms are carried out to determine the specific surface area and pore size distribution of 2DPs.^96,102,108 Thermogravimetric (TGA) analysis is employed to detect the thermal stability of 2DPs,^160,193 dynamic light scattering (DLS) is used to analyze the dispersion size of 2DPs in solution,¹⁰¹ and water contact angles (WCAs) and zeta potential can investigate the hydrophilicity and surface charging characteristics of 2DPs films.¹⁸⁶ Moreover, theoretical studies on the structural properties of 2DPs also provide important information for the characterization and application of 2DPs. Material characterization technology plays an irreplaceable role in the development of new materials science. It is believed that with the continuous progress of science and technology, the development and innovation of material characterization technology will provide better technical support for the development of new materials.

5.4. Electrical characterization

With the continuous development of polymer material technology, more and more polymer materials show good electrical conductivity, and have a wide range of applications in the fields of electronics, optoelectronics, sensors and so on. The research on the electrical properties of materials provides theoretical basis and technical support for improving the performance and reliability of electronic devices and developing new energy-related materials. As a kind of unique organic polymer materials, 2DPs also exhibit excellent electrical properties. For instance, Wang et al. investigated the electrical transport properties of the TIIP films via pre-patterning and shadow-mask approaches methods, and performed conductivity measurements using both top and bottom contact geometries of a probe station at room temperature(Fig. 22a and b).⁹⁷ The test results showed that the films had reproducible conductivity properties, as measured by the top and bottom contact geometries, the conductivity σ is (3.5 ± 0.7) × 10⁻⁵ S m⁻¹ and (2.5 ± 0.8) × 10⁻⁵ S m⁻¹, respectively, which is one of the highest inherent conductivity in 2D COFs. Fu et al. reported a semiconductor 2D COF film with high photoconductivity in which the band-like charge transport was confirmed by temperature-dependent terahertz (THz) photoconductivity measurements.²⁰¹ The results were of great significance for the application of 2DPs in the electronic field. Moreover, the conductivity of 2DPs can also be measured by flash-photolysis time-resolved microwave conductivity (FP-TRMC), Van der Pauw, two-probe, four-probe and other characterization methods.²⁰²

Field-effect transistor (FET) is an electronic component that uses electric field effects to regulate electrical behavior. Due to the inherent advantages of large specific surface area, high crystallinity and high π-conjugation, 2DPs can be effectively used as active materials in FET.^17,173,203 Our group constructed the FET devices by using the as-prepared highly crystalline triazine-based 2DP as the active semiconductor layer of the FET.¹¹³ It could be seen from the typical transfer curve that the device exhibited a slight bipolar behavior with a high on/off ratio of 10³ and a significant mobility of 0.15 cm² V⁻¹ s⁻¹. Kim's group used solution-processable ultrathin π-conjugated 2DPs (C2Ps) as an active semiconductor layer in organic FET (OFET).¹⁷⁵ The typical transfer curve showed clear field-effect behavior, and its carrier mobility was as high as ∼4 cm² V⁻¹ s⁻¹, which was much higher than that of the early synthetic 2DPs. In another typical case, Li et al. used the as-prepared free-standing 2DPTTI thin film as a semoconductor active layer in polymer FETs (PFETs), and the result of the transfer and output curves indicated that it has typical p-type semiconductor properties (Fig. 22e and f).¹⁸⁰ Due to the good semiconductor properties of 2DPTTI and its unique absorption characteristics, it could be effectively used as a photodetector. As can be seen from the typical transfer curve in Fig. 22h, the photocurrent was significantly enhanced with the increase of light intensity. And the transfer curve moved in the direction of the high drain current and the threshold voltage shifted in the positive direction, which was attributed to the 2D extended π-conjugation for efficient charge transport and rapid photoelectron conversion. In addition, Hu's group employed highly crystalline single layer imine 2DPs for the preparation of molecular memristors (Fig. 22g and h).²⁰⁴ It could be seen from the typical I–V curve that the device had obvious non-volatile bipolar resistance switching characteristics, and it also presented low variability, outstanding reliability and stability, as well as excellent scalability.

6. Energy-related applications of two-dimensional polymers

As a valuable new organic 2D material, 2DPs have a series of unique physical, chemical and electronic properties due to their ultrathin 2D structure. Their advantages such as high specific surface area, abundant active sites, and structural diversity made them have great potential in the field of energy storage and conversion. In this section, we mainly focus on the application of 2DPs in batteries, SCs, electrocatalysis, and photocatalysis.

6.1. Batteries

Batteries are considered to be one of the most efficient energy storage devices, and the current growing demand for efficient power storage systems has stimulated the development of high-capacity, low-cost secondary batteries. As we all know, the storage performance and service life of batteries mainly depend on the electrode materials, so exploring new high-performance electrode materials is critical for their further development. In recent years, 2DPs have been explored as potential electrode materials for lithium-ion batteries (LIBs), sodium-ion batteries (SIBs), potassium ion batteries (PIBs) and lithium–sulfur batteries (LSBs) due to their unique 2D properties, functionalizable covalent backbone, and tunable pore sizes.

6.2. Supercapacitors

Supercapacitors have been widely studied because of their high power density, fast charge and discharge rate and long cycle life. The electrochemical performance of the supercapacitors is closely related to the nature of the electrode. The porous structure, electrical conductivity, effective contact area with the electrolyte, and special functional groups of the electrode material have great influence on the performance of the supercapacitors.^222,223 2DPs have the advantages of layered conjugated porous structure, abundant surface functional groups, environmental protection and low cost, which have been potential electrode materials for supercapacitors.^224,225 For example, Loh's group quantitatively synthesized 2D CAPs by solid-state polymerization, in which CAP-2 prepared with 3,6,14,17-tetrabromodibenzo[a,c]-dibenzo[5,6:7,8]-quinoxalino-[2,3-i]phenazine (3-TBQP) as monomer had a lamellar structure, and the thickness of the exfoliated CAP-2 nanosheets was ∼4 nm (Fig. 25a and b).¹⁰⁸ Thanks to the 2D layered structure, nanoporous morphology and high N content, the CAP-2 could effectively reduce the diffusion resistance of ions when used as the positive electrode of asymmetric supercapacitors, so that the ions could rapidly diffuse into its nanopores and improve the surface wettability of electrolyte ion transport, thus exhibiting excellent capacitance performance. It had a specific capacitance of 233 F g⁻¹ at 1.0 A g⁻¹ in 2 M KCl when used as the positive electrode of an asymmetric supercapacitor, and showed excellent cyclic performance (∼80% capacitance retention over 10000 cycles) (Fig. 25c and d). Fang et al. reported a series of capacitor cells composed of exfoliated layered mesoporous 2D COFs (named JUC-510, JUC-511 and JUC-512, JUC namely Jilin University China).²²⁶ Among them, JUC-511 and JUC-512 had large surface areas (842 and 1170 m² g⁻¹, respectively) and pore sizes (both 3.4 nm), indicating ideal DL charge storage and fast ion transport (Fig. 25e and f). At high charge–discharge rates, the JUC-511 and JUC-512 capacitor cells possessed high areal capacitance of 5.46 and 5.85 mF cm⁻² at 1000 mV s⁻¹, respectively, and both exhibited excellent cycle performance (nearly 100% capacitance retention after 10000 cycles) (Fig. 25g–i). In another study, to address the problem that pure graphene and 2D COFs tend to form thick laminates and limit electron transfer and ion diffusion, Wei and co-workers assembled 2D redox-active COF nanosheets with rGO by simply hydrothermal mixing (Fig. 25j).¹⁹² The atomically thin 2D COF acted as porous spacers to prevent the accumulation of rGO nanosheets, resulting in excellent capacitive energy storage performance (Fig. 25k–m). Besides, β-ketoenamine-linked 2D COF,⁷⁷ anthraquinone-linked 2D COF,¹⁹⁷ and hexaaminobenzene derived 2DP,²²⁷ were also effectively employed as active materials of supercapacitor devices.

6.3. Electrocatalysis

The high specific surface area of 2DPs is conducive to contact with electrolytes, the abundant active sites exposed on the surface can be used as catalytic active centers, and the structural regularity and good crystallinity often make them have high electrical conductivity.²²⁸ At the same time, the highly adjustable structure of 2DPs enables some specific atoms or groups with catalytic activity to be conveniently integrated on the surface of 2DPs.^229–232 Therefore, in recent years, 2DPs have shown great development potential in the field of electrocatalysis. For example, the multilayer 2DP synthesized from cobalt-porphyrin monomer could effectively be used as an electrocatalyst for water splitting, in which porphyrin cobalt at Co–N₄ served as the active sites for electrocatalytic hydrogen evolution reaction (HER) (Fig. 26a and b).¹⁵⁹ The as-prepared Co-2DP showed excellent electrocatalytic activity, with the overpotential of ∼367 mV to reach the current density of 1.0 mA cm⁻² in 1.0 M KOH, and a Tafel slope of 126 mV (Fig. 26c and d). In addition, the introduction of metal atoms (such as Co) in 2D COFs could also be used for electrocatalytic CO₂ reduction, and the electronic connectivity between the active sites and the remote functional groups was promoted by reticular electronic tuning of the porphyrin active sites in 2D COFs, thus showing excellent electrocatalytic CO₂ reduction performance.²³³

Electrocatalytic oxygen evolution reaction (OER) is a four-electron transfer process with slow kinetic, so it is very important to develop high-performance and robust electrocatalysts. Recently, Hu et al. designed and synthesized a series of bimetallic monolayer 2DPs, and investigated the synergistic effect of the bimetals in electrocatalytic OER (Fig. 26e and f).²³⁴ The electrochemical test results showed that the as-prepared bimetallic 2DP had the best performance, suggesting that the relative location and coordination environment could affect the intrinsic activity of the catalysts (Fig. 26g and h). Then, the authors further discussed the effect of the coupling strength between the two metal centers on the performance of electrocatalytic OER by adjusting the spacing of Co and Ni and the conjugation degree of the bridge skeleton in the monolayer bimetallic Co-Ni 2DPs, which provided ideas for the design and preparation of high-performance bimetallic OER catalysts.²³⁵

6.4. Photocatalysis

Ultrathin 2D conjugated polymers have become advanced materials for converting solar energy into chemical energy in recent years.²³⁶ 2DPs have significant advantages as photocatalysts:^237–243 (i) 2DPs have a large specific surface area, which can provide more effective active sites for photocatalysis; (ii) the ultrathin thickness and planar conjugated structure of 2DPs enable higher light absorption and faster-photogenerated carrier migration; (iii) the monomer molecules and structural units of 2DPs are highly adjustable, and their electronic structure and physical/chemical properties can be regulated through the design and synthesis at the molecular level to meet different photocatalytic needs; (iv) the layered structure of 2DPs is easy to be combined with other materials to form hybrid structures or heterostructures, thus expanding the design and application range of polymer catalysts. Therefore, 2DPs show broad application prospects in the field of photocatalysis.

Photocatalytic water splitting technology is a clean energy technology for sustainable development in the future. Our group has prepared a series of highly efficient 2D CTFs photocatalysts through structural design and component regulation, including single-layer crystalline 2D-TPs,⁹⁹ 2D-TP-PB nanosheets,¹³⁰ single-layer/few-layer 2D CTF-DCB,¹³³ and r-CTF NSs.¹³⁵ Particularly, the as-synthesized atomically thin crystalline 2D-TPs with a band gap of 2.36 eV and abundant triazine active groups showed a significant photocatalytic hydrogen evolution rate of 1321 μmol h⁻¹ under visible light irradiation, and the apparent quantum yield at 420 nm was as high as 29.5% (Fig. 27a–c).⁹⁹ Moreover, 2D-TPs also demonstrated remarkably photocatalytic overall water splitting, surpassing all molecular framework materials and among the best metal-free photocatalysts ever reported (Fig. 27d and e). The excellent photocatalytic activity of 2D-TPs was mainly due to their fully conjugated nitrogen-rich structure, suitable band structure and strong chemical stability, which could significantly improve the generation, separation and transportation of photoexcited carriers. And early in 2015, Zhao et al. confirmed that 2D-CTFs were a new class of visible-driven organocatalyst materials for water splitting through first-principles calculations.²⁴⁴ Xu's group has also made outstanding contributions to the use of 2DPs as photocatalysts for water splitting. They have successively prepared ultrathin layered PTEPB and PTEB nanosheets,²⁴⁵ as well as aza-CMP/C₂N van der Waals heterostructures,¹⁹¹ and achieved photocatalytic overall water splitting under visible light irradiation. In particular, pure aza-CMP nanosheets could be used as photocatalytic OER catalysts (Fig. 27f).²⁴⁶ It was found that low-bandgap aza-CMP could effectively achieve photocatalytic O₂ generation under visible light (>420 nm) and near-infrared light (>800 nm) illumination, which was also the first reported polymer semiconductor photocatalyst with O₂ production performance in near-infrared region (Fig. 27h–j). In addition, the photocatalytic O₂ evolution of the exfoliated ultrathin aza-CMP nanosheet was increased by about three times deriving from the enhanced photoresponsicity and more accessible reaction sites. The photocatalytic O₂ evolution performance of aza-CMP was further improved by introducing Co(OH)₂ cocatalyst (Fig. 27g and k).

Photocatalytic H₂O₂ production from H₂O and O₂ is a potentially viable green and sustainable method for H₂O₂ synthesis. Xu and co-workers reported that a class of metal-free acetylene and diacetylene functionalized CTFs achieved high-efficiency H₂O₂ production through photocatalytic water oxidation and oxygen reduction under visible light irradiation.²⁴⁷ The experimental and theoretical results suggested that acetylene and diacetylene moieties could not only promote efficient charge separation in conjugated structures, but also facilitate the formation of suitable H₂O₂ intermediates during the photocatalytic reaction. It is noteworthy that photocatalytic CO₂ reduction technology has been considered one of the effective ways to achieve carbon neutrality. Jiang et al. employed COF-367-Co NSs as a representative to explore the application of 2D COF materials in the photocatalytic CO₂ reduction (Fig. 27l).¹¹⁹ The experimental results indicated that the photocatalytic CO₂-to-CO conversion by COF-367-Co NSs had a yield of 10162 μmol g⁻¹ h⁻¹, a selectivity of ∼78%, and excellent cycling stability (Fig. 27m and n).

7. Conclusions and perspectives

As a new type of organic 2D materials, 2DPs inherit the common advantages of 2D materials and polymers, and have potential application prospects in many fields, becoming one of the new growth points in polymer science. The 2D structural characteristics of 2DPs (e.g., large plane size and ultra-thin thickness) endow them with excellent physical, chemical, optoelectronic and mechanical properties, such as high specific surface area, abundant active sites, fast mass transfer, efficient charge separation and migration, excellent photoexcitation response, as well as excellent mechanical strength and flexibility. Their inherent polymer properties endow them with a clear structure, permanent porosity, favorable chemical and thermal stability, as well as excellent processability. Importantly, 2DPs can meet the needs of various applications through careful selection of organic building blocks and various covalent bonds. Therefore, making full use of the advantages of tunable pore size and functional modification of 2DPs is an important direction for the future application of polymer materials. This article reviews the design, synthesis, and latest application progress of 2DPs in energy-related fields. We briefly introduce the development history and unique advantages of 2DPs and list typical linkages for synthesizing 2DPs. Then, a systematic overview is provided of the “top-down” and “bottom-up” strategies for fabricating 2DPs. In addition, the assembly and processing of 2DPs and their applications in energy-related fields (i.e. batteries, supercapacitors, electrocatalysis, and photocatalysis) are discussed. Although some interesting progress and achievements have been made, the research on 2DPs is still in its infancy, and several issues need to be addressed for future development.

(1) Explore more new structures and compositions of 2DPs. At present, most 2DPs are based on COFs or CTFs, and it is necessary to develop new efficient synthesis routes and reasonably manufactured building blocks to explore other possibilities, enrich the types of 2DPs, and expand their application range.

(2) Single or few layered 2DPs with uniform size and highly ordered atomic structures remain extremely challenging. Although various synthesis methods have been reported as effective methods for preparing 2DPs, how to utilize more organic reactions to control the ordered polymerization of monomers within the 2D planes and avoid strong accumulation between planes is still a major issue in the synthesis of 2DPs.

(3) The large-scale preparation of highly crystalline 2DPs with uniform and controllable thickness is the focus of future research. Large quantity and high quality are directly related to their further practical application. Future research still needs to focus on efficient preparation methods of 2DPs and accelerate their development and utilization.

(4) Assembly and processing ability of 2DPs needs to be further improved. More robust methods should be developed to realize designed and arbitrary process of 2DPs into various assemblies and macroscopic materials with controlled nano/micro-structures. Although many studies have shown that functionalized 2DPs can be obtained through element doping, cocatalyst deposition or heterostructure construction, more effective strategies are needed to make full use of the easy modification and composite characteristics of 2DPs, so as to improve their properties and optimize their function.

(5) In comparison with inorganic 2D materials, the research of ultrathin 2DP materials in energy-related fields is still in its infancy, the application range needs to be expanded, and the performance needs to be further improved. More in-depth and essential research on reaction mechanisms and reaction processes needs to be further carried out. In addition, the existing 2DPs materials available for these applications are still limited, and more effective 2DPs need to be developed.

(6) It is still important to develop more advanced and intuitive in situ characterization techniques and more realistic theoretical simulation methods to elucidate the reaction mechanism, clarifying the structure–property relationship, and designing and preparing 2DP materials more reasonably.

2DPs represent an exciting class of materials with important fundamental research opportunities, and their unique 2D topology gives them interesting new properties as well as untapped applications. We believe that with continuous research and development, 2DPs with various functionalities can be obtained to enrich the polymer world.

Abbreviations

2DPs	Two dimensional polymers
TMDs	Transition metal dichalcogenides
LDHs	Layered double hydroxides
SCs	Supercapacitors
COFs	Covalent organic frameworks
CTFs	Covalent triazine frameworks
SCSC	Single-crystal-to-single-crystal
FTIR	Fourier transform infrared spectroscopy
XPS	X-ray photoelectron spectroscopy
NMR	Nuclear magnetic resonance
UV-vis	Ultraviolet-visible
CONs	Covalent organic nanolayers
PXRD	Powder X-ray diffraction
E-FCTF	Exfoliate fluorinated CTF
FCTF	Fluorinated CTF
2D-TPs	2D triazine-based polymers
PB	Sodium 1-pyrenebutyrate
DAAQ-ECOF	Redox-active exfoliated COFs
E-CIN-1	Exfoliated covalent imine network-1
E-SNW-1	Exfoliated Schiff base networks-1
CTF NSs	CTF nanosheets
r-CTF NSs	Reduced CTF NSs
DCB	1,4-Dicyanobenzene
iCONs	Ionic covalent organic nanosheets
MIMAs	Mechanically interlocked molecular architectures
TPTC	9,10-Position of tritycene tricatechol
Da	2,6-Diaminoanthracene
Tp	2,4,6-Triformylphloroglucinol
f-CTF-1	Functionalized CTF-1
NMP	1-Methyl-2-pyrrolidone
2D CAPs	2D conjugated aromatic polymers
LB	Langmuir–Blodgett
DAA	1,8-Diazanthracene
BTPHA2	Triphenylamine
SMAIS	Surfactant-monolayer-assisted interface synthesis
PI-2DP	Polyimine-based 2DP
LAP	Laminar assembly polymerization
HOPG	Highly oriented pyrolytic graphite
sCOFs	Single-layer COFs
STM	Scanning tunneling microscope
SAMNs	Self-assembled molecule networks
BDBA	1,4-Benzenediboronic acid
BPDBA	4,4′-Biphenyldiboronic acid
TBA	2,4,6-Trimethylbenzaldehyde
2DPI	2D polyimide
SAT-2DPs	Secondary amine-linked triazine-based 2DPs
A-1DP	Amphiphilic rigid 1D polymer
2D-SP	2D supramolecular polymers
2D-CP	2D covalent polymers
2DAPAs	2D aromatic polyamides
2DPA-1	2D polyaramid material
2D BECOF	2D boronate ester covalent organic framework
L-2D-PI	2D polyimine
TAPB	1,3,5-Tri(4-aminophenyl) benzene
PDA	Terephthalic acid
PTTI	Polytriethyltriindole
HHTP	2,3,6,7,10,11-Hexahydroxytriphenylene
PBBA	1,4-Phenylenebis(boronic acid)
DHTA	2,5-Dihydroxyterethaldehyde
IPC-COF	Intrinsic proton-conducting COF
TFP	1,3,5-Triformylphloroglucinol
DHF	9,9-Dihexylfluorene-2,7-diamine
LS	Langmuir–Schaefer
XANES	X-ray absorption near-edge structure
2DPI-G	2DPI-graphene
EG	Exfoliated graphene
2D CMPs	2D conjugated microporous polymers
CGA	COF/graphene aerogel
PL	Photoluminescence
GID	Grazing incidence diffraction
WAXD	Wide-angle XRD
MA	Melamine
GIWAXS	Grazing incidence wide-angle X-ray scattering
SAXS	Small-angle X-ray scattering
OM	Optical microscopy
SAED	Selected area electron diffraction
EDS	Energy-dispersive spectrometry
2-TBQP	2,7,13,18-Tetrabromodibenzo[a,c]dibenzo[5,6:7,8]quinoxalino-[2,3-i]phenazine
BAM	Brewster angle microscopy
PLM	Polarized light microscopy
PLE	PL excitation
TGA	Thermogravimetric
DLS	Dynamic light scattering
WCAs	Water contact angles
THz	Terahertz
FP-TRMC	Fash-photolysis time-resolved microwave conductivity
FET	Field-effect transistor
C2Ps	π-conjugated 2DPs
OFET	Organic FET
PFETs	Polymer FETs
LIBs	Lithium-ion batteries
SIBs	Sodium-ion batteries
LSBs	Lithium–sulfur batteries
E-TFPB-COF	Exfoliated few-layered 2D TFPB-COF
CV	Cyclic voltammetry
TThPP	2D COF polyporphyrin linked by 4-thiophenephenyl groups
LiPSs	Lithium polysulfides
PI-CONs	2D polyimide nanosheets
3-TBQP	3,6,14,17-Tetrabromodibenzo[a,c]-dibenzo[5,6:7,8]-quinoxalino-[2,3-i]phenazine
HER	Hydrogen evolution reaction
OER	Oxygen evolution reaction

Author contributions

All authors contributed to the discussion of contents and the editing of the manuscript prior to submission.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22022510) and the “Pioneer” and “Leading Goose” R&D Program of Zhejiang (2022SDXHDX0004).

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