Challenges and opportunities in 2D high‐entropy alloy electrocatalysts for sustainable energy conversion

Two‐dimensional (2D) high‐entropy alloys (HEAs) have emerged as promising electrocatalysts due to the benefits of polymetallic coordination and robust electrical conductivity. However, the multiple elements in 2D HEAs pose challenges in achieving a uniform composition and maintaining a 2D limit morphology, complicating their structural characterization. Furthermore, even minor adjustments to the composition can significantly alter the properties of 2D HEAs, underscoring the need for a deeper understanding of their structure–property relationships to advance synthesis and application. Therefore, this review critically examines the intrinsic factors influencing synthesis methods and the practical applications of 2D HEAs in electrocatalysis for sustainable energy conversion. The urgency is emphasized for developing new synthesis techniques, enhancing advanced characterization methods, and gaining profound insights into the functional mechanisms of 2D HEAs.

5][6][7] Notably, a new class of nanomaterials, 2D high-entropy alloys (HEAs), has recently gained significant attention in the field of electrocatalysis.These innovative materials combine 2D materials and polymetallic elements, further broadening their potential applications. 8,91][12][13][14][15] Traditionally, HEAs were narrowly defined as alloys comprising five or more elements in equal molar ratios. 16,179][20] It is worth noting that in previous concepts, incorporating more metals often led to embrittlement in the alloy.In contrast, the new HEAs stands as a distinct material that avoids embrittlement issues. 21s a result, research interest in HEAs has surged due to their unique multielement nature and multiple magical properties.19 Due to the unique composition, arrangement, and interaction of elements, HEAs carry some magical characteristics that are significantly different from those of conventional alloys, namely high-entropy effect in thermodynamics, lattice distortion effect in structure, hysteresis diffusion effect in dynamics, and cocktail effect in properties.This unique property endows HEAs multiple excellent properties, such as favorable low-temperature mechanical properties, corrosion resistance, high-temperature resistance, excellent soft magnetic properties, and so forth, and show great prospects for development in the field of industrial applications.As a novel nanomaterial developed based on chemical disorder, 2D HEAs show higher stability in performance by mixing multiple components, and equip with superconductivity and magnetism, which is expected to be applied in the field of electrocatalysis.
With the continuous advancement of high-entropy materials, the concept of high-entropy oxides was introduced in 2015, 22,23 and later extended to non-metallic compounds, 24 showcasing exceptional performance in energy catalysis.The intriguing possibility of blending five or more metal cations with an equal atomic ratio in monophasic oxide systems was determined. 25Later, in 2017, Niu et al. synthesized nanocrystals of CoCrCuNiAl HEAs with an average grain size of 14 nm, pioneering the idea of high entropy at the nanoscale. 26High-entropy phenomenon makes it possible for sulfides, 27 nitrides, 28 and carbide 29 to accommodate several metal atoms in solid solution.As a result, the same metal composition in the different compounds give rise to diverse intrinsic properties.Currently, the exploration of new applications for HEAs as functional materials has garnered attention from both theoretical and experimental points of perspectives, especially in the realm of 2D HEA catalysts.1][32] The combination of "2D" and "high entropy" has led to the emergence of various types of 2D HEAs.For instance, Nemani et al. utilized four transition metals to synthesize multi-principal-element high-entropy M 4 C 3 T x MXenes, expanding the compositional diversity within the MXene family to fine-tune their properties. 33This innovation was followed by the introduction of high-entropy layered double hydroxides (HE-LDHs) and high-entropy transition metal disulfides (HE-TMDs).Among these, Gu et al. were the first to report a series of ultrathin HE-LDHs with five nearly equimolar components, achieved through a hydrothermal process assisted by an environmentally friendly plasma strategy. 34Silva et al. proposed a systematic analysis for the phase behavior of substituting 2D alloys at both the metal and chalcogenide sites within the TMD group, to design HE-TMDs. 27These 2D HEAs are rapidly gaining recognition and application in various fields. 35,36Figure 1 summarizes the development and classification of 2D HEA nanomaterials.
1][42] The high-entropy effect leverages the enhanced compatibility between elements due to high mixing entropy, facilitating the formation of solid solutions and providing exceptional stability.The electron interaction among the multiple elements uniformly dispersed within the material supplies the necessary binding energy for reaction intermediates, resulting in reduced overpotential in 2D HEAs.The random occupation of various atoms and local fluctuations in chemical composition significantly impact nucleation, dislocation nucleation, expansion, and layering processes, leading to lattice distortion in HEAs.This lattice distortion can influence the material's resistivity, subsequently affecting its electrocatalytic performance.The increase in lattice potential energy results in slower element diffusion and higher activation energy, leading to a delayed diffusion effect in HEAs.This slower diffusion effect can mitigate the impact of diffusion within the catalyst itself to some extent, thus accelerating the reaction process.The cocktail effect is akin to the highentropy effect, as different elements bring their unique properties, and when combined, they synergistically harness the advantages of each element.Within HEAs, the elements mutually influence one another, achieving multifunctional catalysis.4][45] These 2D HEAs hold significant potential for application in crucial catalytic reactions, such as hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and carbon dioxide reduction reaction (CO 2 RR).However, the preparation and characterization of 2D HEAs face severe challenges.The synthesis of HEA materials with 2D characteristics demands precise control.Conducting fundamental research to understand the dynamic evolution of 2D HEAs under different catalytic conditions necessitates accurate structural characterization.Hence, 2D HEAs offer exciting prospects for catalysis but require sophisticated approaches to overcome the challenges in their preparation and characterization.Further research and advancements in this area are crucial for harnessing the full potential of 2D HEAs in diverse catalytic applications.
Hence, the review provides a comprehensive overview of the current research progress in the field of 2D HEAs, emphasizing their crucial role in electrocatalysis applications.The synthesis methods of 2D HEAs are thoroughly analyzed with a particular focus on the influencing factors of their synthesis.Furthermore, the potential factors affecting the electrocatalytic performance of 2D HEA nanomaterials are briefly introduced, highlighting their significance in enhancing catalytic activity and efficiency.

The entropy
HEAs are generally characterized with a mixing entropy greater than 1.5 R, where R is the gas constant.Mixing entropy quantifies the degree of randomness or chaos in a material's atomic arrangement.Entropy is considered to be a thermodynamic variable that reflects the degree of disorder of a material, which depends on the structure, magnetic moments, atomic and electronic vibrations; among them, the former usually dominates the determination of entropy. 15,41,46The greater the degree of chaos, the higher the entropy.When the mixing entropy is between 1.2 R and 1.5 R, it is commonly classified as a medium entropy, while mixing entropy less than 1.2 R falls into the low entropy category. 14,39,47,48The mixing entropy can be calculated using Equation (1): where R is the gas constant (8.314J [mol K] −1 ), x i is the mole fraction of each element in the alloy, and ln is the natural logarithm function.By calculating the mixing entropy based on the composition of elements in an alloy, one can categorize it as low, medium, or high entropy.From the formula, the mixing entropy is controlled by the proportion of elements in the alloy.As a direct influencing factor, ensuring a uniform distribution of each metal element and preventing element segregation and phase separation are critical factors in achieving desirable HEAs.Liu et al. have investigated the synthesis of HEAs with different morphologies by controlling the amount and rate of precursor addition. 48Figure 2A illustrates that the mixing entropy rapidly increases and then decreases when all precursors are added simultaneously.In contrast, Figure 2B demonstrates successful synthesis of HEAs when the precursors are added in a controlled drip manner.Therefore, researchers can optimize the mixing entropy and achieve a uniform distribution of elements by carefully manipulating the addition process, contributing to the formation of high-quality 2D HEAs with enhanced electrocatalytic properties.

The enthalpy
The other critical influencing factor of HEAs is enthalpy, including 2D HEA nanomaterials.The value of enthalpy plays a significant role in determining the stability and properties of these 2D HEAs. 51,52orral and Chen proposed a ternary canonical solution model. 52The model demonstrates that the addition of immiscible components to the binary system can reduce the transition temperature of isoatomic alloys, although the addition increases the enthalpy.In this work, this model is later extended to high-entropy fiveand six-component isoatomic alloys, and a definition of HEAs is confirmed.The relationship between HEAs and enthalpy value is therefore established.The enthalpy can be increased by adding different metallic elements of the alloy, and the choice of elements can have varying effects on the enthalpy due to their distinct properties.You et al. conducted statistical analysis to understand the relationship between entropy and enthalpy in alloys, and provided insights into the relationship between entropy, enthalpy, temperature, and alloy composition. 49It directly reflects that these factors need to be carefully considered in the process of controllable production of HEAs.From the above Equation (1) and Figure 2C, the elemental components in the alloy are crucial influence to the formation of the HEAs.
Cavin et al. proposed a temperature parameter T 0 to evaluate the stability of HE transitions metal dichalcogenides (TMDs). 36T 0 refers to the temperature of an equimolar HEA.Beyond this temperature, where the Gibbs free energy (ΔG) of a solid solution is lower than that of all other possible mixtures, it is only necessary to calculate the enthalpy of mixing of the equimolar binary alloy doped with TMDs. Figure 2D plots the enthalpy of mixing for six possible binary alloys at multiple molar concentrations.This concept of entropy stability can also be observed in the case of 2D HE layers, which can be stabilized under severe operating conditions such as corrosion, high temperature, and high electrochemical potential.As a result, precise control over the composition of the alloy and its enthalpy value is crucial in tailoring the properties of 2D HEA nanomaterials for specific applications.

2.3
The component type and ratio The morphology of HEAs can be finely tuned by the proportion of elements added, leading to improved electrocatalytic performance.In the catalytic reactions, the synergistic effect of elements plays a crucial role in the performance optimization of HEAs.However, the amorphous nature of HEA allows catalyst design to focus on the selection of elements rather than the specific structure. 53,54heoretical calculations can aid in selecting suitable elements, streamlining the design process, and reducing the reliance on trial-and-error methods.For example, Wang et al. investigated the effect of different element ratios on the catalytic performance of 2D HEAs by adjusting the cobalt content. 50Figure 2E examines the effect of elemental proportions on lattice strain, and the slope of all samples is positive, indicating the presence of tensile strain in all HE samples.To gain a better understanding of the crystallographic properties of the HE samples, Figure 2F analyzes the relationship between P-P bond length and Co content (x).The tunable P-P bond length suggests that a high-entropy strategy can modulate the local electronic structure of the P sites in metal phosphorus trichalcogenides (MPCh 3 ), potentially leading to a redistribution of electron density in each P center, and making them promising electrocatalytic active sites for HER.This study demonstrates that altering the element ratio can modify the electronic structure and affect the catalytic activity (Figure 2F).Thereupon, the desired catalytic properties can be achieved by manipulating the composition of the HEA nanomaterials, unlocking their potential for a wide range of electrocatalytic applications.

Other factors
2D HEA nanomaterials exhibit unique properties resulting from their combination of 2D material characteristics and high-entropy effects.Their properties are influenced by factors such as unique temperature, 55 thickness, 56 and ratio of element of 2D materials.Additionally, the atomic size differences in the alloy can have a significant impact on their behavior.When different atoms are incorporated into the lattice, lattice distortion often occurs due to the varying atomic radii of the elements. 57,58Guo and Liu extensively discussed the effect of atomic size differences on the structural stability of HEA. 59Their findings revealed that the atomic size difference is too large, the excess entropy will increase, and the atomic size difference is an important parameter in determining the formation of solid solution or amorphous state.Wang et al. developed a stepwise alloying strategy using Pt-rich nuclei formed during the first phase reaction as the second thermal diffu-sion seed.This approach successfully led to the synthesis of HEAs containing a number of strongly repulsive elements, such as Bi-W. 60The intricate interplay between the 2D material properties, high-entropy effects, and atomic size differences in these nanomaterials presents exciting opportunities for tailored design and control over their properties.

SYNTHESIS OF 2D HEA NANOMATERIALS
When creating nanoscale materials, nanotechnologists typically employ two main strategies: bottom-up and topdown.Each approach offers distinct advantages and is used based on the specific requirements of the desired nanomaterial.The bottom-up approach involves building nanomaterials from the ground up, starting with basic building blocks like atoms or molecules.This method relies on self-assembly processes, chemical reactions, or physical deposition techniques to create nanostructures.The process typically allows for precise control over the composition and structure of the nanomaterials, enabling the design of highly tailored and intricate nanostructures.Examples of bottom-up approaches include molecular self-assembly, chemical vapor deposition (CVD), and sol-gel synthesis.While the top-down approach, on the other hand, involves reducing bulk materials to nanoscale dimensions.Building 2D nanomaterials can be accomplished through the use of lithographic tools (i.e., physical top-down) or through a chemics-based process (i.e., chemical top-down). 61

Top-down method
3][64] Mechanical stripping and liquid phase stripping are common top-down methods used to achieve nanosheets of small size.In mechanical stripping, mechanical forces such as shear or ultrasonic treatment are applied to break down the bulk HEA nanosheets into smaller fragments.This process generates nanosheets with increased specific surface area, leading to enhanced catalytic performance.On the other hand, liquid phase stripping involves treating the bulk HEA nanosheets with chemical agents that selectively dissolve or exfoliate the material into nanosheets.This chemical approach can produce nanosheets with controlled size and thickness, further optimizing the catalytic properties.Therefore, a top-down approach is expected to achieve nanosheets of small size, mainly including mechanical stripping and liquid phase stripping.

Mechanical stripping
Since the first isolation of graphene in 2004, the interest in 2D materials has been steadily growing.Mechanical stripping has emerged as an important method for preparing 2D materials. 31This process involves using various techniques such as medium grinding stripping, ultrasonic stripping, water jet stripping, homogenizer stripping, jet mill stripping and other methods to strip HEAs into 2D nanomaterials. 65or example, Ying et al. successfully obtained a series of single-crystal HE van der Waals (HEX) materials. 66These are a series of new materials with various metals and internal layered structures, such as (Ti,V,Zr,Nb,Hf)Te 2 , (Mn,Fe,Co,Ni)PS 3 , (Ti,V,Cr,Nb,Ta)S 2 , (Ti,V,Cr,Nb,Ta)Se 2 and other disulfides, halides, and phosphorus trisulfides.As a prominent feature of van der Waals materials, HEX single crystals can be easily stripped into several layers by conventional scotch tape (right panel of Figure 3A).The rainbow coloration is the light interference caused by the different layers of the film on the SiO 2 /Si substrate.However, this method has a low yield and is challenging to achieve a single layer, making it unsuitable for obtaining thinner 2D HE nanomaterials.Though mechanical stripping is a valuable technique for preparing 2D materials, achieving monolayer or few-layer HE nanomaterials through this method remains challenging.Researchers continue to explore alternative approaches to optimize the synthesis of thinner and more uniform 2D HEAs, aiming to harness their unique properties for a wide range of applications in catalysis, electronics, and beyond.

Liquid phase stripping
Liquid phase stripping is usually suitable for large-scale preparation of 2D HEAs.liquid phase stripping remains a valuable and widely used technique for obtaining 2D HEAs due to the scalability and ease of application.This method allows to produce a large quantity of 2D HEA nanomaterials, making it suitable for various industrial and research applications.However, in the process of liquid phase stripping, the external force will lead to the fragmentation of 2D HE materials, resulting in a smaller size, less than 5 μm.
Liquid phase stripping is also a common method for the preparation of 2D HEAs. 67,68or example, Wang et al. produced Co 0.6 (VMnNiZn) 0.4 PS 3 nanosheet samples (NSs) by solid-state reaction combined with ultrasonic stripping. 50umps of high-entropy MPCh 3 (50 mg) were added to the water-ethanol solution (100 mL) and ground for 120 min to ensure that the large particles were sufficiently ground into smaller particles.The dispersion was treated for 2 h in an ultrasonic cleaning machine at room temperature.The obtained 2D high-entropy MPCh 3 nanosheets show good crystallinity (Figure 3B).In addition, the lattice expansion of the high-entropy NSs can be seen through highresolution transmission electron microscope (HRTEM) images (Figure 3C), and the random distribution of atomic column strength in Figure 3C shows the disorder of excessive metal atomic scale.The random distribution of atomic column strengths in the HRTEM image further highlights the disorder at the atomic level, characteristic of HEAs.
Beyond that, Cavin et al. successfully synthesized layered high-entropy TMDs by chemical vapor transport and liquid phase stripping. 36Taking (Mo, W, V, Nb, Ta) as an example, Figure 3D shows the scanning electron microscope-energy dispersive X-ray spectrometry (SEM-EDS) mapping of these high-entropy TMDs, which determines the uniform spatial distribution of each element.The SEM-EDS analysis also confirmed that S and transition metals are distributed throughout the wafer without phase separation.Figure 3E shows an enlarged high angle annular dark field (HAADF) image of the highlighted area in Figure 3F.In the HAADF image, the strength of the atom is approximately proportional to the square of the atomic number (≈Z 2 ).Thus, the heavy elements W and Ta are the brightest, followed by Mo and Nb, followed by V and S with the lowest intensity.Furthermore, scanning TEM (STEM)-HAADF analysis confirmed the random distribution of elements in the high-entropy TMD alloy (Figure 3G), further supporting its potential for high electrocatalytic performance in CO 2 reduction reactions.

Bottom-up method
Compared with the top-down method, the bottom-up approach can produce nanosheets of relatively finer size and thickness, mainly including wet chemical methods and CVD.The wet chemical methods are particularly advantageous, as the preparation reaction occurs in a liquid environment, allowing for precise control over the thickness and size of 2D HEAs by adjusting the reactant precursor and experimental conditions.Additionally, the 2D HEAs prepared using the wet chemical meth-ods exhibit good dispersion.Two commonly used wet chemical methods for synthesizing 2D HEA nanomaterials are the solvothermal method and the co-precipitation method.The solvothermal method involves subjecting reactant precursors to high temperature and pressure in a solvent, leading to the formation of well-dispersed 2D HEA nanomaterials. 34The co-precipitation method entails the simultaneous precipitation of multiple elements from a solution, resulting in the formation of 2D HEA nanomaterials with controlled sizes and properties. 38owever, CVD offers several advantages, making it a popular choice in various industries and research fields.First, it allows for precise control over the deposition process, enabling the production of thin films with uniform thickness and high purity.Moreover, CVD can be used to deposit materials on large substrates or in high-throughput processes, making it suitable for industrial-scale manufacturing.It also enables the growth of materials with complex structures, such as nanostructures and thin films, with excellent uniformity and reproducibility.Furthermore, CVD is a versatile technique, capable of depositing a wide range of materials, from insulating films like silicon dioxide to metallic materials and metal alloys, including HEAs.This flexibility makes CVD a valuable tool in materials science and engineering, as it can be tailored to meet specific material requirements for various applications.Overall, the bottom-up approach offers precise control over nanosheet size and dispersion, making it a valuable and versatile technique for synthesizing 2D HEA nanomaterials with enhanced properties for various applications.

Solvothermal method
In the bottom-up construction of 2D HEA nanomaterials using hydrogenation/solvothermal reactions, the reactant precursor is dissolved under high temperature and pressure, with a specific solvent or water as the reaction medium.During this process, precursor ions, solvent molecules, or surfactants can be inserted between layers, resulting in an increased layer spacing in the final nanomaterial.
In solvothermal reaction, the synthesis of 2D HEA nanosheets with a uniform distribution of elements can be prepared by simply adjusting the precursor composition.As the exact shape or morphology displayed by nanocrystals is determined by the relative rates of atomic deposition (v deposition) and surface diffusion (v diffusion), the morphology of the HEAs can be changed by controlling the synthesis temperature, modifying the types of reducing agents and precursors, adjusting reagent concentrations and pH, and introducing additives. 69,70Meanwhile, the choice of solvent system is crucial in solvothermal reactions to achieve the desired properties and morphology of the 2D HEA nanomaterials.Proper selection and optimization of the solvent system, along with other reaction parameters, can significantly impact the final characteristics and performance of the synthesized nanomaterials.
Bondesgaard et al. reported a general method for synthesizing HEAs at a low temperature using a solvent autoclave at 200 • C with a reaction time of 4-24 h. 71This method allows for good control over the reduction kinetics of a range of noble metal acetylpyruvate precursors dissolved in acetone-ethanol mixtures.Figure 4A illustrates the synthesis scheme, where several metal precursors are mixed and pyrolyzed to achieve a controllable production of HEAs by controlling the reaction time and temperature. 72dditionally, Figure 4B also demonstrates the solvothermal synthesis of HEAs, which offers a straightforward approach to synthesizing HEAs at low temperature. 73Li et al. attempted to synthesize 2D layered HEAs by the bottom-up polyol method. 74In this method, they utilized ethylene glycol as a solvent and conducted solvent heat treatment at 200 • C for 2 h, involving complexation, hydrolysis, and inorganic polymerization of polyols and metal cations.Through this process, 2D layered high-entropy hydrotalcites (HEHs) with large area, high homogeneity, and designed stoichiometric properties were successfully synthesized.
Huang et al. prepared a FeCoNiRu HEA electrocatalyst derived from a high-entropy metal-organic framework (HE-MOFs) precursor (Figure 4C). 75The carbon skeleton derived from the MOF precursor exhibited a porous structure with ample channels, enabling rapid transfer of the reactive material.This unique structure helped to slow down the growth of nanoparticles (NPs) and prevent their aggregation.By using the HE-MOFs as a precursor, the researchers were able to create a well-structured and porous carbon skeleton that served as an excellent support for the FeCoNiRu HEA electrocatalyst.The presence of such a porous structure with efficient channels facilitated the diffusion of reactants and products, enhancing the catalytic performance of the resulting material.

Co-precipitation method
In co-precipitation, various cationic metal salts are added to the solution, allowing them to exist in a uniform phase. 77his method enables the uniform precipitation of 2D HEA nanomaterials from a variety of components.The chemical co-precipitation method not only achieves the refinement and uniform mixing of raw materials, but also has the advantages of simple process, low calcination temperature, short calcination time, and good product quality.The rapid synthesis kinetics of the co-precipitation process can over-come the thermodynamic solubility limitations of various elements and promote the formation of 2D HE materials. 78ing et al. utilized the differences in the solubility products of different metal ions, as shown in the volcanic diagram in Figure 4D, to design a co-precipitation strategy for the preparation of high-entropy layered hydroxides (HELHs). 76In this approach, they replaced the lye with Zn and Co-ZIF solutions (Figure 4E).The organic ligands in ZIF are weakly bound to metal ions and are easily degraded in aqueous solution.Consequently, during the process, ZIF continuously consumes H + ions generated by the addition of ionic hydrolysis, and the metal ions released in the original ZIF rapidly combine with the hydrolyzed ions to grow into HELHs.In Figure 4F, TEM images and energy spectrum (EDS) show that the five metal elements and O elements are evenly distributed throughout the frame.This co-precipitation strategy demonstrates the effective use of differences in solubility products and the rapid kinetics of the process to synthesize 2D HEAs with desirable structures and properties.

Chemical vapor deposition
CVD is the most widely used technique for depositing various materials, including insulation materials, metallic materials, and metal alloys.In CVD theory, two or more gaseous precursors are introduced into a reaction chamber, where they undergo chemical reactions between atoms and molecules to form a new material that is deposited onto the surface of a substrate.This process is the basis of a traditional thin film technology. 60CVD encompasses various methods, including atmospheric pressure chemical vapor deposition, plasma-assisted chemical vapor deposition, laser-assisted chemical vapor deposition, metal organic compound deposition, among others.][84][85] This establishes a close relationship between the substrate and the final morphology of the product (such as nucleation density, nucleation size, and epitaxial orientation).For example, Qu et al. successfully synthesized HE metal disulfide (MoWReMnCr)S 2 using a low temperature (500 • C) and fast (1 h) single-source precursor method. 86The preparation of the HE transition metal disulfide is illustrated in Figure 5A.Gao et al. proposed a fast-moving bed pyrolysis strategy for obtaining denary (MnCoNiCuRhPdSnIrPtAu) HEA nanoparticles (HEA-NPs) at 923 K. 87 In this approach, the HEA-NPs with a narrow size distribution of 2 nm were fixed on a granular support.The fast-moving bed pyrolysis strategy ensures that the mixed metal precursors are rapidly and simultaneously pyrolyzed at high temperatures to obtain small-sized cores.Figure 5B,C presents the schematic diagram of the synthesis strategy.Due to rapidly reaching a high temperature, the high oversaturation of the monomer results in the formation of smaller nucleus clusters and the formation of HEA-NPs without phase separation (Figure 5C).

APPLICATIONS OF 2D HEA NANOMATERIALS TO ELECTROCATALYSIS
9][90] In particular, the outstanding electrocatalytic performance has made them valuable in the field of energy conversion and storage.They show excellent performance in electrocatalytic reactions such as HER, OER, ORR, CO 2 RR, and ethanol oxidation reaction (EOR).The unique properties of 2D HEA nanomaterials, including their high surface area and abundant active sites, contribute to their enhanced catalytic activity in these reactions.

Application of 2D HEA nanomaterials in HER
The application of 2D HEA nanomaterials in HER has shown great promise. 50,86Meanwhile, the preparation of 2D HEA containing these active metals has proven to be a cost-effective approach while maintaining high catalytic activity.][93] In HER, the high entropy of mixing effect and disordered atomic structure of HEAs offer a simple solid solution and unique properties such as larger lattice distortion, slow diffusion, and phase stability, resulting in ultrahigh mechanical strength and corrosion resistance both in acidic and alkaline electrolytes.
HEAs have excellent properties in HER.Zhang et al. found that the low charge transfer resistance is mainly related to the electrocatalytic activity and good conductivity of the alloy surface, which facilitates the charge transfer and electron transport in the electrochemical reaction process. 91Therefore, the authors speculated that the excellent electrocatalytic performance of HEAs is not only due to its metallic properties with good electrical conductivity, but more importantly due to its unique structure (atomic disorder and simple structure).5][96][97][98] Feng et al. reported on ultra-small HEA (us-HEA) NPs, which possess the best HER performance. 99The average diameter of the us-HEA (NiCoFePtRh) NPs is 1.68 nm.The us-HEA/C obtained a high mass activity of 28.3A mg −1 noble metals at −0.05 V (vs. the reversible hydrogen electrode [RHE]).This was 40.4 and 74.5 times higher than that of commercial Pt/C and Rh/C catalysts, respectively.Therefore, 2D HEAs are widely used in HER.For instance, Ding et al. successfully prepared a 2D HE FeNiCoMnVO x oxide array that exhibited excellent electrocatalytic HER activity. 100They have found the sample with best electrocatalytic performances under the Ar plasma of 100 W, 15 min, and gas flow rate of 120 mL/min, which thus was denoted as Ar-15-FeNiCoMnVO x .And the linear sweep voltammetry (LSV) of Ar-15-FeNiCoMnVO x demonstrated outstanding activity comparable to commercial Pt/C catalysts (Figure 6A).Furthermore, Ar-15-FeNiCoMnVOx electrodes show excellent electrocatalytic durability, as evidenced by a 100-h long-term amperage test at a constant current density of 10 mA cm −2 .The long-time amperage test revealed excellent stability with low current attenuation (<2.5%) (Figure 6B).
In addition, Fu et al. also identified the Pt-free combination PdMoGaInNi as the best hydrogen-binding energy (HBE) through computational screening. 101Through computational prediction, the screened components are plotted on a volcano map based on the calculated HBE (Figure 6C).Among them, Pt is the single element closest to the peak of the volcano map (i.e., optimal HBE), but there is still a lot of room to improve the HBE by slightly weakening Pt.Alloying Pt with another element proved to be an effective way to adjust the HBE of Pt, and as an exploratory example of 2D HEAs for HER, PdMo-GaInNi HEA nanosheets were synthesized.PdMoGaInNi HEA nanosheets exhibit high HER activity with an overpotential of 13 mV at 10 mA cm −2 , which is superior to commercial Pd/C and Pt/C catalysts (Figure 6D).Qu et al. have successfully synthesized high-entropy metal disulfide (MoWReMnCr)S 2 using a low-temperature and fast single-source precursor method. 86Mixing the original (MoWReMnCr)S 2 with carbon black (CB) results in a low initial potential of −127 mV and significantly lower potential of MoS 2 @20%CB (Figure 6E,F).These examples demonstrate the potential of 2D HEA nanomaterials as efficient and cost-effective catalysts for HER.Further research and development in this area are expected to lead to more advancements in the field of electrocatalysis and renewable energy technologies.

Application of 2D HEA nanomaterials in OER
The application of 2D HEA nanomaterials in the OER has shown great potential for improving the efficiency of water decomposition.OER is another semi-reaction that produces O 2 , a complex with four-electron/four-proton reaction process.A high kinetic energy overpotential can significantly inhibit the rate of water decomposition to a large extent.The design of OER catalysts aims to enhance the electron transport efficiency and improve the adsorption processes of active intermediates produced during OER, such as OOH* and O*.Currently, the most used OER catalysts are precious metal oxides like IrO 2 and RuO 2 .To reduce the cost of catalysts, alloy in nonprecious metals to convert metals into low-cost materials is a reasonable approach. 102The rich electronic effects generated by the coupling of multiple metal atoms in 2D HEAs enable to provide a variety of catalytic active sites, achieve nearcontinuous intermediate adsorption energy, and improve the electron transport in the OER process.In addition, the 2D HEAs with unique electronic structures can increase the electrocatalytic activity of OER. 103tudies have shown that hydroxides or (oxygen) hydroxides of bimetals, such as NiFe and CoFe, are highly active low-overpotential OER electrocatalysts. 104For example, Nguyen et al. reported a novel high-entropy glycolic acid (HEG), whose main components are Co, Cr, Fe, Mn, and Ni, which served as a new high-performance electrocatalyst for water oxidation. 105Metal glycerates have a layered structure consisting of stacked metal-oxygen sheets separated by glycerate anions.The layered structure is similar to that of anion-intercalated hydroxides, providing the interlayer spacing required for reactant regulation.The open layered structure of metal glycerates allows rapid transport of reactants and provides additional catalytic active sites.In a 1 M KOH aqueous solution, the HEG electrode exhibited an overpotential of 320 mV at 100 mA cm −2 (Figure 7A,B).A variety of active components in HEG, combined with a high-entropy stabilizing effect, show excellent electrochemical OER performance.
Gu et al. introduced the concept of high entropy in LDHs, and first reported a series of ultrathin HE-LDHs, denoted as HE-LDHs-Ar-x (where x is the Ar plasma processing time, x = 10, 20, and 30 min), by using hydrothermal assisted environmentally friendly plasma strategies. 106SV in Figure 7C indicates that Fe-Cr-Co-Ni-Cu HE-LDHs-Ar-20 has higher catalytic activity.In Figure 7D, when the applied potential is increased to 1.50 V, an obvious semicircular curve appears in the Nyquist diagram, confirming the presence of OER, which is consistent with the potential obtained in the LSV polarization diagram (Figure 7C).Yu et al. reported a series of HEHs prepared through co-precipitation method, including quinary, septenary, and even novenary metallic elements. 38In OER, LiMoFeCoNi HEHs exhibited an overpotential 187 mV lower than that of low-entropy hydrotalcite (LEH) at 10 mA cm −2 (Figure 7E).This significant reduction in overpotential indicates improved electrocatalytic performance in OER for the high-entropy hydrotalcite.The apparent activation energy in Figure 7F suggests that the energy barrier for the OER process in HEHs is lower compared to conventional low-entropy hydrotalcites.This lower energy barrier further supports the enhanced catalytic activity and efficiency of HEHs in promoting the OER.

4.3
Applications of 2D HEA nanomaterials to other catalytic reactions 2D HEA nanomaterials have demonstrated excellent catalytic performance in various catalytic reactions, owing to their unique multiatom coordination ability and high stability.Moreover, due to its rich elemental composition, it offers a very wide range of combinations in the design of new materials.While lattice distortions resulting from the arbitrary ordering of metal atoms in the lattice will not only increase the hardness of HEAs, but also their electrical conductivity. 107,108In addition, the creation of lattice distortions can increase the barrier to atomic diffusion at the surface, which is conducive to the formation of nanoscale HEAs. 109,110The significant lattice distortion arising from atomic size mismatch places HEAs in a thermodynamically nonequilibrium state.Consequently, HEAs possess elevated potential energy, leading to a lower energy barrier during catalytic processes.Beyond catalytic activity, the tunability of HEAs' electronic structures, such as the dband center, contributes to enhanced catalytic selectivity.More importantly, different elements in 2D HEAs exhibit distinct properties.The combination of these elements allows for the exploitation of each element's advantages, as they mutually influence one another, achieving the goal of multifunctional catalysis.
For instance, Cavin et al. have successfully synthesized layered HE-TMDs containing four to five Group V and VI transition metals. 36The authors anticipated that four alloys, specifically (MoWNbV)S 2 , (MoWNbTa)S 2 , (MoWVNbTa)S 2 , and (MoVNbTa)S 2 , could be reliably grown at lower growth temperatures.Following this prediction, they effectively synthesized the first three nearly equimolar stoichiometric alloys and assessed their electrocatalytic performance in the electrochemical CO 2 reduction reaction.The catalyst showed excellent catalytic performance in the reduction of carbon dioxide (Figure 8A).The catalysts also exhibited remarkable stability during the CO 2 RR reaction (Figure 8B).2D HEAs have also been used in propane dehydrogenation.For example, Nakaya et al. reported a new class of heterogeneous catalysts using high-entropy intermetallic compounds (HEIs). 111These catalysts showed excellent stability and did not deactivate during prolonged propane dehydrogenation (Figure 8C).
In addition, 2D HEA nanomaterials have found applications in EOR.For instance, the HEA-PdPtCuPbBi ultrathin nanorings (UNRs)/C catalyst prepared by Li et al. exhibited excellent catalytic performance in EOR. 112 the HEA-PdPtCuPbBi UNRs/C presents the highest mass activities at 0.45 and 0.6 V RHE (Figure 8F).Overall, these examples demonstrate the versatility and potential of 2D HEA nanomaterials as effective catalysts in various catalytic reactions, making them valuable candidates for advancing catalysis research and application in diverse fields.

SUMMARY AND OUTLOOK
These 2D HEA nanomaterials have diverse compositions, ultrathin surfaces, highly exposed surfaces, rich surface chemistry, and tunable physical and chemical properties.The high surface area of 2D HEA nanomaterials can prevent aggregation of transition metal atoms and enhance element utilization.The advantages of 2D HEA nanomaterials make them highly promising candidate in electrocatalysis.However, their preparation and characterization pose significant challenges due to elemental differences and compositional complexity.(i) In terms of synthesis, achieving uniform mixing and thin thickness in 2D HEA nanomaterials requires non-equilibrium methods such as temperature, force, pressure, and energy field control.Also, balancing unbalanced compositions with subtle structural or morphological control is essential for achieving desired properties in terms of size, phase, shape, section, and surface decoration. 39(ii) In addition to the challenges involved in preparing 2D HEA nanomaterials, the inherent complexity of the 2D layered structure in HEAs can pose difficulties in their characterization.The available technology for characterizing such layered structures is limited, making it challenging to discern the specific role of each element in relation to its applications and high-entropy properties.Furthermore, traditional characterization techniques may prove inadequate in fully understanding the properties and behaviors of these materials at the nanoscale.(iii) The assessment of 2D HEA nanomaterials' performance often entails a consideration of multiple properties, including catalytic activity, mechanical strength, electrical conductivity, and optical properties.Defining appropriate performance metrics that accurately capture the unique characteristics of 2D HEA nanomaterials can be a complex task.A key challenge lies in determining the individual contribution of each element to the overall properties and performance, as the synergistic effects among elements can complicate their isolation.(iv) 2D HEA nanomaterials may exhibit distinct stability and degradation behaviors compared with single-component materials, necessitating understanding of long-term stability and degradation mechanisms for practical applications.
In terms of economic and energy benefits, the research on 2D HEA-based electrocatalysts is still in its early stage.(I) The number of possible components of 2D HEAs is huge, and it is highly difficult to design a new 2D HEA based on the traditional trial-and-error strategy.Advanced computational tools (such as machine learning) need to be further combined to accelerate the exploration of new 2D HEA electrocatalysts, thereby reducing the research cost.(II) In addition, harsh synthesis conditions are necessary for 2D HEAs, such as high pressure, high temperature, and inert gas environment, and it is urgent to develop a simple, high-yield and mild synthesis method to improve the energy utilization efficiency.(III) In order to deeply understand the structure-activity relationship, it is necessary to combine advanced in situ characterization techniques and establish an accurate correlation of the structure-activity relationship.Although there are still many problems in the research of materials and catalytic mechanism of 2D HEAs, it can be predicted that it will have a broad application prospect in the field of electrochemical catalytic energy conversion.Integrating 2D HEA nanomaterials into real-world applications and evaluating their performance under realistic conditions is a complex task that requires a multidisciplinary approach.
Despite these challenges, the potential applications and properties of 2D HEAs make them highly promising in the context of electrocatalysis.In the future, 2D HEAs urgently need to make breakthroughs in the following aspects for practical industrial applications:

F I G U R E 1
Development of HEAs into 2D HEA nanomaterials and classification of 2D HEA nanomaterials.The figures are reprinted from refs.20, 35, 37-39 with permission.

F I G U R E 2
Influence factors of 2D HEA nanomaterials.(A and B) The instantaneous percentages of Pd, Pt, Rh, Ir, and Ru atoms generated at different adding modes, and the instantaneous entropy of mixing ∆S mix as a function of reaction time.Reproduced with permission from ref.48 Copyright 2023, CC BY-NC.(C) The mixing entropy of the equi-atomic ratio alloys as a function of the number to the components.Reproduced with permission from ref. 49 Copyright 2023, Elsevier.(D) The mixing enthalpies for six possible pairwise binary alloys corresponding to (Mo, W, V, Nb) with the boxed x representing the equimolar HEA enthalpy.Reproduced with permission from ref. 36 Copyright 2023, Wiley.(E) Lattice strains (ε) obtained and extracted from the plots of β cos θ as a function of 4 sin θ. (F) Plots of the P-P bond length as a function of the Co(x).Reproduced with permission from ref. 50 Copyright 2022, American Chemical Society.

F I G U R E 3
Top-down method.(A) Optical images of HEX single crystals on a millimeter paper and the exfoliated few layers on a SiO 2 /Si wafer.Reproduced with permission from ref. 66 Copyright 2021, American Chemical Society.(B) TEM image, inset: selected area electron diffraction (SAED) pattern, and (C) HRTEM images of Co 0.6 (VMnNiZn) 0.4 PS 3 .Reproduced with permission from ref. 50 Copyright 2022, American Chemical Society.(D) EDS chemical maps showing V, Nb, Mo, Ta, W, and S distribution for the flake.(E) STEM-HAADF image of a flake, where the highlighted region (red box) shows the [001] projection of a typical transition metal dichalcogenide structure.(F) Atomic-resolution STEM-HAADF image showing local variations in the intensity of atomic columns due to varying composition of cations.(G) Intensity profile of the region illustrated in (f) as a green box, where the legends correspond to the predominant elements in each atomic column.Reproduced with permission from ref. 36 Copyright 2023, Wiley.

F I G U R E 4
Synthesis of 2D HEAs by solvothermal method and co-precipitation method.(A) Schematic illustration of solvothermal synthesis of HEAs.Reproduced with permission from ref. 72 Copyright 2020, American Chemical Society.(B) Schematic of the synthesis of CNT-supported convex cube-shaped Pt 34 Fe 5 Ni 20 Cu 31 Mo 9 Ru HEA.Reproduced with permission from ref. 73 Copyright 2023, Small.(C) Schematic of the synthesis for FeCoNiRu HEA composites.Reproduced with permission from ref. 75 Copyright 2023, Wiley.(D) The pK sp and the pH value at which ions (0.1 mol L −1 ) begin to precipitate.(E) Schematic diagram of synthesis of high-entropy layered hydroxides (HELHs) through co-deposition and in situ conversion from zeolitic imidazolate framework (ZIF).(F) The morphology and element distribution of ZnCoNiFeMg HELH, scale bar: 200 nm.Reproduced with permission from ref. 76 Copyright 2023, Wiley.

F I G U R E 5
Synthesis of 2D HEAs by CVD.(A) Schematic of the preparation of the HE transition metal disulfide, where five individual precursors are decomposed in tandem to form the HE material and crystal structure of the HE transition metal disulfide, showing variable occupancy of the metal sites within the 2H-MoS 2 structure (indicated by the multicolored spheres).Reproduced with permission from ref. 86 Copyright 2023, Wiley.(B) Schematic diagram of the fast-moving bed pyrolysis (FMBP) experimental setup for synthesis of HEA-NPs.(C) Schematic diagrams for synthesis of homogeneous and phase-separated HEA-NPs by FMBP and fixed bed pyrolysis (FBP) strategies, respectively.Reproduced with permission from ref. 87 Copyright 2020, CC BY.

F I G U R E 6
Electrocatalytic properties of 2D HEA for HER.(A) LSV plots of Ar plasma modulated FeNiCoMnVO x at the scan rate of 5 mV s −1 .(B) The chronoamperometric testing up to 100 h at current density of 10 mA cm −2 ; the inset of (B) shows the corresponding LSV plots before and after durability test.Reproduced with permission from ref. 100 Copyright 2023, Elsevier.(C) A volcano plot constructed from experimentally derived exchange current densities and computational calculated HBE (in terms of Gibbs free energy, ΔGH*) of single elements.(D) LSVs in 0.5 M H 2 SO 4 with infrared (iR)-compensation at a scan rate of 10 mV s −1 .Reproduced with permission from ref. 101 Copyright 2022, American Chemical Society.(E) LSV within the HER potential range at a scan rate of 5 mV s −1 .(F) η 10 Values of the as-prepared materials.Reproduced with permission from ref. 86 Copyright 2023, Wiley.

F I G U R E 7
Electrocatalytic properties of 2D HEAs for oxygen evolution.(A) LSV curves of HEG, (B) overpotentials at 100 mA cm −2 .Reproduced with permission from ref. 105 Copyright 2021, Wiley.(C) LSV curves in O 2 -saturated 1 M KOH solution at a scan rate of 5 mV s −1 .(D) Nyquist plots of Fe-Cr-Co-Ni-Cu HE-LDHs-Ar-20 at various voltages versus RHE.Reproduced with permission from ref. 106 Copyright 2023, Elsevier.(E) LSV curves at a scan rate of 5 mV s −1 with 90% iR compensation.(F) Arrhenius plots of the kinetic currents.Reproduced with permission from ref. 38 Copyright 2023, Wiley.
Figure 8D shows the cyclic voltammetry (CV) of different catalysts under the conditions of 1.0 M KOH and 1.0 M ethanol, among which HEA-PdPtCuPbBi UNRs/C catalyst has the highest EOR activity.Specifically, under forward scanning, the EOR initial oxidation potential of HEA-PdPtCuPbBi UNRs/C (at mass activity of 0.2 A mg −1 Pd + Pt ) decreased to 261 and 234 mV, respectively, compared to commercial Pd/C and Pt/C catalysts (Figure 8E).Furthermore, F I G U R E 8 Electrocatalytic performance of 2D HEA in other reactions.(A) The current density of (Mo, W, V, Nb, Ta) and Ag nanoparticles plotted against V versus RHE using LSV with simultaneous measurements of partial pressure of gaseous products in the inset.(B) Long-term chronoamperometry experiments (20 h) at two different potentials.Reproduced with permission from ref. 36 Copyright 2023, Wiley.(C) Catalytic performances of the PtGe, HEI (0.25), single-atom alloy (SAA), and HEA in propane dehydrogenation at 600 • C without co-feeding H 2 .Reproduced with permission from ref. 111 Copyright 2022, American Chemical Society.(D) CVs of different catalysts in N 2 -saturated 1 M KOH + 1 M C 2 H 5 OH at a scan rate of 50 mV s −1 .(E) Local magnification of the curves during the initial period of EOR in (D) from 0.0 to 0.6 V RHE .(F) The mass activity of different catalysts at 0.45 and 0.60 V RHE .Reproduced with permission from ref. 112 Copyright 2023, American Chemical Society.
(i) Theoretically, machine learning based on big data would accelerate the exploration of 2D HEA electrocatalysts.(ii) In terms of synthesis strategies, understanding the compatibility and diffusion rate of different metal atoms can aid in synthesizing target-oriented 2D HEAs effectively.(iii) In terms of synthesis methods, it is urgent to develop a simple, high-yield and mild synthesis method to produce high-quality 2D HEAs in large quantities.(iv) In terms of advanced characterization techniques, advanced electron microscopy, spectroscopy, diffraction methods, and so forth, can analyze atom arrangement, defects, and interfaces of 2D HEAs.(v) In terms of accurate perfor-mance evaluation, there is an urgent need for precision testing techniques to accurately evaluate the exact role of each metal in 2D HEAs.(vi) In the structure-activity relationship, it is necessary to combine advanced in situ characterization techniques (in situ XPS, in situ TEM, etc.) and establish an accurate correlation of the structureactivity relationship.Further research and development in synthesis, characterization, and applications of 2D HEAs will unlock their full potential and enable their practical applications to diverse areas.How to cite this article: Lu D, Fu X, Guo D, et al.Challenges and opportunities in 2D high-entropy alloy electrocatalysts for sustainable energy conversion.SusMat.2023;3:730-748.https://doi.org/10.1002/sus2.168A U T H O R B I O G R A P H I E S Die Lu graduated from Shenyang University of Chemical Technology in 2022 with a Bachelor's degree in Chemical Engineering and Technology.She is currently pursuing a Master's degree in Materials and Chemical Engineering at Zhengzhou University, where her primary focus is on the development and application of two-dimensional high-entropy alloy electrocatalysts.