Recent Progress in High‐Entropy Alloy Electrocatalysts for Hydrogen Evolution Reaction

High‐entropy alloys (HEAs) materials, as promising nanomaterials, have garnered significant attention from researchers due to their excellent performance in the field of hydrogen evolution reaction (HER). The four core effects of HEAs, including the high‐entropy effect, severe lattice distortion effect, sluggish diffusion effect, and cocktail effect, are pivotal in underpinning their remarkable mechanical and thermodynamic properties. Nevertheless, the intricate geometric and electronic structures of HEAs make their catalytic mechanisms exceptionally complex and challenging to decipher. In particular, a thorough analysis of the underlying factors responsible for the outstanding catalytic activity, selectivity, and the ability to maintain stable hydrogen production, even at high current densities, in HEAs is lacking. To provide a systematic exploration of the design and application of HEAs in HER systems, this review commences with an examination of the physicochemical properties of HEAs. It covers a wide range of topics, including the synthesis methods of HEAs, and the major reaction mechanisms of HERs, and presents innovative methods and approaches for designing HEAs specifically in the context of HERs.


Introduction
High-entropy alloys (HEAs) showcase exceptional characteristics, excelling in terms of strength, toughness, corrosion resistance, and thermal stability. [1]The HEAs have been found to DOI: 10.1002/admi.202301020 have a wide range of applications in various fields, including serving as high-temperature materials for hypersonic aircraft engine components, radiation-resistant materials for nuclear energy, lightweight materials in the aerospace sector, and catalysts for different reactions. [2]The development of HEAs can be traced back to 1788.A metallurgist named Achard published a French book titled "Recherches sur les Propriétés des Alliages Métallique," documenting over 900 alloy structures, including multi-principal element alloys with 5-7 different elements. [3]In 1981, Cantor et al. achieved a world record by crafting an alloy consisting of 20 distinct elements, each contributing 5% to the composition.In 2004, Cantor et al. [4] published a series of articles on multi-component alloys in public journals, capturing the attention of researchers.In that same year, Yeh et al. [5,6] independently investigated multi-component alloys and coined the term "high-entropy alloys."This marked the initial presentation of the high-entropy alloy concept.
The distinctive characteristics of HEAs can be attributed to four core effects resulting from the combination of multiple components: high-entropy effect, severe lattice distortion effect, sluggish diffusion effect, and cocktail effect. [7]The high-entropy effect is characterized by the observation that the entropy of the hybrid configuration increases proportionally with the number of alloying components. [8]The greater the entropy of the mixed configuration, the more significant the impact on the formation of the alloy phase becomes. [9,10]The mixing entropy (S mix ) exhibits a notable increase as an equal-atomic ratio alloy system with n elements transitions from a pure metal to a randomly mixed phase state. [11]As an illustration, the mixing entropy of a fivemember alloy, measuring 1.61 R, surpasses that of a low-entropy alloy (0.69 R) and even surpasses the entropy of melting observed in most conventional metal alloys (1 R). [12] At a given pressure, a five-membered alloy system theoretically has six equilibrium phases according to the Gibbs phase rule.However, in practice, the five-membered alloy system often forms a stable single solid solution phase, which is significantly fewer in number than what is predicted by the Gibbs phase rule.Therefore, the high entropy effect provides an explanation for the discrepancy between the actual number of phases formed in high-entropy alloys and the higher number predicted by theoretical calculations. [10]Moreover, the high mixing entropy plays a crucial role in reducing the free energy of the alloy and enhancing its stability.This is primarily due to the fact that a high mixing entropy promotes the formation of random solid solutions.When the mixing entropy of an alloy surpasses the mixing enthalpy, it favors the development of high-entropy solid solution alloys.These alloys tend to adopt simple crystal structures such as face-centered cubic or body-centered cubic, comprising multiple major elements in a random arrangement.The high mixing entropy in the alloy significantly enhances the degree of mixing.This leads to a random distribution of metal atoms within lattice positions, reducing the likelihood of atomic ordering and segregation.Consequently, the alloy system exhibits improved stability and a reduced propensity for phase separation or the formation of detrimental intermetallic compounds. [13]n high-entropy alloys, the random occupation of different elements within the lattice matrix, characterized by variations in atomic sizes, bond configurations, and lattice potentials, can lead to significant lattice distortion influenced by atomic size, chemical bonding, and electron distribution between constituent elements. [14]The imbalance in atomic positions within the lattice due to random distribution results in pronounced lattice distortions, as larger atoms displace neighboring atoms to occupy more space, while smaller atoms fill in the remaining spaces.This leads to compressive strains caused by larger atoms and tensile strains caused by smaller atoms within the lattice. [15]As a result, the elevation in free energy caused by lattice distortions leads to decreased stability, thereby influencing a wide range of physical and chemical properties exhibited by the lattice. [16]The bonding energy between atoms is an additional factor contributing to lattice distortion, where stronger bonds result in smaller bond distances compared to weaker bonds.Consequently, if the nature and structure of the principal elements differ significantly, the material system may not sustain its original stable lattice, leading to collapse and the formation of an amorphous structure.Furthermore, lattice distortion induces atomic-scale fluctuations in residual strain, which can result in the shifting of the center of the d-band in the alloy.This, in turn, influences the adsorption patterns and catalytic selectivity of intermediates. [17]p until now, the profound lattice distortions present in HEAs have been directly observed through advanced techniques such as high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and neutron scattering, providing valuable insights into their structural characteristics. [18,19]n phase transitions governed by diffusion, the formation of new phases usually requires the coordinated diffusion of elements to achieve an equilibrium distribution of phases. [17]In solid solutions with high concentrations of multiple elements, the diffusion of atoms becomes more challenging, primarily due to fluctuations in the bonding environment within the lattice. [20]he principal elements in HEAs can be considered as both solute and solvent atoms, leading to increased resistance and decreased rate of atomic diffusion.This hinders phase separation, as well as the formation and growth of new phases, resulting in the sluggish diffusion effect on the kinetics of HEAs. [17]Consequently, each site in the lattice possesses a distinct local energy attributed to various local atomic configurations and their corresponding bonding characteristics.Numerous sites with low lat-tice potential energy act as atom traps, impeding atom diffusion.Therefore, when an atom transitions to a low-potential state, the probability of it escaping from that state is considerably reduced.On the other hand, if an atom moves to a high potential state, it is more likely to return to its original position.This is because the energy barrier and activation energy for diffusion increase under such conditions.As a result, both scenarios described aboveatoms trapped in low potential states and atoms facing high energy barriers-contribute to hindering atomic diffusion and impeding particle coarsening. [17]Low-energy sites exhibit considerably longer occupation times, ≈1.73 times longer than highenergy sites, owing to the substantial energy differences between them. [20]The sluggish diffusion effect during the solidification process results in low nucleation and growth rates, leading to the formation of ultrafine crystals or even amorphous phases. [14]Furthermore, the sluggish diffusion effect causes supersaturation and promotes fine precipitation, resulting in an elevated recrystallization temperature and reduced grain coarsening.These advantages facilitate better control over the microstructure, thereby influencing the catalytic properties of the material.
The cocktail effect, initially proposed by Ranganathan [21] arises from the fundamental characteristics of the constituent elements and their interactions.This phenomenon considers the impact of multiple primary elements on the macroscopic properties of an alloy at the atomic scale.Essentially, the microscopic properties of the principal elements in an alloy are directly reflected in the macroscopic properties of the alloy, encompassing a wide range of characteristics such as density, strength, oxidation resistance, and corrosion resistance. [22,23]The overall properties of an alloy depend on various factors, including the shape, distribution, and boundaries of the phases present, as well as the collective contribution of each phase to the material's properties.These properties are not solely determined by the individual characteristics of the constituent elements according to the mixing rule, but also by the interactions between these elements.This synergistic effect is prominently observed in the thermoelectric response, mechanical properties, and magnetic properties of alloys. [24]Scientists widely acknowledge that the cocktail effect in HEAs is a complex synergistic mechanism that contributes to their exceptional catalytic properties.By incorporating highly active metal components, such as noble metals or transition metals, into alloys, the catalytic activity is significantly enhanced.This enhancement is attributed to the synergistic mixing of elements and the ability to easily adjust the electronic and geometric structure, all of which are attributed to the "cocktail" effect in high-entropy alloys.It is worth highlighting that the mechanism underlying the multicomponent synergy in HEAs is still not fully understood.Further investigation is needed to explore potential synergistic mechanisms from the perspectives of lattice distances and electron distributions. [17][27][28] Particularly in HERs, the HEAs demonstrated outstanding catalytic activity.Moreover, HEAs enable efficient and stable hydrogen production even at high current densities.The distinctive physicochemical properties of HEAs hold the potential for them to become star materials in HER.While there have been relevant reviews on the use of HEAs in the field of the HER, a comprehensive review is still lacking.Several crucial questions remain unaddressed.For example, how to identify the reaction sites of HEAs catalysts, why these catalysts can maintain high stability even at high current densities, and how to analyze the structure-activity relationships of HEAs with complex elemental compositions and sites.Therefore, this review will thoroughly explore these questions, offering potential solutions and conclusions.Through this review, we aspire to offer fresh perspectives and methods for the design and utilization of HEAs in the context of HER.

Methods for Synthesis of HEAs
In various fields, researchers have devised numerous synthesis strategies for HEAs, generally categorized into the following main routes: solid-, liquid-, gas-phase, electrochemical, and other approaches, as can be seen in Figure 1.

Liquid-Phase Methods for Synthesis of HEAs
Liquid-phase methods are often employed to fabricate HEAs, such as vacuum arc melting, conventional melting, directional solidification, laser cladding, and thermal spraying for coating applications.The melting and casting method is typically conducted in ambient air, making it susceptible to metal element oxidation and the introduction of impurities, thus presenting certain limitations.In contrast, vacuum arc melting boasts rapid preparation, minimal alloy loss, and a reduced risk of introducing impurities, making it a widely favored technique for synthesizing HEAs. [29]he process for producing HEAs via vacuum arc melting follows these steps: 1) choose the appropriate elemental components to achieve the desired HEA composition, 2) prepare consumable metal electrodes using the selected elements, 3) establish a vacuum chamber or a controlled atmosphere environment, 4) create an electric arc between the consumable metal electrode and a water-cooled copper crucible, 5) high-temperature arc melts and vaporizes the electrode, 6) vaporized metal condenses and solidifies on a water-cooled copper collector, forming the HEAs.
Yang et al. [30] successfully pioneered the synthesis of nanoporous Cu-Au-Pt-Pd quasi-HEA microspheres by employing a novel approach that combines melt spinning and dealloying.As depicted in Figure 2a, this method primarily accomplishes the controllable preparation of nanoporous quasi-HEA through three steps.Initially, metal ingots (Cu 97 Au 1 Pt 1 Pd 1 ) were obtained through the vacuum melting method.Subsequently, the metal ingots were transformed into metal Reproduced with permission. [30]Copyright 2004, Elsevier Ltd. b) The synthesis method consists of the following stages: following ultrashort-pulse laser irradiation of bulk HEAs, bulk atomization/ionization to form a plume, followed by nucleation and condensation of the ablative substance in the liquid vapor phase, and electrostatic stabilization of colloidal high entropy alloy nanoparticles in ethanol.c) The progressive synthesis of nanoscale particles from a single metal.Five individual metal micropowders were successively weighed, ground and mixed, pressed into secondary particles, heat treated particles, laser ablation of metal sheets and deposition of nanoparticles on carbon black.Reproduced with permission. [52]Copyright 2019, Royal Chemistry Society.
microspheres through the utilization of the melt-spinning technique.Finally, in a 2 m nitric acid solution, the excess copper was dissolved through the process of dealloying, ultimately resulting in the formation of a quasi-HEA.This method allows for the production of diverse porous HEA materials, showcasing a no-table degree of versatility.Brif et al. [31] were pioneers in reporting the fabrication of HEAs using selective laser melting, specifically creating the FeCoCrNi HEA.The findings demonstrated a substantial improvement in performance when compared to conventional casting alloys.Similarly, Yu et al. [32] prepared Al 0.6 CoCrFeNi and Al 1.2 CoCrFeNi HEA at an extraction rate of 150 μm s −1 through arc melting and directional solidification.Upon comparing the microstructure, elemental distribution, and compression properties of HEAs produced through these two methods, the researchers observed a more orderly dendrite arrangement following directional solidification.Notably, the third dendrite in Al 1.2 CoCrFeNi HEA became more pronounced, possibly attributed to the lower cooling rate during directional solidification.
Laser cladding technology typically serves as a surface modification method wherein cladding materials are added to the substrate's surface. [33]This process utilizes a high-energy density laser beam for rapid melting, expansion, and solidification, resulting in the formation of cladding layers with distinctive physical, chemical, or mechanical properties on the substrate's surface.Noteworthy advantages include high energy density, rapid solidification, and fast cooling speeds.Furthermore, laser alloy coating exhibits a refined microstructure, excellent metallurgical bonding properties, and minimal mixing with the matrix. [34,35]ence, it is particularly well-suited for crafting HEAs coatings.The element type, process parameters, heat treatment, and ultrasonic assistance play pivotal roles in guaranteeing the quality of the coating. [36][41][42][43] For instance, Chen et al. prepared AlxCoFeNiCu 1−x HEA on AlSi 3 O 4 substrate, observing an increase in material hardness with rising aluminum content.
Thermal spraying technology involves utilizing heat from a heat source to elevate the coating material to its melting point. [44]ubsequently, the molten coating material is atomized into fine particles by a high-speed airflow.With the assistance of external thrust, these fine particles are propelled onto the surface of the substrate, creating a modified coating.Plasma spraying, a prominent technique within thermal spraying, employs an HEA heated to a molten or semi-molten state.The molten plasma is then rapidly sprayed onto a selected metal substrate, resulting in the formation of a protective layer with a smooth surface.
In this intricate process, heat is generated either through a combustible gas or an electric arc within a thermal spray gun.This causes the finely refined HEA powder to undergo melting on a specified base, leading to the formation of a spray deposit.As the target material gradually reaches a molten state through the application of compressed gas, the controlled-flow accelerated plasma is expelled onto the substrate.Upon impact, the plasma flattens and shapes the substrate surface, creating irregularly compatible sheets that integrate seamlessly with the prefabricated substrate surface.The thermal spraying process facilitates the application of HEAs as coatings, leveraging their distinctive microstructure to confer exceptional physical and mechanical properties.These properties encompass outstanding resistance to wear, corrosion, oxidation, and thermal stability even in challenging environmental conditions.The growing need for innovative protective coatings in demanding engineering environments has driven advancements in the development of thermal spraying HEA coatings. [44]Within the thermal spraying domain, the AlCoCrFeNi HEA, with an equal proportion composition, has been employed as a raw material for various processes, including plasma spraying, [45][46][47][48] high-velocity oxygen fuel (HVOF), [49] and cold spraying. [50]Extensive focus is placed on evaluating the microstructure, mechanical characteristics, and oxidation properties of these coatings.Wang et al. [51] innovatively synthesized a range of HEA-NPs by fine-tuning the type and proportion of metal precursors through the application of constrained auxiliary arc and plasma shock (APS) techniques.Notably, TiNbTaCrMo HEA-NPs exhibited distinctive corrosion resistance, establishing it as a promising candidate material for the electrocatalytic HER in natural seawater.This study not only introduces a novel method for crafting refractory HEA-NPs but also significantly broadens the material's potential applications.
Laser ablation stands out as a prominent technique for crafting HEAs through liquid-phase synthesis.Waag et al. [52] demonstrated the synthesis of CoCrFeMnNi HEA colloidal nanoparticles with equimolar stoichiometry and diameters less than 5 nm.This was achieved by ultrashort-pulse laser ablation of five metal micropowders in a liquid medium.The synthesis method and underlying principles are illustrated in Figure 2b.Under the influence of ultrashort-pulse laser ablation, the CoCrFeMnNi HEA undergoes a series of stages to transition into colloidal particles: plume-cavitation bubble-condense.The resulting HEAs colloidal NPs exhibited high stability, and the preparation process eliminated the need for any ligands.Moreover, these HEAs colloidal NPs were successfully employed in the basic OER for heterogeneous catalysis.In order to ascertain the industrial applicability of this method, researchers accomplished the conversion of individual metals into colloidal particles using the aforementioned approach (Figure 2c).

Solid-State Methods for Synthesis of HEAs
The preparation of HEAs through powder metallurgy is a crucial method involving two key steps: mechanical ball milling (or mechanical alloying) followed by subsequent sintering. [53]In the initial stage of HEAs powder preparation, mechanical ball milling is categorized into low energy ball milling and high energy ball milling based on the energy input.Low-energy ball milling primarily aims to uniformly mix the original powder, while high energy ball milling serves the dual purpose of reducing particle size and forming alloy powder with a nanostructure.The internal stress in the ball-milled powder can be alleviated through annealing treatment.As the product of ball milling is in powder form, further sintering is necessary to obtain a consolidated solid.Presently, various sintering methods are employed, including hot-isostatic-pressing (HIP), [54] vacuum-hot-pressing (VHP), [55] and spark-plasma-sintering (SPS). [56]Notably, HEAs produced through powder metallurgy exhibit superior properties compared to other methods, showcasing minimal component segregation, a stable microstructure, and reduced internal stress.
Presently, powder metallurgy finds extensive applications in various fields such as transportation, machinery, electronics, aerospace, weapons, biology, new energy, information, and the nuclear industry. [57]Powder metallurgy is favored for its advantages, including low energy consumption, high precision, and excellent stability. [58]Notably, the HEAs prepared through powder metallurgy exhibit a uniform composition distribution and fine structure, facilitating convenient powder sintering processes. [59]The mechanical alloying stands out as a key advantage within powder metallurgy technology, notably enhancing the solid solubility of HEAs. [60][62] Unlike conventional mechanical milling, mechanical alloying at the atomic level induces structural changes, such as the disintegration of particle aggregates or alterations in particle shape and surface characteristics, providing distinct advantages. [59]Zhao et al. [63] reported the influence of HEA content on the microstructure and mechanical properties of TiB 2 -HEA ceramics.Under low-temperature sintering conditions at 1500 °C, TiB 2 -HEA ceramics with varying initial HEA loading rates demonstrated a dense microstructure and outstanding mechanical properties.This underscores the significant potential of metal sintering aids in the sintering process for high-performance ceramics.Furthermore, in a study by Zhao et al., [64] Co x CrCuFeMnNi HEA powder was prepared using the mechanical alloying method.The researchers explored the effect of milling time, Co content, and vacuum annealing on the structure evolution, thermal stability, and magnetic properties of the alloy.Notably, after grinding for 50 h, the Co 0.5 CrCuFeMnNi HEA powder exhibited a predominantly face-centered cubic (FCC) phase with a small amount of body-centered cubic (BCC) phases, whereas the other three HEAs powders consisted of a single FCC phase.The findings reveal that augmenting Co content or extending pellet forming time contributes to the enhancement of the soft magnetic properties of HEA powder.Duan et al. [66] demonstrated the successful synthesis of flake-structure FeCoNiAlCr x HEA containing both BCC and FCC phases using a mechanical alloying method, as depicted in Figure 3a.The particle size distribution analysis indicates that the increased Cr element enhances the likelihood of particles with a larger surface area in FeCoNiAlCr x alloy powders.The insights gained from this study hold significant implications for the exploration of innovative absorbent materials in the realm of electromagnetic wave absorption.Similarly, in the work by Zhang et al. (Figure 3b), AlCoCr-FeNi HEA powders were successfully synthesized through highenergy ball milling. [30]Employing additional wet ball milling further optimized the powder size and shape for Electromagnetic Interference (EMI) applications.The HEAs sample exhibited a maximum total shielding (SET) of 20 dB, surpassing the 8.44 dB of the HEAL sample.This improvement was primarily attributed to the surface enhanced absorption (SEA), which increased from 6.8 dB for HEAL to 18.0 dB for HEAs.Consequently, the smallersized HEAs, prepared through wet ball milling, emerges as an appealing candidate for microwave shielding applications.Sharma et al. [68] has successfully fabricated nanocrystalline Cu-Pb alloys, along with Cu-Pb-TiB 2 and Cu-Pb-cBN nanocomposites using the SPS.In addition to establishing a correlation between microstructure and properties, the research extensively delved into the tribological properties of these materials.

Gas Phase Methods for Synthesis of HEAs
The gas-phase methods for preparing HEAs primarily involve techniques such as reactive magnetron sputtering, chemical vapor deposition, and the conversion of metal salt precursors into a gaseous state, followed by solidification under specific atmospheres.Examples of gas-phase methods include molecular beam epitaxy (MBE), atomic layer deposition (ALD), pulsed laser deposition (PLD), and others. [67]n recent years, notable advancements have been reported in the synthesis of HEA materials using pulsed laser deposition and chemical vapor deposition, resulting in materials with commendable properties.These techniques involve resistance heating, arc, and ion bombardment sputtering of HEA materials. [68,69]For example, Wang et al. [70] utilized the magnetron sputtering method to deposit a FeCoNiCuPd film with a single FCC structure on carbon fiber cloth, as depicted in Figure 3c.In alkaline conditions, the newly developed HEA/carbon fiber cloth (CFC) system exhibited superior HER and OER activities compared to commercially available catalysts.Kim et al. [73] employed the magnetron sputtering method to successfully produce TiZrHfNiCuCo HEA thin films with adjustable thickness.The key parameters in magnetron sputtering are power, time, and amount of nitrogen gas flow (R N ).As depicted in the diagram (Figure 3d-f), maintaining a constant deposition time (30 mins) and R N (0.25), the thickness of the HEA film notably escalated from 87 to 419 nm with the progressive augmentation of sputtering power.Subsequently, while keeping the sputtering power constant, the sputtering time was increased.The results revealed that as the sputtering time increased from 60 to 90 mins, the thickness of the TiZrHfNiCuCo HEA thin film elevated from 827 (Figure 3g) to 1210 nm (Figure 3h).Moreover, in the absence of nitrogen gas (R N = 0), the thickness of the thin film increased to 2320 nm (Figure 3i).This observation demonstrates the effective control of the thickness of HEA thin films by manipulating these three parameters.

Electrochemical Methods for Synthesis of HEAs
Electrochemical methods have gained widespread application in the designing of the HEAs.The electrochemical deposition method entails connecting a power supply to the anode and cathode of an electrolyte, which can be either water-soluble or organic-soluble, to establish an electrical circuit.This setup induces an electrochemical reaction driven by an electric field.Through a redox reaction, ions precipitate to form dense, pure metal, or alloy coatings on a substrate, thereby achieving the desired coating.This method proves versatile in generating multifunctional coatings on material surfaces.By meticulous control of electrochemical deposition process parameters, including current, solution pH, deposition time, temperature, and concentration, precise management of the chemical composition, thickness, and structure of the deposited layer can be achieved.Additionally, the electrochemical deposition of HEA thin films is a straightforward and rapid process.However, it is essential to note that controlling the growth rate of crystal nuclei on the substrate surface poses challenges, often resulting in predominantly polycrystalline or amorphous composite films with limited performance.Wang et al. [72] devised and synthesized a polymetallic nanotube array (NTA) catalyst consisting of PdNiCoCuFe HEA using an efficient template-assisted electrodeposition method, depicted in Figure 4a.Despite the lower Pd content, the electrocatalytic activity of the synthesized PdNiCoCuFe alloy NTA was found to be significantly enhanced when compared to both Pd NTA and commercial Pd/C catalysts.The electroreduction  [65] Copyright 2020, Elsevier Ltd. b) Schematic diagram of the synthetic mechanism of the planetary ball mill to form HEA powders.Reproduced with permission. [30]opyright 2019, MDPI Ltd. c) Schematic diagram of key steps of FeCoNiCuPd film preparation.Reproduced with permission. [70]Copyright 2022, Elsevier Ltd.With the gradual increase of sputtering ability, the thickness of the HEA film increased significantly from d) 87 to e) 323 nm and then to f) 419 nm.As the sputtering time increases, the thickness of the TiZrHfNiCuCo HEA film increases from g) 827 to h) 1210 nm.In the absence of nitrogen (R N = 0), the thickness of the film increases to i) 2320 nm.Reproduced with permission.Reproduced with permission. [71]Copyright 2019, Elsevier Ltd.
[75] Electrochemical methods have been employed for the synthesis of HEAs using oxide precursor systems in fused chlorides, such as FeCoNiCrMn, [76] TiNbTaZrHf, [77] and AlCrNbTaTi. [78]Huang et al. [79] utilized a stable Ni 11 Fe 10 Cu inert oxygen evolution anode to electrolyze metal oxides in a Na 2 CO 3 -K 2 CO 3 molten solution.The synthesis principle was illustrated in Figure 4b, where a homogeneous FCC phase Fe 0.5 CoNiCuZn x HEA was successfully prepared.The use of oxide precursors and low synthesis  [72] Copyright 2014, Elsevier Ltd. b) Schematic diagram of Fe 0.5 CoNiCuZn x HEAs prepared by molten salt electrolysis.Reproduced with permission. [79]Copyright 2019, Springer Nature.
temperature is advantageous for the efficient preparation of HEA containing zinc and other highly volatile elements, offering a straightforward and environmentally friendly method for the preparation of highly volatile HEAs.By regulating the structure and composition of HEAs, the OER activity can be effectively modulated.
Electrodeoxidation is a relatively novel metallurgical technique employed for the direct extraction of metals and alloys from solid oxides. [74,80,81]General metal oxide is used as the cathode, graphite as the anode, and molten CaCl 2 salt as the electrolyte.In this process, a typical setup involves utilizing a general metal oxide as the cathode, graphite as the anode, and molten CaCl 2 salt as the electrolyte.The cathode polarization potential is adequate to expel oxide ions from the oxide into the molten salt, while avoiding the electrolytic molten salt.[84] Sure et al. [85] pioneered the synthesis of HEAs through electrochemical reduction in molten salts.They achieved it by subjecting premixed solid metal oxides to cathodic polarization in a molten CaCl 2 salt electrolyte at 1173 K.This innovative approach resulted in the production of isoatomic refractory alloys, specifically TiNbTaZr and TiNbTaZrHf, in a single-step alloying process.Importantly, this study serves as a crucial link between the concepts of molten salt electrodeoxidation and the synthesis of HEAs, offering a bridge between these two metallurgical methodologies.Glasscott et al. [86] implemented a novel approach by confining the metal salt precursor within a suspended water nanodroplet of dichloroethane (DCE).This innovative method allows for the isolation of a specific number of precursor salt molecules during local nucleation and growth when the nanodroplet collides with the conductor. [87,88]In this method, known as nanodroplet-mediated electrodeposition, the outcome of transferring precursor atoms to the substrate is a droplet/electrode contact radius of less than 10 nm at the time of nanodroplet collision.This setup is utilized to observe the real-time nucleation and growth of individual Pt NPs on the microelectrode.It was determined that mass transfer within the nanodroplet, involving the rapid reduction of chloroplatinate to Pt 0 , played an indispensable role in the formation of porous Pt NPs.The stoichiometry and microstructure control of NPs in nanodroplet-mediated electrodeposition were illustrated in Figure 4c-f, while the evaluation of electrocatalytic performance was depicted in Figure 4d,g.

Other Preparation Methods for Synthesis of HEAs
Yao et al. [91] introduced an innovative approach to synthesize HEAs through a robust heat treatment method.In their pioneering work, they developed a technique involving the blending of up to eight distinct elements to create single-phase solid solution nanoparticles.This process includes subjecting metal precursors, positioned on an oxygen-containing carbon substrate, to rapid heating and cooling.Operating at a scorching temperature of ≈2000 K, with a shock duration of ≈55 ms and ramp rates of ≈105 K s −1 , they successfully produced HEAs nanoparticles incorporating up to eight different metal elements.
The carbon thermal shock (CTS) synthesis of HEA nanoparticles on a carbon support is illustrated in Figure 5a-d.The polymetallic nanoparticles produced through CTS synthesis exhibit small size and a uniform dispersion on the carbon carrier.The combination of high temperature and the catalytic activity of liquid metal plays a crucial role in promoting rapid particle "fission" and "fusion."This phenomenon leads to the homogeneous mixing of various elements within the nanoparticles.For a detailed explanation of the catalytic-driven particle fission/fusion mechanism, referred to Figure 5e.
Subsequent to this process, the rapid cooling rate assumes a crucial role in governing the kinetics of the thermodynamic mixing state, enabling the formation of crystalline solid solution nanoparticles, as illustrated in Figure 5f.By modifying the input electrical pulse parameters, it is also possible to produce phase separation nanoparticles by reducing the cooling rate.The versatile capabilities of CTS technology open up extensive opportunities for the synthesis of alloys and nanocrystals, showcasing substantial potential for a wide range of applications.
Xu et al. [90] synthesized HEA nanoparticles using electric pulses, as illustrated in Figure 6a.This method demonstrated the capability of uniformly depositing HEA nanoparticles on carbon nanofibers (CNF) and explored the variation in HEA nanoparticle growth with the direction of CTS current.By means of the CTS method, FeNiCoMnMg nanoparticles of ≈30 nm and FeNi-CoMuCu nanoparticles of ≈50 nm were obtained.
[101][102][103][104][105][106] Qiao et al. [107] utilized microwave heating to synthesize HEA nanoparticles, as depicted in Figure 6b.They employed a partially reduced graphene oxide (rGO-570) film in argon at ≈570 K as a substrate for the model.This scalable and straightforward high-temperature synthesis method induced by microwave heating can be adapted to a continuous coil process, as illustrated in Figure 6c.This adaptability allows for the scalable manufacturing of nanomaterials, suitable for various applications such as catalysis and energy.
While the high-temperature methods mentioned above are effective for synthesizing HEAs, they come with high demands for laboratory equipment and safety measures.Consequently, researchers are actively exploring more environmentally friendly and safer synthesis methods.As an example of more environmentally friendly synthesis methods, Wu et al. [108] attempted to obtain extremely small metal nanoparticles.They added six types of metal precursors to a preheated triethylene glycol solution (TEG) containing poly (N-vinyl-2-pyrrolidone) (PVP) as a protective agent at 230 °C, maintaining an equal molar ratio (≈16.7%) of mixed water solution.The simplicity of this wet chemical method offers the flexibility to adjust the composition and structure of HEA to accommodate various complex reactions.Zhang et al. [109] synthesized RuFeCoNiCu HEA nanoparticles at low temperatures (≤250 °C) and atmospheric pressure, applying it to nitrogen reduction reaction (NRR).The RuFeCoNiCu/CP configuration exhibited remarkable NRR performance in a 0.1 m KOH solution.This study not only introduces a new method for the synthesis of HEAs but also extends their application to the field of NRR, showcasing a novel NRR mechanism.Xu et al. [110] devised a straightforward in situ reduction method, employing guanine and transition metal nitrate as precursors for the synthesis of NiCeLaFeCo and NiCeLaFeCu mesentropic alloy (MEA) nanoparticles on nitrogen and oxygen co-doped carbon carriers.The synthesis method was depicted in Figure 6d.The resulting FCC NiCeLaFeCo MEA nanoparticles exhibited a small particle size (≈21.1 nm) and a moderate mixing entropy (1.31 R).To broaden the application of MEA nanoparticles, the researchers conducted toluene cracking experiments to assess toluene conversion and hydrogen generation.This study presents a simple and viable method for the synthesis of functionalized carbon-based polymetallic nanoparticles and demonstrates their excellent catalytic performance in toluene cracking and hydrogen production.Mc-Cormick et al. [111] successfully obtained colloidal nanoparticles Reproduced with permission. [89]Copyright 2018, American Association for the Advancement of Science.
of high entropy metal sulfide at low temperatures by introducing multi-ion exchange as a post-synthetic modification strategy.The synthesis method is depicted in Figure 6e.The stoichiometric ratio of different metal components in the nanoparticles can be adjusted by varying the ratio of cations exchanged in the solution.The cation exchange process, which leads to the formation of high entropy material, is favorable in terms of both entropy and enthalpy.It results in an ordered structure through cation randomization separation.This method offers a new approach for designing and synthesizing HEM.
Recognizing the limitations of traditional solvothermal methods in terms of complex processes and low yields for preparing HEAs, [112] researchers are exploring the development of a simpler but high-throughput method for synthesizing HEA catalysts.The metal-organic framework (MOF) derived method has proven to be an effective approach for producing binary/ternary alloy@(N-doped) carbon composites.The resulting materials exhibit a high specific area and an adjustable crystal structure. [113]he (N-doped) carbon carrier accompanying the material significantly improves stability.These polymetallic MOF derived Reproduced with permission. [90]Copyright 2020, Elsevier Ltd. b) Schematic diagram of roll-to-roll process for synthesizing HEA NP using microwave heating.c) Schematic diagram of HEA NP formed on rGO by microwave heating.Reproduced with permission. [107]Copyright 2021, Royal Society Chemistry.d) Synthesis of Ni-rich MEA nanoparticles on oxygen and nitrogen co-doped carbon carrier.Reproduced with permission. [110]Copyright 2022, Elsevier Ltd. e) High entropy wurtzite metal sulfide Zn 0.25 Co0 .22Cu 0.28 In 0.16 Ga 0.11 S formation, the metal sulfides are composed of a (but not all) Cu + cation exchange at the same time in the green pyroxene Cu 1.8 S + , Zn 2+ , Co 2 In 3+ and Ga 3+ form.Reproduced with permission. [111]Copyright 2021, American Chemistry Society.materials are considered promising electrocatalysts. [114]Wang et al. [115] prepared a nano-sized HEA@C electrocatalyst (CoN-iCuMnAl@C) using a simple and scalable method, as illustrated in Figure 7a.The MOF precursor is synthesized through room temperature precipitation, making it suitable for practical applications, unlike traditional solvothermal methods. [112]n addition to MOF derivations, the mixing of polymetallic elements in hollow structured nanoparticles is a promising strategy for the synthesis of highly efficient and cost-effective catalysts.
However, the synthesis of polymetallic hollow nanoparticles is currently limited to two or three elements due to the challenges in controlling morphology under harsh alloying conditions.Wang et al. [116] reported the rapid and continuous synthesis of hollow HEA nanoparticles using the continuous "droplet to particle" method.The evolution diagram of the droplet to particle during the formation of hollow HEA particles is illustrated in Figure 7b.The formation of these hollow HEA nanoparticles is achieved by the decomposition of a gas foaming agent, where a large amount  [115] Copyright 2022, Elsevier Ltd. b) Schematic diagram of droplet to particle evolution during the formation of hollow HEA particles.c) HAADF-STEM images and HAADF-EDS elemental images of a single HEA-RuIrFeCoNi nanoparticle.Reproduced with permission. [116]Copyright 2020, Wiley-VCH.d) Schematic diagram of synthesis process of HEA@air@NiO and HEA@air @Ni-NiO microspheres.Reproduced with permission. [118]Copyright 2019, Elsevier Ltd.
of gas is generated in situ, "suctioning" droplets during heating, followed by the decomposition of metal salt precursors and the nucleation/growth of polymetallic particles.The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image and the HAADF-EDS element image of a single HEA-RuIrFeCoNi nanoparticle were shown in Figure 7c.
The high active site/mass ratio of this hollow HEA nanoparticles makes it an effective candidate for energy and electrocatalytic applications.This work provides a feasible strategy for the continuous fabrication of hollow HEA nanomaterials, which can have a wide range of applications in energy and catalysis.Tao et al. [117] have also employed the template method as a synthesis approach.
They have developed a new general synthesis method to manufacture ultrathin 2D HEA subnanoribbon (SNR) composed of up to eight metallic elements by using Ag nanowire (NW) as a template.The formation of HEA Subnanoribbon (HEA-SNR) has been demonstrated through 1) the nucleation of different metal atoms via electrical exchange reactions between various metal precursors and Ag nanowire templates, 2) co-reduction of different metal precursors on the nanowire templates, and 3) removal of internal Ag nuclei.Similarly, utilizing a two-step hydrothermal method followed by calcination, Wu et al. [118] successfully synthesized HEA@air@NiO and HEA@air@Ni-NiO core-shell microspheres.The schematic diagram of the synthesis process is illustrated in Figure 7d.This marks the first report on preparing coreshell micro/nano structures from HEAs to achieve robust electromagnetic absorption properties.The HEA core contributes to magnetic loss, while the nickel oxide housing provides dielectric losses and improves impedance matching.This rational design allows pure HEA microspheres with almost no electromagnetic wave absorption ability to attain excellent electromagnetic wave absorption capability.
In summary, the remarkable properties of HEAs make them an intriguing subject for comprehensive investigation by researchers.Alongside the synthesis methods discussed earlier, numerous alternative approaches are currently in development, each with its own set of advantages and limitations (Table 1).The key to successful HEAs preparation lies in the careful selection and refinement of the appropriate method, considering the specific elements, the desired alloy performance, and the environmental conditions.Presently, HEAs research is in full swing, and as preparation methods for HEAs continue to improve and technological advancements occur, the application of this innovative alloy is expected to become even more widespread.

Hydrogen Evolution Reactions and Mechanisms
Hydrogen (H 2 ) energy boasts a remarkable mass-energy density of 120-142 MJ kg −1 , alongside high conversion efficiency, zero carbon emission, and a nontoxic, harmless nature.Starting from 1820, hydrogen energy has progressively shifted its focus from aviation propulsion to ground-based applications.It emerges as a crucial path in the global shift toward sustainable energy development.The production technology of H 2 stands as a crucial direction for the widespread application of hydrogen energy.It encompasses primarily hydrogen generation from fossil fuels, electrolysis, industrial by-product hydrogen, and hydrogen production from renewable energy sources, among others.Up to this point, more than 95% (≈105 million tons) of global annual hydrogen production has been derived from fossil fuels.Nevertheless, this production method, reliant on fossil fuels, not only exhibits low efficiency but also gives rise to substantial carbon dioxide (CO 2 ) emissions.Utilizing renewable energy for water electrolysis (electrochemical water splitting), characterized by its cleanliness, eco-friendliness, and high efficiency, stands as a vital pathway for future hydrogen production technology.
Currently, there are three primary technologies employed in the field of water electrolysis: alkaline electrolysis cells (AEC), proton exchange membrane electrolysis cells (PEMEC), and solid oxide electrolysis cells (SOEC). [119]AEC, being the most prevalent electrolysis technology, finds widespread application in large-scale industrial settings owing to its remarkable stability and comparatively low cost.Nevertheless, challenges like low current densities and operating pressures have an adverse impact on the system size and cost of hydrogen production. [120,121]PEMEC has gained recognition among researchers and is primarily used for small-scale applications.It outperforms AEC in terms of higher current density and cell efficiency.PEMEC also offers advantages such as fast response times and a wider range of power requirements for operation.However, the use of noble metal catalysts like Ir and Pt, along with their limited lifespan, has hindered its broader application.The principle of operation for a SOEC can be considered as the reverse of a Solid Oxide Fuel Cell (SOFC).Both devices consist of a dense electrolyte layer sandwiched between two porous electrodes.This design allows the same device to function as both an SOFC and an SOEC concurrently, leading to the term "solid oxide cell" (SOC). [122]ER as a half-reaction of water in cathodic electrolysis (Figure 8a).Efficiency in the HER relies heavily on the choice of appropriate catalysts and electrolytes.Due to their exceptional proton transfer efficiency, acidic electrolytes are extensively employed in the HER process.However, in an acidic electrolyte environment, the requirement for costly Pt-group catalysts leads to substantial production expenses.Furthermore, the reaction vessel necessitates the use of an acidic proton exchange membrane (Figure 8b,c).Additionally, it is worth noting that acidic electrolytes exhibit a notably strong corrosive nature.Acidic electrolytes not only tend to corrode the electrolysis cell easily but also result in the release of acidic gases, which can contaminate the produced hydrogen gas.Employing non-precious metal catalysts in alkaline electrolytes can substantially slash production expenses.Alkaline electrolytes can effectively enhance system stability.Regrettably, the slow kinetic processes in alkaline electrolytes pose a significant limitation to their reaction efficiency.Research findings demonstrate that the reaction kinetics in acidic electrolytes are elevated by 2-3 orders of magnitude compared to alkaline electrolytes.Hence, significant research endeavors are directed toward the development of highly efficient and stable electrocatalysts for the effective production of hydrogen in acidic electrolytes. [123,124]n general, HER is a chemical process involving two electrons.However, when the electrolyte varies at different pH values, the electrode reaction processes also become more complex.As shown in Figure 8d (left), the reaction steps primarily involve two pathways in acidic electrolyte.In general, the initial step of the HER, referred to as the Volmer reaction, entails protons in the electrolyte acquiring electrons from the electrode surface to form adsorbates (H ads ).Furthermore, protons in acidic electrolytes tend to exist in the form of H 3 O + .The H 3 O + facilitates the coupling of electrons to form H ads , while concurrently releasing H 2 O.The next step in the hydrogenation process primarily involves two pathways.When the coverage of adsorbed hydrogen is low, the adsorbed hydrogen combines with protons in the electrolyte to generate H 2 (Heyrovsky reaction).Conversely, two adjacent adsorbed H atoms is more likely to combine and produce H 2 at high coverage of adsorbed hydrogen (Tafel reaction).
Unlike acidic electrolytes, water is the primary reactant in alkaline or neutral electrolytes (Figure 8d, right).In the Volmer reaction, the initial step of the HER involves the breakdown of water molecules to generate adsorbed hydrogen.After adsorption,

Impregnation adsorption method
Aerosol droplet-mediated method Hydrothermal hydrogen combines with a hydrogen atom from a water molecule to produce H 2 .In another scenario, two adjacent adsorbed hydrogen atoms couple to generate H 2 .It is worth noting that in alkaline electrolytes, the adsorption and desorption of hydroxide groups (OH − ) are frequently involved.In particular, the competitive adsorption between OH − and water molecules greatly constrains the dissociation process of water.When the adsorption free energy of hydrogen approaches zero (∆G *H = 0 eV), it indicates the thermodynamically most favor-able condition for hydrogen desorption, making it a key criterion in the assessment of catalyst activity.Up to this point, Pt-based metals (e.g., Pt, Ru, Ru, Ir, and Pd) remain the optimal catalysts for facilitating hydrogen production.However, despite their remarkable catalytic activity, these precious metals encounter stability challenges that hinder the achievement of sustained and reliable hydrogen production.Hence, an extensive body of research has been undertaken to improve catalyst stability, employing techniques such as alloying, encapsulation, and substrate  [125] Copyright 2018, American Chemical Society.b) Electrolyzer setup with AEM, CEM and middle buffer chamber.Reproduced with permission. [126]Copyright 2017, Wiley-VCH.c) Overall water electrolysis based on bifunctional nonprecious electrocatalysts (or pre-catalysts).Reproduced with permission. [127]Copyright 2022, Royal Society of Chemistry.d) Schematic illustration of the hydrogen evolution reaction mechanism in acidic and alkaline media.Reproduced with permission. [128]Copyright 2022, MDPI Ltd. e) Tafel slope of HER performance.Reproduced with permission. [129]Copyright 2022, Wiley-VCH.f) Tafel slopes of various electrodes obtained from the LSV polarization curves.Reproduced with permission. [130]Copyright 2019, Elsevier Ltd. g) Tafel slopes of HEA-NPs, Ti, Nb, Ta, Cr, and Mo powder.Reproduced with permission. [51]Copyright 2022, Elsevier Ltd. modification.It is widely acknowledged that the following factors play a role in diminishing catalyst stability.Catalyst detachment from the electrode is a primary cause of active component loss.The dissolution and subsequent loss of metal in acidic solutions can further contribute to a reduction in catalyst stability.Under large current density, the catalyst loss becomes significantly noticeable.The presence of a substantial amount of H 2 bubbles on the catalyst surface not only hinders the activity of the catalyst's active sites but also leads to catalyst erosion.It is often overlooked that large current density can result in elevated electrode temperatures, leading to thermal deactivation of the catalyst.Catalyst thermal deactivation is a crucial factor leading to a decrease in catalyst stability and can be categorized into two types: Ost-wald ripening and particle migration and coalescence.Raising the melting temperature, while preserving activity, represents a crucial research direction in mitigating catalyst thermal deactivation.
In order to evaluate the catalytic efficiency of the HER, various evaluation approaches are comprehensively reviewed.Based on current literature, these parameters mainly encompass overpotential (h), Tafel plot (b), electrochemical impedance spectroscopy (EIS), Faradaic efficiency (FE), turnover frequency (TOF), electrochemical active surface area (ECSA), mass activity and volcano plot.
The overpotential () can be thought of as the extra electrical potential that promotes the smooth progress of the HER.It represents the difference between the Nernst potential (EHER) and the actual applied potential (E), can be expressed as  = E HER -E.To be more precise, the overpotential arises from the necessity to surmount a substantial activation energy for the HER.Linear sweep voltammetry (LSV) measurement is a common method employed for the analysis of overpotentials.Several distinct h values are utilized to differentiate the activity of catalysts.h1 (h at 1 mA cm −2 ) signifies the commencement of the HER and is also commonly referred to as the "onset potential."To benchmark against solar water splitting devices with an efficiency of 12.3%, h10 (h at 10 mA cm −2 ) is frequently utilized for comparing the activity of different catalysts.Considering its practical applications in production, h1000 (h at 1000 mA cm −2 ) emerges as a crucial performance indicator.
The Tafel slope (b) unveils the inherent activity of the HER and showcases its reaction kinetics characteristics.The Tafel slope (b) can be derived through fitting overpotential (), current density (j) and exchange current density (j 0 ), with the equation expressed as  = blogj/j 0 +a.Faster electron-transfer kinetics suggest a smaller b and high j 0 .Based on various reaction mechanisms, the b can be classified into three categories: According to the Figure 8e, it is evident that the Pt 34 Fe 5 Ni 20 Cu 31 Mo 9 Ru and PtFeNiCuMo HEAs have a stronger inclination toward hydrogen production through the Tafel process.While the PtFeNiCuMoRu-1 h and Pt/C exhibited a greater propensity for hydrogen synthesis through the Heyrovsky process.Based on Figure 8f, it is apparent that with the exception of Pt rods and Ni foam, which utilize the Tafel and Heyrovsky processes respectively, the majority of catalysts predominantly undergo hydrogen synthesis through the Volmer process.In Figure 8g, it is evident that apart from TiNbTaCrMo, which employs the Heyrovsky process, the other catalysts primarily undergo hydrogen synthesis through the Volmer process.Hence, researchers can deduce the HER pathway based on the Tafel slope values.
The electrochemical impedance spectroscopy (EIS) can provide insights into the HER kinetics processes occurring at the electrode/electrolyte interface.The charge transfer impedance (R ct ) is associated with the charge transfer process at the electrode interface, and it can be distinguished by the diameter of the semicircle in the high-frequency region (EIS Nyquist plots).Research suggests that as R ct decreases, the rate of the HER increases, and the overpotential decreases.In the low-frequency range, adsorption impedance (R ad ) plays a dominant role.R ad represents the adsorption of species on the electrode surface and is associated with the onset potential.If the onset potential is lower, it indicates a lower R ad .
The faradaic efficiency (FE) is another crucial parameter used for evaluating the hydrogen evolution activity of catalysts.Generally, in the context of the HER, FE is defined as the ratio between the theoretically predicted (galvanostatic or potentiostatic electrolysis) and experimentally observed (gas chromatography) hydrogen generation.
The TOF, as a vital metric for evaluating catalyst activity, can effectively mitigate discrepancies stemming from diverse variables (e.g., electrocatalyst loading mass, effective number of active sites).Costentin et al. [133] first proposed to utilize TOF (i.e., the number of molecules reacting per site per second) as a means to evaluate the catalytic rate of the HER.So far, the TOF has also been defined as the total number of reaction molecules converted to the target product per unit of time at each catalytic site, defined as TOF = jA/4 nF.A is the area of the working electrode (W E ).The number of moles of active materials, denoted as n, is determined through the utilization of the electrochemically active surface area (ECSA) of catalyst.However, it should be noted that not all atoms on the catalyst's surface are active sites, leading to potential errors and limitations in this method.As a result, there is an ongoing need to develop more precise testing methods for assessing the TOF of catalysts.
Specific activity and mass activity are both parameters employed in assessing the catalytic activity.The specific activity is the current density per unit real surface area of the catalyst, and it can be determined by normalizing the current to the ECSA.The current normalized by the loading mass is known as mass activity, which is typically employed for comparing materials of a similar nature.Hence, when the catalyst possesses a greater surface area, it generally demonstrates higher mass activity.
In recent years, the volcano plot has become one of the main methods for describing catalyst activity, and it is based on the Sabatier principle.The Sabatier principle embodies the age-old doctrine of the Golden Mean: the principle of moderation.In the context of catalytic reaction, when reactant adsorption is too strong, it impedes product desorption.Conversely, when reactant adsorption is too weak, it hampers the progression of the reaction.In the HER systems, when the adsorption free energy of hydrogen approaches zero (∆G *H = 0 eV), the catalyst attains its peak activity.Additionally, when the adsorption free energy reaches zero, the HER achieves its highest current density.

Noble-Metal HEAs
Precious metal-based (e.g., Pt, Pd, Ru, Rh, Ir, and Au) HEA materials not only exhibit excellent catalytic activity but also enhance system stability, providing a broad range of research possibilities.The distinctive geometric and electronic structural effects of HEAs offer limitless potential for the HER. [132]Pt-based HEAs demonstrate outstanding performance in the HER, attracting extensive attention from researchers. [108]esearchers utilized five precious metal elements to fabricate HEAs with the goal of maintaining a high density of active sites while simultaneously enhancing stability.As shown in Figure 9a-c, the PtPdRuRhAu HEA, synthesized through an ultrasonically assisted wet-chemical method, has been demonstrated to possess exceptional hydrogen production performance under alkaline conditions. [133]At a current density of 30 mA cm −2 in a 1 m KOH electrolyte, the PtPdRuRhAu HEA demonstrated an overpotential of only 190 mV, a value markedly lower than  [133] Copyright 2019, Wiley-VCH.d) HAADF-STEM image of the as-prepared PGMHEA and the corresponding EDX maps.e) Comparison of the EOR j specific values of PGM-HEA with those of the as-synthesized monometallic NPs and the commercial catalysts.The arrows show the scan directions of CVs.Comparison of the j specific and j mass values of PGM-HEA at potentials of 0.45 and 0.6 V in the forward scan with those of the monometallic catalysts and the reported Au@PtIr/C, respectively.The current densities of Pt/C are magnified ten times for easy comparison.Comparison of the initial CV curve for EOR and the CV curve after 50 cycles of PGM-HEA.The current is normalized by geometric area of the electrode.CV curves in the positive scan of PGM-HEA and other monometallic NPs showing different electrochemical CO stripping behaviors.The peak potentials are given.Note that the peaks of Ru and Os might come from dissolution rather than CO stripping away.Reproduced with permission. [108]Copyright 2020, American Chemical Society.
that observed in medium-entropy alloy materials (260 mV for PtAuPdRh and 600 mV for PtAuPd).Furthermore, at this current density, the presence of multiple reaction mechanisms for the HER is indicated by a Tafel slope of 62 mV dec −1 .It is worth noting that further research is still needed to fully understand the synergistic catalytic mechanisms among noble metal alloys.As shown in the Figure 9d,e, the PtPdRuRhIr HEA material, synthesized through a one-pot polyol method, exhibits exceptional stability and activity under both acidic and alkaline conditions. [108]esearchers have demonstrated that the PtPdRuRhIr catalyst Reproduced with permission. [134]Copyright 2020, Springer Nature.exhibits a significantly higher TOF compared to commercial Pt, with a 9.5-fold increase in acidic conditions (0.05 m H 2 SO 4 ) and a 7.8-fold increase in alkaline conditions (1.0 m KOH).By analyzing with high-energy X-ray photoelectron spectroscopy (HAX-PES), researchers have unveiled that the valence band spectrum of HEAs reveals a dense electron density of states, which provides a guarantee for the efficient advancement of HER.

Noble-Metal Based HEAs
In comparison to elemental composition entirely of precious metals, HEAs formed with non-precious metals are not only more cost-effective but also hold greater potential for various applications.Recently, HEAs consisting of non-precious transition metals in conjunction with Pt have seen widespread use in the HER.
As can be seen in the Figure 10a-c, in an alkaline electrolyte (1 m KOH), PtPdIrFeCo@GO HEA-NPs demonstrated an overpotential of just 42 mV at a current density of 10 mA cm −2 . [134]he performance tests for various indicators of HER also demonstrated exceptional excellence.The ECSA for PtPdIrFeCo@GO reached 1462.5 mA cm −2 , indicating the presence of larger active sites for efficient HER.The PtPdIrFeCo@GO boasts an impressive FE of 99.4%, and it also boasts a modest Tafel slope of 82 mV dec −1 .Furthermore, the PtPdIrFeCo@GO catalyst demonstrated remarkable stability, maintaining its performance for 150 h at 10 mA cm −2 without any noticeable changes.Researchers posit that this is the result of the "cocktail effect" brought about by HEAs.Nevertheless, it's worth pointing out that the synergistic mechanism of oxidized graphene (GO) is not further elaborated upon.To further reduce the noble metal content, researchers fabricated a PtNiFeCoCu HEA through a straightforward low-temperature oil-phase reduction method, ) and Gibbs free energy (ΔG *H ) profiles for the HER at the Fe, Co, Ni, Cu, and Mn sites.Reproduced with permission. [136]opyright 2023, Royal Society of Chemistry.e) Electrochemical characterization of nanoporous electrodes.f) Electrochemical performance in neutral electrolyte.Reproduced with permission. [137]Copyright 2020, Wiley-VCH.
similar to Figure 10d-g.In an alkaline medium, the PtNiFe-CoCu displays a minimal overpotential of only 10 mV at a current density of 10 mA cm −2 . [135]The mass activity of the HER reached 10.96A mg −1 Pt at −0.07 V versus reversible hydrogen electrode.As shown in the Figure 10e, theoretical calculations suggested that minor changes in the stoichiometric ratios of the primary elements in the HEAs have no impact on the electronic structure.Therefore, researchers established the most stable surface structure and conducted an indepth analysis of the reaction mechanism.Through an analysis of electronic density of states, the researchers have determined that Pt-5d occupies the deepest electron energy levels.It suggests that Pt plays the role of electron reservoir in the HER.We have compiled a list of representative Ptbased HEAs and presented them in Table 2. It's worth noting that these catalysts necessitate an alkaline electrolyte in the HER.

Non-Noble Metal HEAs
Non-precious metal HEAs, without the addition of noble metals, have also been continuously developed.Elements with closely matched atomic radii are more likely to form homogeneous solid solutions.Researchers prepared HEA of FeCoNiCuMn through electrospinning technology, as illustrated in Figure 11a-d. [136]he FeCoNiCuMn HEA with a particle size distribution within a range of 6.3-19.5 nm, and no aggregation was observed.Figure 11c shows the HEA exhibited excellent catalytic activity for HER (281 mV at 100 mA cm −2 ).Theoretical results suggested that coordinating the charge distribution between Cu and Mn can enhances the kinetics of the HER.Charge density difference analysis of FeCoNiCuMn HEA showed an increase in local electron density at the Ni and Cu sites, whereas a decrease was observed in the local electron density at the Mn, Co, and Fe sites.Inactive Cu can be activated by strong local electron interaction [mV] [mA cm −2] [mV dec  [mV] [mA cm −2] [mV dec −1  induced by electronegativity differences.To enhance the electronic transfer capability of Cu elements, researchers utilized a hierarchical nanoporous Cu scaffold to construct a CuAlNiMoFe HEA. [137]The surface of the CuAlNiMoFe HEA, composed of varying Cu, Ni, Mo, and Fe metals, offers dual-function electrocatalytic sites, greatly improving the kinetics of water dissociation and the adsorption/desorption kinetics of hydrogen intermediates.As shown in Figure 11d,e, in a 1 m KOH buffer electrolyte, the CuAlNiMoFe HEA exhibits an overpotential of 240 mV even at high current density (≈1840 mA cm −2 ).Jia et al. reported an L1 2 -type high-entropy intermetallic (HEI) compound FeCo-NiAlTi.This particular periodic structure displays a site-isolation effect that enhances the efficiency of HER.In an alkaline electrolyte, the HEI not only facilitates the adsorption of H 2 O but also promotes the desorption of H*.Compared to noble metals, the HEI demonstrates exceptional performance, with an overpotential of just 88.2 mV at a current density of 10 mA cm −2 , and a Tafel slope of 40.1 mV dec −1 .

Conclusions and Outlook
The HEAs unquestionably possess significant potential in catalytic reactions and are on the brink of experiencing rapid and explosive growth in the near future.A thorough understanding of the physicochemical properties of HEAs is essential for their effective utilization in various catalytic reactions.Hydrogen energy serves as a guiding light for future energy development and stands as a pioneer in this field.An essential element of hydrogen production technology is the advancement of high-performance industrial catalysts.Fortunately, the HER involves relatively few reaction steps, making it convenient for examining the structure-property relationships of HEAs in hydrogen production.In simpler terms, the HER provides a more accessible means of uncovering the physic-ochemical properties of HEAs.Hence, incorporating HEAs into the hydrogen production process not only aids in the development of efficient hydrogen production catalysts but also holds the potential to reveal the enigmatic aspects of the complex physicochemical properties of HEAs.
To date, numerous unresolved challenges persist in the utilization of HEA materials in HERs.Notably, the synthesis procedures for HEA materials have historically prioritized their mechanical characteristics, with a shift in focus toward catalytic reactions occurring only in recent years.Consequently, it is imperative to thoughtfully engineer HEA materials to imbue them with catalytic activity tailored for specific catalytic reactions.Presently, existing preparation methods pose significant difficulties in achieving precise control over the dimensions, structure, reaction sites, constituent concentrations, and elemental distribution of HEA materials.
In the realm of understanding HER mechanisms, the critical first step involves identifying the atomic coordination environments of catalyst surfaces.However, the complex atomic arrangements on HEA surfaces inevitably pose significant challenges for in-depth research into these reaction mechanisms.On the one hand, the primary methods for discerning surface coordination environments at the atomic scale include techniques such as synchrotron radiation and aberration-corrected electron microscopy.On the other hand, the potential for significantly enhancing the accuracy of reaction mechanism studies lies in the use of methods like machine learning and density functional theory for computational analysis of surface-coordinated atoms.
It's crucial to emphasize that one of the primary criteria for assessing a catalyst's suitability for industrial applications is its ability to efficiently and reliably generate hydrogen at high current densities.However, based on current stability tests, existing HEA catalysts still fall short of meeting industrial demands.Thermodynamic analysis indicates that high current densities inevitably lead to significant joule heating at the electrode, as Joule heating is directly proportional to the square of the current density.The heating effect is particularly pronounced at the interface between the catalyst material and the electrode, rendering catalyst materials more susceptible to thermal deactivation at high current densities.Balancing the exceptional activity of small-sized HEAs with their susceptibility to melting deactivation is currently one of the foremost challenges.Utilizing methods such as encapsulation and loading to enhance the thermal stability of HEA materials without compromising the active sites represents a viable solution for addressing catalyst deactivation at high current densities.
In conclusion, the potential for designing and developing HEA materials remains substantial.By harnessing the HER as a mediator and conducting a comprehensive analysis of the physicochemical properties of HEAs, followed by the design of highperformance catalysts, we can employ a highly effective approach.However, the vast diversity of HEA materials in terms of types and quantities makes it impractical to identify suitable catalysts through traditional trial-and-error methods.Hence, the integration of intelligent techniques like machine learning and theoretical calculations can swiftly pinpoint appropriate HEAs for catalytic reactions.Furthermore, the establishment of a HEA materials database through a fusion of experimental and intelligent methods is of paramount importance, providing a robust platform for the development of efficient and stable catalysts.

Figure 2 .
Figure 2. a) Synthesis of nanoporous quasi-HEA microspheres.Reproduced with permission.[30]Copyright 2004, Elsevier Ltd. b) The synthesis method consists of the following stages: following ultrashort-pulse laser irradiation of bulk HEAs, bulk atomization/ionization to form a plume, followed by nucleation and condensation of the ablative substance in the liquid vapor phase, and electrostatic stabilization of colloidal high entropy alloy nanoparticles in ethanol.c) The progressive synthesis of nanoscale particles from a single metal.Five individual metal micropowders were successively weighed, ground and mixed, pressed into secondary particles, heat treated particles, laser ablation of metal sheets and deposition of nanoparticles on carbon black.Reproduced with permission.[52]Copyright 2019, Royal Chemistry Society.

Figure 3 .
Figure 3. a) Schematic of SPS set-up, cut-away view of SPS chamber and cross-sectional view of the die.Reproduced with permission.[65]Copyright 2020, Elsevier Ltd. b) Schematic diagram of the synthetic mechanism of the planetary ball mill to form HEA powders.Reproduced with permission.[30]Copyright 2019, MDPI Ltd. c) Schematic diagram of key steps of FeCoNiCuPd film preparation.Reproduced with permission.[70]Copyright 2022, Elsevier Ltd.With the gradual increase of sputtering ability, the thickness of the HEA film increased significantly from d) 87 to e) 323 nm and then to f) 419 nm.As the sputtering time increases, the thickness of the TiZrHfNiCuCo HEA film increases from g) 827 to h) 1210 nm.In the absence of nitrogen (R N = 0), the thickness of the film increases to i) 2320 nm.Reproduced with permission.Reproduced with permission.[71]Copyright 2019, Elsevier Ltd.

Figure 4 .
Figure 4. a) Preparation method of PdNiCoCuFe alloy NTAs.Reproduced with permission.[72]Copyright 2014, Elsevier Ltd. b) Schematic diagram of Fe 0.5 CoNiCuZn x HEAs prepared by molten salt electrolysis.Reproduced with permission.[79]Copyright 2021, Elsevier Ltd. c) The current transient of a single nanodroplet collides on a carbon fiber with a bias of −0.4 V relative to Ag/AgCl and representation of nanodroplet collision events, highlighting the rapid formation of NP at the water/substrate interface and the charge balance guaranteed by TBA transfer at the oil/water interface.d) Correlation ICP-MS and EDX results of Co 0.5 Ni 0.5 , Co 0.25 Ni 0.75 , and Co 0.75 Ni 0.25 MG-NPs demonstrate the precise control of NP stoichiometry.e) Confirmation of amorphous microstructure by the lack of crystallinity at high resolution and the presence of dispersion rings on the SAED map.f) Electrodeposition of alloy films by aqueous solutions of equimolar metal salt precursors, showing phase and stoichiometric inhomogeneity.g) Evaluation of electrocatalytic performance of CoFeLaNiPt HEMG-NP electrocatalyst.Reproduced with permission.[86]Copyright 2019, Springer Nature.
Figure 4. a) Preparation method of PdNiCoCuFe alloy NTAs.Reproduced with permission.[72]Copyright 2014, Elsevier Ltd. b) Schematic diagram of Fe 0.5 CoNiCuZn x HEAs prepared by molten salt electrolysis.Reproduced with permission.[79]Copyright 2021, Elsevier Ltd. c) The current transient of a single nanodroplet collides on a carbon fiber with a bias of −0.4 V relative to Ag/AgCl and representation of nanodroplet collision events, highlighting the rapid formation of NP at the water/substrate interface and the charge balance guaranteed by TBA transfer at the oil/water interface.d) Correlation ICP-MS and EDX results of Co 0.5 Ni 0.5 , Co 0.25 Ni 0.75 , and Co 0.75 Ni 0.25 MG-NPs demonstrate the precise control of NP stoichiometry.e) Confirmation of amorphous microstructure by the lack of crystallinity at high resolution and the presence of dispersion rings on the SAED map.f) Electrodeposition of alloy films by aqueous solutions of equimolar metal salt precursors, showing phase and stoichiometric inhomogeneity.g) Evaluation of electrocatalytic performance of CoFeLaNiPt HEMG-NP electrocatalyst.Reproduced with permission.[86]Copyright 2019, Springer Nature.

Figure 5 .
Figure 5. a) Microscopic images of tiny precursor salt particles on a carbon nanofiber (CNF) carrier before thermal shock, and well-dispersed (PtNi) nanoparticles synthesized after CTS.b) Time evolution of temperature during sample preparation and thermal shock at 55 ms.c) Diagrams of low-rate and single-particle elements, HAADF images, and corresponding atomic diagrams of binary PtNi alloys.d) Elemental diagram of HEA-NP composed of eight different elements (Pt, Pd, Ni, Co, Fe, Au, Cu, and Sn).e) Description of the catalytic driven particle fission/fusion mechanism for the synthesis of uniformly dispersed HEA NP. f) Schematic comparison of phase separation heterostructures synthesized by conventional slow reduction procedures (slow kinetics) with solid solution HEA NP (fast kinetics) synthesized by the CTS method.Reproduced with permission.[89]Copyright 2018, American Association for the Advancement of Science.

Figure 6 .
Figure 6.a) Schematic diagram of HEA NPs formation on CNFs substrate by CTS method.Reproduced with permission.[90]Copyright 2020, Elsevier Ltd. b) Schematic diagram of roll-to-roll process for synthesizing HEA NP using microwave heating.c) Schematic diagram of HEA NP formed on rGO by microwave heating.Reproduced with permission.[107]Copyright 2021, Royal Society Chemistry.d) Synthesis of Ni-rich MEA nanoparticles on oxygen and nitrogen co-doped carbon carrier.Reproduced with permission.[110]Copyright 2022, Elsevier Ltd. e) High entropy wurtzite metal sulfide Zn 0.25 Co0 .22Cu 0.28 In 0.16 Ga 0.11 S formation, the metal sulfides are composed of a (but not all) Cu + cation exchange at the same time in the green pyroxene Cu 1.8 S + , Zn 2+ , Co 2 In 3+ and Ga 3+ form.Reproduced with permission.[111]Copyright 2021, American Chemistry Society.

Figure 7 .
Figure 7. a) Schematic diagram of preparation steps of electrocatalyst.Reproduced with permission.[115]Copyright 2022, Elsevier Ltd. b) Schematic diagram of droplet to particle evolution during the formation of hollow HEA particles.c) HAADF-STEM images and HAADF-EDS elemental images of a single HEA-RuIrFeCoNi nanoparticle.Reproduced with permission.[116]Copyright 2020, Wiley-VCH.d) Schematic diagram of synthesis process of HEA@air@NiO and HEA@air @Ni-NiO microspheres.Reproduced with permission.[118]Copyright 2019, Elsevier Ltd.

Figure 9 .
Figure 9. a) Schematic illustration of synthesis of HEA-NPs/carbon (PtAuPdRhRu supported on XC-72 carbon) catalysts and their application in HERs.b) HAADF image and EDS elemental maps.c) Polarization curves and Tafel plots of HEA-NPs/carbon-700 °C, PtAuPdRh/carbon-700 °C, PtAuPd/carbon-700 °C, and commercial Pt/C (Pt loading = 20 wt.%) catalysts in 1.0 m KOH.Reproduced with permission.[133]Copyright 2019, Wiley-VCH.d) HAADF-STEM image of the as-prepared PGMHEA and the corresponding EDX maps.e) Comparison of the EOR j specific values of PGM-HEA with those of the as-synthesized monometallic NPs and the commercial catalysts.The arrows show the scan directions of CVs.Comparison of the j specific and j mass values of PGM-HEA at potentials of 0.45 and 0.6 V in the forward scan with those of the monometallic catalysts and the reported Au@PtIr/C, respectively.The current densities of Pt/C are magnified ten times for easy comparison.Comparison of the initial CV curve for EOR and the CV curve after 50 cycles of PGM-HEA.The current is normalized by geometric area of the electrode.CV curves in the positive scan of PGM-HEA and other monometallic NPs showing different electrochemical CO stripping behaviors.The peak potentials are given.Note that the peaks of Ru and Os might come from dissolution rather than CO stripping away.Reproduced with permission.[108]Copyright 2020, American Chemical Society.

Figure 10 .
Figure 10.a).CV curves for the quinary (FeCoPtPdIr) HEA-NPs.Cdl measurements for the quinary HEA-NPs in HER.Chronopotentiometry of FeCoPdP-tIr@GO at constant 100 mA cm −2 .b) Electrochemical performance comparison of samples.c) Elemental maps for FeCoPdIrPt supported on GO.Reproduced with permission.[134]Copyright 2020, Springer Nature.d) CV curves of Pt/C and Pt 18 Ni 26 Fe 15 Co 14 Cu 27 /C.Peak values of mass activity and area activity.Chronoamperometric tests for MOR at 0.65 V versus RHE.CV curves of the Pt 18 Ni 26 Fe 15 Co 14 Cu 27 /C before and after 1000 cycles.e) The side view of the structural configuration of HEA.The side view of HEA structural configuration and the real spatial contour plots for bonding and anti-bonding orbitals near EF.And the top view of the real spatial contour plots for bonding and anti-bonding orbitals near EF for the HEA.The PDOSs of the HEA.f) The corresponding elemental mapping of Pt 18 Ni 26 Fe 15 Co 14 Cu 27 /C nanoparticles.g) CV curves.HER polarization curves (geometrical area).Pt mass loading normalized (mass activity) LSV curves.Comparison of area activity and mass activity values for HER at −70 mV versus RHE.Tafel slope.HER polarization curves (geometrical area) for Pt 18 Ni 26 Fe 15 Co 14 Cu 27 /C with different CV cycle.Reproduced with permission.[135]Copyright 2020, Springer Nature.
Figure 10.a).CV curves for the quinary (FeCoPtPdIr) HEA-NPs.Cdl measurements for the quinary HEA-NPs in HER.Chronopotentiometry of FeCoPdP-tIr@GO at constant 100 mA cm −2 .b) Electrochemical performance comparison of samples.c) Elemental maps for FeCoPdIrPt supported on GO.Reproduced with permission.[134]Copyright 2020, Springer Nature.d) CV curves of Pt/C and Pt 18 Ni 26 Fe 15 Co 14 Cu 27 /C.Peak values of mass activity and area activity.Chronoamperometric tests for MOR at 0.65 V versus RHE.CV curves of the Pt 18 Ni 26 Fe 15 Co 14 Cu 27 /C before and after 1000 cycles.e) The side view of the structural configuration of HEA.The side view of HEA structural configuration and the real spatial contour plots for bonding and anti-bonding orbitals near EF.And the top view of the real spatial contour plots for bonding and anti-bonding orbitals near EF for the HEA.The PDOSs of the HEA.f) The corresponding elemental mapping of Pt 18 Ni 26 Fe 15 Co 14 Cu 27 /C nanoparticles.g) CV curves.HER polarization curves (geometrical area).Pt mass loading normalized (mass activity) LSV curves.Comparison of area activity and mass activity values for HER at −70 mV versus RHE.Tafel slope.HER polarization curves (geometrical area) for Pt 18 Ni 26 Fe 15 Co 14 Cu 27 /C with different CV cycle.Reproduced with permission.[135]Copyright 2020, Springer Nature.

Figure 11 .
Figure 11.a) Reaction pathways of the alkaline HER and OER at the Cu sites in the HEA NPs.The d-orbital PDOS of Fe, Co, Ni, Cu, and Mn and the total value for FeCoNiCuMn.b) STEM-EDX mapping images of the HEA NPs supported on CNFs.c) Polarization curves of the FeCoNi/CNF, FeCoNiMn/CNF, FeCoNiCu/CNF, and FeCoNiCuMn HEA/CNF catalysts in a 1 m KOH aqueous electrolyte for the HER and OER.Polarization curves of various electrocatalysts in a 1 m KOH aqueous electrolyte for the HER and OER based on the geometric electrode surface area and ECSA.The long-term HER and OER stability test of the HEA/CNF catalyst at current densities of −170 and 230 mA cm −2 under alkaline conditions.Overall water splitting LSV curves of the HEA/CNFs||HEA/CNF and Pt/C||IrO 2 electrocatalysts in an alkaline electrolyte.Comparison of current densities (at 1.7 V) with other reported alkaline electrolyte catalysts d) Reaction pathways of the alkaline HER and OER at the Cu sites in the HEA NPs.The calculated energy barriers of water dissociation (ΔG *H2O -ΔG *OH ) and Gibbs free energy (ΔG *H) profiles for the HER at the Fe, Co, Ni, Cu, and Mn sites.Reproduced with permission.[136]Copyright 2023, Royal Society of Chemistry.e) Electrochemical characterization of nanoporous electrodes.f) Electrochemical performance in neutral electrolyte.Reproduced with permission.[137]Copyright 2020, Wiley-VCH.

Table 1 .
Summary of the pros and cons for HEAs synthesis methods.

Table 2 .
Summary of high-entropy alloy catalysts for the HER.