High entropy alloys: a comprehensive review of synthesis, properties, and characterization for electrochemical energy conversion and storage applications

Selvaganapathy Ganesan† ^ab, Diksha Bhatt†^c, Divyadharshini Satheesh†^d, Thangavelu Kokulnathan*^e, Prabhat Pant^c, Dhivya S. Bharathi^f, Leena Baskar^d, Nanda Gopal Sahoo^c, Arunkumar Palaniappan^b and Kovendhan Manavalan^d
aDepartment of Chemistry, School of Advanced Sciences, Vellore Institute of Technology, Vellore 632 014, Tamil Nadu, India
^bHuman Organ Manufacturing Engineering Lab, Centre for Biomaterials, Cellular and Molecular Theranostics, Vellore Institute of Technology, Vellore 632 014, Tamil Nadu, India
^cPRS-NSNT Centre, Department of Chemistry, D.S.B. Campus, Kumaun University, Nainital, Uttarakhand-263002, India
^dDepartment of Physics and Nanotechnology, SRM Institute of Science and Technology, Kattankulathur, Chengalpattu – 603203, Tamil Nadu, India
^eDepartment of Electro-Optical Engineering, National Taipei University of Technology, Taipei 106, Taiwan. E-mail: kokul49@gmail.com
^fDepartment of Physics, School of Advanced Sciences, Vellore Institute of Technology, Vellore 632 014, Tamil Nadu, India

First published on 5th August 2025

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1. Introduction

Recent innovations have dramatically transformed essential sectors such as technology, manufacturing, and transportation, creating a world that is both exciting and complex.¹ However, this remarkable evolution has brought to light a critical energy crisis that has deepened over the last two decades. This predicament largely stems from our persistent dependence on limited fossil fuels, coal, oil, and natural gas. The use of these resources not only fuels our daily lives but also leads to severe environmental repercussions, including climate change, air pollution, and acid rain, which pose significant challenges for societies worldwide. In response to this energy dilemma, renewable energy has emerged as a compelling and viable alternative. A substantial commitment of resources, including skilled professionals and financial backing, is being devoted to the advancement and implementation of renewable energy technologies. The primary goals in this pursuit include improving the efficiency and longevity of energy storage and conversion systems, ultimately steering us toward a more sustainable and resilient future.

As a result, there has been a significant increase in the development of innovative materials, particularly high-entropy alloys (HEAs) and their derivatives.^2,3 These materials are distinguished by their exceptional stability and activity, placing them at the forefront of research in this field. Unlike conventional alloys, which are typically based on one or two principal elements, HEAs are engineered by combining five or more elements, each with atomic concentrations ranging from 5% to 35%.⁴ Current phase diagrams and principles of physical metallurgy indicate that multicomponent alloys can form intermetallic compounds and various phases. However, this complexity often leads to brittle microstructures, which can limit their practical applications. Experimental studies suggest that increasing the mixing entropy of these alloys promotes the formation of a solid solution phase characterized by a more random and simplified structure. Unlike intermetallic compounds and other complex phases with intricate chemical compositions, the crystalline structures of HEAs consist of straightforward solid solution phases, such as Body-Centered Cubic (BCC), Face-Centered Cubic (FCC), Hexagonal Close Packing (HCP), or combinations of these phases (Fig. 1).^5,6

Research on HEAs is consistently advancing beyond standard alloys due to their superior properties, including increased strength and hardness, enhanced ductility and fatigue resistance, improved stability at high temperatures, and greater resistance to wear and corrosion.^8,9 Each HEA, with minor elemental variations, represents a new alloy system, suggesting the potential for an endless array of materials.¹⁰ This review provides an overview of the historical background, fundamental concepts, synthesis strategies, and key factors associated with the properties of HEAs. It offers significant insights for researchers interested in exploring the captivating field of energy conversion and storage applications using HEAs. Additionally, the review discusses current and forthcoming challenges in this area. It presents a comprehensive analysis of hydrogen evolution reactions (HER), oxygen evolution reactions (OER), oxygen reduction reactions (ORR), battery technology, hydrogen storage, supercapacitors, and future perspectives for HEAs.

2. History of HEAs

As human society advanced, the demand arose for materials that possessed specific and often conflicting properties–qualities that nature does not provide directly. Over time, humanity devised a solution by combining one metal with others or with nonmetals to meet these specific needs. This process of combining metals to create new materials with distinct properties is known as alloying, sometimes referred to as conventional alloying. Bronze, composed of copper and tin, is the earliest known alloy in human history, marking the beginning of the ‘Bronze Age around 4000 BCE.^11,12 Iron (Fe) emerged ∼3000 years later, leading to the Iron Age around 1500 BCE, as it proved sharper and harder than bronze.¹³ The mid-1800s saw the industrial revolution propelled by the mass production of inexpensive steel, achieved through careful control of carbon and impurities during large-scale production.

In the 12^th century, an advanced alloy was discovered that combined lightweight properties with high-temperature stability, playing a crucial role in the evolution of the aerospace sector and transforming society through air transportation and satellite technology.¹⁴ The concept of HEAs was pioneered by Cantor in the late 1970s and early 1980s, although his work was not published until 2004. He referred to these unique materials as “multicomponent alloys” rather than “HEAs”.^15–18 Cantor derived the equiatomic FeCrMnNiCo base alloy, commonly known as the “Cantor alloy,” which remains the focus of extensive research. This alloy exemplifies a successful combination of a single-phase FCC structure and a solid solution.¹⁹ In 2016, a nanoparticle (NP) library comprising various elemental combinations was developed; however, due to thermodynamic immiscibility, the NPs underwent phase segregation. Following this, several monophasic, HE NPs were produced, expanding the range of elements used.^20,21 Since the discovery of entropy-stabilized metals in 2015, numerous HEAs designed for energy-related applications have been developed. Modern computational techniques can elucidate the relationship between local electronic structures and material properties. Coupled with advanced synthesis methods, these approaches present vast opportunities for the efficient exploration of novel HEAs. Depicts the timeline of HEAs, as shown in Fig. 2.²²

3. Definition and theoretical concept of HEAs

HEAs can be defined from two perspectives: composition and entropy. Below are concise definitions based on these two viewpoints.

3.1 Composition-based HEAs definition

The first composition-based definition was introduced in 2004. HEAs are broadly defined as alloys that consist of at least five primary elements, with each element present in an atomic percentage ranging from 5% to 35%. Additionally, the atomic percentage of each minor component must be below 5%. This can be mathematically expressed as follows:

nmajor ≥ 5, 5 at% ≤ ci ≤ 35 at%	(1)

nminor ≥ 0, cj ≤ 5 at%	(2)

where n_major and n_minor represent the quantities of major and minor elements, and, c_i and c_j represent their respective atomic proportions.

3.2 Entropy-based definition of HEAs

The primary goal of HEAs, composed of multiple principal elements, is to optimize the formation of solid-solution phases by achieving a high mixing entropy. The configurational entropy of an ideal random n-component solid solution can be expressed as:

	(3)

where ΔS_conf represents the ideal configurational entropy, R is the gas constant, c_i is the mole fraction of the ith element, and n is the number of components.²³ In the case of an equimolar alloy in a liquid or regular solid-solution state, the configurational entropy can be similarly expressed as:

ΔSconf = RInn	(4)

Based on configurational entropy, HEAs can be categorized as follows:²⁴

• Low-entropy alloys (LEAs): ΔS_conf < 0.69R.

• Medium-entropy alloys (MEAs): 0.69R < ΔS_conf < 1.61R.

• HEAs: ΔS_conf > 1.61R.

The multicomponent effect in HEAs describes the combination of several components (usually five or more elements) in nearly equal proportions, resulting in significant deviations from the properties of conventional alloys. The retention of the crystal structure and phase of HEAs, as well as their mechanistic aspects, can be distinguished by four core effects.

4. HEAs structural property correlation and design consideration

Developing high-performance HEAs catalysts with unique physical and chemical characteristics depends on an awareness of the correlations between structure and electrocatalytic performance since site-specific factors are quite important. In this regard, important considerations in context are the characteristics of HEAs, e.g., components, composition, and crystal structures.

4.1 Factors affecting phase formation

Although the entropy-driven effect theoretically favors the development of solid solutions, multi-element systems with high mixing entropy do not always lead to a single crystalline solid solution phase. Consequently, mixing entropy becomes a significant factor influencing whether a solid solution or an amorphous phase forms. Previous research has identified five key characteristics that can predict phase formation in multi-element alloys:

(1) Mixing entropy,

(2) Atom radius difference,

(3) Mixing enthalpy,

(4) Electronegativity, and

(5) Valence electron concentration.

These factors collectively influence the stability and formation of phases in HEA, guiding the design and optimization of new materials.

4.2 Elemental selection

The enhancement of catalytic performance heavily relies on the strategic selection of components. Typically, the choice of elements is dictated by the target reactions. For example, in the HER and OER, noble metals such as ruthenium (Ru), iridium (Ir), and platinum (Pt) are often found to be active.^46,47 Transition metals, including nickel (Ni), iron (Fe), cobalt (Co), manganese (Mn), chromium (Cr), and copper (Cu), are predominantly employed in HEAs. The selection of these elements, particularly in conjunction with noble metals, is typically associated with ligand effects on the active sites. On the other hand, optimizing the overall electronic structures such as d-band centers and charge distribution is crucial in designing non-noble metal catalysts, as these factors significantly influence interactions with reactants and catalytic intermediates. Fig. 4 shows elemental selection for HEAs in electrocatalysis. The electrical characteristics of HEAs can be modified through deliberate element selection. Additionally, the arrangement of multiple elements generates numerous active sites, creating a diverse spectrum of adsorption energies. This variability results in an adsorption energy distribution curve, which can be fine-tuned by refining the fundamental elements and their mixtures to achieve desired adsorption energies for specific reactions. Furthermore, variations in electronegativity between neighboring atoms influence charge distribution and adsorption energy. Valence electrons also play a significant role in determining the adsorption strength of particular intermediates, thereby facilitating the chemical processes involved.

5. Synthesis methods for HEAs

The exploration of bulk amorphous alloys has paved the way for the development of HEAs. As a result, the synthesis methods for HEAs are largely derived from those used for amorphous alloys, which can be categorized into three primary strategies.

5.1 Liquid-state synthesis

The principal method for synthesizing HEAs is liquid-state synthesis, as illustrated in Fig. 5a and b. This category includes techniques such as:

• Arc melting.

• Electric resistance melting.

• Induction melting.

• Selective laser melting.

• Selective electron beam melting.

• Laser-engineered net shaping.

While these methods are popular for HEAs synthesis, they require very high processing temperatures to achieve uniformity.^52–54

5.2 Solid-state techniques

Solid-state techniques, shown in Fig. 5c and d, include methods such as:

• Mechanical alloying.

• Spark plasma sintering.

These techniques, however, can be energy-intensive and may suffer from susceptibility to oxidation. As a result, achieving adequate homogeneity can be challenging, often necessitating additional pressing and sintering.⁵⁵

5.3 Gas-state synthesis

Gas-state synthesis methods, depicted in Fig. 5e and f, encompass techniques like:

• Magnetron sputtering deposition.

• Pulsed laser deposition.

• Atomic layer deposition.

• Vapor-phase deposition.

These approaches require sophisticated and costly equipment, making large-scale operations challenging. Consequently, the advancement of new applications for HEAs relies on the development of innovative synthesis methods that yield distinct properties.⁵⁶

5.4 Electrochemical synthesis

Electrodeposition is one of the most efficient methods for synthesizing pure metal NPs. This technique allows for easy regulation of the nucleation and growth of NPs with various morphologies. Additionally, electrolytic deposition is straightforward, and the compositions can be modified by altering specific parameters (Fig. 6a).^57,58 In a conventional electrolytic cell, a metal oxide serves as the cathode, graphite functions as the anode, and molten salt acts as the electrolyte. The principal parameters for electrodeposition include the geometry, density, and porosity of the oxide precursor, as well as the operating temperature, applied potential, and electrolysis duration.⁵⁹ Sure et al.⁶⁰ were the first to investigate the use of electrochemical reduction to complex HEAs in molten salts. In their study, a pre-mixed solid metal oxide underwent cathodic polarization in a molten CaCl₂ electrolyte at 1173 K, resulting in the formation of isotonic refractory alloys TiNbTaZr and TiNbTaZrHf in a single alloying operation. Another study by Sure et al.⁶¹ successfully synthesized the ultra-high temperature and high-entropy carbide (TiNbTaZrHf)C using a straightforward electrochemical method. This demonstrated the feasibility of producing nanometer-scale high-entropy ceramics powder with a uniform chemical composition through molten salt electrode oxidation. The simplicity and versatility of the deoxidation procedure enable the preparation of HECs with various components, making them suitable for specific applications.

5.5 Laser-based synthesis

In recent years, HEAs have emerged as significant foundational materials for research and applications. However, due to the lack of a scalable, direct synthesis method for HEA-NPs, most research has focused on HEA-bulk materials rather than HEA-NPs. Techniques such as laser ablation, pulsed laser deposition (PLD), and laser irradiation synthesis have recently been employed to synthesize HEA-NPs (Fig. 6b). Chen et al.⁶⁶ demonstrated the use of PLD to fabricate CrMnFeCoNi thin films on carbon cloth (CC) and nickel foam (NFs) substrates for the OER. The authors reported an overpotential of 315 mV and 287 mV to achieve a current density of 10 mA cm⁻² for these CrMnFeCoNi on CC and NFs, respectively. They showed that electrochemical redox over 500 cycles, utilizing the cyclic voltammetry (CV) technique, facilitated the redissolution and redeposition of surface metals, resulting in advantageous stability outcomes. The HEAs exhibited remarkable stabilities of 300 and 360 hour (h) on CC and NFs substrates, respectively. Additionally, the authors utilized a magnetic field to enhance the kinetics of the OER through spin polarization effects. Lu et al.⁶⁷ synthesized FeCoNiCuPtIr using laser irradiation for the HER and OER performance. They presented an efficient technique for depositing a mixed metal precursor solution onto multi-walled carbon nanotubes (MWCNTs) and exposing it to ultra-fast laser irradiation. This process resulted in the breakdown of metal precursors, yielding uniformly sized NPs on the surfaces. The authors demonstrated that laser irradiation elevated the temperature to approximately 1600 K from room temperature in about 50 ms, followed by ultra-fast cooling to ambient temperature, which facilitated the breakdown of the precursor solution and the creation of HEA-NPs. The dimensions of the NPs were regulated by modifying the duration of laser exposure; extended exposure times resulted in larger particles due to diffusion, while shorter durations yielded smaller particles. The total synthesis process was completed in under one millisecond without any phase separation. The HEA-based electrocatalyst exhibited superior electrocatalytic activity, with overpotentials of 21 mV at −10 mA cm⁻² for the HER and 255 mV at 10 mA cm⁻² for the OER, surpassing standard Pt/C and RuO₂ benchmarks for these processes. The catalyst reportedly maintained stability at 10, 20, and 30 mA cm⁻² after 20 h of catalysis. This synthesis approach proved to be highly rapid and economical.

5.6 Carbothermal shock treatment

HEAs were initially created using melting or plasma techniques that yielded atomically mixed phases. In contrast, wet chemical methods often synthesize nanostructured alloys by co-reducing metal salts with capping agents that inhibit crystal growth and aggregation. The potential to fuse immiscible multi-elements into a single NPs offers significant technological promise; however, traditional synthesis techniques often face challenges in this regard. Developing a synthetic method that accurately controls alloy composition, atomic size, and phase could yield numerous new HEAs with unprecedented functionalities.⁶⁸ Yao et al.⁶⁴ developed a standardized method for combining up to eight distinct elements into single-phase solid solution nanomaterials. This process involved loading metal salt precursors onto a carbon carrier at approximately 2000 K for a duration of 55 ms, achieving a heating rate of ∼1 × 10⁵ K s⁻¹. By adjusting the carbothermal shock parameters—such as substrate, temperature, impact duration, and heating/cooling rate of multicomponent NPs with the desired chemical composition, size, and phase was synthesized (Fig. 6c). Zhang et al.⁶⁹ employed metal oxides, B₄C, and graphite as raw materials to create (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)B₂, (Hf_0.2Zr_0.2Mo_0.2Nb_0.2Ti_0.2)B₂, and (Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂. The hexagonal structure of the boride entropy crystallization yielded target phases calculated to contain over 93.0 wt% of sintered ceramics, without compromising fracture toughness and high hardness.

The immiscible elements are alloyed on carbon support to generate single-phase NPs with the following characteristics:^69,70

High entropy mixing: polymetallic combinations yield solid solution NPs characterized by maximal configurational mixing entropy.

Non-equilibrium processes: rapid quenching enables the formation of HEA-NPs within milliseconds, circumventing the initial segment of the time–temperature–transformation curve and preventing phase separation among immiscible elements.

Uniform dispersion: at elevated temperatures, catalytically facilitated carbon metabolism produces NPs that are homogeneous, well-dispersed, and size-controllable, in contrast to particle coarsening.

5.7 Fast-moving bed pyrolysis

The fast-moving bed pyrolysis (FMBP) method, developed by Gao et al.⁶⁵ in 2020 is based on the principles of carbothermal shock treatment to produce (MnCoNiCuRhPdSnIrPtAu) HEA-NPs on granular substrates. Similar to CTS, this technique involves the rapid pyrolysis of precursors, including mixed metal salts, at elevated temperatures to generate uniform and well-dispersed nano-sized HEA-NPs. It demonstrated that the FMBP approach enables the attainment of temperatures up to 923 K within just 5 seconds in a vacuum furnace, facilitating the synthesis of single-phase quinary, senary, septenary, and octonary HEA-NPs. This effective synthesis method produced denary MnCoNiCuRhPdSnIrPtAu-based HEA-NPs with an approximate size of 50 nm (Fig. 6d). Furthermore, the authors proposed that the minimal energy required for nucleation, combined with the short heating period, contributed to the formation of smaller-sized particles.

6. High entropy materials for energy conversion

Hydrogen is regarded as one of the most abundant resources, serving as a promising energy carrier and fuel source with zero carbon emissions. HEAs play a crucial role in energy conversion by facilitating hydrogen production through water-splitting, utilizing various eco-friendly electric sources such as solar photovoltaic and wind power. Recent reports indicate that HEAs exhibit superior electrocatalytic properties, particularly for electrochemical reactions including the HER, OER, and ORR. The application of electrochemical water-splitting has gained global recognition as an effective means to increase hydrogen production, accompanied by the cathodic half-cell reaction. This section will provide a detailed overview of various classes of HEAs utilized in electrochemical applications.⁷¹ The HER contributes to high energy efficiency, zero carbon emissions, and a non-toxic environment.⁷² It is part of the electrocatalytic process involved in water-splitting, as illustrated in Fig. 7a. The mechanisms of water oxidation and reduction have been understood since the 18th century.

On surfaces, absorbed hydrogen exists in two forms: underpotentially deposited hydrogen (UPDH), which occurs on noble metals such as Pt, Rh, Ru, Ir, and Pd, and over potentially deposited hydrogen (OPDH), which refers to other forms of adsorbed hydrogen.⁷⁵ Generally, the HER involves a two-electron transfer process, and the electrode reaction becomes more complex as the electrolyte varies with different pH levels. The primary goal of water-splitting is to produce hydrogen fuel, thereby reducing reliance on fossil fuels, which currently account for 95% of hydrogen production,⁷⁶ predominantly through coal and natural gas steam reforming, according to a 2020 survey. Understanding the HER mechanism is essential for optimizing the water-splitting reaction rate. As shown in Fig. 7b, the use of HEAs is a burgeoning field for HER applications. A common pathway for the HER in acidic media involves the adsorption and desorption of the hydrogen intermediate (H*) through either the Volmer–Heyrovsky or Volmer–Tafel mechanisms:⁷⁷

H+ + e− → H* (Volmer step)	(12)

H* + H+ → H2 (Heyrovsky step)	(13)

H* + H* → H2 (Tafel step)	(14)

This reaction mechanism slightly changes in alkaline media, as shown in the following equations (Fig. 7d):⁷⁴

H2O + e− → H* + OH− (Volmer step)	(15)

H2O + e− + H → H2 + OH− (Heyrovsky step)	(16)

H* + H* → H2 (Tafel step)	(17)

As illustrated in Fig. 7b, the pathways for the HER in acidic and alkaline conditions differ, with the energy required to drive the overall reaction varying according to the specific pathway, each characterized by its distinct energy barrier. The HER is more active in acidic media due to the higher concentration of H⁺ ions in the electrolyte compared to alkaline conditions. The volcano plot shown in Fig. 7c depicts the activity of catalysts for HER as a function of M–H bond strength, featuring both ascending and descending branches. Materials are classified based on their Gibbs free energy change (ΔG_H) values. An ideal catalyst composed of inexpensive transition metals should exhibit M–H bond strengths comparable to that of Pt.⁷³ Consequently, alloys that incorporate Ni along with other transition metals such as Co, Fe, Cu, Zn, and Cr are of particular interest.⁷⁸ HEAs, also known as multi-principal element alloys, possess a disordered atomic structure and exhibit numerous desirable properties that conventional alloys cannot achieve.⁷⁹ The exceptional catalytic properties of HEAs are often attributed to the synergistic effects intrinsic to their composition, commonly referred to as the “cocktail effect,” as well as electronic effects associated with their unique atomic structures.⁸⁰ Given their excellent corrosion resistance, highly tunable compositions, and metastable characteristics, HEAs with enhanced surface chemistry are promising candidates for catalytic applications, particularly in water-splitting HER.⁸¹

6.1 HEAs for HER

HEAs are recognized as highly active and stable electrocatalysts for the HER under acidic and alkaline conditions. Also referred to as multi-principal element alloys, HEAs possess a distorted atomic structure that imparts several advantageous properties, making them more effective than traditional alloys. A key focus in HEAs research is to enable the HER across various electrolytes using metal-based catalysts, thereby enhancing their superiority in electrochemical applications.^33,82 Recently, Chen et al. reported a RuRhPdPtAu through electrodeposition by controlling the cathodic potential. This alloy was utilized as an electrocatalyst without further processing. Linear sweep voltammetry (LSV) method was employed to evaluate its electrocatalytic activity in a 1 M KOH solution, demonstrating an overpotential of 275 mV at 100 mA cm⁻² which is significantly lower than that of monometallic Pt NPs (370 mV) and commercial Pt/C (463 mV). Notably, at −10 mA cm⁻² the quinary HEA exhibited an even lower overpotential of 88 mV, compared to 99 mV for monometallic Pt and 93 mV for commercial Pt/C. The Tafel slopes were calculated to be 150 mV dec⁻¹ for the quinary RuRhPdPtAu, 162 mV dec⁻¹ for Pt NPs, and 217 mV dec⁻¹ for commercial Pt/C. Long-term stability was assessed using chronopotentiometry at −10 mA cm⁻² for 24 h.⁸³ In another study, Choi et al. reported that the FeCoNiCuIr exhibited a low overpotential of 71 mV at −10 mA cm⁻², with an even lower overpotential of 50 mV at high mass activity. This alloy also demonstrated stable performance in chronopotentiometry for 48 h at a constant current density of 100 mA cm⁻².⁸⁴

A monolithic multielement alloy, CuAlNiMoFe, prepared using a facile arc melting method, exhibits lower overpotential due to electron transfer along the interconnected copper ligaments. This structure enables electroactive sites through small nanoporous interpenetrating lamellar channels, resulting in superior HER activity, characterized by reduced Tafel slope, overpotential, and excellent durability in alkaline and buffered conditions.⁸⁵ These materials were studied for HER in 0.5 M H₂SO₄, and the key values are summarized in (Table 1). The results validate the catalyst design strategy, which suggests that alloys composed of group IV–VI and group VIII metallic elements yield promising HER electrocatalysts.⁸⁶ Gao et al. proposed a hypothesis regarding Ni-based HEA, as illustrated in Fig. 8.⁸⁷ They suggest that these alloys can enhance water adsorption by weakening hydrogen adsorption properties, which promotes the formation of hydronium ions near the catalyst surface. This mechanism subsequently results in faster kinetics during the HER process. Fig. 8a–g displays the HEAs that achieved the best results across all tested combinations. The NiFeCo-based alloys investigated for HER in alkaline media include NiFeCo, NiFeCoCr, CuNiFeCo, CuNiFeCoCr, and CuNiFeCoCrTi. These were synthesized through a two-step process, where the precursors were first ball-milled, followed by preparation using a DC arc plasma method.

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The electrochemical performance of these HEAs was evaluated using a three-electrode system with a 1 M KOH electrolyte. Fig. 8a displays the LSV results, while (Fig. 8b) shows the Tafel plots. Fig. 8c presents LSV data with iR compensation, and Fig. 8d and e includes bar graphs indicating the values of overpotential and Tafel slope. As the number of alloy components increases, the catalytic properties also improve. Notably, the quaternary alloy CuNiFeCoCrTi demonstrates the lowest overpotential of 117 mV at −10 mA cm⁻². In contrast, the quinary alloy CuNiFeCoCr@NF shows an overpotential of 155 mV, followed by NiFeCoCr@NC at 181 mV, and the ternary alloy NiFeCo@NC NPs exhibiting the highest overpotential of 302 mV. Comparative analysis of the EIS results, shown in Fig. 8f, indicates high conductivity in the electrolyte, supporting the notion that a higher number of entropy elements correlates with lower overpotential. Stability assessments for the quaternary alloy, identified as the optimal catalyst among the combinations, are presented in Fig. 8g.⁸⁷

The volcano-based plot (Fig. 7c) on the metal–hydrogen bond energy, developed by Tarasati, aids in selecting candidate metallic elements for HEA electrocatalysts. Metals and alloys such as Pt and Ni–Mo are commonly employed as catalysts for HER applications. The H–FeCoNiMnW was prepared using the electrodeposition technique and subsequently coated onto CC, with an exposed area of 0.5 cm × 0.5 cm, serving as the working electrode for electrochemical studies related to HER. The resulting overpotential and Tafel slope values are detailed in (Table 1).⁸⁹ Ma et al.⁹⁰ fabricated an CoCrFeNiAl HEA sheet using mechanical alloying and spark plasma sintering methods. Typically, HEA are prepared by mixing the constituent chemicals and milling them in a ball mill at 250 rpm for 60 h in an argon-filled atmosphere, utilizing stainless steel balls for the milling process. This study highlighted the self-supported CoCrFeNiAl, which were investigated under acidic conditions both before and after treatment with hydrofluoric acid. Upon adding HF acid, the surface of the HEAs-tends to shrink, transforming from a semi-hydrophobic to a hydrophilic nature, which facilitates the migration of protons from the electrolyte to the electrode surface. This transformation results in favourable HER activity, characterized by a reduced overpotential of 125 mV at a current density of −10 mA cm⁻². After 4000 cycles of CV, the HF acid-treated HEA exhibited improved HER activity, with a further decreased overpotential of 73 mV at −10 mA cm⁻². Additionally, the calculated Tafel slope was 39.7 mV, highlighting the superior performance of the HF acid-treated self-supported HEAs.

The researchers pursued HEA due to their unique disordered atomic structure and high mixing entropy, providing properties that surpass traditional alloys, including greater lattice distortion energy.^95–98 Another study explored the electrocatalytic performance of HER activity with FeCoNiPC ribbons, which were designed by varying the concentrations of iron and carbon. For this investigation, a standard three-electrode system was employed, with a saturated calomel electrode as the reference and a CC as the counter electrode. The overpotential of the FeCoNiPC alloy was examined, with the optimal composition designated as Fe₃₅Co₂₅Ni₂₅P_7.5C_7.5, demonstrating outstanding HER performance with a comparatively low overpotential of 88 mV at −10 mA cm⁻². Chronoamperometric tests conducted for 12 h at a current density of −10 mA cm⁻² confirmed that the prepared high-entropy alloy is stable in an acidic medium. LSV measurements were performed for 1000 cycles, indicating no change in the LSV response before and after these cycles, thus demonstrating the stability of the HEAs as a working electrode in an acidic environment.⁹⁹

A free-standing nanoporous high-entropy alloy NiCoFeMoMn (Fig. 9) was developed by varying the dealloying time to 0, 3, 4, 5, 6, and 7 h. The optimal properties were achieved after 6 h of dealloying, resulting in a minimal overpotential of 14 mV at −10 mA cm⁻², which is lower than the commercially available Pt/C electrode, which exhibits an overpotential of 32 mV at the same current density (Fig. 9a). The nanoporous HEA with 6 h of dealloying also demonstrated an extremely low Tafel slope of 29 mV (Fig. 9b). The double-layer capacitance (C_dl) values derived from the slope of the current density vs. scan rate plot, as show in Fig. 9c, reflects the electrochemical active surface area (ECSA) of all the prepared catalysts. Overpotential at current densities of 500 and 900 mA cm⁻² are plotted against the dealloying time (Fig. 9d). The minimum overpotential is observed at 6 h of dealloying, which confirms the optimal duration for HER activity. Fig. 9e presents the chronopotentiometry curves at 100 and 500 mA cm⁻² over 150 h there shows a negligible degradation, which confirms the excellent long-term operational stability of the NiCoFeMoMn 6 h catalyst. The on-set potential of the optimal catalyst NiCoFeMoMn 6 h is significantly lower than that of commercial Pt/C indicating faster initiation of HER as shown in Fig. 9f. The price activity of both NiCoFeMoMn 6 h and Pt/C catalysts is 3642.8 A per $ and 4.6 A per $ respectively, as shown in Fig. 9g, indicating the superior cost effectiveness, and making it as a promising alternative to the noble metal catalysts. In Fig. 9h, a comparative bar chart shows the overpotential and the Tafel slope of this catalyst, against various HER catalysts. The NiCoFeMoMn 6 h outperforms many literature reports with one of the lowest overpotential and Tafel slopes, which emphasizing its competitive edge.⁹² Another study highlights a series of HEAs, specifically FeCoCrNiMo_x, created through the arc melting method and subsequently enhanced by heat treatment. This study investigates the influence of varying molybdenum content on the HER activity as the melted HEA ages.⁹³ A multi-element alloy is used to tune the bandgap to facilitate the HER and electron transfer kinetics, which inspires the investigation of the HER activity, the obtained overpotential for the FCC HEAs obtained from the LSV at −10 mA cm⁻² is found to be 355 mV and the Tafel slope value obtained from the potentiostat is 68 mV dec⁻¹ this reflects the excellent electron transfer. This study explains corrosion resistance complex alloy which improves the HER effectively. The HEAs have significantly demonstrated as a potential candidate for HER. HEAs have emerged as promising candidates for HER applications due to their unique properties, including multi-element composition, tunable electronic structures, and enhanced stability. These characteristics contribute to superior catalytic performance. The high-entropy configuration fosters a synergistic effect among the constituent elements, optimizing active sites for HER and enhancing overall catalytic activity. Additionally, HEAs exhibit improved durability and chemical stability compared to traditional alloys, making them suitable candidates for sustainable and scalable hydrogen production technologies. Furthermore, understanding the mechanistic pathways of HER on HEAs through advanced characterization techniques and computational modelling will provide deeper insights into the design and development of next-generation electrocatalysts.

6.2 HEAs for OER

The OER is the anodic semi-reaction involved in the electrocatalytic decomposition of water. The kinetic rate of electron transfer in this reaction is extremely slow, leading to high overpotential, which presents a significant barrier to improving the efficiency of electrocatalytic water decomposition. Generally, HEAs are recognized as powerful electrocatalysts with a wide range of applications in energy storage and conversion. However, HEAs also face limitations, such as a deficiency in low specific surface area, which restricts the exposure of active sites. Due to its inherently slow kinetics, the OER is frequently regarded as the main bottleneck in the overall water-splitting process.¹⁰⁰ This delayed reaction time severely restricts the system's energy conversion efficiency, posing a substantial challenge to the development of effective and sustainable energy systems.¹⁰¹ The OER (Fig. 10a and b) is a complex process that generates molecular oxygen by oxidizing water molecules, involving the transfer of four electrons at a time. The high energy barrier associated with the OER results in significant anodic overpotential, which decreases the overall efficiency of the water electrolysis process. As a consequence of this large overpotential, more energy is required to drive the reaction. In recent decades, considerable research efforts have focused on designing and developing anodic materials with enhanced electrocatalytic activity compared to conventional and earth-abundant materials, including HEAs.^102,103

The four-electron transfer mechanism for the OER under alkaline conditions is represented by the following overall reaction:

4OH− → O2 (g) + 2H2O (l) + 4e−	(18)

This reaction typically proceeds through four elementary steps:

* + OH− → *OH + e−	(19)

*OH + OH− → *O + 2H2O + e−	(20)

*O + OH− → *OOH + e−	(21)

*OOH + OH− → O2 (g) + 2H2O (l) + 4e−	(22)

In contrast, the water oxidation reaction under acidic conditions is represented by the overall equation:

2H2O (l) → O2 (g) + 4H+ + 4e−	(23)

This reaction is generally believed to proceed through the following four steps:

H2O (l) + * → *OH + H+ + e−	(24)

*OH → *O + H+ + e−	(25)

H2O (l) + *O → *OOH + H+ + e−	(26)

*OOH → * + O2 (g) + H+ + e−	(27)

In the above equation * represents the active sites of the catalyst, (l) refers to the liquid phase, (g) refers to the gas phase and *O, *OH, *OOH represent species adsorbed on the active sites. The OER mechanisms under acidic and alkaline conditions are outlined in the above equations.¹⁰⁴

Catalysts play a crucial role in reducing the energy barriers of the OER and enhancing its efficiency. Heterogeneous catalysts are commonly used in practical applications, making their development a dynamic area of research. HEA, particularly those with FCC structures, have emerged as promising electrocatalysts due to their unique properties.¹⁰⁵ Their distinctive atomic arrangement and high configurational entropy create a dense array of active sites, significantly enhancing catalytic activity. Moreover, the composition and stoichiometry of HEA can be tailored to optimize their properties for the OER, allowing for the design of more efficient catalysts. The complex atomic arrangements in HEAs provide diverse active sites, facilitating the electrochemical reactions involved in the OER. However, it is important to note that the OER remains an active research area, and an ideal catalyst with optimal activity and stability has yet to be identified. Ongoing studies and advancements in HEA and OER catalysts are essential to fully realize their potential and improve the efficiency of energy conversion systems.¹⁰⁶ The activity of an electrocatalyst is influenced by the strength with which reaction species bind to its surface. For certain metallic catalysts, a volcano plot illustrating catalytic activity versus the adsorption energy of atomic oxygen shows that moderate binding strength yields the highest catalytic activity. In OER applications, weak binding complicates the stabilization of intermediates, while overly strong binding hinders O₂ release. Binding strength is affected by several factors, primarily the electronic structures of the electrocatalyst. Transition metals with d-band vacancies can form intermediate species that lower activation energy, with an optimal number of d-vacancies being ideal for OER applications.¹⁰⁷ Additionally, in heterogeneous reactions, the lattice spacing of the electrocatalyst must align with that of the reaction species. If the interatomic distance is too large, it hinders the adsorption of diatomic reactants and the bonding between oxygen atoms. Conversely, if atoms are too closely packed, strong repulsions between intermediates can inhibit product formation. Some intrinsic structural defects may also enhance catalytic performance. Compared to conventional metals and alloys, carefully engineered HEAs with the right composition are likely to integrate the advantages of each component element, fulfilling the criteria for improved OER performance.¹⁰⁸

Dai et al.¹⁰⁹ reported that the MnFeCoNi HEA demonstrates significant potential as an electrocatalyst for the OER. Electrochemical tests reveal that the MnFeCoNi electrode exhibits a low overpotential, a small Tafel slope, and excellent stability, often comparing favorably or even surpassing several current catalysts, including the state-of-the-art RuO₂ catalyst. Further research has identified that the superior OER performance of the MnFeCoNi HEA is attributed to the oxidation of the HEA powders during the electrochemical CV method activation process. This oxidation leads to the formation of MO_x (where M represents Mn, Fe, Co, and Ni) nanosheets on the surfaces of the MnFeCoNi HEA particles, resulting in a core–shell structure. The core–shell structure of the MnFeCoNi HEA demonstrates a low overpotential of 302 mV to achieve a current density of 10 mA cm⁻², alongside a small Tafel slope of 83.7 mV dec⁻¹ and remarkable long-term stability, maintaining electrolysis for over 20 h in a 1 M KOH alkaline solution. These findings are significant as they introduce a novel and promising electrocatalyst for the OER based on the MnFeCoNi HEA. The excellent performance of this HEA as an OER catalyst, characterized by low overpotential, small Tafel slope, and exceptional stability, opens new possibilities for using HEAs as functional materials in various applications. The oxidation of the HEA during the electrochemical CV activation process enhances the growth of MO_x nanosheets, further improving OER performance. These insights could inspire further research into the utilization of HEAs as effective functional materials.

The exceptional electrocatalytic performance and stability of the FeCoNiPB electrocatalyst can be attributed to several key factors. Firstly, the formation of amorphous sheets with thin (FeCoNi)OOH crystal layers in situ at the edges during the prolonged OER process enhances both its efficiency and stability.¹¹⁰ Additionally, the synergistic effect of the multiple components in the FeCoNiPB catalyst, along with its amorphous nanostructure, contributes to its superior performance. Notably, the FeCoNiPB catalyst outperformed other oxides, such as FeCoPB, FeNiPB, and CoNiPB, as well as the commercial RuO₂ catalyst, in terms of overpotential and Tafel slope. Furthermore, the FeCoNiPB exhibited excellent stability, with negligible overpotential increase over a span of 40 h.

The AlCoCrFeNi requires a lower overpotential of approximately 260 mV to achieve the current density of 10 mA cm⁻²,¹¹¹ indicating its high efficiency in driving the OER. With a Tafel slope of about 58 mV per decade, the AlCoCrFeNi showcases efficient catalytic activity, as a lower Tafel slope suggests better performance. This alloy reaches a current density of 10 mA cm⁻² at an overpotential of 260 mV, highlighting its capability to deliver high current at low overpotential. The AlCoCrFeNi also exhibits good stability under OER conditions, maintaining its performance over extended periods, making it a promising candidate for water splitting and other electrochemical applications. Likewise, the CoCrFeMnNi exhibits an overpotential of 220 mV at a current density of 10 mA cm⁻², with a Tafel slope of around 100 mV per decade, demonstrating stability for up to 44 h.¹¹²

Nanoporous Ir-based quinary HEA with the composition AlCoNiIrX (where X can be Mo, Cu, Cr, V, or Nb) have been created using a straightforward alloying-dealloying method. These alloys contain a low Ir content of 20 atomic percent, significantly lower than the typical 450 atomic percent found in common Ir-based binary and ternary alloys. The nanoporous structure is formed through dealloying, which selectively removes one or more elements from a precursor alloy. In this case, Al-based precursor alloys are utilized to incorporate Ir and four other metal elements, resulting in a single nanostructured phase with 20 atomic percent Ir. This nanoporous HEA structure offers extensive possibilities for adjusting the electronic properties of the alloy and enhancing its catalytic activities. One specific quinary nanoporous HEA, AlNiCoIrMo, has exhibited record-high OER activity in acidic environments which presented in Fig. 11a and b. The formation of HEAs significantly enhances the structural and catalytic durability of the alloys, regardless of their compositions. Additionally, AlCoNiIrMo demonstrates substantially enhanced cycling stability, making it a promising material for OER applications.¹¹³ Fig. 11c and d, shows the performance comparison of AlNiCoFe-based high entropy compositions with different metal dopants such as Mn, Zn, Cu, V, Nb, Cr. The sample AlNiCoFeCr exhibits a low Tafel slope as low as 46 mV dec⁻¹, revealing improved catalytic kinetics better than the benchmark material IrO₂ in some cases.¹¹⁴ A schematic representation illustration of room temperature synthesis of high-entropy MOF based material (Fig. 11e) formatted by integrating multiple cations such as Co²⁺, Mn²⁺, Cu²⁺, Ni²⁺, and Fe³⁺ etc.¹¹⁵ LSV and Tafel plots for alkaline metal assisted fluorides systems such as K_xNa_y(MgMnFeCoNi)F_x demonstrated (Fig. 11f and g) that dual alkali assisted system shows enhanced current density and lower overpotential when compared with the standard benchmark IrO₂ at 10 mA cm⁻². The stability of the catalyst investigated using chronoamperometry curves indicating the long term for the catalyst at fixed overpotential and maintains excellent stability for 12 h as shown in Fig. 11h.¹¹⁶ These alloys show great potential for future applications as OER electrocatalysts. The composition of the alloy plays a crucial role in enhancing OER performance. The values of OER activity for various HEAs are summarized in (Table 2). These materials hold significant potential for various applications in electrocatalysis and energy storage.

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6.3 HEAs for ORR

The ORR on noble and non-noble metal surfaces is one of the most extensively studied electrochemical processes. In acidic media, platinum-based materials serve as the backbone catalysts for the ORR. Conversely, in alkaline electrolytes, a diverse range of non-noble metals and their oxides demonstrate sufficient stability for practical applications. The ORR pathway is similar in both acidic and alkaline environments when utilizing Pt-based materials. An important rate-determining step (RDS) was proposed by Yeager et al.¹¹⁷ suggesting that the ORR on platinum surfaces likely involves the dissociative chemisorption of molecular O₂, with the initial adsorption of O₂ serving as the RDS. In alkaline solutions, the presence of exclusive HO₂⁻ species generated on the platinum surface results in lower catalytic activity and efficiency of the ORR compared to acidic media. Notably, gold (Au) exhibits the most active crystalline facet for the ORR in alkaline media, specifically the Au (100) facet; however, it performs poorly as a catalyst in acidic environments. The fundamental pathways of the ORR in acidic, alkaline, and non-aqueous solvents follow a 4-electron (4e⁻) process:

Alkaline aqueous solution: O2 + H2O + 4e− → 4OH−

Acidic aqueous solution: O2 + 4H+ + 4e− → H2O

The rate-determining step for the charge transfer of adsorbed oxygen molecules can be described by the following eqn (30):

M + O2 → M – O2	(28)

M – O2 + H+ + e− → M – O2H	(29)

M – O2 + H + 3H+ + 3e− → 2H2O + M	(30)

here, M denotes the Pt catalyst. In the dissociative model, the first step involves the dissociation and chemisorption of oxygen, which is accompanied by charge transfer:

M + ½O2 + e− → M – O−	(31)

The ORR is central to transformative energy technologies, playing a crucial role in the efficiency and performance of fuel cells, metal–air batteries, and other electrochemical devices. Traditional catalysts, such as HEAs, are less explored due to limited resource availability.

A complex functional system like the reduction process requires an elaborate catalyst with multiple active surfaces for the adsorption, activation, and reaction of various species. HEAs show great potential as multifunctional catalysts, particularly for the ORR. The sluggish kinetics and complex multi-electron processes are crucial for improving fuel cell efficiency in the ORR. This reaction typically involves a 4e⁻ transfer process that reduces oxygen to water, producing hydrogen peroxide in acidic media. In alkaline media, the four-electron transfer pathway yields hydroxide ions, while a 2e⁻ pathway generates peroxide ions. Wang et al.¹¹⁸ evaluated CrMnFeCoNi HEAs for the ORR in 0.1 M KOH to develop high-performance catalysts using a Rotating Ring-Disk Electrode (RRDE). CV measurements were conducted in the Ar and O₂-saturated electrolytes. The LSV results, obtained at a rotation speed of 1600 rpm, compared the CrMnFeCoNi, CuMnFeCoNi, and MnFeCoNi alloys with commercially available Pt/C. Among these, the CrMnFeCoNi electrode demonstrated the highest current density. The kinetics of this HEAs were analyzed using a Koutecky–Levich (K–L) plot, which plots the inverse current density against the square root of the rotation speed. The half-wave potential of the CrMnFeCoNi alloy was found to be 0.788 V, with an onset potential of 0.88 V. This electrode exhibited high selectivity towards total ORR, with an average selectivity of 0.2%, confirming a dominant four-electron pathway for the reaction. Notably, the LSV results showed no significant variation in current density before and after 1000, 2000, and 5000 CV cycles.

Wu et al.¹¹⁹ the preparation of this HEAs typically involves grinding and pyrolysis at high temperatures (Fig. 12a). The ORR activity of this alloy was rigorously tested by CV in N₂-saturated 0.1 M KOH solution at room temperature (Fig. 12b). Two key parameters for evaluating ORR are the onset potential (E_onset) and the half-wave potential (E_1/2). The most positive onset potential was obtained for FeCoNiMnV/N-CNTs, with E_onset = 0.99 V and E_1/2 = 0.85 V, as shown in Fig. 12c. The Tafel slope for FeCoNiMnV/N-CNTs (77.22 mV dec⁻¹) is much lower than that of other prepared composites (Fig. 12d). This comparison indicates that the alloy has significantly higher efficiency towards ORR due to its rich active sites and optimal catalytic activity. The LSV of the optimal alloy FeCoNiMnV/N-CNTs was tested at different rotation rates (400 to 2500 rpm), as shown in Fig. 12e. The K–L plots are presented in Fig. 12f. To evaluate the ECSA values were calculated from the C_dl recorded from CV curves in the non-faradaic region at different scan rates. As shown in Fig. 12g, the largest C_dl value was obtained for FeCoNiMnV/N-CNTs (C_dl = 25.97 mF cm⁻²). All the above-mentioned data convincingly demonstrate the excellent ORR catalytic properties of FeCoNiMnV/N-CNTs.

The electrocatalytic performance of the ORR for the PtRhNiFeCu catalyst was studied by Hu and his team, utilizing a LSV test in 0.1 M HClO₄ with a working electrode was composed of the HEA (Fig. 13).¹²⁰ In comparison to PtRhNi/C (E_1/2 = 0.875 V versus RHE) and Pt/C (E_1/2 = 0.844 V versus RHE), the PtRhNiFeCu/C catalyst exhibited the highest half-wave potential (E_1/2 = 0.9 V versus RHE), indicating its superior catalytic performance toward the ORR. When compared to the RHE, the mass activity and specific activity of the PtRhNiFeCu/C catalyst at 0.9 V versus the acidic ORR reached 1.23 A mg⁻¹ Pt and 2.67 mA cm⁻², respectively. These values are 6.5 and 9.9 times higher than those of commercial Pt/C.

The electrocatalytic performance of the Pt(FeCoNiCuZn)₃/C catalyst was assessed using a three-electrode RDE setup in an O₂-saturated 0.1 M HClO₄ solution. To achieve optimal activity, electrochemical dealloying cycles were conducted between 0.05 V and 1.2 V across various potential cycles prior to evaluating the ORR performance. The formation of a Pt-rich shell was confirmed through CV and LSV recorded in N₂-saturated 0.1 M HClO₄ (Fig. 13a and b). These curves exhibited clear hydrogen adsorption and desorption peaks, along with peaks corresponding to Pt oxide formation and reduction following cycling. Notably, upon activating the Pt(FeCoNiCuZn)₃/C, a distinct peak at 0.35 V was observed, associated with hydrogen desorption from the Pt (100) plane. The half-wave potential (E_1/2) of 0.922 V is higher than that of PtCu₃/C (0.902 V) and Pt/C (0.885 V), indicating that Pt(FeCoNiCuZn)₃/C exhibits the highest ORR activity among the tested catalysts. Fig. 13c and d illustrates the Tafel slope alongside the specific and mass activities of the material. The CV test results depicted in Fig. 13e and f show no significant difference in the performance of Pt/C and PtRhNiFeCu/C alloys before and after 1000 potential cycles, when compared to commercial Pt/C.¹²⁰ The quinary alloy AlNiCoRuMo demonstrated the highest ORR activity, achieving a half-wave potential of approximately 0.875 V, thereby outperforming commercial Pt/C (0.86 V). Other effective alloys, including AlNiCoRuCu, AlNiRu, and AlNiCoRuV (Fe), were also noted; however, alloys containing Ru, such as AlNiCoMo, exhibited the lowest activity. The presence of Ru is crucial for achieving high ORR activity, and adding a fifth element, particularly Mo, further enhances ORR performance, even with reduced Ru content. These findings indicate significantly improved ORR performance while minimizing Ru usage, despite Ru not traditionally being considered a strong ORR electrocatalyst.¹²¹ Table 3 summarizes the electrocatalytic performances of HEAs in terms of ORR activity. HEAs have emerged as promising materials for catalytic applications in ORR, playing a critical role in fuel cells and metal–air batteries. These alloys, characterized by their multi-component nature with five or more principal elements in near-equiatomic ratios, exhibit unique properties that enhance catalytic performance. The synergistic interactions among the diverse elements in HEAs create a high density of active sites and favorable electronic structures, significantly improving ORR kinetics. Importantly, HEAs demonstrate exceptional stability and durability under operating conditions due to their inherent corrosion resistance, thermal stability, and mechanical strength. Addressing these challenges through advanced characterization can help realize the potential of HEA-based catalysts for practical applications in energy conversion technologies.

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7. HEAs for energy storage

HEAs in single-phase forms, have shown promise in energy storage applications. Their diverse elemental integration leads to enhanced properties such as increased mechanical strength, exceptional ionic conductivity, and outstanding thermal stability, making HEAs attractive for advanced energy applications.^124–126

7.1 HEAs for hydrogen storage

HEAs have emerged as promising materials for hydrogen storage, demonstrating remarkable hydrogen retention as hydrides, with hydrogen-to-metal (H/M) ratios as high as 2.5.^127–129 Each developed HEA possesses a unique composition and microstructure, leading to superior properties compared to traditional alloys. Reports indicate that HEAs exhibit improved heat resistance, superior corrosion resistance, greater ductility, enhanced strength at elevated temperatures, effective magnetization response, and other significant functional attributes.¹³⁰ The technologically significant properties of HEAs are linked to four primary effects: high entropy effect, sluggish diffusion, severe lattice distortion, and the cocktail effect. Each of these effects contributes uniquely to achieving an optimal hydrogen storage system.^131,132 For example, incorporating 10% tantalum (Ta) into the Ti_0.3V_0.25Zr_0.1Nb_0.25Ta_0.1 results in a notable enhancement in hydrogen absorption/desorption capacity and cycling stability compared to the quaternary alloy Ti_0.325V_0.275Zr_0.125Nb_0.275.¹³³ Furthermore, the choice of alloying elements and the ability to control their proportions offer significant opportunities for the development of HEA hydrides. Key practical features of these hydrides include their reversible hydrogen storage capacity and electrochemical discharge capacity when interacting with hydrogen gas or serving as metal hydride anodes. These features are highly influenced by operating conditions such as temperature, hydrogen pressure, and electrode potential, determined by the thermodynamics of hydrogen's interaction with the base materials.¹³⁴ Metal hydrides are formed by exposing HEAs to H₂ gas under specific pressure and temperature conditions. The hydrogen-metal reaction occurs in several stages. Initially, the gaseous hydrogen molecule interacts with the metal surface through van der Waals forces, resulting in a physical adsorption process known as physisorption (Fig. 14). This interaction leads to an exothermic reaction in which H₂ gas is absorbed by the HEAs, forming metal hydride, as described by the equation:

	(32)

In this process, the gas molecule interacts with multiple atoms on the solid surface. The attractive forces diminish as the distance between the hydrogen molecule and the solid metal increases.

The potential energy of the hydrogen molecule reaches its lowest point at approximately one molecular radius. In the next phase of the hydrogen–metal interaction, the hydrogen must overcome an activation barrier to dissociate and form a hydrogen–metal bond, a process known as dissociation and chemisorption. When a metal hydride is subjected to the appropriate pressure and temperature, it decomposes into metal and hydrogen, releasing heat, as described by the equation:

	(33)

The energy associated with chemisorption is significantly greater than that of physical adsorption. After dissociation on the metal surface, hydrogen atoms migrate into the bulk material, forming a metal-hydrogen solid solution known as the α-phase. In HEAs, hydrogen occupies interstitial sites within the metal lattice, which can be either tetrahedral or octahedral. When the hydrogen concentration in the metal exceeds a specific threshold (typically H/M > 0.1), significant interactions between hydrogen atoms occur, leading to lattice expansion. Chemical bonds form between hydrogen and metal atoms, resulting in the nucleation and growth of the hydride phase, referred to as the β-phase.¹³⁵ Fig. 15 illustrates simplified one-dimensional potential energy profiles for hydrogen in both molecular and atomic forms interacting with a metal.

From eqn (32) and (33), we can express the equilibrium of hydride formation as:

	(34)

The thermodynamics of a system provide insights into the temperature and pressure ranges in which the hydride material is functional. A chemical process is thermodynamically favourable if it results in a decrease in Gibbs free energy, represented as ΔG < 0. The change in ΔG is determined by the enthalpy change (ΔH), entropy change (ΔS), and temperature (T), according to the equation:

ΔG = ΔH − TΔS	(35)

The direction of the reversible hydride formation or decomposition process is dictated by the sign of ΔG. When ΔG < 0, hydride formation occurs; conversely, when ΔG > 0, the reverse reaction takes place, leading to the release of hydrogen or decomposition of the hydride. The temperature T₀ at which ΔG is zero can be calculated by:

	(36)

Hydride formation occurs when the temperature is below T₀ (i.e., T < T₀), while decomposition occurs when the temperature is above T₀ (T > T₀). The thermal stability T₀ is influenced by both ΔH and ΔS. The thermodynamic characteristics of hydride formation from gaseous hydrogen are illustrated by pressure-composition isotherms (Fig. 16). It is important to note that an ideal hydrogen storage material undergoes a phase change between the hydrogen solid solution (α-phase) within the metal matrix and the hydride phase (β-phase).¹³⁵ The presence of both phases is indicated by a plateau in the pressure-composition isotherms, reflecting the quantity of hydrogen stored.

Based on the thermodynamic properties of the alloys, the desorption pressure of the metal hydride changes with temperature according to the Van't Hoff eqn (37):

	(37)

The plateau pressure, which varies with temperature within this two-phase region, indicates the equilibrium dissociation pressure of the hydride and reflects its stability. Additional hydrogen absorption and increased pressure occur after the full conversion to the hydride phase.

The characteristics of hydrogen storage in HEAs were first explored by Kao et al.¹³⁶ Using a vacuum arc melting approach, they developed a range of CoFeMnTi_xV_yZr_z by varying the parameters x, y, and z within predetermined limits. Their results indicated that the CoFeMnTiVZr_2.3 achieved the highest hydrogen storage capacity, ∼1.8% at room temperature.^136,137 Kunce et al. investigated the TiZrNbMoV and ZrTiVCrFeNi, which were produced using a laser-engineered net shaping technique. After activation and high-temperature treatment, the ZrTiVCrFeNi alloy—primarily composed of the C14 Laves phase—exhibited a maximum hydrogen storage capacity of 1.81 wt%. In contrast, the TiZrNbMoV alloy, characterized by a BCC phase, demonstrated a hydrogen storage capacity of 2.3 wt%.^138,139 Sahlberg et al.¹⁴⁰ reported notable hydrogen storage capacities of 2.7 wt% (2.5H/M) in a BCC TiNbVZrHf produced via arc melting. Zlotea et al.¹⁴¹ synthesized the BCC TiZrNbHfTa using the same technique, achieving an ultimate hydrogen storage capacity of approximately 1.65 wt%. In another study, Xiangfeng et al.¹⁴² examined an equimolar TiVZrNbFe and found that it consisted of a solid solution based on Nb and a C14 phase. This alloy demonstrated rapid hydrogen absorption, achieving 1.60 wt% hydrogen uptake within 100 seconds at around 50 °C and 1 MPa hydrogen pressure. The mechanism of hydrogen absorption in the TiVZrNbFe is schematically represented in Fig. 17a and b, which also displays the pressure-composition isotherm (PCI) hydrogenation curves at various temperatures.

Kumar et al.¹⁴³ synthesized the TiZrVCrNi through arc melting in an argon atmosphere. This alloy comprises a C14-type hexagonal Laves phase, confirmed by XRD analysis. It reliably stores hydrogen reversibly at a capacity of 1.52 wt%, with only a minor capacity drop over ten cycles (Fig. 17c). Dangwal et al.¹⁴⁴ developed a novel TiV₂ZrCrMnFeNi, featuring an AB-type structure conducive to effective hydrogen storage. This alloy, with both C14 Laves and BCC phases, requires no activation and can absorb 1.6 wt% hydrogen at ambient temperature. Fig. 17d illustrates the alloy's capacity to reversibly absorb and release hydrogen, as demonstrated by PCT isotherms. The alloy exhibits rapid kinetics and complete reversibility in hydrogen absorption and desorption. Chen et al.¹⁴⁵ synthesized the hexanary TiZrFeMnCrV, which forms a single C14 Laves phase via arc melting and ball milling. This TiZrFeMnCrV displays exceptionally fast hydrogen absorption kinetics, absorbing 1.80 wt% of hydrogen at a mild temperature of 30 °C. Moreover, it demonstrates excellent cycling stability, maintaining a capacity of 1.76 wt% after 50 cycles of hydrogen uptake and release. The performance of hydrogen uptake and release for the TiZrFeMnCrV HEA is shown in Fig. 17e, using PCT isotherms at temperatures of 0, 15, and 30 °C. The results of evaluating absorption/desorption reversibility over 50 cycles are depicted in Fig. 17f. Lastly, Floriano et al.¹⁴⁶ investigated a TiZrNbCrFe alloy, which exhibited a small disordered BCC phase (1.6%) and a dominant C14 Laves phase (98.4%) after arc melting. This alloy demonstrated reversible hydrogen absorption and desorption, achieving a hydrogen-to-metal atomic ratio (H/M) of 1.32 and a hydrogen storage capacity of 1.9 wt% at 473 K.

Wu et al.¹⁴⁷ designed a Mg₁₀Ti₃₀V₂₅Zr₁₀Nb₂₅, which is well-suited for hydrogen storage. This alloy demonstrates an impressive hydrogen absorption rate of 1.196 wt% at room temperature, showcasing excellent hydrogen absorption kinetics. Cheng et al.¹⁴⁸ investigated the hydrogen storage properties of Ti₂₅V₃₀Nb₁₀Cr_35−xMo_x, where x takes values of 2, 4, and 6. The pressure–composition–temperature (PCT) curves obtained for these three TiVNbCrMo alloys at different temperatures are displayed in Fig. 18 a–c. The Van't Hoff equation applied and calculated the thermodynamic parameters like (enthalpy and entropy changes), alloy (Fig. 18d and e). The calculated bulk modulus of Ti₂₅V₃₀Nb₁₀Cr_35−xMo_x (x = 2, 4 and 6) HEA correlating with hydrogen absorption/desorption plateau pressures results bulk modulus increases, the absorption/desorption plateau pressure rises (Fig. 18e and f). The Ti₂₅V₃₀Nb₁₀Cr₃₅Mo₄ alloy demonstrated a reversible hydrogen storage capacity of 2.24 wt% at 323 K, establishing a new benchmark for BCC–HEA. Montero et al.¹⁴⁹ reported on a refractory TiNbVZr alloy with trace amounts of Zr, produced using the ball-milling process. This material exhibited good stability and a hydrogen storage capacity of 2 wt%. Serrano et al.¹⁵⁰ engineered a TiVNbCrMn alloy for hydrogen storage using the CALPHAD technique. They fabricated alloys with three different compositions via arc melting: Ti₃₂V₃₂Nb₁₈Cr₉Mn₉, Ti₃₅V₃₅Nb₂₀Cr₅Mn₅, and Ti_27.5V_27.5Nb₂₀Cr_12.5Mn_12.5. These alloys developed FCC hydrides that stored hydrogen capacities of 2.09, 2.47, and 3.38 wt% when exposed to room temperature and 2 MPa H₂ pressure.

Montero et al.¹⁵¹ designed a BCC phase HEA with the composition Ti_0.30V_0.25Zr_0.10Nb_0.25Ta_0.10 using an arc melting approach. This alloy demonstrated a maximum hydrogen absorption capability of 2.5 wt% at 100 °C and 33 bar H₂ pressure, achieving 98% of its maximum capacity within just two minutes after a 2 h activation at 600 °C under vacuum. Edalati et al.¹⁵² synthesized the TiZrCrMnFeNi alloy, which comprises 95 wt% of the C14 Laves phase. This material exhibited a maximum hydrogen absorption capacity of 1.6 wt% at ambient temperature without any activation treatment during the initial hydrogenation, increasing to 1.7 wt% by the third hydrogenation cycle. The incorporation of hydrogen atoms into the lattice resulted in the hydride maintaining the C14 Laves structure, with a unit cell volume 40% greater than that of the metallic phase. Andrade et al.¹⁵³ employed plasma procedures to produce the TiZrNbCrFeNi (AB-type) HEA, which predominantly consisted of two distinct C14 Laves structures. At ambient temperature, the non-activated TiZrNbCrFeNi absorbed 1.5 wt% of hydrogen but released only 0.9 wt%. Upon increasing the temperature to 473 K, the alloy could reversibly capture and fully discharge 1.1 wt% of hydrogen. Floriano et al.¹⁵⁴ investigated the hydrogen storage capabilities of non-equimolar Ti₂₀Zr₂₀Nb₅Fe₄₀Ni₁₅ and equimolar TiZrNbFeNi alloys. With less than 1.4 wt% of a cubic phase, both materials primarily consist of the C14 Laves phase. The initial maximum hydrogen storage capacities were 1.64 wt% for Ti₂₀Zr₂₀Nb₅Fe₄₀Ni₁₅ and 1.38 wt% for TiZrNbFeNi, both measured at room temperature without prior activation. Notably, Ti₂₀Zr₂₀Nb₅Fe₄₀Ni₁₅exhibited better hydrogen storage capacity and reversibility during repeated adsorption and desorption cycles, with capacities of 1.14 wt% compared to 0.72 wt% for TiZrNbFeNi. Table 4 summarizes the electrocatalytic activity in terms of hydrogen storage capacity.

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7.2 HEAs for battery

The increasing demand for large-scale energy storage devices and renewable energy sources is driven by rising energy consumption, leading to environmental degradation and resource depletion.¹⁵⁵ For decades, lithium-ion batteries (LIBs) have been the core technology behind rechargeable energy storage systems. Between 1991 and 2023, commercial battery packs have seen a substantial increase in specific energy, ranging from 80 Wh kg⁻¹ to approximately 360 Wh kg⁻¹.¹⁵⁶ LIBs are widely adopted due to their high energy storage capacity, reliability, and versatility. However, with the ongoing electrification across various economic sectors, there is a growing need for innovative battery materials that can surpass existing limitations and deliver exceptional energy storage capacities beyond the current incremental advancements.^40,157 Graphite has traditionally been the predominant anode material in lithium-ion batteries. Nonetheless, its limited theoretical capacity (372 mA h g⁻¹) and suboptimal safety characteristics during high-current density charging have prompted the search for alternative negative electrode materials.¹⁵⁸ This search has led to a surge in research focused on HEAs. In energy storage applications, HEAs show promise as anodes, electrolytes, cathodes, and in catalytic processes. Their potential stems from elevated capacities derived from multiple metal ions, improved ionic conductivities, and superior structural stability, all of which contribute to extended cycle life.¹⁵⁹ To achieve these objectives, HEAs have driven the development of new materials across various technologies. Design approaches for HEAs have proven highly beneficial for battery technology in electrochemical energy storage, including strategies such as minimizing volume changes, adjusting the energy landscape, reducing undesirable short-range ordering, and lowering the reliance on critical metals.¹⁶⁰

Apart from compositional variety, new studies emphasize the advantages of high configurational entropy in terms of mechanism. In order to inhibit structural phase transitions during cycling and enhance long-term durability, high-entropy materials promote the creation of stable single-phase solid solutions by reducing the Gibbs free energy of mixing. High-entropy materials' atomic-scale disorder causes lattice distortion, which helps to disperse internal stress uniformly and lowers the chance of microcrack development and structural deterioration. Furthermore, ionic and electronic conductivity are improved by the synergistic “cocktail effect” that results from many primary elements, which promotes effective charge transfer and reduces polarization. Additionally, by buffering interfacial reactions and inhibiting dendritic development, high-entropy compositions improve interfacial stability, which is especially advantageous in solid-state systems. Furthermore, the abundance of defects promotes ion transport, which enhances high-rate performance.^161,162

HEAs exhibit a sophisticated lithium storage mechanism due to their numerous electroactive sites. Unlike transition metal oxides, which experience structural degradation after multiple electrode cycles, HEAs demonstrate superior structural stability. The HEAs lattice creates a spatial matrix that preserves structural integrity during the intercalation and de-intercalation of lithium ions, resulting in exceptional cycling stability. To examine the phase transitions of HEAs anodes during electrochemical cycling, in situ XRD is a common method used to analyze phase transformations throughout the cycling process.¹⁶³ Research on advanced anodes for LIBs focuses on materials that operate through three primary electrochemical mechanisms: intercalation–deintercalation, alloying/dealloying, and phase transformation. The intercalation–deintercalation mechanism (Fig. 19) includes transition metal oxides and similar materials with either two-dimensional layered or three-dimensional network structures. In this mechanism, lithium ions can be reversibly inserted and extracted without causing structural degradation. When lithium ions occupy interstitial sites within metals, the metals undergo alloying or form intermetallic compounds with lithium. The reversible capacity observed during lithium cycling is governed by these alloy formation and subsequent dealloying mechanisms.¹⁶⁴ The conversion or phase transformation mechanism applies to transition metal oxides, sulfides, fluorides, and similar compounds, involving reactions with lithium to produce corresponding lithium oxides, fluorides, sulfides, and reduced metals. When transition metal oxides are in the nanoscale range, lithium oxide can decompose into metal and oxygen, enabling effective lithium cycling and achieving substantial reversible capacity at optimal electrochemical potentials.¹⁶⁵

In HEA-based anodes, metallic elements engage in alloying reactions with lithium or sodium ions. This process involves the formation of metal-lithium or metal-sodium intermetallic compounds during the charging phase, followed by their disintegration during the discharging phase. During charging, lithium or sodium cations intercalate into the interstitial sites within the HEAs crystal lattice. The metallic elements in the HEAs then participate in alloying reactions with these cations, resulting in the formation of a metal-ion intermetallic phase or metal-ion alloy. This alloying mechanism enhances electrochemical capacity but requires careful control of volumetric fluctuations. During the discharging phase, the alloy undergoes decomposition, releasing the intercalated cations back into the electrolyte. This dealloying process restores the alloy to its original metallic structure, allowing the electrode material to regain its initial configuration. A typical alloying/dealloying reaction involving lithium in HEAs can be represented as follows:

M + xLi+ + xe− ↔ MLix	(38)

HEAs can experience multiple phase transformations when lithium ions are intercalated, which can enhance the stability of the electrode structure. These phase transitions accommodate the material's volumetric expansion and contraction, thereby improving cyclic stability. Furthermore, certain HEAs contain redox-active elements that participate in reversible redox reactions. These reactions contribute to the overall charge storage capacity and can increase the energy density of the battery. The host matrix is crucial for preventing structural expansion, preserving conductive channels, and promoting effective ion transport in addition to mechanically anchoring HEA particles. According to Yang et al., active materials embedded in a structurally strong, pre-formed carbon matrix greatly increased electrode durability, which in turn improved cycle stability and sustained capacity in potassium-ion battery systems. This technique efficiently reduced mechanical stress, avoided particle disintegration, and maintained electrical connectivity, all of which are equally relevant to HEA-based electrodes, particularly those that are going through alloying/dealloying operations or conversion reactions.¹⁶⁶

Li deposition, particularly in the presence of non-lithiophobic interphases, hinders the growth of Li-free anodes. This challenge contributes to increased energy density and reduced Li dendrite growth in lithium batteries. To achieve uniform Li nucleation and deposition, Wang et al.¹⁶⁷ developed a NiCdCuInZn HEA (20 nm) on carbon fibers (CFs), referred to as HEA/CFs. This alloy provides multiple Li⁺ transport pathways and interaction sites with a range of absorption energies from −3.18 to 2.03 eV. The HEA/C anode demonstrated an impressive 99.6% coulombic efficiency over 2000 cycles at a current density of 5 mA cm⁻² and a capacity of 1 mA h cm⁻². It also maintained strong reversibility for over 7200 h at a current density of 60 mA cm⁻² and a capacity of 60 mA h cm⁻². Notably, lithium metal batteries appear promising, as shown by an anode-free complete cell using HEA/CFs that retained 99.5% efficiency at a 1C charge rate after 160 cycles. In another report, Wei et al.¹⁶⁸ designed a GeSnSbSiFeCuP HEA for Li-ion batteries, achieving an initial charge efficiency (ICE) of up to 91%, with a reasonable voltage plateau around 0.5 V and an impressive capacity exceeding 1448 mA h g⁻¹. This HEA is encased in a carbon matrix, forming a dragon-fruit-like HEA/carbon composite that exhibits outstanding cycle stability over 1600 h and impressive rate performance (787 mA h g⁻¹ with 63% capacity retention at a current density of 2000 mA g⁻¹). This HEA represents a novel approach to developing advanced energy storage materials and could serve as a replacement anode for the next generation of Li-ion batteries. Fig. 20 illustrates the design, along with SEM, TEM, and elemental mapping of the synthesized GeSnSbSiFeCuP HEA, and presents the electrochemical characteristics of the GeSnSbSiFeCuP HEA and its composite with carbon.

Lithium–carbon dioxide (Li–CO₂) batteries offer carbon neutrality and enhanced energy storage, but they require effective cathode catalysts. Using recycled waste batteries, Yi et al.¹⁶⁹ developed hierarchical nanosheets of FeCoNiMnCuAl@C, as a new cathode material. The FeCoNiMnCuAl@C-based battery demonstrated a minimal overpotential of 1.05 V at 100 mA g⁻¹, endurance over 134 cycles, and a discharge capacity of 27664 mA h g⁻¹. Tests and density functional theory (DFT) revealed the adsorption capabilities of various HEAs sites. For lithium–oxygen (LiO₂) batteries, Zhang et al.¹⁷⁰ created a core-satellite electrocatalyst, PtRuFeCoNi@Pt designed as a cathode material. This heterostructure exhibits regions of high and low electron density, enhancing the kinetics of the ORR and OER. Since the HEA NPs and Pt share the FCC structure, Pt can align with the HEA lattice through the homogeneous growth of Pt dendrites (23 nm) surrounding them. After 10000 cycles, the PtRuFeCoNi@Pt core-satellite catalyst showed only a minimal decrease in half-wave electrode potential for ORR of 7 mV, and after 5000 runs, an increase in OER overpotential of 40 mV. With a polarization potential of 0.46 V, it achieved a discharge capacity of 8400 mA h g⁻¹ at 100 mA g⁻¹ and maintained performance over 210 cycles at the same current density (100 mA g⁻¹) with a cut-off capacity of 1000 mA h g⁻¹. Sulfur has an exceptionally high theoretical capacity; however, its slow kinetics and conversion issues limit its use in lithium–sulfur batteries. To enhance sulfur cathode performance, Wang et al.¹⁷¹ engineered a novel HEAs, Fe_0.24Co_0.26Ni_0.10Cu_0.15Mn_0.25 as a catalytic host. This HEAs exhibits strong electrocatalytic activity for converting solid sulfur and lithium polysulfides (LiPSs) when dispersed on nitrogen-doped carbon. Even with a low electrolyte volume of 3 μL mg⁻¹ and significant sulfur loading of 27.0 mg cm⁻², the S/HEA-NC electrode (cathode) achieves a high reversible charge storage capability of 1079.5 mA h g⁻¹ cathode with 89.4% utilization, maintaining a discharge capacity of 868.2 mA h g⁻¹ cathode. This method presents a novel approach to enhancing sulfur utilization in lithium–sulfur batteries.

To enhance the stability of lithium metal in anode-free lithium metal batteries, Wang et al.¹⁷² developed a three-dimensional CFs architecture incorporating CuInNiSnCd NPs. This framework improves lithium kinetics, offers a large surface area, and its three-dimensional structure effectively restricts volume expansion. An anode-free cell achieved a stable coulombic efficiency of approximately 99.5% over 800 h at a current density of 2 mA cm⁻² and a capacity of 1 mA h cm⁻². Furthermore, a symmetrical cell, Li@HEAs/CF, demonstrated minimal hysteresis and stability for over 3000 h at high current densities. After 160 cycles at 1C, the full cell maintained a coulombic efficiency of around 99.2% when paired with an NCM-811 cathode. In another study, Yao et al.¹⁷³ employed a freeze-drying-calcination process to fabricate FeCoNiCrMn NPs, hybridizing them with reduced graphene oxide (rGO) to create HEA/rGO composites. The FeCoNiCrMn HEA, with its FCC structure, exhibits synergistic interactions among various metal cations, resulting in strong chemical confinement and rapid kinetics for converting soluble LiPSs. This design significantly enhances Li⁺ transport and reaction kinetics in conjunction with the conductive rGO matrix. The HEAs/rGO cell demonstrated impressive discharge capabilities of 1344.6 mA h g⁻¹ at 0.1C and 758.5 mA h g⁻¹ at 2C. Wang et al.¹⁷⁴ developed a three-dimensional porous electrode by embedding AgCuInCdZn NPs into CFs. This design enhances lithium reaction kinetics due to its 3D structure, which mitigates anode volume expansion and promotes uniform deposition. The HEA-NPs serve as active sites, lowering the lithium nucleation barrier. The HEA/PCF@Li anode exhibits remarkable stability, achieving over 4500 h of cycling at a current density of 40 mA cm⁻² and a capacity of 40 mA h cm⁻² in a symmetrical setup. When combined with an NCM-811 cathode in a complete prototype cell assembly (HEA/PCF@Li‖NCM-811), the system demonstrates an impressive 99.5% coulombic efficiency over 200 cycles, showcasing its robust performance in lithium metal batteries.

Csik et al.¹⁷⁵ created a composite with high entropy by subjecting the AlCrFeCoNi HEA to high-temperature oxidation in a pure oxygen environment. The dual-phase configuration, identified through XRD, including a metallic phase with a FCC structure and a spinel-structured oxide with high entropy, led to improvements in electrical conductance and electrochemical characteristics. Long-term rate capability studies demonstrated its substantial potential for Li-ion battery applications, which yielded 98% regeneration efficiency and a specific energy release capacity of 543 mA h g⁻¹ at a current density of 500 mA g⁻¹ following 1000 cycle. Edalati et al.¹⁷⁶ studied Ti_xZr_2−xCrMnFeNi alloy with varying Ti/Zr ratios as potential anode materials for Ni–MH batteries. They discovered that changes in the Ti to Zr ratio affected discharge capacity, indicating that the chemical structure of the material influences its electrochemical properties. The alloys demonstrated no capacity degradation, predominantly exhibiting the C14 Laves structure, with a minor presence of the B2 phase. For Ti/(Ti + Zr) ratios of 0.2, 0.4, 0.5, 0.6, and 0.8, the corresponding capacities were approximately 50, 80, 57, 28, and 14 mA h g⁻¹, respectively, after being charged for 6 h at a current density of 100 mA g⁻¹ and discharged for 50 cycles at a current density of 50 mA g⁻¹ to −0.6 V (versus Hg/HgO). In another study, Liu et al.¹⁵⁵ developed a HEA anchored on copper foam (CF) to create a self-sustained catalytic electrode for Ni–H₂ batteries. This catalyst significantly enhances hydrogen evolution and oxidation reactions due to its improved electronic structures, as evidenced by both experimental and theoretical data. The Ni–H₂ battery with the NNM–HEA@CF anode exhibited excellent rate capability and over 1800 h of continuous cycling stability at 15 mA h cm⁻² without any capacity loss. Compared to commercial Pt/C-based Ni–H₂ batteries, a larger-scale Ni–H₂ battery with a capacity of 0.45 Ah achieved a reduced cost of about $107.8 per kWh and a high energy storage density of 109.3 Wh kg⁻¹. Fig. 21a–f shows the electrochemical properties of the Ni–H₂ battery equipped with the NNM–HEA@CF catalytic electrode. This work paves the way for developing high-performance, low-cost bifunctional catalysts made from non-noble metals for extensive hydrogen storage applications.

Dangwal et al.¹⁷⁷ investigated two AB-type HEAs, TiV₂ZrCrMnFeNi and TiV_1.5Zr_1.5CrMnFeNi as potential negative electrodes for Ni–MH batteries. Vanadium was incorporated into both alloys due to its beneficial properties for hydrogen storage. The TiV₂ZrCrMnFeNi alloy features both C14 and BCC phases, while TiV_1.5Zr_1.5CrMnFeNi is a single-phase C14 Laves alloy. The dual-phase TiV₂ZrCrMnFeNi, enriched with vanadium and possessing interphase boundaries that enhance hydrogen nucleation and diffusion, demonstrated an energy release capacity of 150 mA h g⁻¹. In contrast, the single-phase TiV_1.5Zr_1.5CrMnFeNi showed potential as an efficient anode material for Ni–MH batteries, albeit with a lower capacity of 60 mA h g⁻¹ and delayed activation. ZABs require effective bifunctional catalysts for the ORR and OER. He et al.¹¹⁸ introduced the CrMnFeCoNi HEA as a bifunctional catalyst, synthesized using a low-temperature solution approach. Thanks to twinned defects, lattice distortions, and electrical synergy, this HEA exhibited superior OER performance, achieving a 265 mV overpotential at 10 mA cm⁻² and a Tafel slope of 37.9 mV dec⁻¹, surpassing RuO₂. It also performed exceptionally well in ORR, with a half-wave potential of 0.78 V, an onset voltage of 0.88 V, and a small potential gap of 0.734 V, closely matching the performance of Pt/C. With a CrMnFeCoNi air cathode and a zinc metal sheet as the positive electrode, the ZABs achieved a specific capacity of 836 mA h g⁻¹, an open-circuit voltage of 1.489 V, and a maximum power density of 116.5 mW cm⁻². This battery demonstrated remarkable stability for over 10 days (720 cycles at a current density of 8 mA cm⁻²) and 16.6 days (1200 cycles at 5 mA cm⁻²). Cao et al.¹⁷⁸ reported a low-cost method for coating Fe₁₂Ni₂₃Cr₁₀Co_55−xMn_x multi-element entropy alloy nanoscale particles onto CNT structures. The Fe₁₂Ni₂₃Cr₁₀Co₃₀Mn₂₅/CNT catalyst outperformed most previously reported catalysts in 0.1 M KOH, exhibiting a low bifunctional oxygen overpotential (ΔE) of 0.7 V. This catalyst enabled the air electrode to demonstrate long-term cycle stability for over 256 h, achieving a remarkable specific capacity of 760 mA h g⁻¹ and an energy density of 865.5 Wh kg⁻¹ in a zinc–air battery. Table 5 summarize the electrocatalytic performance of HESs in battery performance.

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7.3 HEAs for supercapacitors

Energy storage is crucial for addressing global economic needs. The rapid expansion of the global economy, coupled with the accelerated depletion of fossil fuels, has led to two significant concerns: the exhaustion of existing fossil fuel reserves and a dramatic increase in greenhouse gas emissions, contributing to widespread environmental pollution. Therefore, it is essential to advance and promote renewable energy sources that are environmentally sustainable, along with their associated technologies, on a global scale.¹⁷⁹ In this context, supercapacitors offer a compelling alternative to traditional batteries and energy storage technologies due to their superior power and energy density, extended cycle stability, strong chemical and thermal stability, ease of maintenance, and environmental friendliness. Their diverse frameworks and high specific surface areas make them particularly suitable for energy storage applications.¹⁸⁰ Additionally, supercapacitors enable significantly faster charging of electric vehicles compared to lithium-ion batteries.¹⁸¹ HEAs, characterized by a unique combination of multiple metals in nearly equal proportions, are gaining popularity for supercapacitor applications. These materials enhance structural stability, conductivity, and electrochemical performance, making them ideal candidates for improving supercapacitor efficiency and effectiveness.¹⁸²

Supercapacitors can be classified into three categories based on their charge storage mechanisms: Electric Double Layer Capacitors (EDLCs), pseudocapacitors, and hybrid supercapacitors (Fig. 22a–c). EDLCs store electric charge electrostatically through the reversible absorption and desorption of positive and negative ions at the electrode–electrolyte interface, without direct charge transfer reactions. When electric polarization occurs at this interface, an electric double layer is formed. Charges at the double-layer interface are distributed and separated by just a few angstroms and remain stationary. EDLCs demonstrate greater cyclic stability compared to rechargeable lithium-ion batteries.¹⁸³ Common materials used in EDLCs include carbon-based substances such as graphene, activated carbon, and MWCNTs. These materials provide high power density and extended cycle life, although they exhibit limited energy density.¹⁸⁴ In contrast, pseudocapacitors store energy in the electrode materials through a faradaic process, involving rapid, reversible redox reactions occurring at or near the surface of the electrodes. This mechanism places pseudocapacitors between traditional batteries and supercapacitors, hence their name. The charge storage mechanism in pseudocapacitors relies on faradaic processes, such as oxidation–reduction reactions, where charge is reversibly transferred between the electrolyte and the electrode. When a potential is applied, reduction and oxidation reactions occur in the electrode material, facilitating charge transfer across the double layer and generating a faradaic current within the supercapacitor cell. As a result, pseudocapacitors can achieve higher specific capacitance and energy densities than EDLCs, although they typically have a reduced cycle life. Electrode materials commonly used in pseudocapacitors include metal oxides, metal hydroxides, metal chalcogenides, and conducting polymers.^40,185,186

Hybrid supercapacitors integrate EDLC and pseudocapacitive mechanisms for charge storage. This combination addresses the limitations associated with individual electrode materials used in EDLCs and pseudocapacitors. Hybrid supercapacitors achieve the energy density of a battery-like electrode and the power density of a capacitor-like electrode within a single device, utilizing both faradaic and non-faradic processes. In these devices, pseudocapacitive materials function as the anode, while EDLC materials serve as the cathode. The cathode stores charge through surface adsorption and desorption, representing a non-faradic process, whereas the anode stores charge via intercalation, a faradaic process.^187,188 Hybrid supercapacitors are cost-effective and offer enhanced performance characteristics. They operate across a broad temperature range and exhibit high energy density, power density, mechanical strength, cyclic stability, and low equivalent series resistance (ESR).¹⁸⁹ With prolonged cycling, however, the capacitance of the supercapacitor tends to decrease while the ESR increases.¹⁹⁰ Current research and advancements in the field of electrochemical supercapacitors aim to enhance energy density while maintaining high power density. Additional objectives include developing novel electrode materials with high surface areas to improve specific capacitance, utilizing natural resources for fabricating cost-effective electrodes, and increasing cell voltage through existing electrolytes or new electrolyte formulations.¹⁹¹

The primary challenges lie in engineering supercapacitor electrode materials that possess outstanding attributes, including high electrical conductivity, elevated energy density, high power density, and resilience to harsh environmental conditions.^192,193 HEAs, characterized by a unique blend of multiple metals in nearly equal ratios, are gaining popularity for supercapacitor applications due to their ability to enhance structural stability, conductivity, and electrochemical performance.¹⁸² The distinctive effects associated with HEAs, such as cocktail effects, lattice distortion, and high entropy—boost electrochemical catalytic activity and charge storage properties. These enhancements are particularly pronounced in disordered multimetallic complexes.⁴⁰ HEAs typically consist of near-equiatomic proportions of five primary metallic elements, which can form one or more stable solid solution phases. The presence of FCC or BCC solid solution phases in HEAs results from the increased mixing entropy compared to conventional alloys.³⁸ This can lead to improved electrochemical stability and electrical conductivity. Many HEAs demonstrate high electrical conductivity due to their diverse metal components, which enhance conduction pathways and facilitate efficient charge transfer. This reduces resistance and increases power density. The high entropy in HEAs contributes to their structural stability, corrosion resistance, and durability, improving the lifespan and reliability of supercapacitors, particularly under severe operating conditions.¹⁹⁴ In supercapacitors, HEAs primarily exhibit both EDLC and pseudocapacitive mechanisms. Their substantial surface area and excellent electrical conductivity make them well-suited for EDLC applications, while their ability to incorporate redox-active elements supports pseudocapacitive mechanisms. The integration of these mechanisms can lead to improved performance attributes, enhancing the energy and power densities of supercapacitors.

There are various techniques to analyze the electrochemical properties of electrode materials. One commonly used method is CV, which assesses the electrochemical behavior of the electrode material within an optimized voltage range. The specific capacitance can be calculated from the CV curves using the following equation:

	(39)

In the following equation, C_s denotes the specific capacitance, A represents the area under the CV curve, m indicates the mass of the electrode material deposited on the current collector (measured in mg), K is the scan rate (expressed in mV s⁻¹), and V signifies the voltage range spanned by the CV curves.

Additionally, the GCD technique is typically conducted at different current densities within fixed voltage ranges to evaluate the charge–discharge performance and cyclic stability of the electrode materials and assembled SC cells. The specific capacitance can be determined from the charge–discharge tests using the following equation:

	(40)

where m represents the mass of the electrode material deposited on the current collectors in mg, Δt denotes the discharge time in seconds, and ΔV signifies the potential window.

The performance of supercapacitors is primarily characterized by two critical factors: energy density and power density. Energy density quantifies the amount of energy stored by a device and is calculated using the following formula:

	(41)

where ΔV represents the voltage window, and C denotes the specific capacitance at a given current density.

Power density refers to the rate at which the device can release its stored energy. The following formula is employed to calculate the power density of the fabricated device:

	(42)

ED signifies the device's energy density at a particular current density, and Δt is the duration of the discharging time.

Mohanty et al.¹⁹⁵ synthesized bulk CoCrNiFeMn using a straightforward induction melting technique. These samples were then ball-milled into NPs to increase their surface area for supercapacitor applications. In a three-electrode setup, the CoCrNiFeMn demonstrated a maximum specific capacitance of 386.66 F g⁻¹ maximum specific capacitance at a scan rate of 5 mV s⁻¹ with 3 M KOH aqueous electrolyte (Fig. 23a–f). The role of the metals' d-orbital electrons in electrochemical interactions was examined through DFT computations. A liquid-state asymmetric energy storage device (ASC) was assembled using activated carbon (AC) for the anode and CoCrNiFeMn as the cathode. This device, powered by a 1.5 V LED, showcased an energy density of 21 Wh kg⁻¹, a power density of 307 W kg⁻¹ and capacitance value of 104.85 F g⁻¹ at a scan rate of 5 mV s⁻¹, maintaining approximately 87% cyclic stability over 5000 cycles. In another report, Mohanty et al.¹² developed a FeCoNiCuMn to serve as a cathode component, achieving a capacitance of 388 F g⁻¹ at a scan rate of 5 mV s⁻¹. For the positive electrode, they produced eco-friendly graphene derived from rice husks via pyrolysis, which exhibited the highest specific capacitance of 560 F g⁻¹ under the same measurement conditions. They fabricated an asymmetric liquid-state device using these materials as electrodes with a 3 M KOH electrolyte, operated up to a 1.7 V potential window. This ASC device achieved a charge capacity of 83.22 F g⁻¹ charge capacity at a current density of 2 A g⁻¹, along with an impressive power density of 1.7 kW kg⁻¹ and energy density of 33.4 Wh kg⁻¹, exhibiting 94% cyclic stability after 5000 cycles. Kong et al.¹⁹⁶ synthesized a nanoporous AlCoCrFeNi through selective dissolution and employed it as a binder-free supercapacitor electrode. This electrode demonstrated remarkable cyclic stability of 97% over 3000 cycles and exhibited a high volumetric charge storage capacity of 700 F cm⁻³ at a current density of 1 mA cm⁻² with 6 M KOH aqueous electrolyte.

Abid et al.¹⁹⁷ developed CoCrFeNiMn with varying levels of porosity, demonstrating exceptional electrochemical performance with a 2 M KOH aqueous electrolyte due to the synergistic effects of the components. They achieved an areal charge storage capacity of 1.56 F cm⁻² at a current density of 2 mA cm⁻², along with a remarkable capacitance durability of 114.6% after 5000 cycles at 8 mA cm⁻², indicating excellent stability. Furthermore, Mohanty et al.¹⁹⁸ synthesized a FeCoNiCuZn via arc melting and reduced its dimensions through ball milling. A positive electrode was fabricated using the ball-milled FeCoNiCuZn for supercapacitor applications. This HEA achieved a high specific capacitance of 325.17 F g⁻¹ at a current density of 1 A g⁻¹ in a single-electrode setup with a 3 M KOH aqueous electrolyte. The asymmetric assembly displayed an energy density of 23.82 Wh kg⁻¹, along with exceptional specific capacity retention of 70.54% after 10000 cycles. Xu et al.¹⁹⁹ utilized a carbothermal shock synthesis approach to generate quinary FeNiCoMnMg and FeNiCoMnCu NPs on ultra-aligned carbon nanofibers (A-CNFs) produced by electrospinning, which enhanced NPs formation and improved electron transport efficiency. They proposed an asymmetric design using FeNiCoMnMg NPs as the positive electrode and aligned CNFs as the negative electrode, denoted as FeNiCoMnMg NPs/A-CNFs. With a precursor concentration of 5 mM, the asymmetric device exhibited a significant specific capacitance of 203 F g⁻¹ and a specific energy storage capacity of 21.7 Wh kg⁻¹ with a 6 M KOH aqueous electrolyte. Additionally, this asymmetric design demonstrated impressive retention of 89.2% of its specific capacitance after 2000 cycles. Verma et al.²⁰⁰ fabricated a multi-structured NiCuFeCoMn as a electrode material. At a current density of 3 A g⁻¹, the NiCuFeCoMn exhibited an impressive gravimetric charge storage capacitance of 1241 F g⁻¹ demonstrating outstanding cyclic stability by maintaining 84.7% of its capacitance after 5000 cycles. They further assembled a hybrid electrode material using the nanostructured multi-element alloy as the anode and graphene as the cathode. This device showcased a substantial energy storage capacity of 61 Wh kg⁻¹ and a specific power release capacity of 1017 W kg⁻¹ at current density of 2 A g⁻¹. The hybrid device also maintained 88.5% of its capacity after 5000 cycles and successfully powered an LED. Siddique et al.²⁰¹ successfully generated and characterized NbTiVZrY foams with three-dimensional interconnected porosity. The developed foam demonstrated an areal specific charge storage capacity of 85 mF cm⁻² at a scan rate of 2 mV s⁻¹ and exhibited remarkable capacity retention of 104% after 5000 cycles using a 1 M Na₂SO₄. Shen et al.²⁰² designed HEA NPs with a thin metal oxide layer across the surface of hyper-crosslinked polymer-based carbon (HCPC). This was achieved by adsorbing five metal cations, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, and Sn²⁺ into the hyper-crosslinked polymers, followed by in situ reduction and carbonization. In a 1.0 M KOH electrolyte, the final composites demonstrated an exceptional gravimetric charge storage capacity of 495.4 F g⁻¹ at a current density of 0.5 A g⁻¹ along with outstanding cycling stability, retaining 94.7% of the original capacitance after 15000 charge–discharge cycles at a current density of 10 A g⁻¹.

The electrochemical performance and durability of HEAs-based electrodes are significantly influenced by the interface between the electrode and electrolyte, particularly in systems that involve redox reactions or phase transformations. In potassium-ion batteries, Wen et al. showed that altering the electrolyte formulation, especially by using a weakly solvating solvent, makes it easier to create a stable, ion-conductive solid electrolyte interphase (SEI), which greatly enhances rate capability and thermal stability. Since HEAs systems frequently have chemically varied surfaces due to their multi-component composition, this observation is especially relevant to them. These surfaces may interact with the electrolyte in unpredictable ways, resulting in irregular SEI production and potential interfacial instability. Therefore, to fully use the potential of HEAs in energy storage technologies, electrolyte solutions that promote uniform SEI production, improve ionic conductivity, and guarantee interfacial chemical stability must be designed.²⁰³ Table 6 summarizes the electrocatalytic performance of HEAs in supercapacitors applications.

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8. Summary and future perspective

The field of HEAs has experienced significant growth in scholarly activity, highlighting the increasing interest and potential of these materials.¹⁹⁴ Despite this progress, there remains a notable gap in comprehensive studies focusing on the mechanical properties of HEAs, particularly regarding the regulation and analysis of their microstructures.²⁰⁴ This gap arises from the wide compositional range of HEAs, which presents vast opportunities for novel alloy configurations.²⁰⁵ Researchers are motivated to explore this field, yet only a small fraction of these potential compositions has been thoroughly examined. HEAs are recognized for their broad range of applications in energy conversion and storage, providing a framework for designing new materials with tailored properties.²⁰⁶ The inherent randomness of HEAs allows for manipulation in both compositional and structural disorder, facilitating the exploration of mechanical and functional properties across various applications. Defined as alloys composed of five or more chemical elements, each between 5% and 35%, HEAs aim to achieve a solid solution enhanced by ideal entropy configurations.²⁰⁷ The formation of multiphase microstructures is commonly observed in these alloys, occurring more frequently than in single-phase solid solutions.

HEAs can be classified into subfamilies based on their component elements.²⁰⁸ A well-studied subfamily includes 3d transition metal HEAs, which often combine elements such as Fe, Ni, Co, Al, Cr, Cu, V, and Ti, typically forming BCC solid solutions. However, challenges persist regarding the distribution of atomic species—whether they are randomly dispersed or exhibit short-range order. The connection between short-range order and mechanical properties remains largely unexplored, aside from basic understandings derived from traditional alloy ordering. The connection between short-range order and mechanical properties remains largely unexplored, aside from basic understandings derived from traditional alloy ordering, which contributes to thermal strengthening.²⁰⁹ HEA catalysts have demonstrated exceptional performance, making them suitable as both cathodes and anodes for various electrocatalytic applications, including HER, OER, and ORR.²¹⁰ HEAs have also been investigated for applications such as methane combustion, ammonia decomposition, CO oxidation, CO₂ reduction, and dye degradation. Noble-metal-based HEAs have shown higher activity and stability compared to commercially available platinum group catalysts, attributed to the synergistic effects of various elements that enhance atomic coordination and active surface area.²¹¹

Currently, research on HEAs in electrocatalysis is expanding, with a focus on improving catalytic activity for HER, OER, and ORR. The publication rate in this area is rapidly increasing, and the growing use of HEAs in electrochemical applications is crucial for advancing technologies like metal–air batteries and fuel cells. HEAs with a single-phase BCC structure have shown promise for hydrogen storage due to their ability to accommodate lattice distortion.²¹² Compared to FCC or hexagonal close-packed structures, BCC HEAs exhibit significantly high hydrogen adsorption capacities. Enhancing hydrogen storage performance requires the design of stable single-phase BCC structures. Recent advancements using semi-empirical parameters have primarily focused on 3d transition metal-based HEAs,²¹³ with further exploration needed regarding the effects of individual constituent elements. This review aims to inspire researchers to develop HEA-based materials with high cyclability and low desorption temperatures. HEAs offer unique advantages for hydrogen storage, including high surface area, mechanical strength, and excellent solubility, making them suitable for various applications. Their multi-element composition facilitates a large surface area, enhancing electrochemical processes and improving energy storage efficiency. Additionally, HEAs exhibit remarkable electrical conductivity, contributing to effective charge transfer and minimal internal resistance, both of which are critical for the longevity of supercapacitors.

In battery applications, HEAs provide compelling benefits due to their high energy density customization, allowing substantial energy storage in compact forms—ideal for electric vehicles and portable devices. Their strong mechanical properties and resistance to phase changes contribute to cyclic stability, enabling efficient operation over numerous charge–discharge cycles. Moreover, HEAs' ability to achieve lower hydrogen desorption temperatures enhances battery kinetics, facilitating quicker charging and more effective energy release.²¹⁴ In conclusion, research into HEAs electrocatalysts is still in its early stages. As investigations progress, HEAs are expected to play an increasingly important role in electrocatalysis, potentially leading to significant breakthroughs. To address these challenges, more advanced characterization techniques must be introduced, such as a new transscale imaging method for AFM and scanning microlens associated microscopy,²¹⁵ a terahertz scanning tunnelling microscope system,²¹⁶ small-molecule serial femtosecond X-ray crystallography,²¹⁷ in situ liquid transmission electron microscopy,²¹⁸ and so on. More information, such as surface atomic structure evolution, lattice strain, atomic diffusion behavior, and each atom's electronic structure, can be obtained using more advanced techniques, providing a solid foundation for theoretical simulation/calculation and understanding of the formation mechanism. HEAs exceptional thermal stability allows them to operate reliably at elevated temperatures, while their inherent corrosion resistance enhances the longevity and reliability of energy storage devices. The versatility of HEAs to function with various electrolyte chemistries in diverse environments broadens their applicability in energy and environmental-related fields.²¹⁹

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The author would like to thank the National Science and Technology Council (NSTC) of Taiwan for their financial support under grants NSTC 114-2221-E-027-120.

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Footnote

† These authors contributed equally.

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