Synthesis of High-Entropy Alloys and Their Application in Electrocatalytic Water Splitting (https://doi.org/10.63386/619919)

Zhou Xu^1,a,*,  Fanqi Gao^2,b, Danxuan Pan^1,c, Jiaman Wu^1,d

¹ School of Chemistry and Chemical Engineering, Anhui University of Technology, Maanshan24300, Anhui Province, China

² School of Energy and Environment, Anhui University of Technology, Maanshan24300,Anhui Province, China

^a13866424857@163.com

^b1249319136@qq.com

^cpdx31347@163.com

^d19836958807@163.com

*Corresponding author: Zhou Xu

Email: 13866424857@163.com

 Abstract

Energy shortage and environmental pollution are two major global challenges. The development and utilization of new energy green hydrogen, which is characterized by environmental protection and sustainability, has attracted more and more attention. Accordingly, the research and development of high-performance electrolytic water catalysts has become a hot research topic. In the past few years, high entropy alloys have attracted extensive attention from scholars due to their excellent mechanical properties, corrosion resistance, and outstanding thermal stability. With its unique high entropy characteristics and surface structure, high entropy alloy nanostructures have greater advantages in the field of catalysis and energy storage. The composition of HEAs with multiple elements can be easily adjusted by Sabatier’s principle to optimally adsorb reaction intermediates, thereby further improving HER/OER activity. In this work, we summarize and summarize the research results of high entropy alloys in electrolytic water in recent years, and put forward the opportunities and challenges of high entropy alloy electrolytic water in the future.

Keywords: High entropy alloy, hydrogen evolution reaction, oxygen evolution reaction, electrolysis of water.

1. Introduction

The U.S. Department of Energy released the report “The Basic Science of Carbon Neutral Hydrogen Energy Technology.” This report is key to zero-carbon hydrogen energy. Such energy may solve environmental and energy problems.The core of zero-carbon hydrogen energy is green hydrogen production^[1],[2]. However, to date, hydrogen production from fossil fuels and industrial by-products still predominates in the hydrogen production structure due to its lower cost. Nevertheless, this process generates significant carbon dioxide emissions as a by-product, which is detrimental to environmental protection; consequently, it necessitates the development of new technologies and materials^[3]–[4][5].

With the decline in fossil fuel production and the deepening of the concept of sustainable development, electrocatalytic water decomposition is considered to be the greenest and most sustainable method for renewable energy storage^[6],[7].  However, to date, hydrogen production from fossil fuels and industrial by-products still predominates in the hydrogen production structure due to its lower cost. Nevertheless, this process generates significant carbon dioxide emissions as a by-product, which is detrimental to environmental protection; consequently, it necessitates the development of new technologies and materials. In order to achieve the goal of net zero CO₂ emissions by 2050, renewable energy driven electrolyzed water decomposition must become popular as the main hydrogen production technology^[8]. In addition, the technology also serves as an essential energy storage approach to address the intolerable intermittency and unreliability of renewable energy. However, the popularity of this hydrogen production technology is limited by the high cost of electricity. Therefore, the development of economically efficient and stable electrocatalysts to drive the electrolyzed water decomposition reaction is key^[9],[10]. In order to improve the activity, selectivity and stability of the catalytic reaction, it is necessary to develop high-performance advanced catalysts that can meet the demands of rapid development^[11],[12].

In the past few years, high entropy alloys have attracted extensive attention in the scientific community due to their excellent properties, such as excellent mechanical properties^[13], corrosion resistance^[14], excellent thermal stability^[15] and so on. As an alloy with randomly mixed constituent elements and high configuration entropy, the concept of high entropy alloys was first proposed and defined by Cantor and Ye in 2004^{[16]–[17][18][19][20]}. High entropy alloys are increasingly applied in industry, aerospace^[21], welding^[22], and so on^[23].

It is noteworthy that, with its unique high entropy characteristics and surface structure, the nanostructure of high entropy alloys occupies a greater advantage in the field of catalysis and energy storage^[24]. The “high entropy” of high entropy alloys refers to the chemical or topological disorder at the atomic scale, that is, the atoms of the alloy are arranged in a disordered state. Due to its unique element composition, arrangement and interaction potential field, high entropy alloys produce some characteristics significantly different from those of traditional alloys^[25],[26]. Benefiting from greater configurational entropy, the composition of HEAs with multiple elements can be easily adjusted by Sabatier principle to optimally adsorb reaction intermediates, thereby further improving HER/OER activity^[27],[28]. Due to the high entropy property of stable structure, many high entropy alloys can show good structural stability, thereby further improving HER/OER stability^[29],[30].

In this review paper, the recent research achievements of high entropy alloys in water electrolysis are summarized and summarized, and the opportunities and challenges for the future of high entropy alloys in water electrolysis are proposed^{[31],[32][33]}. It not only includes the definition and characteristics of high entropy alloys, but also includes the various synthetic methods of high entropy alloys and their performance in water electrolysis. We will discuss the current trends, future directions and ongoing controversies in this field. We hope that this review will stimulate further interest in high entropy alloys by showing their many advantages in a wide range of catalysis-related applications (Fig. 1).

2. Properties of high entropy alloys

Figure 1. The design, preparation and application of high entropy alloys

HEA is a material category containing five or more relatively high concentration elements (5-35 at.%) ^[34]. Traditional metal alloys are usually composed of several specific elements, while high entropy alloys increase their complexity and entropy by increasing the number of elements and random distribution, so they have unique structure and properties^[35]. The preparation and application of high alloys are being studied and applied more and more widely, such as high temperature materials, high strength materials, corrosion resistant materials, etc. Different from traditional alloys, the unique composition characteristics of high entropy alloys make them have four core effects, including the high entropy effect in thermodynamics, the slow diffusion effect in dynamics, the lattice distortion effect in structure, and the cocktail effect in performance^[36],[37].

2.1 The high entropy effect in thermodynamics

The high entropy effect of high entropy alloys refers to the enhanced disorder in the preferential arrangement of different atoms when multiple atoms are mixed together, leading to the attainment of maximum entropy and thereby achieving higher thermodynamic stability^[38]. The entropy value of high entropy alloys surpasses that of traditional alloys, thereby reducing the free energy associated with solid solution formation and facilitating solid solution formation, particularly at elevated temperatures.

2.2 The slow diffusion effect in dynamics

The slow diffusion effect, also known as hysteretic diffusion effect, refers to the phenomenon where high entropy alloys exhibit a significantly slower diffusion rate at elevated temperatures compared to traditional metals. This is due to the presence of multiple elements in high entropy alloys that restricts atomic movement through various factors such as lattice defects and impurity atoms. Additionally, the complex atomic structure of high entropy alloys with synergistic effects from multiple elements further impedes atom mobility. This effect can lead to changes in material mechanical properties under extreme conditions or even result in material failure. Therefore, understanding and addressing the hysteretic diffusion effect in high entropy alloys is crucial for materials science research^[39],[40].

2.3 lattice distortion effects on the structure

The lattice distortion effect in high entropy alloys, arising from the irregular arrangement of atoms within the lattice structure, is a significant research focus due to its impact on material performance. This distortion occurs as a result of variations in atom size, electronegativity, and lattice parameters. Moreover, the presence of multiple elements in high entropy alloys leads to greater lattice distortion and defects due to differences in surrounding environments and atomic occupancy. Consequently, the mechanical properties, thermal stability, and corrosion resistance of high entropy alloys are influenced by this lattice distortion effect. It should be noted that a small degree of distortion may result in an FCC(Face-Centered Cubic) structure formation while larger degrees can lead to BCC(Body-Centered Cubic) structures or even amorphous structures^[41],[42].

2.4 Cocktail effect on performance

The “cocktail” effect of high entropy alloys refers to the phenomenon that the alloy formed by blending various elements exhibits properties that are not attainable in pure metals of any single element. This effect implies that the performance of the alloy can be effectively regulated by adjusting its composition and proportion^[43]. HEAs exhibit multi-dimensional characteristics, enabling multiple deformation mechanisms. This leads to exceptional comprehensive properties, including mechanical performance, thermal stability, corrosion resistance, and magnetic behavior, consequently offering significant application potential^[44].

Figure 2. Four properties of high – entropy alloys and synthesis methods

3. Preparation method of high entropy alloy

Various physical and chemical synthetic methods have been extensively investigated and reported to manipulate thermodynamic or kinetic factors for the production of solid-solution phase high entropy alloys. These high entropy alloys possess significant research value in the field of materials science, owing to their distinctive physical and chemical properties that offer numerous possibilities for diverse applications. To precisely control the size, phase structure, and composition distribution of nano-sized high entropy alloys, it is imperative to thoroughly explore the effects of synthetic methods and conditions^[45]. Particularly in catalytic applications, high entropy alloys have garnered considerable attention due to their exceptional performance. In order to meet the demand for high entropy catalysts, reducing catalyst size to the nanoscale is necessary to enhance specific surface area and thereby improve catalytic efficiency. The conventional wet chemical method has been widely accepted as a prevalent technique for preparing bimetallic/trimetallic alloy nanocrystals in scientific research endeavors^[46].

Fortunately, recent years have witnessed successful reports on non-traditional approaches that enable effective synthesis of multiphase high entropy catalysts (Fig. 2). These novel preparation methods not only facilitate fabrication of diverse-phase structured and compositionally varied high entropy catalysts but also allow precise control over catalyst size, morphology, and crystal phase – thus further optimizing their catalytic performance. Moreover, through comprehensive exploration of synthetic methods and conditions, structural regulation can be achieved alongside enhanced performance characteristics for high entropy alloys – offering expanded possibilities for their application across fields such as catalysis, magnetism electronics optics^{[42]–[47][48]}.

3.1 Carbon thermal shock synthesis

In recent years, carbon thermal shock (CTS)^[49] technology has emerged as a promising and versatile synthesis method for preparing high-performance nanomaterials^[50]. By rapidly reaching temperatures in the range of thousands Kelvin within milliseconds, this technique allows for the creation of unique non-equilibrium structures that cannot be achieved through traditional heating methods. As a result, there is significant research interest in utilizing carbon thermal flushing synthesis to produce high entropy alloys^[51].

The mixture of metal precursor salts was loaded onto carbon nanotubes, subjected to a rapid thermal shock of up to 2000K for 55ms at a rate of approximately 105K/s(Fig. 3a). High entropy alloy nanoparticles with 8 different elements were successfully prepared .By precisely controlling the parameters of the carbon thermal shock (including substrate, temperature, shock time, and heating/cooling rate), this method enables the production of high-entropy alloy nanoparticles with varying sizes and phases. Yao et al^[52]. successfully utilized this approach to prepare five distinct types of ammonia oxidation catalysts that exhibited exceptional stability and selectivity properties. Moreover, this preparation method is scalable since the input ratio of metals is faithfully replicated in the final product (Fig. 3b-d). However, this technique is restricted in its application for large – area uniform heating due to its reliance on conductive support materials and the fact that Joule heating efficiency is influenced by the quality of carbon.

The team led by Cha et al.^[53] has innovatively developed a photothermal synthesis method. This method does not rely on the selection of specific materials, is compatible with ambient air, and is applicable to large – scale and remote operations. Through single – flash irradiation (temperature T > 1800°C, duration 20 ms, heating/cooling rate > 10^. K s^-1) – induced transient high – temperature annealing, high – entropy alloy nanoparticles containing up to nine elements have been successfully fabricated (Fig. 3e-f), demonstrating promising potential for large – scale synthesis. In particular, the specially – designed six – component high – entropy alloy nanoparticles (PtIrFeNiCoCe) have demonstrated excellent activity (ηoverall = 777 mV) and remarkable long – term stability (over 5000 cycles) during water electrolysis, opening up new perspectives for the development of efficient water – splitting catalysts. CTS has strong controllability, and its unique processing under high temperature and rapid heating/cooling rates, as well as the induced particle splitting/fusion mechanisms, differ from other HEA synthesis methods and can avoid phase separation.

Figure 3. CTS synthesis of HEA-NPs on carbon supports. (a) Time evolution of temperature during sample preparation and 55-ms thermal shock. (b-d) Microscopic images of microprecursor salt particles on carbon nanofiber (CNF) carriers before thermal shock, and of well-dispersed nanoparticles (PtNi) synthesized after thermal shock^[52]. Copyright 2018, Science. (e) Ultrafast Joule heating and spontaneous cooling plotted against mS pulse spacing. (f) Illustration of the sample temperature during the passage of electric current^[53]. Copyright 2022, Wiley-VCH GmbH.

3.2 Wet chemical methods

As a pivotal strategy for the synthesis of HEA NPs, the wet chemical method enables the precise construction of multi-component solid-solution alloys through the co-reduction of metal precursors and the regulation of reaction kinetics^[54]. Based on solution-phase reactions, this method leverages the synergistic control of solvent polarity, reducing agent strength, and stabilizer coordination to direct the nucleation-growth process of nanocrystals under mild conditions^[55]. This allows for fine-tuned control over crystallographic orientation, size distribution, and morphology (such as polyhedra, dendritic, and hollow structures).

Unlike traditional alloys, which are predominantly governed by the thermodynamics of intermetallic compound formation, the synthesis of HEA NPs must overcome the tendency for phase separation arising from differences in reduction potentials and lattice mismatches among components within the high-entropy system^[56]. Therefore, the wet chemical method represents a highly promising strategy for synthesizing high-entropy alloys with desired nanostructures.

Wei et al.^[57] first employed the wet chemical method to fabricate Pt (Co/Ni) MoPdRh high-entropy alloy nanofibers (NFs) with a thickness of approximately 1.8 nm, which exhibited excellent HER performance in alkaline environments (Fig. 4a-b). Their mass activity was 6.38 times that of commercial Pt/C catalysts. The high-entropy alloy, with its abundant active sites, enabled effective reduction of the water dissociation energy barrier and facilitated hydrogen adsorption. Combined with the lattice distortion effect induced by its unique structure, it collectively enhanced the catalytic efficiency and long-term stability of the HER. In addition, Liu et al^[58]. prepared PtAuPdRhRuhigh- HEAs-NPs using an ultrasonic synthesis method (Fig. 4c-d). Ultrasound can induce the formation, expansion, and collapse of bubbles in a liquid, generating local hotspots through the conversion of kinetic energy from liquid motion. During the ultrasound process, local regions can reach temperatures of 5,000 ℃ within a timescale of 9 to 10 seconds, which can directly accelerate the reduction of metal ions. This achievement not only broadened the application scope of the wet chemical method in the preparation of complex alloy systems but also further confirmed its great potential in the synthesis of nanostructured high-entropy alloy electrocatalysts.

Recently, Cao et al.^[59] developed a wet chemical method of ultraviolet-induced radical reduction under ambient conditions for the synthesis of HEA nanoparticles (Fig. 4e-f). A simple and robust wet chemical method for the synthesis of HEA nanoparticles under ambient conditions was reported by utilizing highly reducing carbon-centered isopropyl alcohol radicals generated by ultraviolet irradiation. These isopropyl alcohol radicals were verified by electron paramagnetic resonance spectroscopy. By applying a very large overpotential, different metal ions were reduced into HEA nanoparticles containing 5 to 7 different elements.

Although the wet chemical method plays an important role in the synthesis of nanostructured high-entropy alloy electrocatalysts due to its simplicity and low equipment requirements, its eco-friendliness still needs to be improved. Relevant studies have shown that the wet chemical method is effective in preparing nanoscale HEAs because it can promote the simultaneous reduction of all metal precursors to form solid-solution alloys. However, when there is a significant difference in the redox potentials of the individual components, it can lead to severe phase separation in the resulting NPs.

Figure 4. (a) The synthesis scheme of PtCoMoPdRh/PtNiMoPdRh nanoframes (NFs). (b) HAADF-STEM image and (f1-5) the corresponding EDS mapping of PtCoMoPdRh NFs^[57]. Copyright 2023, The Royal Society of Chemistry. (c) Schematic illustration of the synthesis of PtCoMoPdRh NFs. (d) HAADF-STEM image of PtCoMoPdRh NFs and (f1-5) corresponding EDS elemental distribution maps^[58]. Copyright 2019, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (e) Free radical-assisted wet chemical synthesis of HEA nanoparticles on rEGO support: Schematic diagram of the synthesis process. (f) STEM-EDS elemental maps of Pt, Pd, Ir, Rh, Au, Ag and Cu in the PtPdIrRhAuAgCu-rEGO sample (Pt Lα, Pd Lα, Ir Lα, Rh Lα, Au Lα, Ag Lα and Cu Kα) ^[59]. Copyright 2025, American Chemical Society

3.3 Laser synthesis method

Laser synthesis utilizes focused laser-induced photothermal effects to rapidly generate localized high-temperature zones under specific conditions. It has been proven effective for the fabrication of monometallic and binary alloys. Moreover, it represents a novel approach for preparing HEA NPs, offering excellent reproducibility and scalable elemental composition.

WAAG et al.^[61] sequentially mixed single metals, pressed, and heated the single-metal powders to fabricate a laser ablation target (Fig. 5a). In an ethanol solution, an ultrashort picosecond-pulsed laser was irradiated onto the surface of the ablation target, thereby exciting a large number of metal atoms and forming a plume through atomization and ionization. Cavitation bubbles were formed at the solid-liquid interface, and within these bubbles, CoCrFeNiMn HEA NPs were grown (Fig. 5b). As the cavitation bubbles burst, the NPs were released into the ethanol and stabilized by electrostatic interactions. This demonstrates that the method used for synthesizing binary alloys is also applicable to HEAs (Fig. 5c). With the development of machine learning, the use of laser synthesis for the efficient exploration and optimization of the composition and properties of high-entropy alloys has become more widespread. Xie et al^[62]. proposed a method for synthesizing oxygen-free high-entropy alloy nanoparticles (HEA-NPs) in air using laser scribing (Fig. 5d), successfully preparing a library of single-phase solid-solution HEA-NPs. Furthermore, by combining active learning with high-throughput synthesis strategies and through multiple iterations, the quinary Pt₂₇Pd₂₄Fe₂₁Co₂₀Ni₈ HEA-NP catalyst composition was proven to have the lowest overpotential for Li-CO₂ batteries (Fig. 5e). This work provides the possibility of designing oxygen-free HEA NPs with precise compositions across a wide range of elements and significantly accelerates the potential applications of HEA NPs.

Laser synthesis can achieve precise compositional control and offers high reproducibility, holding promise for the large-scale production of high-quality high-entropy alloy nanocatalysts.

Figure 5. Qualitative representation of the laser-based synthesis of high-entropy alloy nanoparticles. The synthesis method consisted of the following stages: ultrashort-pulsed laser irradiation of the bulk HEA (a),the atomization/ionization of the bulk causing the formation of a plume, and subsequent nucleation and condensation of the ablated matter in the vapor phase of the liquid (b) and the colloidal high-entropy alloy nanoparticles electrostatically stabilized in ethanol (c) ^[61]. Copyright 2019 The Royal Society of Chemistry. (d) Schematic diagram for the synthesis of single-phase HEA solid-solution NPs and phase-separated oxides on different hosts via the FLASH strategy.(e) Schematic diagram of active learning optimization of Li-CO₂ batteries using the PtPdFeCoNi cathode database.^[62] Copyright 2024, American Chemical Society

3.4 Electrodeposition method

Electrospinning is a versatile and practical method for producing ultrafine fibers. The development of electrospinning technology and nanofiber engineering has made great strides in enabling a wide range of applications. Carbon nanofibers (CNFs) have excellent electrical and thermal conductivity, surface area and tailoring capabilities and are commonly used as support materials^[63],[64].

In view of the above methods, in order to solve the dissolution, oxidation and amorphization process of Ir-based electrocatalysts, Zhu et al.^[65]  prepared ultra-small FeCoNiIrRu HEA nanoparticles by electrostatic spinning, Fe,Co,Ni,Ir,Ru (Fig. 6a) salts in polyvinylpyrrolidone (PVP) solution, mixed homogeneously by electrostatic spinning technique to obtain FeCoNiIrRu/CNFs (Fig. 6b-d), and EDS mapping further verified that the elements uniformly formed high-entropy alloy nanoparticles (Fig. 6e), and this prepared high-entropy alloy could effectively utilize the mass transfer channels of nanofibers to fully expose the active sites, which resulted in the excellent catalytic activity of the prepared catalysts, and thanks to the hysteresis-diffusion effect of high-entropy alloy This property strongly inhibits the leaching and dissolution of metals, resulting in excellent durability of the catalysts. The limited understanding of the relationship between metal sites and reaction intermediates in HEA hinders the solution of these fundamental problems of HEA electrocatalysts, and for this reason Zhu et al. similarly combined low electronegativity Mn and high-electronegativity Cu with Fe, Co, and Ni transition metals to form face-centered cubic-phase FeCoNiCuMn HEA NPs by electrospinning method to manipulate the electrocatalytic performance by tuning the competing adsorption sites in the HEA NPs(Fig. 6f). This high-entropy atomic environment tuning strategy proposed by Zhu et al. Not only successfully converts the inactive sites into active sites for electrocatalysis but also provides a good basis for understanding the mechanism of electrolysis of water in high-entropy alloys (Fig. 6g-j).

In the field of high-entropy alloy synthesis, the production of ultrathin nanofibers using electrospinning is a common practice. High entropy alloy nanoparticles can be produced in situ on ultrathin carbon fibers by electrospinning appropriate proportions of mixed metal precursors. This method is an efficient way to synthesize high-entropy alloy nanoparticles.

Figure 6. The combination of electrostatic spinning and graphitisation techniques allows the fabrication of HEAs. (a) the electrostatic spinning process for the synthesis of FeCoNiIrRu HEA. (b) the FE-SEM image of the FeCoNiIrRu/CNFs composite. (c) the HRTEM images of randomly selected FeCoNiIrRu HEA nanoparticles. Finally, (d) the TEM images of the FeCoNiIrRu HEA nanoparticles. FeCoNiIrRu/CNFs. (e) STEM-EDS mapping image of a FeCoNiIrRu NP supported on CNFs^[65]. Copyright 2022, Elsevier. (f) Synthesis procedure of HEA/CNF hybrid nanomaterials through a polymer fiber nanoreactor. (g) FE-SEM and (h) HAADF-STEM images of FeCoNiCuMn HEA/CNFs. (i) STEM-EDX mapping images of the HEA NPs supported on CNFs. (j) XRD patterns of the HEA/CNFs^[65]. Copyright 2023, Royal Society of Chemistry.

3.5 Electrodeposition method

Nanomaterials or alloys prepared by electrodeposition have the advantages of excellent binding, high density, and high potential for application and production. In particular, catalysts for many high melting point elements can be prepared by this method. In this process, various coating properties can be achieved by changing the current density, applied potential and plating solution conditions. The preparation of HEA coatings by electrodeposition was first reported by Yao et al.^[67] After the development in recent years, the electrodeposition strategy has gradually become an effective technique for the preparation of self-supported high-entropy electrodes.

Different from the energy-intensive arc melting and mechanical alloying methods, the Lu  et al.^[68] research adopted a fast, simple, low-temperature and scalable pulsed current electrodeposition method. Using a properly configured metal lacquer to lift the electrolyte, an equimolar FeCoNiCuMo high-entropy alloy was in-situ grown on nickel foam (Fig. 7a). Due to the bubble generation from the hydrogen evolution side reaction during electrodeposition, the synthesized high-entropy alloy exhibits a three-dimensional dendritic structure. FeCoNiCuMo has a solid solution FCC phase structure without any phase separation (Fig.7b-c). The synergistic effect among uniformly distributed atoms in FeCoNiCuMo enables it to exhibit excellent catalytic activities for hydrogen evolution reaction (HER) and OER in both alkaline and acidic media, while demonstrating remarkable stability and strong corrosion resistance. In contrast to the above work, Chen et al. ^[69] also prepared a high entropy alloy on nickel foam by electrodeposition, and the CoFeNiCrMn high entropy alloy is P-doped and amorphous, and shows excellent performance in HER and hydrazine oxidation HzOR. The five elements closely and uniformly form nanosheet morphology, and the XRD characterization better illustrates the high entropy disorder induced by strong intermetallic effect, which makes the generated high entropy alloy amorphous and amorphous.

Figure 7. a. Preparation schematic illustration of H-FeCoNiCuMo b. SEM image c. HRTEM image and (inset) selected area electron diffraction image ^[68] Copyright 2022, Elsevier B.V.  d. The schematic representation for the synthesis of CoFeNiCrMnP/NF sample. e. HRTEM image of CoFeNiCrMnP/NF h. elemental mapping images of CoFeNiCrMnP i. The XRD pattern of CoFeNiCrMnP/NF.^[69] Copyright 2023, Wiley‐VCH GmbH

3.6 Other synthesis methods

The preparation techniques for HEA catalysts exhibit diverse characteristics, such as rapid moving bed pyrolysis^[70], microwave-assisted method^[71], and mechanical milling^[72] alloying. The selection of actual process routes requires comprehensive consideration of specific application scenario requirements and alloy composition design indicators. In-depth exploration of the performance advantages and application boundaries of various synthesis technologies is of critical value for optimizing the preparation processes of HEA catalysts and expanding their innovative applications in multiple fields.

Table 1. Main synthesis methods for HEA catalysts and their respective advantages and disadvantages.

4. Applications of High Entropy Alloys in Water Electrolysis

Currently, hydrogen production through water electrolysis accounts for less than 5% of the total industrial hydrogen production, but it has significant potential in terms of future technological development and field promotion^{[73]–[74][75]}. In this regard, electrocatalytic water splitting is considered a very safe and efficient method for hydrogen production. electrocatalytic water splitting involves two half-reactions (Fig. 8) ^[76], namely the HER at the cathode and the OER at the anode. Both reaction processes require a certain overpotential to drive the continuous progress of the overall reaction. In order to improve the efficiency of the electrocatalytic process and reduce energy consumption, the assistance of electrocatalysts is usually required to overcome the relatively large overpotential^[77],[78].

High entropy alloys possess a large number of grain boundaries and interfaces in their microstructure, thus providing abundant opportunities for hydrogen evolution during the process of water electrolysis. In addition, high entropy alloys exhibit outstanding stability, strong corrosion resistance, rich resource availability, and recyclability, which makes them highly promising catalysts for water electrolysis^[79]. The catalytic effect of high entropy alloys in water electrolysis is closely related to their unique microstructure. A large number of grain boundaries and interfaces endow these materials with a great quantity of active sites, promoting the electrochemical reactions involved in water electrolysis. Meanwhile, the synergistic interactions among multiple elements in high entropy alloys further enhance their catalytic activity^[80],[81].

Figure 8. Schematic Diagram of Electrocatalytic Device^[76] Copyright 2015, Royal Society of Chemistry.

4.1 Hydrogen evolution reaction

According to different media, the reaction process of HER in acidic and alkaline electrolytes can be expressed by the following formulas, where * represents the active sites on the surface of the catalyst:

HER under acidic conditions:

HER under acidic conditions:

The HER, as a crucial step in water electrolysis, involves a complex multi-step electrochemical process on the surface of the catalyst^[82]. Although the reaction mechanisms in acidic and alkaline media are slightly different, they both start with the Volmer step, in which electrons are transferred to the catalyst surface and combine with protons to form adsorbed hydrogen atoms (H*). This step is generally regarded as the rate-limiting step of the hydrogen evolution reaction^{[83]–[84][85]}.

In the field of water electrolysis catalysis, Pt-group noble metal catalysts still remain the most superior. However, the inherent scarcity and high cost of Pt-group noble metals severely limit their large – scale application in the water electrolysis field^[86]. Therefore, researchers have developed a series of Pt-based high – entropy alloy catalysts, aiming to achieve the dual goals of cost reduction and catalytic activity improvement through multimetallic synergistic effects.

Glasscott et al. ^[87] successfully prepared CoFeLaNiPt HEA NPs with equal elemental proportions using an electric shock method and applied them to HER catalysis. Studies show that under a current density of 10 mA·cm^-2, the overpotential of this catalyst is lower than that of single – metal catalysts such as Fe, Pt, and Co (Fig. 9a). In addition, this high-entropy alloy also exhibits the ability to catalyze OER, serving as a bifunctional catalyst for water electrolysis, whose performance benefits from the synergistic effect among various elements.

Ultra-small high-entropy alloy nanoparticles can exhibit excellent hydrogen evolution performance, which is mainly attributed to their extremely high atomic utilization efficiency, synergistic interaction among multiple components, and tunable electronic structure properties for hydrogen production^[88]. Furthermore, the catalytic performance can be further enhanced by reducing the catalyst size to increase the specific surface area. Xia et al.^[89] successfully synthesized the currently smallest high-entropy alloy nanoparticles (us-HEANPs) via chemical co-reduction, with an average diameter of 1.68 nm. Benefiting from the synergistic effect among the five elements and the tunable electronic structure, the desorption of H* at the Pt, Rh sites and Fe/Co/Ni sites on the (111) crystal plane of us – HEA is easier than that on the Pt (111) surface. Therefore, NiCoFePtRh exhibits extremely high hydrogen evolution activity in acidic environments, with a mass activity of up to 28.3 A·mg^-1(Fig. 9b-c). It still demonstrates excellent catalytic activity after 10,000 cycles (Fig. 9d).

The application of high-entropy materials composed entirely of non-noble metals to HER has become a recent research hotspot. Lang et al.^[90] research team described the application of self-supported nanoporous NiFeCoCuTi HEAs prepared in situ on Ni foam. Its 3D interconnected columnar nanostructure promotes electron/mass transfer in the hydrogen evolution reaction. The self-supported NiFeCoCuTi electrode exhibits excellent performance in alkaline hydrogen evolution reaction, achieving a current density of 2 A·cm^-2 with only a 200 mV overpotential (Fig. 9e). the Ti element significantly enhances the hydrogen evolution reaction activity, due to the adsorption of *OH by Ti atoms, which forms an electrocatalytic core with multiple active sites on the alloy surface (Fig. 9f).

Figure 9. (a) Electrocatalytic evaluation of a CoFeLaNiPt HEMG-NP electrocatalyst.^[87] Copyright 2019, The Author(s) (b) ECSA of noble-metal mass loading. (c) Comparisons between the mass activities. (d) Modern noble metal catalysts with state-of-the-art mass activity evaluations of us-HEA/C at 0.05 V with RHE have been revealed in latest literature. (e) Polarization curves for alkaline HER. (f) Comparison of specific activities for nanoporous alloys, and bare Ni sites at the overpotential of 200 mV. Reproduced with permission from Re.^[90] Copyright 2020, Elsevier Inc.

4.2 Oxygen evolution reaction

According to different media, the reaction processes of OER in acidic and alkaline electrolytes can be expressed by the following formulas, where * represents the active sites on the surface of the catalyst:^[91]

OER under acidic conditions:

OER under acidic conditions:

The reaction process of OER electrocatalysts mainly involves the reaction between the active sites and four OH⁻ ions. Generally, the activity of OER electrocatalysts is related to the magnitude of∆G_OOH*−∆G_OH*, which is the process shown in Equation (14) As indicated by the above equation, the OER process is more complex than the HER^[93]. The entire reaction involves various reaction intermediates in acidic and alkaline electrolytes, including OH*, O*, and OOH*. OER involves the coupled transfer of multiple protons and electrons, which also leads to slow kinetics and the necessity of overcoming a relatively large overpotential. Therefore, OER is a crucial process in technologies such as water splitting, carbon dioxide reduction, and metal-air batteries^{[94]–[95][96]}.

Currently, the most advanced OER electrocatalysts are Ir^[97] and Ru^[98] metals and their corresponding oxides. However, their high cost severely limits their practical applications in water splittings. Therefore, the development of low-cost OER electrocatalysts with multiple active sites and high activity has become an urgent issue. HEAs, by virtue of their inherent multi-component nature, can provide abundant active sites. Through rational selection of elements and precise regulation of component ratios, these materials show great potential for achieving efficient OER performance^[99].

The alloying strategy of Ir can not only significantly reduce the cost of OER catalysts but also enhance the intrinsic electrocatalytic activity through the synergistic effect of alloys with unique electronic structures. Jin et al.^[100] investigated a nanoporous Ir-based quinary AlNiCoIrX (X=Mo/Cu/Cr/V/Nb) HEA library and found that these materials exhibited excellent OER performance in acidic environments. Among them, the catalyst with the composition of AlNiCoIrMo showed the best electrocatalytic activity. When assembled as a bifunctional catalyst in an electrolyzer, its performance outperformed the commercial Pt/C-IrO₂ electrolyzer (Fig. 10a). The outstanding catalytic activity of nanoporous HEAs stems from two aspects: on one hand, the porous structure facilitates gas transport on the surface; on the other hand, the surface-formed oxides/hydroxides adsorb and stabilize a large number of active intermediates during the OER process.

To explore noble-metal-free porous HEA systems and demonstrate the potential advantages of multi-element alloy systems, Qiu et al.^[101] combined precursor alloy design with a dealloying strategy to prepare nanoporous HEAs with surface-coated high-entropy (oxy)hydroxides for OER. The AlNiCoFeX (X=Mo/Nb/Cr) composition showed the highest OER activity. Compared with ternary and quaternary HEAs, the formation of quinary HEAs significantly improved the electrochemical cycle stability (Fig.10 b). CAI et al.^[102] prepared nanoporous ultra-high-entropy alloys (np-UHEAs) with hierarchical porosity through alloying. Composed of 14 metal elements including Al, Ag, Au, Co, Cu, Fe, Ir, Mo, Ni, Pd, Pt, Rh, Ru, and Ti, these elements are highly uniformly distributed. The as-prepared np-UHEAs catalysts exhibited excellent catalytic activity and long-term stability for both HER and OER in acidic media (Fig. 10 c-e).

In promoting the large-scale commercial application of OER electrocatalysts, reducing material costs is one of the core challenges to be addressed. Introducing transition metal elements (such as Fe, Co, Ni, etc.) into HEA electrocatalysts not only effectively reduces the catalyst preparation cost but also facilitates the construction of synergistic effects between different elements. More importantly, transition metal elements are prone to oxidation, which helps in the in-situ formation of metal oxides/hydroxides on the catalyst surface during the reaction. These act as efficient OER active sites, significantly enhancing reaction kinetics and material stability.

Figure 10. (a) OER polarization curves of different AlCoNiIr-based HEAs and IrO₂ in 0.5 M H₂SO₄, Tafel plots, and LSV curves for overall water splitting;^[100] Copyright 2019, WILEY‐VCH (b) OER polarization curves of different AlNiCoFe-based HEAs, Tafel plots, and comparison of the catalytic activity of different component alloys after different CV cycles^[101]. Copyright 2019, American Chemical Society (c) Polarization curves and (d) the corresponding Tafel plots of commercial 20% Pt/G, np-UHEA12, np-UHEA13, and np-UHEA14 electrodes for the HER in 0.5 M H₂SO₄. (e) Chronopotentiometry curves (V–t) of the np-UHEA12 electrode and commercial 20% Pt/G electrode in 0.5 M H₂SO₄ at a constant current density of 10 mA cm^-2[102].

4 Conclusion and Prospects

The research achievements of high entropy alloys in water electrolysis in recent years are summarized and summarized, and the opportunities and challenges for the future of high entropy alloys in water electrolysis are proposed. It not only includes the definition and characteristics of high entropy alloys, but also includes the exploration of various synthetic methods of high entropy alloys and their performance in water electrolysis. It is of practical significance for the development of advanced HE electrocatalysts and their catalytic applications. In addition, the in-depth study of the catalytic mechanism of HE catalysts also helps to better understand the role of metal elements in HEMs, thus greatly promoting the improvement of electrocatalytic performance. Despite great progress, there are still some challenging problems in synthesis, reaction mechanism and performance, which need further study.

Although many effective synthesis methods have been proposed, these methods are complex and energy-wasting. Therefore, it is necessary to develop more convenient and green methods to achieve rapid and green synthesis of advanced HE catalysts.

Due to the high complexity of HEA, the understanding of its structure-activity properties remains unclear. By deepening the understanding of the catalytic sites and considering the synergistic effects brought by different component types, catalysts can be designed more effectively. Therefore, further research on its growth mechanism and reaction mechanism is needed.

The design of high entropy alloy catalyst by means of mechanization can further enhance its research value in electrolysis water and save manpower. Based on the above research and analysis, the following forward-looking considerations and strategic suggestions are put forward regarding the subsequent development of high entropy alloys in the field of electrocatalysis:

1. a) In recent years, density functional theory (DFT) calculations, high-throughput screening, machine learning, and their integrated methodologies have advanced rapidly in the field of catalytic materials research, establishing a sophisticated framework for catalyst discovery and rational design. Advanced computational approaches are instrumental in optimizing the performance of HEA electrocatalysts: efficient algorithms and multi-scale simulations enable the precise identification of active sites at the atomic level, guiding the regulation of composition and structure; first-principles calculations unveil the mechanistic roles of individual elements within multi-component alloys and illuminate their synergistic effects. This line of research provides theoretical underpinnings for elucidating the origin of catalytic activity and reaction pathways in HEA electrocatalysts, facilitating the development of cross-scale correlation models.
2. b) In the field of HEA synthesis, despite the reported development of non-equilibrium preparation techniques that enable the fabrication of nanoscale HEA nanoparticles (NPs), their synthesis still relies on harsh conditions such as high pressure, elevated temperatures, and inert atmosphere protection. Moreover, significant bottlenecks persist in achieving atomic-level control over elemental distribution and arrangement within HEAs.

Currently, there is an urgent need to develop novel synthetic pathways that enable precise control over composition, crystal phase, particle size, and atomic arrangement with uniform homogeneity. Simultaneously, overcoming the technical challenges of large-scale, high-yield synthesis under mild conditions remains a critical frontier. Addressing these bottlenecks will be pivotal for advancing the practical application of HEAs as heterogeneous catalysts in chemical engineering, environmental remediation, and other industrial sectors.

1. c) In the field of catalytic mechanism elucidation for high-entropy alloys (HEAs), the complexity of their catalytic systems increases significantly with the number of constituent elements, far exceeding that of single-metal and bimetallic catalyst systems. Therefore, in-depth analysis of HEA catalytic reaction mechanisms urgently requires the use of high-precision in situ characterization techniques. These techniques enable real-time acquisition of core information such as dynamic evolution of material structure and composition, and state changes of surface-active sites under actual working conditions, providing empirical evidence for precise mechanistic interpretation.

From a technical implementation perspective, in situ transmission electron microscopy (TEM) and scanning electron microscopy (SEM) allow atomic-scale observation of dynamic evolution in morphology, component distribution, and crystal structure of HEAs during growth processes or electrochemical reactions. In situ X-ray photoelectron spectroscopy (XPS) combined with X-ray absorption spectroscopy (XAS) can accurately reveal the evolution laws of elemental composition, chemical valence states, and electronic structures during electrocatalysis. Meanwhile, in situ Raman spectroscopy provides molecular-level structural fingerprint information for identifying true catalytic active sites and parsing reaction pathways by capturing molecular vibration characteristics (such as vibration modes of C-C, C=C, N-O, and C-H bonds).

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