High-entropy catalysts for electrochemical water-electrolysis of hydrogen evolution and oxygen evolution reactions

Abstract

High entropy materials (HEMs) have developed rapidly in the field of electrocatalytic water-electrolysis for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) due to their unique properties. In particular, HEM catalysts are composed of many elements. Therefore, they have rich active sites and enhanced entropy stability relative to single atoms. In this paper, the preparation strategies and applications of HEM catalysts in electrochemical water-electrolysis are reviewed to explore the stabilization of HEMs and their catalytic mechanisms as well as their application in support green hydrogen production. First, the concept and four characteristics of HEMs are introduced based on entropy and composition. Then, synthetic strategies of HEM catalysts are systematically reviewed in terms of the categories of bottom-up and top-down. The application of HEMs as catalysts for electrochemical water-electrolysis in recent years is emphatically discussed, and the mechanisms of improving the performance of electrocatalysis is expounded by combining theoretical calculation technology and ex-situ/in situ characterization experiments. Finally, the application prospect of HEMs is proposed to conquer the challenges in HEM catalyst fabrications and applications.

Graphical abstract

Keywords

high-entropy / electrocatalysis / synthetic methods / water-electrolysis / hydrogen and oxygen evolutions

1 Introduction

With the rapid increasing of consumption fossil fuels for energy, the possible global energy crisis may happen and the associated environment pollution is becoming a great concern. Therefore, it is urgent to develop renewable and environment-friendly energy sources to replace fossil fuels [1–7]. Hydrogen as an ideal alternative to traditional energy sources, which can be electrochemically produced from the electricity generated by sustainable and clean sources such as geothermal, solar, waterfall, wind and water through water-electrolysis process, has come to the foreground due to its zero carbon emissions and high weight energy density [8–11]. Electrochemical water-electrolysis is one of the key methods for green hydrogen energy production [12–17]. Although precious metal-based catalysts are the most advanced electrocatalysts, their cost-prohibitive, poor scalability, and scarcity have hindered their large-scale practical application. Therefore, an urgent task is to develop and design cost-effective and outstanding-performance non-noble metal catalysts [18,19]. Recently, high entropy materials (HEMs) containing several different elements with equal atomic ratio and single-phase characteristics have been found to have structural stability and sound strength, ductility, hardness, and corrosion resistance. As identified, multi-element mixing enables HEMs to have multi-reactive centers, un-saturated coordination, and entropy stability, which is profitable for the adsorption/desorption, electric transfer, and durability of the intermediates [20,21]. Compared with traditional materials, HEMs have a unique high entropy effect, lattice distortion effect, hysteresis diffusion effect, and cocktail effect [22,23]. Due to these four effects, HEMs can show an excellent activity and stability through improving adsorption and chemical energy. Therefore, HEMs have the potential to become late-model catalysts for water-electrolysis with a low energy consumption and long service time.

Owing to their high configuration entropy, HEMs have the characteristics of multi-component “random” distribution and highly disordered structure [24–26]. Regarding this, significantly extending the regulatory range of the catalytic active-sites is beneficial for the formation of the stable monophase solid solution structure [27–29]. Concurrently, the unique entropy stabilization effect can reduce the carrier transport capacity, increase the active sites, and avoid agglomeration of HEMs particles, which in turn makes the improved electrochemical stability of the water-electrolysis process [30–33]. In particular, the random distribution of metal atoms in the lattice can result in the lattice distortion effect by the significant difference in the sizes of the atomically constituting HEMs [34]. The lattice distortion effect will cause larger surface defects and even coordination of unsaturated sites, increase the atomic diffusion barrier, reduce the energy of the whole system, and expose more active centers to promote the activation and transmission of active species to drive water decomposition [35–37]. Meanwhile, the synergistic effect of infinite element combinations and unique components provides additional possibilities for adjusting the catalytic activities and surmounting the limitations of single-atom catalysts [38]. Li et al. [39] reviewed the application of HEMs catalyst in electrolytic water in recent years and proved that the four core effects, structure, composition, and multiple active sites of HEMs catalyst can significantly improve the electrochemical performance of the catalyst.

In addition, compared with bulk HEMs, nanoscale HEMs exhibit a larger specific surface area and a stronger adsorption capacity, which are more favorable for applications in catalysis. Since nanoscale carbon-loaded high-entropy alloys (HEAs) were prepared by carbon thermal shock (CTS) in 2018, researchers have focused on developing more convenient methods to prepare nano-HEMs [40]. It is well known that designing metal-based alloys into nanostructure is an effective method to improve their conductivity, specific surface area, strength, and hardness [41,42]. Therefore, preliminary headway has been made in the preparation of nano-HEMs with different nanostructures and high catalytic activities by mechanical alloying, dealloying, sputtering deposition, dynamically controlled laser synthesis, wet chemistry, solvothermal synthesis and sol-gel method in the field of electrochemical water-electrolysis [43–47]. For example, Zhan et al. [48] synthesized a two-dimensional (2D) medium entropy HEA with PtBiPb as the core and high entropy PtBiNiCo as the shell by precise control of sample morphology through wet chemistry methods. This HEA had an average diameter and thickness of 26.2 ± 3.3 and 8.4 ± 1.5 nm, respectively. The improved catalytic performance of this PtBiPbNiCo HEA hexagonal nanosheet structure could be attributed to the optimization of the surface structure under the synergism of high entropy effect and lattice distortion effect. Sheelam et al. [49] took a simple sol-gel strategy for preparation LaNi_1−xFe_0.5xCo_0.5xO₃ (LNFCO-x) electrocatalyst for both OER and oxygen reduction reaction (ORR). This LNFCO-0.5 showed the largest BET surface area of 14.46 m²/g and the minimum particle size of 80 nm. Based on the molecular orbital theory, the best ORR and OER catalytic activities could be obtained when the filling of e_g orbitals of the perovskite-based catalysts was close to 1.2. Therefore, the existence of Fe^3+Δ- and Co³⁺-sites could shift the e_g-orbital filling (e_g > 1.2) of LNO to an ideal (e_g = 1.2) position, and then promote the redox activity of oxygen.

Concurrently, theoretical research can provide a deeper insight into the HEM formation and structure-activity relationship. For example, Yao et al. [50] predicted and simulated the synthesis of multi-element alloy nanoparticles (MEA-NP) both thermodynamiclly and kineticlly through computation-assisted, entropy-driven mechanisms. The synthesized MEA-NP exhibited excellent thermal stabilities. As indicated, HEMs with an excellent catalytic performance could be reasonably designed and synthesized through computational assistance. Ludwig [51] proposed an automated high-throughput characterization method that could comprehensively determine the structures, the multifunctional properties, and the composition of the materials contained in the database, and then effectively optimized the newly identified materials through combinatorial processing. The combination of theoretical calculation and combinatorial synthesis is more conducive to achieving efficient material discovery and design.

This paper reviews the research progress of electrochemical water-electrolysis for hydrogen generation using HEMs as the electrocatalysts in recent years. First, the concept of HEMs and its four unique effects are introduced to understand the reasons why HEMs are suitable for splitting water catalysts. Then, preparation methods for HEMs are discussed to show the challenges and optimization strategies in preparing HEMs. Afterwards, the application of HEMs as catalysts in HER, OER, and complete water-electrolysis are discussed in terms of combining theoretical calculations and fundamental properties to provide guidance for the choice of the HEMs synthesis technique for enhancing the water-electrolysis process. Finally, the application of HEAs in catalysis is prospected in terms of performance, preparation, and characterization. It is believed that the research progress reviewed in this paper should be able to facilitate the understanding of the basic properties of HEMs and the exploration of the electrocatalytic applications of HEMs for accelerating the design of low-cost, and high-performance catalytic HEMs-based catalysts for water-electrolysis for HER.

2 Definition and specific effects of high entropy materials

2.1 Definition of HEMs

In 2004, Yeh et al. [52] proposed the concept of HEMs after conducting a series of synthesis studies on multi-element alloys with equal and near-equal atomic ratios. HEMs can be defined in terms of composition and entropy, i.e., HEMs refer to single-phase solid solutions containing five or more metal elements uniformly mixed in an equal molar ratio, and the content of each element is in the range of 5% to 35% [53] or the HEMs composed of multiple elements conform to the Gibbs free energy equation

(1)\mathrm{\Delta }{G}_{\mathrm{m}\mathrm{i}\mathrm{x}}=\mathrm{\Delta } H_{\mathrm{m}\mathrm{i}\mathrm{x}}-\mathrm{\Delta } T{S}_{\mathrm{m}\mathrm{i}\mathrm{x}},

where \mathrm{\Delta }{G}_{\mathrm{m}\mathrm{i}\mathrm{x}}, \mathrm{\Delta }{H}_{\mathrm{m}\mathrm{i}\mathrm{x}}, S_{\mathrm{m}\mathrm{i}\mathrm{x}}, and \mathrm{\Delta }T are the mixing Gibbs free energy, mixing enthalpy, mixing entropy, and thermodynamic temperature [54,55], respectively. The relative values of \mathrm{\Delta }{H}_{\mathrm{m}\mathrm{i}\mathrm{x}} and -\mathrm{\Delta }T{S}_{\mathrm{m}\mathrm{i}\mathrm{x}} determine the value of \mathrm{\Delta } G_{\mathrm{m}\mathrm{i}\mathrm{x}}. A positive value indicates the phase separation driven by thermodynamics, while a negative one indicates that the system tends to be in a single-phase state with stochastic distribution of elements. The system tends to be in a low-energy steady state when the entropy value is sufficient to overcome the effect of \mathrm{\Delta }{H}_{\mathrm{m}\mathrm{i}\mathrm{x}} on \mathrm{\Delta }{G}_{\mathrm{m}\mathrm{i}\mathrm{x}}.

Entropy is a very vital consideration in the synthesis of HEMs. The entropy of the system includes the electron randomness entropy (\mathrm{\Delta }{S}_{\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}}), magnetic dipole entropy (\mathrm{\Delta }{S}_{\mathrm{m}\mathrm{a}\mathrm{g}}), vibration entropy (\mathrm{\Delta } S_{\mathrm{v}\mathrm{i}\mathrm{b}}), and configuration entropy (\mathrm{\Delta }{S}_{\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{f}}). In general, \mathrm{\Delta } S_{\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{f}} has a dominant effect on the construction of the material system because HEMs are composed of multiple elements and are uniformly distributed in the same lattice. The \mathrm{\Delta }{S}_{\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{f}} of the system can be approximated to \mathrm{\Delta }{S}_{\mathrm{m}\mathrm{i}\mathrm{x}} of the ideal solid solution. The derivation of their relationship is reflected by Eqs. (2) and (3)

(2)\mathrm{\Delta }{S}_{\mathrm{m}\mathrm{i}\mathrm{x}}=\mathrm{\Delta } S_{\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{f}}+\mathrm{\Delta }{S}_{\mathrm{v}\mathrm{i}\mathrm{b}}+\mathrm{\Delta }{S}_{\mathrm{m}\mathrm{a}\mathrm{g}}+\mathrm{\Delta }{S}_{\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}},

(3)\mathrm{\Delta }{S}_{\mathrm{m}\mathrm{i}\mathrm{x}}\cong \mathrm{\Delta }{S}_{\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{f}}=-R\sum _{i=1}^n{x}_i\mathrm{l}\mathrm{n}{x}_i,

where R and x_i are the gas constant and the mole fraction of the ith component, respectively [56]. Therefore, \mathrm{\Delta }{S}_{\mathrm{m}\mathrm{i}\mathrm{x}} of HEMs in liquid or solid solution with an equal molar ratio of metal elements can be simplified as Eq. (4)

(4)\mathrm{\Delta }{S}_{\mathrm{m}\mathrm{i}\mathrm{x}}=-R \mathrm{l}\mathrm{n}n,

where n is the number of elements in the alloy [54]. Based on the entropy value, materials can be split into three types, low-entropy with one or two elements as the main constituent elements (\mathrm{\Delta }{S}_{\mathrm{m}\mathrm{i}\mathrm{x}} < 0.69R); medium entropy containing two to four main elements (0.69R < \mathrm{\Delta } S_{\mathrm{m}\mathrm{i}\mathrm{x}} < 1.61R); and high entropy containing at least five main constituent elements (\mathrm{\Delta }{S}_{\mathrm{m}\mathrm{i}\mathrm{x}} ˃ 1.61R) [57]. As shown in Fig.1, the lowest standard of high entropy (˃ 1.61R) can only be achieved when at least 5 alloys are mixed in equal proportions [58].

2.2 Four specific effects of HEMs

The distinctive mechanical, physical, chemical, and biological properties of HEMs are attributed to their unique bonding style and atomic arrangement in lattice [59]. Compared with pure metals and traditional binary and ternary materials, HEMs have four specific effects, i.e., cocktail effect on performance, slow diffusion effect in dynamics, high entropy effect in thermodynamics, and lattice distortion effect on structure [60]. Specifically, the high entropy effect obtained by multi-element composition can improve the stability of the material; the lattice distortion effect obtained by different atomic radii can significantly increase the hardness of material; the hysteresis diffusion effect is caused by lattice distortion which hinders the slip deformation of dislocations, contributing to the formation of nanometers of HEMs; and the cocktail effect is caused by the synergistic effect between various elements, which can improve the adsorption and chemical energy of HEMs (Fig.2) [61]. The catalytic performance of HEMs is normally benefited from these four effects through the reconstruction of the electronic structures in multi-element systems [62–64].

2.2.1 High entropy effect

The high entropy effect is mainly used to explain the reasons why single-phase solid solutions can be formed due to multicomponent mutual dissolution in HEAs [62]. The increase of \mathrm{\Delta }{S}_{\mathrm{m}\mathrm{i}\mathrm{x}} at high temperatures can effectively reduce the \mathrm{\Delta }{G}_{\mathrm{m}\mathrm{i}\mathrm{x}} of the alloy system. However, high entropy may reduce electronegativity differences, inhibit compound formation, promote mixing between elements, and form simple FCC or BCC phases. As a result, HEMs can have a good thermodynamic stability [65]. Meanwhile, the physical properties like strength and hardness of HEMs are significantly enhanced because the solid solution phase in HEMs can produce a strong solid solution strengthening effect [66].

2.2.2 Lattice distortion effect

Generally, HEMs are made up of various elements with different atomic sizes. As shown in Fig.2, the atoms of each element in the crystal randomly are assigned to the lattice position with equal probability [36]. The size difference between different atoms in HEMs will lead to lattice point deviation from the ideal position and lattice distortion. The lattice distortion effect of HEMs can be explained by Eqs. (5) and (6).

(5)\delta =\sqrt{{\sum }_{i=1}^n{C}_i(1-r_i/\overline{r}{)}^2},

(6)\overline{r}= \sum _{i=1}^n{C}_i{r}_i,

where C_i is the atomic content of various elements, \overline{r} is the average sizes of alloy element atoms, and r_i is the atomic radius of various elements. The lattice distortion produces uncertainty in atomic positions, leading to the higher enthalpy of formation and microscopic stresses in the material, which increases its strength and hardness [61,67]. Lee et al. [68] increased the degree of lattice distortion by adding Zr to NbTaTiV HEA, thus significantly improved the yield strength under comparable plasticity.

2.2.3 Sluggish diffusion effect

The internal structure of HEMs composed of multiple elements is complex, which hinders the atomic diffusion channel and reduces the atomic diffusion rate on the surface. Therefore, HEMs can serve as catalysts with a good stability [69]. As known, phase transformation requires the collaborative diffusion among the main elements to reach equilibrium phase separation. Therefore, the sluggish diffusion effect will affect the formation of new phases in HEMs [70]. For example, Li et al. [71] designed a high-temperature and high-entropy shape-memory alloy Ti₂₀Hf₁₅Zr₁₅Cu₂₅Ni₂₅ with a larger super-elasticity through slow diffusion and lattice distortion effects in HEAs. As observed, the active atoms were more likely to diffuse into vacancies and gaps because of the variation of melting point and bonding strength of different atoms, so the atoms in HEM mainly diffused through the vacancy mechanism [72]. Meanwhile, the strain generated by the lattice distortion can change the d-band center of the catalyst, causing the adsorption energy of reactants and intermediates to be affected. Huang et al. [27] synthesized MnFeCoNiCu high-entropy nanoparticles with high lattice distortion. They demonstrated that lattice distortion induced strain and optimized the electronic structure of the material, resulting in improved OER performance.

2.2.4 Cocktail effect

The cocktail effect is not a single feature, but a feature produced by multiple alloy elements in multi-components through a large number of interactions or synergistic interactions on the macro level initially proposed by Ranganathan [73]. Five or more alloy elements with similar or equal molar ratios in HEMs can improve the performance of HEMs macroscopically through the interactions between the shape and size distributions of the microscopic grains, and grain boundaries or phase boundaries. Huang et al. [74] significantly improved the electrolytic water performance of noble metal-free equimolar HEA by exploring and maximizing the cocktail effect between atomic-level mixtures of H-FeCoNiCuMo. In general, HEMs are complex multi-component systems. Therefore, the selection of the appropriate elemental composition can lead to the greater synergy among the components, resulting in the good catalytic performance of HEMs [75,76].

3 Synthesis and optimization of HEMs

The extensive research on HEMs has accelerated the development of the corresponding HEMs nanoparticle synthesis methods, which can be parted into top-down and bottom-up strategies, respectively (Fig.3) [25,77]. The former can be used to prepare the desired nanostructures through physical or chemical processes by etching large-sized substances through various etching techniques; while the latter can be used to self-assemble certain smaller constitutional structural units through weak interactions to form comparatively complex and large structural systems at the nanoscale [78–80]. In general, the synthesis scheme and the synthesis process of the above two strategies play the major roles in determining the microscopic morphology, particle size, phase structure, and crystallinity of HEMs nanoparticles [81]. Therefore, in the following subsections, the effects of both top-down and bottom-up strategies on HEMs are discussed along with their influence on electrocatalytic performance.

3.1 Top-down methods

Top-down methods can be used to break, grind, and shape large pieces of metal into required shapes and nanoscale through inputting energy sources [82]. Mechanical alloying, dealloying, sputtering deposition, and kinetically-controlled laser synthesis are common top-down methods for preparing nanometer-level HEM materials [83,84].

3.1.1 Mechanical alloying

The mechanical alloying strategy is a powder metallurgy processing technology. Through mechanical force (high-energy ball milling), the powder of five or more components can be repeatedly cold-welded, broken, and re-welded in the process of high-speed friction and grinding to form HEM powder with different compositions and uniform particle size (Fig.3(a)) [45,85,86]. The mechanized alloying strategy can produce more energy during cold welding, extrusion, and rewelding, allowing more difficult chemical reactions to happen at room temperature [87,88].

Breitung and his team [89] synthesized Pnma and Pa-3 structured high entropy sulfides with polycrystalline nanoparticles as electrocatalysts for OER through a high-energy planetary ball milling process. The high entropy effect and cocktail effect of high entropy sulfide (HES) make TiFe₂Co₂Ni₃MoS₁₀ and MnFeNiCoCuS₁₀ with the Pnma and Pa-3 structure have a better OER performance and stability (Fig.3(b)).

The mechanical alloying strategy has the advantages of providing localized short-range heating, accelerating molecular diffusion, and avoiding the use of solvents and high temperatures. For example, Feng et al. [91] integrated perovskite oxide and heat-resistant halide into a single-phase solid solution through a mechanical alloying strategy, and its configuration entropy was even higher than its high-entropy precursor. The combined high-entropy oxide and fluoride solid solution exhibited a stronger catalytic OER activity and a lower overpotential than its oxide precursor (Fig.3(d)).

3.1.2 Dealloying method

The dealloying method refers to the selective etching of more electrochemically active components from an alloy through chemical or electrochemical methods. This strategy can effectively modulate the surface properties of HEMs to make the increased electrocatalytic activity [20,79,92]. Li et al. [43] used physical metallurgy and dealloying strategies to synthesize HEM with graded porous structures, which showed the coexistence of multi-level mesoporous structures in the microstructure of dealloying HEM. The Cu nanophases of the dealloying HEM showed excellent synergistic effects. It provides an appropriate electronic structure for the electrocatalytic activity toward HER. Thereby, at the current density of 10 mA/cm², a lower overpotential of 42.2 mV was obtained (Fig.4(a)).

3.1.3 Sputtering deposition

Sputtering is a vapor deposition technique in which atoms are sputtered out of a solid target by bombarding it with high-energy particles, which is widely used in the preparation of nanofilms [93–95]. The desired HEMs can be obtained by adjusting the sputtering power. For example, Löffler et al. [96] prepared highly dispersed HEM nanoparticles (CrMnFeCoNi) with flexible sizes and compositions by adjusting the sputtering elements and the individual deposition rate of appropriate ionic liquids using the combined co-deposition method in ionic liquids (Fig.4(b)). Wang et al. [97] prepared FeCoNiCuPd HEM films on carbon fiber cloth by magnetron sputtering with pulsed DC reaction using Fe/Co/Ni/Cu and Pd particle targets, and the formed FeCoNiCuPd HEM films showed a single-sided core-cubic (FCC) structure. The HEM films were well-distributed and smooth with a thickness of 2 µm, and the metal elements were well-distributed in the prepared samples. In 1 mol/L KOH solution, at a current density of 10 mA/cm², the film showed overpotentials of 194 and 29 mV for OER and HER, respectively.

3.1.4 Kinetically controlled laser synthesis

Generally, the kinetically controlled laser synthesis is originated through the interaction between short intense laser pulses and the surface and can promote the formation of single-phase solid solutions of HEMs uniformly distributed on the carrier (Fig.4(c₁)) [99]. Jiang et al. [98] synthesized high-entropy alloy nanoparticles under atmospheric conditions in three steps. In the first step, the generated laser photons were absorbed by carbon carriers and metal ions. In the second step, photon ionization and reaching high temperature conditions lead to the reduction of metal ions and the etching of graphene. In the last step, the reduced metal atoms were aggregated into ultrafine nanoalloys at defective sites in the carbon carriers (Fig.4(c₂)). Due to the characteristic carbon etching and laser-induced hot electron emission, the ultra-fine HEAs stabilized by defective carbon carriers. At the same time, the high entropy nanoparticles were all in the size range of 1 to 3 nm, and the productivity was as high as 7 g/h.

3.1.5 Challenges and opportunities of top-down methods

As shown in Fig.5, the top-down method for preparing nano HEMs has the advantages of easy expansion and simple operation. In addition, HEMs can be prepared into granular or self-supporting catalysts according to different needs. For these reasons, the preparation of catalysts by using top-down methods should have advantages in the development of catalysts in the future. However, the top-down method has some shortcomings such as difficulty in accurately controlling the synthesis morphology, easy aggregation of HEM nanoparticles, and certain requirements on precursor alloys and etching media. All these shortcomings are challenging issues for the fabrication of high-quality nanosized HEM electrocatalysts.

3.2 Bottom-up methods

Bottom-up approaches can precisely regulate the formation and growth processes of precursors through chemical reactions and interactions between atoms or certain molecules, thereby synthesizing finer nanoparticles [100,101]. Carbon thermal shock (CTS), wet chemistry, solvothermal synthesis and sol-gel methods are the main techniques for bottom-up synthesis of HEMs.

3.2.1 Carbon thermal shock

CTS is a method for synthesizing HEA nanoparticles using rapid heating and cooling. The method is a fast and easy high-throughput method that enables the controlled and efficient synthesis of uniformly dispersed single-phase HEM nanoparticles through selecting the composition and adjusting CTS parameters such as heating/cooling rate, temperature, metal precursor concentration, carbon substrate, and duration. Due to the above advantages, the yield of CTS can be approximately 100%. For example, Yao et al. [40] synthesized single-phase solid solutions consisting of Pd, Co, Pt, Fe, Au, Ni, Sn, and Cu by utilizing the CTS method, which were fixed on conductive activated carbon fibers. The transient high temperature in CTS could promote the homogeneous mixing of diverse elements and induce the rapid decomposition of metal salts. The rapid quenching process uniformly dispersed the synthesized catalyst on the substrate and controlled the kinetics of solid solution nanoparticle formation to limit elemental segregation and avoid phase separation (Fig.6(a)). Based on this, Abdelhafiz et al. [102] in situ synthesized a non-noble metal catalyst FeNiCoCrMnV on carbon fiber through CTS. Carbon was easily dissolved in the high-entropy polymetallic droplets and formed amorphous carbon during the heating process, and the amorphous carbon was then deposited at the interface between carbon fibers and HEA nanoparticles during the cooling process. Thereby, the strong metal-carrier interactions between HEA nanoparticles and amorphous carbon were generated, which enhanced the catalytic stability and avoided corrosion of the electrocatalyst (Fig.6(b)). Moreover, the surface reforming and dealloying processes during the test showed that, after 1000 cycles, the overpotential of the FeNiCoCrMnV catalyst decreased by 8 mV at 10 mA/cm² (Fig.6(c)).

3.2.2 Wet chemistry

Wet chemistry is a method of generating nanoscale alloys by the combined action of capping agents, metal salts, and reducing agents in solution to prepare nanoscale alloys with small size, controlled morphology, and uniform dispersion [103]. This method allows the rearrangement of metal atoms in multiple metal precursor salt solutions to form highly active electrocatalysts. Thus, the morphology, size, and crystallographic surface of the HEMs catalyst can be adjusted by changing the solvent, reducing agent, and capping agent [22,104]. Because some elements are not mutually soluble, suitable metal salt solutions need to be selected to avoid phase separation and inhomogeneity during the preparation of HEAs by wet chemical methods. Traditional wet chemistry methods include typical wet chemistry, electrodeposition, and alloying-de-alloying strategies [105,106]. Chen et al. [103] synthesized hexagonally confined (HCP) PtRhBiSnSb entropic intermetallic nanoplates through a simple wet chemical strategy (Fig.7(a)). The projected partial density of states (PDOSs) revealed that the introduction of Rh could enhance the oxidation activity of the catalyst for oxidation of alcohols and effectively increase the charge transfer speed of PtRhBiSnSb. Meanwhile, the coexistence of Sb, Sn, and Bi sites made the electronic structure of the active site more stable (Fig.7(b)). Fu et al. [107] used liquid-phase synthesis to achieve obtained the two-dimensional PdMoGaInNi nanosheets with optimized material utilization. They achieved a fine-tuning of the hydrogen binding energy by modulating the electronic structure of the active metal by modulating the synergistic effect between the components and each component metal in the alloy. The HEM material is brought closer to the peak of the volcano diagram (Fig.7(c)). In 0.5 mol/L H₂SO₄, the overpotential is as low as 13 mV when the current density is 10 mA/cm² (Fig.7(d)).

3.2.3 Solvothermal synthesis

Inspired by the hydrothermal method, the solvothermal synthesis method is developed, and the solvent used for this method is non-aqueous. The crystalline phase, particle morphology, and grain size of the material can be controlled by adjusting the temperature, reaction time, solvent, metal precursors, reducing agents, and surfactants [110]. Tao et al. [108] used Ag nanowires (NWs) as patterns to prepare ultrathin two-dimensional HEM sub-nanoribbons (SNR) synthesized from up to eight metallic elements with a layer thickness of only 0.8 nm, realizing the world’s thinnest HEM metal material up to date. The resulting HEA-PtPdIrRuAg pentamer exhibited a good catalytic activity (Fig.8(a)). Studies on their electronic contributions of HEAs through PDOS showed that Au, Ag, Pt, and Pd were the main reasons for their strong reduction ability and stability. Concurrently, the involvement of Ru, Os, Rh, and Ir could increase charge transfer capacity (Fig.8(b)). On this basis, Li et al. [109] successfully synthesized one-dimensional PtRuRhCoNi HEM ultrathin nanowires (NWs) using oleamine (OAm) as solvents, cetyltrimethylammonium bromide (CTAB) as a structural guide, and molybdenum hexacarbonyl (Mo(CO)₆) as reducing agents, respectively. The complementary electronic structure effect (Fig.8(c)) can induce a higher catalytic HER performance than Pt/C catalyst (Fig.8(d)).

3.2.4 Sol-gel method

The sol-gel method can be devoted to form HEMs by homogeneously mixing compounds containing highly chemically active components as precursors in the liquid phase and carrying out hydrolysis and condensation chemical reactions, which can make HEM nanoparticles uniformly distributed by controlling the condensation and hydrolysis rate of metal precursors in HEM [111,112]. In addition, during the catalytic process, the catalytic active sites can self-regulate through structural reconstruction. For instance, Ma and his team [113] prepared Ni-Co-Fe gel complexes using the sol-gel strategy, which could prevent the formation of non-homogeneous precipitates and ensure homogeneous compositions down to the nanoscale by controlled hydrolysis and condensation reactions catalyzed by formic acid at room temperature (Fig.9(a)). Meanwhile, both the phosphate and borate as anions were introduced to regulate the charge distribution of the active center by rapid proton transfer (Fig.9(b)). Perovskite oxides (ABO₃), due to their high Goldschmidt tolerance factor, could allow equivalent and heterovalent doping, making the catalytic performance tunable. Tang et al. [47] also prepared high-entropy perovskite cobalt oxides synthesized from five equimolar metals (Mn, Mg, Fe, Ni, and Co) at the B-site through a simple sol-gel strategy (Fig.9(c)). The evolution of the electronic configuration of the oxygen ions and the change in the charge states between several B-site cations could combine to promote the intrinsic catalytic OER activity. Notably, cations with different charges and different ionic radius may cause varying degrees of lattice distortion. Similarly, the entropy-induced improvement in the catalytic OER properties could also be modulated through the elementary composition of the B- and A- positions (Fig.9(d)).

3.2.5 Challenges and opportunities of bottom-up methods

As shown in Fig.10, the bottom-up method provides a possibility for the preparation of efficient new catalysts because it can regulate morphology and various combinations, and control the diameter of nanoparticles. However, there are still disadvantages such as complicated operation procedures, many kinds of solvents, special equipment, high-cost and low yield. Therefore, a lot of research work should be conducted to explore its potential.

4 Electrocatalytic applications of HEMs in water-electrolysis

HEM catalysts can be used as electrocatalysts for water-electrolysis because of their more active sites, higher material utilization, and good stability induced by their unique cocktail effect, hysteresis diffusion, high entropy, and lattice distortion. The electrolysis of water contains HER for hydrogen generation at negative electrode and OER at positive electrode for oxygen generation [15,120]. The HER and OER at practically relevant electrolytic current densities include the transport of bubbles, nucleation, growth, and detachment. The above processes lead to an increase in the energy losses related to the ion transport and reaction kinetics. Bubbles in the electrolyte block the ion conduction pathway, increase the effective resistance of the electrolyte and bubbles attached to the surface of the catalyst, and reduce the effective active electrocatalytic region [121–125]. Therefore, selecting a suitable catalyst can better reduce the activation potential barrier and improve the rate of water splitting [75,126]. Excellent catalyst for water-electrolysis demands many exposed active reaction sites with balanced H* adsorption and desorption energies on the catalyst surface. The ligand effect and strain effect of PtM (M = transition metal) can change the electronic structure of Pt, but the small range of its combination hinders the further improvement of catalytic performance [127–129]. Compared with Pt based catalysts and low entropy materials (LEMs), HEMs have the following characteristics to improve their catalytic performance: ① a large number of element compositions and multiple active sites; ② the stronger d-band ligand effect and lattice strain effect can optimize the H* desorption and adsorption energy of the active site [91,130]; ③ the high entropy effect enhances the thermodynamic and kinetic stability of the catalyst [55].

4.1 Electrocatalysis of HEMs in HER

The HER process is a dual electron transfer procedure [8] involving the adsorption as well as the desorption of hydrogen atoms (Fig.11) [131–133]. The process of hydrogenolysis reaction involves three main steps: the Heyrovsky step, the Volmer step, and the Tafel step, as follows [134,135].

Volmer reaction:

{\mathrm{H}}^{+}+{\mathrm{e}}^{-}+\ast = {\mathrm{H}}^{\ast }

Heyrovsky reaction:

{\mathrm{H}}^{\ast }+{{\mathrm{H}}^{+}+\mathrm{e}}^{-}={\mathrm{H}}_2

Tafel reaction:

2{\mathrm{H}}^{\ast }={\mathrm{H}}_2

The Volmer step is the first step in which the reduction of protons occurs at the active site. Typically, electrons are transferred to the catalyst surface and subsequently interact with H⁺ ions to produce adsorbed hydrogen atoms (H*). The H₂ molecule parsed from the electrode surface is then formed by the recombination of a second proton/electron transfer in the Heyrovsky step (Heyrovsky step) or two H* in the Tafel step reductions [136,137]. According to the HER reaction steps, it can be found that the desorption and adsorption of H atoms on the catalyst surface affect the reaction kinetic process of HER, while the adsorption and desorption of hydrogen atoms are competing with each other. From Sabatier’s principle, it can be obtained that a highly active HER reaction catalyst makes the adsorption of H* on its surface too strong, making it difficult for the products to desorb from the catalyst surface and too weak to make it difficult for the reactants to adsorb on the catalyst surface [138,139]. Therefore, the optimization of the Volmer reaction as a rate-limiting step is crucial in achieving higher performance HER [140].

Among HER-related catalysts, Pt-based catalysts have better catalytic activity because their Gibbs free energy for adsorption of hydrogen atoms and hydrogen bond energies are most conducive to HER reaction [141]. Notwithstanding, the insufficient reserves and high-cost of Pt-based catalysts limit their application in practical industrial production [28,117]. The preparation of Pt-based HEMs provides a good method for preparing Pt-based alloy catalysts with a low Pt content. Especially, HEM catalysts with only a small amount of Pt still have excellent stability and catalytic activity because of the cocktail effect between other elements and Pt. Chen et al. [142] prepared carbon-supported ultrasmall high-entropy alloy (us-HEA) nanoparticles (NP) using a chemical co-reduction method. At −0.05 V, us-HEA/C achieved a high-quality activity of 28.3 A/mg precious metal. In addition, the catalyst exhibited an ultra-high TOF (30.1 s⁻¹) at a super-potential of 50 mV which did not decay after 10000 cycles (Fig.12(a)). The main active sites were analyzed by in situ X-ray absorption near edge structure (XANES) and the kinetics of the activation barrier of Tafel reaction in the rate-limiting step was analyzed by using density functional theory (DFT) calculation. It was found that the HER performance improvement is mainly due to the fact that the strong nucleation effect of NP on carbon carrier leads to the reduction of HEM particle size and the uniform distribution, which improves the utilization rate of precious metals; the fact that Rh and Pt as direct active site increase the adsorption and desorption rate to accelerate HER; the fact that Fe/Co/Ni atoms adjust the electronic structure of Pt/Rh atoms to reduce the Fermi energy level and the d-band center position of Pt; and the fact that the high entropy structure and diffusion barrier of HEMs make it have an excellent catalytic stability.

The different electronic structures and atomic sizes of components in the high entropy structure of HEMs lead to lattice distortion. The atomic size mismatch increases the potential energy of HEMs, leading to a decrease in the energy barrier during the electrolysis of water. In addition, different atomic radii will change the strain distribution, where tensile lattice strain makes the d-band move upward to enhance the adsorption capacity of O₂ and H₂, and the compressive strain makes d-band lower to weaken the interaction of adsorbents in the process of water electrolysis. Guo and his team [143] prepared ultrathin Pt-based HEM nanomaterials with both controlled structure and composition of 17 components by using the low-temperature reductive diffusion method. The Pt-based HEM NWs exhibited a significant lattice distortion relative to the low-entropy alloy. The lattice distortion induced both strain distribution and electronic structure modulation, resulting in the lowest over potential of 24 mV of 10-HEA nanowires at 10 mA/cm² (Fig.12(b)).

Due to the cocktail effect of various metal components, HEM catalysts can serve as multifunctional catalysts with excellent performance. However, the preparation of direction-controlled HEMs catalysts, particularly those with high index crystallographic planes, still needs to optimize the functional properties via atomic structure selection and regulate the chemically ordered atom distribution [144]. Liu and his team [115] produced a multicomponent high entropy intermetallic compound (HEI) catalyst through fine-tuning the electronic structure by structural site isolation effects and chemical synergies. The dendritic porous L1₂-type biphasic FeCoNiAlTi HEI catalyst with the increased specific surface area was prepared using physical metallurgy with FeCoNi as a potential active site and AlTi to improve the ordered atomic configuration. DFT calculations can reveal the mechanism of the improved electrocatalytic performance. The (100) and (111) exposed surfaces of L1₂ HEI lattice were compared with Pt-(111) surfaces, which showed that L1₂ HEI had a better ability to adsorb H₂O. Furthermore, the calculation of ΔG_H* for the catalytic sites on the exposed surface indicates that the chemical synergism of HEI weakens the hydrogen atom adsorption (Fig.12(c)). These results show that the ordered structure of L1₂-type can reduce the energy barrier of the hydrogen precipitation process and stabilize the adsorption/desorption procedure due to its isolation effect at specific sites. The Tafel slope was 40.1 mV/dec and the overpotential was observed to be 88.2 mV at a current density of 10 mA/cm².

The strong interaction among various metals can regulate the electronic structure of the nanomaterials and anchor the HEM nanoparticles to the carriers. Meanwhile, the synergistic interaction between metal elements-carriers can facilitate the improvement of catalytic performance. Feng et al. [145] embedded CoNiCuMgZn NPs uniformly onto two-dimensional graphene conductive substrate to form a HEA@C catalyst. The graphene in the catalyst could be used as a reducing agent to convert the mixed metal premise to CoNiCuMgZn HEA nanoparticles at N₂ atmosphere and also acted as a direct template for uniform dispersion of HEA nanoparticles, effectively preventing particle aggregation. The composite of this HEM with ultrathin graphene nanosheets made the \mathrm{\Delta } G_{\mathrm{m}\mathrm{i}\mathrm{x}} of H* close to zero, facilitating the adsorption and desorption of H* and promoting catalytic kinetics (Fig.12(d)).

4.2 Electrocatalysis of HEMs in OER

OER is a four-electron transfer process [75]. The OER process involves the desorption of O₂ and the adsorption of OH*, OOH*, and O* (Fig.13). First, M-O* and M-OH* are formed through the adsorption of oxygen-containing substances to the catalytic active site and deprotonation in the first two reaction steps [147]. Next, two possible reaction paths for O₂ generation: ① direct binding of M-OH* to form O₂ molecules; and ② another M-OOH* intermediate is formed by the reaction of M-OH* with OH or H₂O to form the O₂. Accordingly, OER has sluggish reaction kinetics and a high overpotential, leading to a slow rate of water electrolysis [148,149].

The slow kinetics of OER can lead to a high overpotential, which is the main reason limiting the performance of water electrolysis [150]. For example, a class of noble metal oxides such as RuO₂ and IrO₂ exhibit an extremely good electrocatalytic activity for OER [151]. However, the instability and high manufacturing cost hinder their real application. In the effort to improve the catalytic OER performance, Wang et al. [152] introduced a method for the preparation of core-shell HEA catalysts on a nickel foam substrate using polymetallic MOF as a precursor. The shell was ultra-thin carbon, and the core was HEA with an FCC structure. The formed CoNiCuMnAl@C showed a significantly improved OER performance over commercial RuO₂. The rearrangement of Co and Ni sites in the surface structure was observed during the OER by in situ Raman testing. According to comparative experiments and DFT calculations, Ni/Co-OOH species could accelerate the rate-determining step in the OER step (O* → OOH*) and other species (Cu/Mn/Al), also play a synergism in improving the OER efficiency (Fig.14(a)). Thus, CoNiCuMnAl@C could show a low Tafel slope of 35.6 mV·dec⁻¹ and a lower overpotential of 215 mV at 10 mA/cm².

Transition metal alloys like Ni-, Co-, and Fe-based alloys are very promising OER catalysts because Ni can further remarkably accelerate the electrocatalytic efficiency of Co and Fe for OER simultaneously, Co can usefully reduce overpotential, and Fe can surprisingly improve OER kinetics through reducing Tafel slope [153]. The metal doping of HEA can adjust the adsorption and desorption properties of OER intermediates and improve the performance of OER. For example, Mei et al. [118] constructed FeCoNiMo/C catalysts through adding the fourth transition metal Mo into the FeCoNi-based alloy. The introduction of Mo with noble metal-like properties can improve the OER performance by adjusting the electronic structure of the FeCoNi alloy and reducing the adsorption energy of Ni, Fe, and Co sites to the intermediate OH*. In an alkaline medium, the above catalyst provided an overpotential as low as 250 mV at 10 mA/cm² (Fig.14(b)). The introduction of C, Si, P, and S quasi-metals into HEA is conducive to the formation of amorphous phases. Wang et al. [154] synthesized non-precious metal-based FeCoNiPB amorphous high-entropy oxides by utilizing a simple chemical reduction method. At 10 mA/cm², the overpotential and Tafel slope of FeCoNiPB oxide were 235 mV and 48 mV·dec⁻¹, respectively (Fig.14(c)). The electrocatalytic OER efficiency and stability of the FeCoNiPB catalyst upper stage were mainly due to the formation of amorphous sheets with thin (FeCoNi)OOH crystal layers in situ at the edges during the long-term OER process, the synergism of multiple components, and its amorphous high-entropy nanostructure.

The special properties of high-entropy oxides (HEO) make them promising for clean energy-related electrocatalysis due to their independent cation and anion lattices, which can increase the structural diversity [155]. For example, Sun and his team [47] used high-entropy perovskite cobaltates composed of equimolar Ni, Mg, Co, Mn, Fe at the B-site as electrocatalysts for OER. The high conformational entropy can promote surface reconstruction, facilitate the formation of stable surface oxygen vacancies, and lead to the random occupation of cations. Because of these advantages, the lattice oxygen mechanism was more favorable to the formation of O₂ from a kinetic point of view (Fig.14(d)).

In the process of electrolytic water reaction, electrochemically activated surface self-reconstruction helps to improve the number and activity of surface-active sites and increase OER performance [156,157]. Meanwhile, the abundant active sites generated in situ during the pre-oxidation induced activity process can promote the enrichment of active centers, which significantly enhances the long-term catalysis of the OER process [158,159]. Guo et al. [160] prepared CoFeNiCuCr sulfide nanosheets with rough surfaces and high entropy characteristics. High-entropy sulfide pre-catalysts lead to an improved OER performance through increased surface area and composition and valence changes induced by pre-oxidation induced surface reconfiguration. After 72 h of catalytic reaction, the metal content was detected through inductively coupled plasma emission spectrometry (ICP-OES). It can be found that the leaching of Cr^III substances in the alkaline OER process may be the reason for the reduction of activity during long-term operation. The catalytic active site is formed on the catalyst surface after electrochemical activation. The “pre-catalyst” in the OER process can promote surface self-remodeling to increase the activity and number of active sites. Schweidler and his team [114] prepared high entropy sulfides using transition metal sulfides as pre-catalysts. In the OER process, metal sulfides can be pre-oxidized with alkaline electrolytes to produce free active sites. In the catalytic process, the cocktail effect of HEMs and the strong interaction between metal and sulfur regulate the charge state and enhance the catalytic activity and stability of OER.

4.3 Bifunctional HEMs catalyst for water-electrolysis

Electrochemical water-electrolysis is a bright strategy for large-scale hydrogen production [14,161]. As identified, HEM catalysts with multiple elemental interactions, multiple active sites, good entropic stability and diverse structural compositions are the emerging materials for water cracking [46]. In addition, HEM catalysts can also optimize the adsorption capacity of various reaction intermediates, exhibiting an excellent intrinsic catalytic activity with a series of active centers, and significantly accelerated reaction kinetics [162]. Therefore, most scholars have focused their attention on high-performance of the bifunctionality of the catalysts toward both HER and OER [19,163].

The limited number of active sites in single metal catalysts affects the adsorption and desorption of reaction intermediates at the active sites. Hao et al. [164] designed and synthesized FeCoNiXRu (X: Cr, Mn, and Cu) HEA with two active sites by combining the graphitization process and the electrospinning technology. The distribution of water dissociation energy and ΔG* of adsorbed hydrogen atoms were calculated by DFT. It is proved that Co and Ru sites have the lowest energy to accelerate H₂O dissociation and H adsorption simultaneously. Operando electrochemical Raman spectroscopy was used to prove that Co site can promote the dissociation of H₂O and promote the binding of H* to H₂. The multi-site synergism of FeCoNiMnRu/CNFs HEA was verified theoretically and experimentally (Fig.15(a)). At 10 mA/cm², the overpotentials of FeCoNiMnRu/CNFs for the OER and HER were 71 and 308 mV, respectively (Fig.15(b)).

The highly customizable electrochemical properties and special structure of transition metal sulfides allow for good catalytic property in aqueous electrolysis. Shang and his team [165] prepared synthesized Co-Zn-Cd-Cu-Mn sulfide nanoarrays on carbon fibers (CoZnCdCuMnS@CF) using a mild cation exchange method. Taking advantage of the synergism between the poly-metals and the strong interfacial binding between the high-entropy sulfides and the carbon fiber carriers, the formed CoZnCdCuMnS@CF catalyst exhibited both stability and superior catalytic activity for the electrochemical water-electrolysis in alkaline media (Fig.15(c)).

The performance of electrolytic water catalyst is evaluated mainly from catalytic efficiency and stability. The current research mainly focuses on improving the catalytic performance but seldom pays attention to the catalytic stability [166]. The catalytic performance is mainly determined by the surface structure of the catalyst in contact with the electrolyte, while the catalytic stability is determined by the self-restructuring process that occurs on the surface when the cyclic potential is applied [142]. Huang et al. [116] prepared FeCoNiRu HEA electrocatalyst using metal-organic framework (MOF) with the stability of structure and composition as templates. The reason for the improvement of catalytic efficiency and stability was explored by characterizing the morphology, defects, and element dissolution of the catalyst before and after catalysis. During the catalytic process, the catalyst and electrolyte react to form spinel oxides with intrinsic hollow active sites of the original HEA catalyst. The spinel oxide hollow site consists of one tetrahedral atom and two octahedral atoms. The DFT calculation verifies that Co and Fe tend to occupy tetrahedral sites more preferentially than Ni in the HER process, while Ni tends to occupy tetrahedral sites more preferentially than Co and Fe in OER. The lattice distortion and defects caused by Ru located on the octahedron also accelerate the reaction rate of HER and OER. Therefore, one of the ways to improve the catalytic stability is to retain the intrinsic catalytic active site while increasing the free energy of H₂O adsorption when the surface self-restructuring occurs. Kwon et al. [119] designed ZnNiCoIrX (X = Fe, Mn) HEA. In the process of catalytic reaction, the dissolution rate of the noble metal Ir is minimal due to its corrosion resistance. In addition, projective state density (PDOS) confirmed that the addition of Mn led to the redistribution of electrons in the Ir band, which weakened the binding strength of intermediates and promoted the occurrence of catalytic reactions. Therefore, changing the electronic structure of the catalyst through component engineering can not only reduce the dissolution of the catalyst during the catalytic process to improve the stability but also adjust the d-band center to improve the catalytic performance.

5 Conclusions and prospects

HEMs contain five or more metallic elements, giving them a certain flexibility in composition and structure that can be used to adjust the electronic and geometric structures for electrocatalytic performance optimization. Fig.16 displays the research progress of HEMs as the catalysts for electrochemical water electrolysis in terms of theoretical research, synthesis, and application. The specific properties of HEMs have profound influences on the electrocatalytic performances in water-electrolysis for both HER and OER.

1) The multi-element nature of HEMs can induce optimized active sites for HER and OER through electron redistribution between the components in the lattice. The high entropy lattice structure enables a higher chance to generate balanced adsorption and desorption energy of the active site for intermediates of HER and OER, which reflects the lattice distortion and cocktail effect.

2) The synthesis technique has significant influences on the electrocatalytic performance because of the particle size, shape, microstructure, lattice strain, oxygen vacancies, specific surface area, and exposed surface crystal facets and elements.

3) Theoretical analysis of the electrocatalysis of HEMs indicates that multiple metal cations randomly distributed in the lattice of HEMs play different roles in catalytic water splitting. Therefore, the theoretical analysis can better understand the synthesis-structure-property relationship and effectively improve the catalytic performance of HEMs.

However, the research of HEMs electrocatalysts for water-electrolysis is still in its infancy and still faces many opportunities and challenges:

1) Unlike pure metals and conventional binary and ternary alloys, HEMs have elemental differences and compositional complexity. Although the basic concepts of HEMs have been initially understood, the microscopic active sites and the mechanism during the catalytic reaction are not sufficiently studied, resulting in the failure of HEMs to achieve theoretical catalytic performance. Therefore, further research on the mechanism of catalytic reactions of HEMs is still needed to enhance the water decomposition ability of HEMs catalysts.

2) Innovative HEMs catalyst design and synthesis schemes are prerequisites for achieving large-scale electrocatalytic electrolysis. The synthesis process of HEMs nanoparticles involves conditions such as temperature, pressure, and applied energy fields due to the differences in elements and the complexity of their composition. In addition, the current synthesis methods cannot finely tune the HEMs morphology, size, elemental distribution, etc. Therefore, the development of advanced synthesis strategies is demanded to achieve HEM catalysts with controllable morphological composition and size in a fast and green manner.

3) Using advanced characterization techniques and theoretical calculations (DFT calculations), it is possible to systematically analyze the effect of the complex structure of the catalyst and the synergistic interaction between different elements on the electrocatalytic performance. However, the synergistic effects of HEMs and the functions of each element in the hydrolysis process cannot be determined by the current testing means. In addition, the current chemical parameters about the phase composition of HEMs and the laws affecting the phase formation are not perfect enough, leading to some discrepancies between the results and the actual performance. Therefore, the database and characterization tools should be further improved to enable the rational selection and design of HEMs materials with a high catalytic performance and a better understanding of the reaction mechanisms and activity centers.

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