REVIEW www.advmat.de High-Entropy Materials for Water Splitting: An Atomic Nanoengineering Approach to Sustainable Hydrogen Production Yufei Zhao, Jinhu Wu, Xianjun Cao, Dongfang Li, Peng Huang,* Hong Gao, Qinfen Gu, Jinqiang Zhang,* Guoxiu Wang, and Hao Liu* Green hydrogen production via water electrolysis is pivotal for achieving energy sustainability. However, the inherently sluggish kinetics of the hydrogen evolution reaction and oxygen evolution reaction impede the progress of water-splitting technology. Recently, high-entropy materials (HEMs) composed of at least five elements have garnered significant attention as promising electrocatalysts for water splitting, owing to their compositional versatility, structural robustness, and synergistic interactions among elements. This review comprehensively explores the development of HEMs, tracing their emergence and structural evolution via atomic nanoengineering strategies (i.e., from bulk to nanostructuring, from random distributions to relatively ordered architectures, from bare HEMs to reconstructed HEMs, from intact HEMs to defective structures, from pristine HEMs to functionalized variants) and revealing how these evolutionary steps contribute to the properties and enhance catalytic performance in water splitting. The fundamental roles of individual elements (e.g., active sites, promoters, stabilizers) in shaping the structure, stability, and catalytic activity of HEMs are examined, laying a foundation for the rational design of efficient HEM-based electrocatalysts. The review also highlights recent advances in HEM-based catalysts for water splitting, emphasizing desirable properties and elemental contributions. Finally, the remaining challenges and perspectives on the future directions of HEM-based materials in energy conversion technologies are discussed. 1. Introduction Hydrogen is a clean and sustainable energy resource with sig- nificant potential to replace fossil fuels. It is considered a J. Wu, X. Cao, H. Gao Joint International Laboratory on Environmental and Energy Frontier Materials School of Environmental and Chemical Engineering Shanghai University Shanghai 200444, China The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.202506117 © 2025 The Author(s). Advanced Materials published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. DOI: 10.1002/adma.202506117 promising renewable energy carrier due to its exceptionally high mass-energy den- sity, making it an attractive alternative to conventional fuels.[1–4] However, the large- scale production of hydrogen is still lim- ited by its synthesis approach that relies heavily on steam reforming of natural gas, inevitably accompanied by emissions of carbon dioxide.[5–7] Water splitting, on the other hand, is an alternative way to generate hydrogen with zero carbon emissions.[8–11] Nevertheless, the widespread application of water electrolysis is hindered by its reliance on highly efficient catalysts, which are cur- rently based on precious metals such as platinum (Pt), ruthenium (Ru), and irid- ium (Ir).[12] The high cost and scarcity of these precious metal-based catalysts sig- nificantly restrict the scalability of water- splitting electrolyzers. Therefore, develop- ing active and cost-effective catalysts for water-splitting systems is critical for po- tential large-scale production of hydrogen. Transition metal-based electrocatalysts, such as sulfides, phosphides, carbides, ni- trides, oxides, alloys, etc., have drawn inten- sive attention in the past few decades.[13–18] However, they face limitations in stability, conductivity, selectivity, and scalability, which hinder their long- term efficiency and industrial applicability in water splitting. High-entropy materials (HEMs), defined by comprising typi- cally five principal elements or more in near-equimolar ratios, Y. Zhao, X. Cao, D. Li, J. Zhang, G. Wang, H. Liu School of Mathematical and Physical Sciences Faculty of Science University of Technology Sydney Broadway, Sydney, NSW 2007, Australia E-mail: Jinqiang.Zhang@uts.edu.au;Hao.Liu@uts.edu.au P.Huang School of Chemistry andMaterials Science JiangsuNormalUniversity Xuzhou, Jiangsu221116, China E-mail: huangpeng@jsnu.edu.cn Q.Gu Australian Synchrotron ANSTO 800BlackburnRd, Clayton, VIC 3168, Australia Adv. Mater. 2025, 2506117 2506117 (1 of 39) © 2025 The Author(s). Advanced Materials published by Wiley-VCH GmbH http://www.advmat.de https://doi.org/10.1002/adma.202506117 http://creativecommons.org/licenses/by/4.0/ http://creativecommons.org/licenses/by/4.0/ mailto:Jinqiang.Zhang@uts.edu.au mailto:Hao.Liu@uts.edu.au mailto:huangpeng@jsnu.edu.cn http://crossmark.crossref.org/dialog/?doi=10.1002%2Fadma.202506117&domain=pdf&date_stamp=2025-06-16 www.advancedsciencenews.com www.advmat.de possess significant potential owing to the highly disordered ho- mogeneous single-phase and high configurational entropy of mixing.[19–21] The development of HEMs can be traced back to the early 2000s with two seminal papers published on metallic alloys containing five or six principal components.[22,23] These al- loys quickly attract attention within the materials science com- munity due to their core effects, including the high-entropy effect, lattice distortion effect, and cocktail effect. Specifically, the high-entropy effect refers to the thermodynamic stabiliza- tion resulting from the large configurational entropy gener- ated by mixing multiple principal elements. The lattice distor- tion effect arises from atomic size mismatches and complex chemical interactions among the constituent elements, lead- ing to local lattice strain. Meanwhile, the cocktail effect de- scribes the unexpected synergies and emergent properties that result from the intricate interactions among the diverse ele- ments within HEMs.[24–27] The family of HEMs has expanded from HEAs to high entropy oxides (HEOs), high entropy car- bides (HECs), high entropy nitrides (HENs), high entropy sul- fides (HESs), high entropy borides (HEBs), etc.[28–36] The random distribution of multiple metal elements in HEMs promotes ex- ceptional homogeneity, potentially surpassing the properties of single or unary element-based materials. HEMs leverage syn- ergistic interactions (the combined effect of multiple compo- nents is greater than the sum of their individual effects) among multiple principal elements, leading to a higher density of ac- tive sites, optimized electronic structures, and superior struc- tural integrity.[37–39] These unique features enhance electrochem- ical activity and ensure outstanding stability for water-splitting applications.[40,41] With continuous advancements in nanotechnology, HEMs have undergone significant structural evolution, leading to sub- stantial enhancements in their electrocatalytic activity. Unlike conventional alloys or metal compounds, HEMs possess a di- verse range of unique surface binding sites, providing a versatile platform for precisely tuning binding energies to optimize reac- tion properties.[42] This ability is crucial for water splitting which relies significantly on the interaction of active sites with reactants and intermediates.[43,44] In particular, the structural evolution of HEMs plays a vital role in tailoring the geometric and electronic features to modulate the adsorption energy, thereby enhancing their catalytic efficiency for water splitting.[45,46] The selection and composition of elements in HEMs influence both the density and local environment of active sites, thereby directly impacting the intrinsic catalytic activity.[47–49] Furthermore, downsizing HEMs from bulk to the nanoscale and engineering porous or hierarchical nanostructures can dramatically increase surface area, facilitate electron and mass transport, and improve atomic utilization, all of which contribute to accelerated reaction kinetics and enhanced catalytic performance.[50–54] Another critical structural advancement involves the transition from a randomly distributed atomic arrangement to a relatively ordered configuration, which further optimizes HEMs for water-splitting applications.[55] Other than the structure evolution for the pris- tine HEMs, surface engineering approaches, including creating reconstructed, defective, or functionalized HEMs, offer addi- tional pathways for expanding and refining their properties.[56–58] These surface modifications can introduce new active sites, en- hance stability, and further improve catalytic performance, making HEMs increasingly attractive as next-generation electrocatalysis. Several reviews have summarized the synthesis methods, properties, effects, design strategies, and potential applications of HEMs.[59–63] However, a comprehensive review detailing the evolution of HEMs from structural foundations to functionality, as well as an in-depth understanding of the roles each element plays, is still lacking. This updated review focuses on recent ad- vancements in HEMs for water splitting. In this review, we will discuss the emergency and structural evolution of HEMs, cov- ering the structural progression from bulk forms to nanostruc- tured, from random atomic distributions to more ordered struc- tures, from bare HEMs to reconstructed, from intact to defective structures, and from pristine HEMs to functionalized HEMs, all of which significantly enhance their properties for water split- ting applications. We then elaborate on how each element con- tributes to overall structure, stability, and performance to identify elemental roles in HEMs, providing a foundation for designing efficient HEM-based electrocatalysts. This review will also sum- marize HEM-based catalysts with desired properties and identi- fied elemental roles for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in water splitting. Finally, we address current challenges and future perspectives for HEM- based catalysts in energy conversion applications (Figure 1). 2. The Emergence and Structure Evolution of High Entropy Materials 2.1. The Emergence of High Entropy Materials The development from non-high-entropy materials to HEMs marked the emergence of a new design paradigm, which lever- ages multiple principal elements (≥5) in near-equimolar ratios to harness configurational entropy and unlock unique material properties (Figure 2).[64–69] This transition reflects a shift from simplicity and predictability to controlled complexity and tun- ability, opening a new frontier in functional materials design. HEMs resolve the limitation of the stability, corrosion resistance, and versatility in traditional non-high-entropy materials, display- ing enhanced stability, chemical properties, cocktail effects, etc., making them promising for water splitting.[70,71] Generally, the simplest nanocrystals are monometal-based compounds, consisting of a uniform arrangement of atoms from a single metallic element.[72–75] Their properties are primarily governed by the intrinsic nature of the metal, crystal struc- ture, and particle size. Their relatively simple composition al- lows for precise control over their synthesis and the investi- gation of structure–catalytic properties. Advancements in alloy- ing and doping strategies with additional metal elements fur- ther enable precise engineering of the properties of monometal- based structures, which typically consist of two or three com- positional elements with one primary component.[76–78] This strategy has facilitated the development of numerous materi- als based on a primary component coupled with additional mi- nor elements improved conductivity and catalytic activity. For in- stance, we have designed Ni-doped CoP and Co-doped MnO2 for water splitting, effectively modulating the electronic struc- tures of the primary metals, namely Co and Mn, respectively, Adv. Mater. 2025, 2506117 2506117 (2 of 39) © 2025 The Author(s). Advanced Materials published by Wiley-VCH GmbH 15214095, 0, D ow nloaded from https://advanced.onlinelibrary.w iley.com /doi/10.1002/adm a.202506117 by A nsto, W iley O nline L ibrary on [17/07/2025]. See the T erm s and C onditions (https://onlinelibrary.w iley.com /term s-and-conditions) on W iley O nline L ibrary for rules of use; O A articles are governed by the applicable C reative C om m ons L icense http://www.advancedsciencenews.com http://www.advmat.de www.advancedsciencenews.com www.advmat.de Figure 1. Overview of structural evolution and elemental roles of HEMs for water splitting. Figure 2. Historical timeline of emergency and structural evolution of HEMs for water splitting. Adv. Mater. 2025, 2506117 2506117 (3 of 39) © 2025 The Author(s). Advanced Materials published by Wiley-VCH GmbH 15214095, 0, D ow nloaded from https://advanced.onlinelibrary.w iley.com /doi/10.1002/adm a.202506117 by A nsto, W iley O nline L ibrary on [17/07/2025]. See the T erm s and C onditions (https://onlinelibrary.w iley.com /term s-and-conditions) on W iley O nline L ibrary for rules of use; O A articles are governed by the applicable C reative C om m ons L icense http://www.advancedsciencenews.com http://www.advmat.de www.advancedsciencenews.com www.advmat.de to significantly enhance their catalytic performance.[79,80] Al- though proven effective, these strategies often lead to localized effects, and the central properties of the multicomponent phase space may deviate significantly from the corresponding corners and edges. Moreover, the doped metal compounds often suffer from dopant leaching or phase segregation under harsh reaction conditions. Further development of metal alloys or compounds has progressed from simple monometal-based compounds or doped materials to more complex systems (e.g., ternary or quaternary), allowing for increased tunability of composition, structure, and functionality.[81] For instance, Ni–Fe oxide aerogels synthesized via a modified epoxide route exhibited high catalytic activity for OER, achieving 10 mA cm−2 at an overpotential of 380 mV with an optimized Ni/Fe ratio.[82] Similarly, modulated NiFeX and FeCoX (X = W, Mo, Nb, Ta, and Re) oxyhydroxide catalysts demonstrated excellent OER performance, attributed to the facilitated oxidation of 3d transition metals induced by the incorporation of high-valence modulators.[83] This systematic progression toward multi-metallic systems paves the way for the design of HEMs, which composed of multiple principal elements in nearly equiatomic proportions. HEMs exhibit signif- icantly higher configurational entropy, enhanced multi-element synergy, superior structural stability, and tunable electronic properties, offering unconventional compositions with many possibilities to regulate catalytic performance and overcome the limitations of single-element or doped catalysts. These enable them to be a highly promising alternative for water splitting. Various HEMs have been designed and prepared, for example, PtCoNiRuIr HEA-NPs, PtRuMoFeCoNi ultra-small HEA quantum dots, PtRuPdCoNi HEA nanoparticles, M- RuIrFeCoNiO2, have exhibited unique properties and excellent HER/OER performance.[84–87] Further to precious metal-based HEMs, non-precious metal-derived HEMs, composed of mul- tiple inexpensive elements, offer cost-effective alternatives while maintaining exceptional catalytic performance. HEMs such as CoFeMnCuZn, FeCoMoW, and Co0.6(VMnNiZn)0.4PS3 nanosheets, with high-concentration active sites, have been successfully developed, exhibiting enhanced HER/OER performance.[88–90] 2.2. The Structure Evolution of High Entropy Materials Since the first appearance of HEMs, the structural evolution of HEMs plays a crucial role in expanding their structural, elec- tronic, and catalytic properties, significantly broadening their potential applications. Typically, several significant steps have marked the evolution of HEMs. The first is to introduce nanos- tructures to the bulky HEMs which can significantly enhance the surface-to-volume ratio, exposing a greater density of ac- tive sites and facilitating improved mass transport.[91–93] This evolution optimizes charge transfer dynamics and catalytic effi- ciency, leading to superior performance for water splitting. An- other structural evolution is from the random distribution of metal elements in the crystal lattice to a relatively ordered struc- ture in HEMs which can maintain entropy stabilization from inherent disorder while generating unique electronic proper- ties from controlled structural ordering. Moreover, the devel- opment of reconstructed, defective, and functionalized HEMs represents a significant advancement, compared to the pristine structures, to further optimize the properties and catalytic ca- pabilities of HEMs. Thus, in this section, we will focus on the critical steps of the structural evolution in HEMs with an in- depth understanding of properties and optimized catalytic per- formance, namely from bulk to nanostructuring, from random distribution to relatively ordered structure, from pristine HEMs to their reconstructed, defect-engineered, and functionalized forms. 2.2.1. From Bulk to Nanostructuring While most studies of HEMs mainly focus on adjusting the chemical composition to optimize the catalytic performance, structural engineering, especially the regulation of size and di- mensionality, has also emerged as a critical approach to im- prove catalytic efficiency. HEMs are predominantly synthesized as bulk materials using conventional methods (e.g., melt pro- cessing), due to the challenges associated with mixing ele- ments with vastly different chemical and physical properties, as well as cooling rate constraints.[22,94] Reducing the size of HEMs to the nanoscale with controlled morphologies is sig- nificant for enhancing nanostructural diversity and investi- gating relationships between structures and properties. The typical low-dimensional HEMs are nanoparticles, which fea- ture small size and high specific surface area, exposing abun- dant active sites and maximizing atom utilization.[95–99] A typ- ical example is the well-dispersed sub-10 nm (MoWVNbTa)C HEC nanoparticles, which were successfully synthesized via electrical discharge-induced bulk-to-nanoparticle transformation (Figure 3a,b). These HEC nanoparticles have demonstrated out- standing catalytic activity and long-term durability for HER, which was attributed to their unique microstructure and the en- hanced electronic interactions arising from high configurational entropy (Figure 3c).[100] Similarly, PdCuPtNiCo HEA nanoparti- cles were synthesized using PdCu nanoparticles as seeds to ini- tially form PdCu/PtNiCo core–shell structures, followed by an annealing process.[101] Generally, substrates are critical to con- trolling the size of HEM nanoparticles. The commonly used sub- strates are carbon nanotubes, graphene, and organic materials- derived carbon substrates, among others.[102,103] A carbon shock method has been developed to synthesize HEA and HEO nanoparticles uniformly dispersed on carbon substrates.[104,105] The high synthesis temperature ensured the formation of ho- mogeneous alloy structures while simultaneously strengthening the bonding between the HEA/HEO nanoparticles and the car- bon substrate, thereby enhancing structural durability. In par- ticular, the smallest possible form of nanoparticles is single- atom catalysts (SACs), which represent the ultimate downsiz- ing of HEMs to the atomic level, forming single-atom HEMs. The unique electronic properties and maximum atom utiliza- tion make single-atom HEMs favorable to the catalysis pro- cess. For example, a substrate-mediated SACs formation strat- egy has been employed to prepare single-atom HEMs, lever- aging reversible redox reactions at the TMDs (substrate)/TM ion interface. The as-achieved Pt,Ru,Rh,Pd,Re-MoSe2 delivered excellent hydrogen evolution reaction (HER) performance in Adv. Mater. 2025, 2506117 2506117 (4 of 39) © 2025 The Author(s). Advanced Materials published by Wiley-VCH GmbH 15214095, 0, D ow nloaded from https://advanced.onlinelibrary.w iley.com /doi/10.1002/adm a.202506117 by A nsto, W iley O nline L ibrary on [17/07/2025]. See the T erm s and C onditions (https://onlinelibrary.w iley.com /term s-and-conditions) on W iley O nline L ibrary for rules of use; O A articles are governed by the applicable C reative C om m ons L icense http://www.advancedsciencenews.com http://www.advmat.de www.advancedsciencenews.com www.advmat.de Figure 3. a) Schematic process of proceeding HEC bulk to nanoparticles by WEDM. b) High-Angle Annular Dark-Field Scanning Transmission Elec- tron Microscopy (HAADF-STEM) image of (MoWVNbTa)C HECNPs. c) Comparison of 𝜂10 between the HECNPs and other nonprecious metal-based catalysts. a–c) Reproduced with permission.[100] Copyright 2022, Wiley-VCH. d–f) schematic diagram, Scanning Electron Microscope (SEM) and OER polarization curves of the synthesis process of CoZnCdCuMnS@CF. d–f) Reproduced with permission.[114] Copyright 2022, Springer Nature. g) OER reaction energy diagram on 𝛽-NiOOH and 𝛾-NiOOH models. g) Reproduced with permission.[120] Copyright 2022, Elsevier. h) Aberration-corrected HAADF-STEM image viewed along the [001] zone axis. h) Reproduced with permission.[43] Copyright 2020, Wiley-VCH. alkaline conditions.[106] Further transitioning from nanoparti- cles/single atoms to higher-dimensional structures, such as 1D (nanowires and nanotubes, etc.), 2D (nanosheets, nanoplates, nanospheres, etc.), and 3D (nanoflowers, nanodendrites, hollow structure, etc.), expands the properties and kinetics of HEMs for electrocatalysis.[107–113] For instance, high-entropy sulfide nanoar- rays (CoZnCdCuMnS@CF) were synthesized through a low- temperature cation exchange reaction by using Co9S8@CF with a structure of vertical needle-like nanowires as a precursor, demonstrating exceptional performance for alkaline water split- ting (Figure 3d–f).[114] A growing variety of 2D HEMs have been developed for water splitting, including sub-nanometer ribbons, 2D high entropy oxides, and sulfides, layered double hydroxides (LDHs), high entropy metal–organic frameworks.[115,116] More- over, 3D hollow-structured HEOs (ZnFeNiCuCoRu-O) feature a large surface area and rapid mass transfer kinetics, delivering ex- ceptional OER catalytic performance across a wide pH range.[117] 2.2.2. From Random Distribution to Relatively Ordered Structure HEMs rely heavily on the formation of crystalline or amor- phous solid-solution phases, usually exhibiting a highly disor- dered atomic arrangement. Specifically, multiple principal ele- ments are randomly distributed within the lattice, leading to increased configurational entropy, which stabilizes the single- phase solid solution and prevents phase segregation.[118,119] The atomic disorder significantly influences the catalytic properties, which are crucial for optimizing catalytic activity and stability. Moreover, the inherent randomness in elemental distribution en- ables tunable local environments, enhancing synergistic inter- actions between different elements. For instance, the designed NiFeCoMnAl oxide featured a random distribution of elements, enabling each component to contribute synergistically to OER ac- tivity. In particular, Ni served as the primary active site, while the incorporation of Mn created an electron-rich environment that Adv. Mater. 2025, 2506117 2506117 (5 of 39) © 2025 The Author(s). Advanced Materials published by Wiley-VCH GmbH 15214095, 0, D ow nloaded from https://advanced.onlinelibrary.w iley.com /doi/10.1002/adm a.202506117 by A nsto, W iley O nline L ibrary on [17/07/2025]. See the T erm s and C onditions (https://onlinelibrary.w iley.com /term s-and-conditions) on W iley O nline L ibrary for rules of use; O A articles are governed by the applicable C reative C om m ons L icense http://www.advancedsciencenews.com http://www.advmat.de www.advancedsciencenews.com www.advmat.de Figure 4. a) The HER activity comparison of IrFeCoNiCu HEA and other Ir-based catalysts at different overpotentials. b) High resolution HAADF-STEM image showing an Ir-enriched surface layer (indicated by white arrows) on theHEA core after electrochemical activation. a,b) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license.[132] Copyright 2023, American Chemical Society. c) AFM image and corresponding height profile of the HEA-PdPtRhIrCu metallene. d) HAADF-STEM image and false-color aberration-corrected image of the HEA-PdPtRhIrCu metallene. c,d) Reproduced with permission.[142] Copyright 2023, Wiley-VCH. e) Diagram of FeCoNiMnCr HEA-HEO/CNT catalyst. f) Polarization curves before and after the stability test. e,f) Reproduced with permission.[149] Copyright 2024, Wiley-VCH. enhances the catalytic activity of Ni active sites. Co-doping im- proved the electrical conductivity, and Al doping facilitates the formation of a nanoporous structure via a dealloying process. This random elemental distribution ensures that each element plays a distinct and complementary role in boosting the overall electrocatalytic efficiency (Figure 3g).[120] Although the disordered atomic arrangement of HEMs ex- hibits unique advantages, this characteristic may potentially hin- der the precise control of electronic and catalytic properties. Therefore, understanding how to control the atoms within a chemically ordered distribution and figuring out which atomic configurations benefit the activity remain the challenge in HEM development. Multimetallic alloys or compounds with a partic- ular ordered structure should be promising to overcome this challenge.[121–123] Local short-range chemical ordering has been observed in a concentrated solution of single-phase medium- entropy alloys (MEA) with a composition near the center of the phase diagram.[124] Moreover, the most typical HEMs with rela- tively ordered structures are high entropy intermetallics (HEIs), which are a class of ordered HEMs where multiple principal ele- ments form an intermetallic compound instead of a random solid solution.[125–130] HEIs still retain the high configurational en- tropy while featuring ordered atomic arrangements based on in- termetallic compounds, offering unique electronic and catalytic properties. HEIs can be synthesized using parent binary inter- metallics, such as PtSn, to serve as the initial reactant. In this structure, Pt sites were partially substituted with Ni and Co, while Sn sites were partially replaced with In and Ga.[131] The increased number of constituent metals not only enhances the kinetic sta- bility of the nanoparticles but also suppresses undesired side re- actions. The HEIs combine the merits of both high-entropy and ordered intermetallic properties, potentially enabling specific ef- fects to boost the water-splitting performance. For instance, the dendrite-like porous L12-type HEI, derived from the dual-phase FeCoNiAlTi HEI (where FeCoNi served as potential active sites and AlTi enhanced the formation of an ordered atomic configu- ration), exhibited a specific site-isolation effect that further sta- bilizes H2O/H* adsorption and desorption (Figure 3h).[43] This significantly optimizes the energy barrier for hydrogen evolution. 2.2.3. From Bare HEMs to Surface Reconstructed HEMs Further engineering HEMs to form appealing structures by re- fining their structural, electronic, and surface properties can in- duce additional enhancements, leading to improved catalytic ef- ficiency and stability for water splitting. For instance, selective leaching of 3d transition metals during the electrochemical ac- tivation process leads to the formation of a precious metal-rich surface. A typical example is IrFeCoNiCuHEA, which resulted in an Ir-enriched surface layer that significantly enhanced OER cat- alytic capability for a low overpotential of≈302mV at 10mA cm−2 (Figure 4a,b).[132] Surface reconstruction to form an oxidized or reduced layer on the surface of HEMs is another emerg- ing strategy to dynamically change the atomic arrangement and electronic structure, leading to unconventional properties.[133–135] Adv. Mater. 2025, 2506117 2506117 (6 of 39) © 2025 The Author(s). Advanced Materials published by Wiley-VCH GmbH 15214095, 0, D ow nloaded from https://advanced.onlinelibrary.w iley.com /doi/10.1002/adm a.202506117 by A nsto, W iley O nline L ibrary on [17/07/2025]. See the T erm s and C onditions (https://onlinelibrary.w iley.com /term s-and-conditions) on W iley O nline L ibrary for rules of use; O A articles are governed by the applicable C reative C om m ons L icense http://www.advancedsciencenews.com http://www.advmat.de www.advancedsciencenews.com www.advmat.de Especially, the non-HEOs (e.g., HEAs or HE metal compounds) underwent rapid in situ electrochemical oxidation, into metal (oxy)hydroxides or oxides, which are considered the real OER active sites in alkaline solution.[136–139] For instance, the surface states of HEA sheets (Ti, Cr, Mn, Fe, Co, Ni, Zr, Nb, and Mo) ex- hibited no significant changes between the OCP and HER state, indicating that the metallic nature of the HEA was maintained during the reduction processes.[140] In contrast, during the OER process, all elements in the HEAs were partially oxidized to form oxide layers. Similarly, FeNiCoCrMnS2 also experienced in situ structural self-reconstruction during OER, fully converting into metal (oxy)hydroxide at 1.2 V versus RHE, which served as the real active sites for OER. Additionally, the remaining sulfate an- ion also synergistically boosted the OER catalytic activity.[141] 2.2.4. From Intact HEMs to Defective HEMs Other than surface reconstruction, forming defective HEMs can also significantly influence their physicochemical prop- erties, which directly affect the adsorption of the reac- tants/intermediates. There are various defects, such as lattice distortion, stacking faults, and vacancies, observed in HEMs, which can be formed during the preparation process.[142,143] For instance, an ultrathin HEA-PdPtRhIrCu metallene rich in lattice distortions and defects was synthesized through a simple hy- drothermal method (Figure 4c,d).[142] While atomic-level mixing of multiple elements may already be effective to form defects, developing methods to specifically introduce desired defects for HEMs is more significant. Plasma strategy, electrochemical dealloying, high-temperature treatment, etc., have been applied to create cation and anion vacancies for HEMs. For example, low-temperature plasma techniques facilitated the transfor- mation of LDHs into HEO nanosheets with abundant oxygen vacancies and a high surface area.[144] A low-temperature surface carbonization–decarbonization strategy has also been developed by encapsulating high-entropy oxides with a carbonaceous layer and precisely controlling the decomposition to introduce and modulate oxygen vacancies in CoMnFeNiZn-Ov HEOs. [143] The oxygen vacancies formed in HEOs enhance both *OH adsorp- tion and deprotonation simultaneously, leading to excellent OER activity. 2.2.5. From Pristine HEMs to Functionalized HEMs Similar to conventional catalysts, the surface of pristine HEMs can be functionalized by introducing additional foreign species, enabling appealing properties and higher catalytic activity. Non- metal doping (e.g., N, F, P, and B) is an effective strategy for tailoring the properties of HEMs. For example, N incorporation into NFeCoNiAlMoOx activated lattice oxygen by modulating the hybridization between active site Mo 3d orbitals and O 2p or- bitals, thereby enhancing OER activity.[145] Another effective and straightforward approach is the incorporation of single atoms, clusters, or nanoparticles, which introduces cocktail effects to en- hance the reaction kinetics and durability of HEMs. Alloying Pd with the surface atoms on non-nobleHEA-NPs enhanced catalyst stability while minimizing noble metal usage.[146] Ag clusters in- corporated in high entropy CuCoFeAgMo (oxy)hydroxides could efficiently lower the limiting energy barrier, facilitating improved reaction kinetics, while their higher Fermi levels served as elec- tron donors to activate the metal sites.[147] This cocktail effect op- timizes charge transfer and adsorption energy, ultimately leading to superior OER performance. Beyond metal elements, the sur- face of HEMs can also be functionalized with non-metallic com- ponents or stabilized on carbon substrates. Stabilizing HEMs on conductive carbon-based substrates can significantly improve electrical conductivity, structural integrity, and mechanical sta- bility, ensuring long-term stability for water splitting. A typi- cal example is CuNiFeCoCrTi@N-doped graphene nanoparticles (CuNiFeCoCrTi@NC NPs) with precisely controlled N-doped graphene shell layers.[148] Reducing the graphene layer thick- ness and increasing the number of elements in the alloy sig- nificantly boosted the HER performance. Moreover, construct- ing high-entropy heterostructures can enhance structural stabil- ity and corrosion resistance, contributing to sustained OER activ- ity under harsh electrochemical conditions. A Cr-induced spon- taneous reconstruction strategy was employed to synthesize Fe- CoNiMnCr HEA and HEO heterocatalysts.[149] These materials provide active sites with stable valence states, ensuring long-term durability for OER, even under industrial operating conditions (Figure 4e,f). 3. Elemental Roles in High Entropy Materials The diverse elemental composition of HEMs endows them with a range of unique synergistic properties, making them highly promising for water splitting.[150,151] However, this same com- plexity also presents a significant challenge to understanding the individual role of each element. In HEMs, each element can con- tribute differently, such as serving as active sites, increasing con- ductivity, maintaining corrosion resistance, and promoting sta- bility. Therefore, clearly understanding the influence of each el- ement on the overall structure, stability, and functional perfor- mance of HEMs is critical for the rational design and optimiza- tion of these HEMs. It is well established that the catalytic per- formance of water splitting (HER and OER) is closely related to the adsorption energy of reactants or intermediates at active sites of HEMs. Achieving optimal catalytic activity typically requires an optimized adsorption strength, which is often determined by the active sites. Thus, identifying the active sites of HEMs for wa- ter splitting is considered the first vital approach. By tuning the properties or electronic structure ofHEMs through the structural evolution discussed in Section 2, the adsorption energy can be precisely modulated to enhance catalytic performance in water splitting. This highlights the importance of identifying not only the elements that serve directly as active sites but also those that, while not directly involved in catalysis, still play significant roles in influencing overall catalytic activity. In addition to catalytic ac- tivity, the stability of HEMs is a critical factor influencing overall catalytic performance. Understanding how individual elements contribute to stability is also essential. This session will focus on exploring the atomic-level roles of individual elements within HEMs, aiming to uncover fundamental insights that drive mate- rial innovation. We will examine the elements in terms of three distinct roles: active sites, promoters, and stabilizers (Figure 5). A few approaches have been applied to can identify the el- ement role of HEMs for water splitting. The first approach is Adv. Mater. 2025, 2506117 2506117 (7 of 39) © 2025 The Author(s). Advanced Materials published by Wiley-VCH GmbH 15214095, 0, D ow nloaded from https://advanced.onlinelibrary.w iley.com /doi/10.1002/adm a.202506117 by A nsto, W iley O nline L ibrary on [17/07/2025]. See the T erm s and C onditions (https://onlinelibrary.w iley.com /term s-and-conditions) on W iley O nline L ibrary for rules of use; O A articles are governed by the applicable C reative C om m ons L icense http://www.advancedsciencenews.com http://www.advmat.de www.advancedsciencenews.com www.advmat.de Figure 5. The element role of HEMs as (a,b) active sites, c) promoters and (d) stabilizers for water splitting. the poisoning experiment. The roles of metal species are typi- cally identified using poisoning agents that selectively bind to specific metals, reducing catalytic activity and indicating the functional sites.[152–154] However, HEMs generally contain five or more metal elements, and many show similar properties, which makes it challenging to pinpoint the element roles us- ing this approach. In situ characterization techniques are widely employed to study the water splitting process under realistic op- erating conditions, offering valuable insights into the underly- ing relationship between catalyst structure and active sites. For instance, operando IR and Raman spectroscopy are particularly important for characterizing HEMs, as they enable direct mon- itoring of interactions between intermediates and the catalyst surface (binding sites). Additionally, in situ, X-ray absorption spectroscopy allows the monitoring of the dynamic process of water splitting, revealing changes in electronic states, identify- ing active sites, and uncovering synergistic effects within HEM catalysts.[125,155,156] Meanwhile, due to the complexity of HEMs, DFT calculations play essential roles in identifying the element roles for water splitting.[157] As the HER or OER catalytic activity is largely governed by the inherent electronic structure of HEMs, the d-band theory is developed to predict the bonding interac- tions between HEM surfaces and intermediates. The total den- sity of states (DOS) is commonly used to indicate the conductiv- ity of HEMs, with a higher DOS near the Fermi level facilitating more efficient electron transfer. The projected density of states (PDOS), derived from the total DOS, provides detailed insights into the electronic structure and is strongly correlated with HER and OER activity, which is a good indicator to identify the ele- ment roles. ΔG diagrams are widely recognized as a powerful tool for identifying the rate-determining step (RDS) in the HER and OER pathways. The activity of the active sites directly influ- ences the adsorption energy of reactants or intermediates, as ad- sorption onto these sites is a necessary first step to initiate the catalytic process.[158–160] 3.1. Elements as Active Sites HEMs exhibit a complex composition of multiple metal species, resulting in a diverse array of potential active sites for HER and OER. In particular, precious metals such as Pt, Pd, Ir, and Ru in HEMs have demonstrated their capability to significantly en- hance catalytic activity. In Pt-based HEMs, Pt atoms typically serve as the primary active sites for HER, as observed in composi- tions such as PtIrFeCoNi, PtPdRuIrAu, and PtCoMoPdRh.[161–163] For instance, the investigation for PtCoMoPdRh indicates that the ΔGH* values at most active sites were superior to that of the Pt(111) surface, highlighting the significant support from the high entropy elements to the active sites within the HEA (Figure 6a,b).[58] Especially, hydrogen adsorption on Pt top sites forms stable M─H* bonds with the lowest ΔGH*, confirming that these Pt top sites served as the primary and most active cen- ters during the Tafel step of the HER mechanism. Similar to Pt- based HEMs for HER, in Ir-based HEMs including IrFeCoNiCu HEA nanoparticles and Ir-rich IrRuNiMo shell@IrRuCoNiMo core structures,[53,164] Ir atoms typically act as the primary ac- tive sites for OER. These catalysts exhibit enhanced OER activ- ity compared to monometallic Ir and other highly active Ir-based bimetallic alloys. Other precious metals in HEMs, such as Ru in FeCoNiMoPtRu, Rh in PtRhNiFeCu and PtMoPdRhNi, and Pd in PtPdIrRuAu can act as active sites for HER or OER, either inde- pendently or collaboratively boost the performance.[86,165–167] For example, in situ Raman spectroscopy has identified characteristic Adv. Mater. 2025, 2506117 2506117 (8 of 39) © 2025 The Author(s). Advanced Materials published by Wiley-VCH GmbH 15214095, 0, D ow nloaded from https://advanced.onlinelibrary.w iley.com /doi/10.1002/adm a.202506117 by A nsto, W iley O nline L ibrary on [17/07/2025]. See the T erm s and C onditions (https://onlinelibrary.w iley.com /term s-and-conditions) on W iley O nline L ibrary for rules of use; O A articles are governed by the applicable C reative C om m ons L icense http://www.advancedsciencenews.com http://www.advmat.de www.advancedsciencenews.com www.advmat.de Figure 6. a) Binding configurations and b) Adsorption free-energy landscape of atomic hydrogen on of Pt30Mo5Pd25Rh25Ni15(111). a,b) Reproduced with permission.[58] Copyright 2023, Elsevier. c) In situ SERS with different applied potentials of Pt28Mo6Pd28Rh27Ni15NCs. c) Reproduced with permission.[166] Copyright 2023, Royal Society of Chemistry. d) Standard free energy diagram of the OER process at 0 V of FeCoNiPdWOOH for various active sites. d) Reproduced with permission.[172] Copyright 2024, Royal Society of Chemistry. e) Scheme of the different adsorption strengths of interme- diates on metal sites in HEAs and Cu catalysts and reaction pathways of the alkaline HER and OER at the Cu sites in the HEA NPs. e) Reproduced with permission.[110] Copyright 2023, Royal Society of Chemistry. f) Edge models of Co, P, and S sites for CoVMnNiZnPS3 and CoPS3. g) Gibbs free-energy profile of HER occurring on the characterized edge configurations (f). f,g) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license.[88] Copyright 2022, American Chemical Society. h) Adsorption sites of Pd-Au bridging on the surface of HEA-1.4 and adsorption energies of hydrogen atoms. h) Reproduced with permission.[176] Copyright 2025, Wiley-VCH. i) Optimal adsorption intermediates and j) Adsorption free-energy on HEAs-Co2RuO4-Ru. i,j) Reproduced with permission.[181] Copyright 2025, Wiley-VCH. peaks at 1912 and 2048 cm−1, corresponding to Rh─H and Pt─H bonds, respectively. These findings confirm that Pt and Rh served as the primary active sites for H* adsorption and desorption in Pt28Mo6Pd28Rh27Ni15 NCs (Figure 6c). [166] Moreover, both catalytically active transition metals (such as Fe, Co, and Ni) and typically inactive ones (such as Cu and Zn) can serve as effective active sites in HEMs for HER and OER. This is attributed to the unique high-entropy structures that create a highly disordered atomic environment to modu- late the local electronic structure and facilitate synergistic in- teractions among multiple elements, which may enhance the catalytic capability these metal species as active sites for HER and OER. For example, the active transition metals Ni in NNM- HEA@CF, Ni in FeCoNiMoWHEA, Fe/Ni in CoFeNiCrMnP/NF, Fe/Co/Ni in CrMnFeCoNi, Fe/Co/Ni in (FeNiCoCrMnV) HEO, etc. have demonstrated catalytic contributions to the HER or OER process.[36,67,168–172] Specifically, FeCoNiPdWP high-entropy phosphide (HEP) nanoparticles undergo surface reconstruction to form a FeCoNiPdW high-entropy oxyhydroxide, in which Fe, Co, and Ni with high oxidation states act as active sites for OER. As a result, this catalyst exhibits an exceptionally lowOER overpo- tential of 227 mV at a current density of 10 mA cm−2 (Figure 6d). In addition to active transition metals, traditionally inactive met- als in HEMs can also function as catalytic active sites for HER and OER. This unexpected activity arises from the unique local chemical environments and electronic structure modulation in- duced by the high-entropy configuration, which can activate in- ert metals by altering their adsorption energies. For instance, the electronegativity differences among Mn, Cu, Fe, Co, and Ni in- duce local electron redistribution, increasing electron density at the Ni and Cu sites. (Figure 6e) This electron enrichment acti- vated Cu sites to serve as effective active sites for both HER and OER. Therefore, the FeCoNiCuMn HEA nanoparticles demon- strated excellent electrocatalytic performance, achieving an over- potential of 281 mV at 100 mA cm−2 for HER and 386 mV at 200 mA cm−2 for OER.[110] Moreover, in the H-FeCoNiCuMo or Fe FeNiCuWRu catalyst, the inactive Cu could collaborate with Co or Ni to promote the HER process, resulting in exceptional Adv. Mater. 2025, 2506117 2506117 (9 of 39) © 2025 The Author(s). Advanced Materials published by Wiley-VCH GmbH 15214095, 0, D ow nloaded from https://advanced.onlinelibrary.w iley.com /doi/10.1002/adm a.202506117 by A nsto, W iley O nline L ibrary on [17/07/2025]. See the T erm s and C onditions (https://onlinelibrary.w iley.com /term s-and-conditions) on W iley O nline L ibrary for rules of use; O A articles are governed by the applicable C reative C om m ons L icense http://www.advancedsciencenews.com http://www.advmat.de www.advancedsciencenews.com www.advmat.de electrocatalytic activity and long-term stability.[173] In addition to metal sites, non-metallic sites can also be identified as active cen- ters. For example, in Co0.6(VMnNiZn)0.4PS3 nanosheets,Mn sites play a crucial role in facilitating water dissociation. Simultane- ously, optimized sulfur sites at the edges and phosphorus sites on the basal plane serve as active sites for hydrogen adsorption (Figure 6f,g).[88] This synergistically enhancesHERperformance, achieving a low overpotential of 65.9 mV at 10 mA cm−2. Beyond the commonly recognized top sites, bridge and hol- low sites can also serve as effective active sites to enhance HER or OER performance.[50,86,163,174,175] For example, Pd-Au bridge sites have been shown to contribute significantly to HER activity (Figure 6h).[176] Among various PdPtRuRhAu HEAs with differ- ent particle sizes, the sample with an average size of 3.14 nm ex- hibited the highest proportion of bridge sites (18.97%), resulting in the best catalytic performance and achieving an overpotential of just 70.07 mV at a current density of 10 mA cm−2. Similarly, in the (RuSnSbReF)Ox electrocatalyst, Ru–Re bridge sites enabled electron transfer from Ru to Re via oxygen bridges, thereby tun- ing the electronic structure and oxidation states.[177] These bridge sites served as active centers for acidic OER, playing a key role in determining the energy barrier of the RDS.Hollow sites have also been identified as active centers, offering additional adsorption configurations that further promote reaction kinetics. In the Pt- NiFeCoCu HEA, Fe sites actively promote water adsorption, ini- tiating the alkaline HER process.[44] The resulting OH species were stabilized by adjacent hollow sites, while hydrogen adsorp- tion preferentially occurred at hollow sites near Ni and Co atoms, facilitating efficient HER activity under alkaline conditions. Reconstructed surface species can also serve as active sites for water splitting.[178–180] Ag-decorated Co-Cu-Fe-Ag-Mo (oxy)hydroxide (Ag@CoCuFeAgMoOOH) electrocatalysts were synthesized via an electrochemical reconstruction method.[147] Remarkably, the Ag sites within the Ag clusters acted as com- parably active sites for OER, synergistically enhancing the cat- alytic performance by achieving a low overpotential of 270 mV at 100 mA cm−2. Moreover, the optimized self-reconstructed PtRu2.9Fe0.15Co1.5Ni1.3HEA, featuring an interface with cobalt ruthenate (HEAs-Co2RuO4), demonstrated enhanced HER cat- alytic activity (Figure 6i,j).[181] In this heterostructure, Co2+ and Ru3+ species in Co2RuO4 functioned as active sites for both H2O dissociation and hydrogen adsorption, which is different from pure HEA, where Ru atoms within the alloy were the main ac- tive sites. This synergistic interface significantly boosted alkaline HER performance, withHEAs-Co2RuO4 achieving a current den- sity of 41.8 mA cm−2 at 0.07 V versus RHE, which is 2 times higher than PtRu2.9Fe0.15Co1.5Ni1.3 HEA. 3.2. Elements as Promoters While some elements in HEMs may not act as direct active sites for water splitting, they can play a crucial role as promoters by modulating the geometric configuration and electronic structure of neighboring active sites. These promoter elements can influ- ence factors such as electron density, oxidation states, adsorption energy, and coordination environment, thereby enhancing the in- trinsic catalytic activity of the true active sites. Their presence con- tributes to the synergistic effect which is often observed inHEMs, enabling fine-tuning of surface properties and improved overall catalytic performance. One of themost commonmechanisms for the rest elements as promoters to enhance catalytic performance in HEMs is through electron transfer between the promoter ele- ments and the active sites due to their close interaction.[26,182,183] For instance, in the PtFeCoNiCuCr@HCS, Cr primarily acted as an electron donor, modulating the electronic configuration of the surrounding active sites to enhance their catalytic properties (Figure 7a).[184] Co served as the principal active site for water dissociation to generate hydrogen intermediates, which prefer- entially migrated toward Pt, Fe, and Cu sites, where hydrogen molecule formation occurred. In addition, the size mismatch be- tween different elements and active sites in HEAs can induce compressive strain on the catalyst surface, which in turn en- hances catalytic activity. For example, in the PtFeCoNiCu HEA, the atomic radii of Fe, Co, Ni, and Cu are all smaller than that of Pt.[185] When these smaller atoms were incorporated into the HEA matrix, they exerted compressive strain on the surround- ing Pt atoms at the surface. This strain effect modified the lo- cal electronic structure of the Pt active sites, resulting in an op- timized H* adsorption on surface Pt sites, which promotes the HER process (Figure 7b–d). Non-metal can also work as promot- ers, including N, P, F, Te, and B, to facilitate the enhancement of the OER process. F species increased the overlap between Co/Fe 3d and O 2p energy bands, thereby enhancing the covalency of Co─O and Fe─O bonds, which promoted more efficient electron transfer between Co/Fe and the OER intermediates, contribut- ing to the high intrinsic catalytic activity of 𝛾-FeCoNi2F4(OH)4 (Figure 7e).[186] The incorporation of non-metallic Te in CoFeNi- MoWTe N-HECGs facilitated efficient electron transfer between Te and metal species, leading to increased valence states of metal cations that promote OER activity.[187] Additionally, nitrogen (N), another non-metallic element, activated lattice oxygen by mod- ulating the hybridization between Mo 3d orbitals (active sites) and O 2p orbitals in NFeCoNiAlMoOx. [145] This modification en- ables a shift in the reaction pathway from the adsorbate evolution mechanism (AEM) to the lattice oxygen mechanism (LOM) dur- ing the formation of *OOH in step 3. The synergistic effect of AEM and LOM significantly boosts OER activity. Defects or vacancies can act as promoters to enhance the cat- alytic performance of HEMs.[143,188] In (MoWVNbTa)C, W, and Mo serve as themost active sites for the HER.[100] Further investi- gations reveal that these active sites, when adjacent to vacancies, exhibit more favorable ΔGH* values than those in a defect-free structure. This improvement is attributed to the shielding effect of vacancies on electron transfer, leading to optimized ΔGH* and high HER activity. Similarly, the presence of phosphorus vacan- cies (VP) can effectively activate the surrounding Co atoms, which are the active sites in VP-CoNiCuZnFeP. [189] Twin boundaries, as planar defects, also play a key role in modulating surface elec- tronic structures. When combined with the promoter effect from electron-rich B atoms in B-doped FeCoNiCuMoB HEA film, they lead to optimized atomic configurations that synergistically pro- mote a favorable pathway for both HER and OER.[116] The incorporated species or forming heterostructure can also act as promoters, modifying the electronic structure of the ac- tive sites. In VxCuCoNiFeMn, the incorporation of V alters the electronic environment of neighboring elements (particularly Cu and Fe) through electron sharing from the high-valent V.[190] Adv. Mater. 2025, 2506117 2506117 (10 of 39) © 2025 The Author(s). Advanced Materials published by Wiley-VCH GmbH 15214095, 0, D ow nloaded from https://advanced.onlinelibrary.w iley.com /doi/10.1002/adm a.202506117 by A nsto, W iley O nline L ibrary on [17/07/2025]. See the T erm s and C onditions (https://onlinelibrary.w iley.com /term s-and-conditions) on W iley O nline L ibrary for rules of use; O A articles are governed by the applicable C reative C om m ons L icense http://www.advancedsciencenews.com http://www.advmat.de www.advancedsciencenews.com www.advmat.de Figure 7. a) HER polarization curves of PtFeCoNiCuCr@HCS. a) Reproduced with permission.[184] Copyright 2024, Wiley-VCH. b) ΔGH* distribution on 5.9%-HEA (5.9% compressive strain) showing adsorption sites (hcp, fcc, bridge). Red dashes mark Volmer/Heyrovsky (or Tafel) active sites; green indicates H* diffusion (DR). c) Volmer–Heyrovsky mechanism and d) Volmer–Tafel mechanism of HER on 5.9%-HEA (111) and Pt (111). b–d) Repro- duced under the terms of the CC-BY Creative Commons Attribution 4.0 International license.[185] Copyright 2024, Springer Nature. e) Active site model of HEA LDH and its electrochemical properties. e) Reproduced with permission.[186] Copyright 2024, Wiley-VCH. f) The LHB center positions and (g) Computed free energies (ΔG) of OER steps of AuSA-MnFeCoNiCuOOH and MnFeCoNiCuOOH. f,g) Reproduced under the terms of the CC-BY Cre- ative Commons Attribution 4.0 International license.[24] Copyright 2023, Springer Nature. h) ToF-SIMS image of the FeCoNiMnMo SPHEA after 1000 h of continuous stability testing. h) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license.[195] Copyright 2022, American Chemical Society. i) Active site model of HE-MOFs. i) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license.[196] Copyright 2024, American Chemical Society. j) current density–time curve of FeCoNiRu-450. j) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license.[197] Copyright 2022, Wiley-VCH. This electronic modulation shifts the preferred active site for H2O adsorption and dissociation from Fe (in the pristine Cu- CoNiFeMn) to Cu sites (in VxCuCoNiFeMn). The resulting H+ is then readily adsorbed on adjacent Fe sites, where the binding energy barrier is reduced due to the optimized electronic struc- ture of V1.0-HEA, facilitated by efficient electron transfer. As a result, the engineered VxCuCoNiFeMn exhibits an overpotential of 250 mV at 50 mA cm−2, which is ≈170 mV lower than that of non-engineered HEA (422 mV). Incorporating Au single atoms into MnFeCoNiCu (AuSA-MnFeCoNiCu LDH) promotes the re- lease of lattice oxygen, thereby favoring the LOM and enhancing the intrinsic catalytic activity (Figure 7f,g).[24] AuSA-MnFeCoNiCu LDH achieves a low overpotential of 213 mV at 10 mA cm−2 and an impressive mass activity of 732.925 A g−1 at an overpotential of 250 mV for alkaline OER. Carbon materials, including carbon nanotubes (CNTs) and graphene, can also act as effective pro- moters for HEMs in water splitting. The hybridization between the metal d-orbitals of the RuPdIrPtAu HEA and the 𝜋-electrons Adv. Mater. 2025, 2506117 2506117 (11 of 39) © 2025 The Author(s). Advanced Materials published by Wiley-VCH GmbH 15214095, 0, D ow nloaded from https://advanced.onlinelibrary.w iley.com /doi/10.1002/adm a.202506117 by A nsto, W iley O nline L ibrary on [17/07/2025]. See the T erm s and C onditions (https://onlinelibrary.w iley.com /term s-and-conditions) on W iley O nline L ibrary for rules of use; O A articles are governed by the applicable C reative C om m ons L icense http://www.advancedsciencenews.com http://www.advmat.de www.advancedsciencenews.com www.advmat.de of CNTs induces localized charge redistribution at the interface between CNTs and HEA, highly facilitating charge transfer and electronic coupling.[191] This synergy enables ultralow overpoten- tials of 30.7mV forHER and 330mV for OER at a current density of 10 mA cm−2. Moreover, the heterostructure facilitates electron transfer, thereby enhancing the intrinsic catalytic activity of the active sites. The engineered core–shell FeCoNiMoAl-based HEA features a shell that lowers electron transfer resistance and of- fers supplementary active sites, with a crystalline core that sup- ports improved conductivity and long-term stability.[192] More- over, charge redistribution at the core–shell interface elevates the valence states of Ni and Co species, further promoting the OER process. 3.3. Elements as Stabilizers One disadvantage of HEMs is their sluggish diffusion effect on kinetics, which significantly influences the phase transitions. The presence of multiple elements with varying atomic radii and chemical activities in HEMs leads to lattice distortion, which im- pedes atomic movement and limits diffusion rates.[193,194] The suppressed diffusion impacts not only phase stability but also plays a pivotal role in governing the overall performance of the HEM system. For instance, the as-prepared FeCoNiMnMo HEMs demonstrate excellent electrocatalytic performance for OER, achieving a low overpotential of 279mV at 10mA cm−2 and maintaining remarkable stability for over 1000 h.[195] The results reveal Mo as a key contributor to the enhanced catalytic activity and long-term durability. The high-entropy effect plays a vital role in suppressingMo leaching, thereby preserving the structural in- tegrity and thickness of the active catalytic layer throughout the OER process (Figure 7h). Moreover, the incorporation of high- valence metals such as Mo and W into CoFe-based oxides impart self-healing properties to the electrocatalysts, enabling them to maintain high OER activity and long-term stability.[90] In addition to the intrinsic high-entropy effect, which pro- vides enhanced structural and chemical stability compared to single-metal electrocatalysts, manyHEMs are further engineered through the incorporation of specific stabilizing elements. These added elements can improve resistance to corrosion, suppress the dissolution of active sites under harsh electrochemical con- ditions, and maintain the integrity of the active phase during long-termwater-splitting operations. The atomic-level dispersion of Ru and Mo within HE(Ru,Mo)-MOF nanosheets leads to the formation of densely packed O-bridged RuMo dual-atom active sites.[196] The incorporation of Mo promotes the stabilization of Fe and Ni in lower oxidation states (+2), contrasting with the ef- fect of Ru, which tends to induce higher oxidation states of Fe and Ni. Additionally, in Mo-doped HE(Ru,Mo)-MOF samples, the Ru 3p peaks shift toward higher binding energies, indicating a de- crease in electron density around Ru atoms due to electronic in- teractions with Mo. These findings suggest that the high-valent Mo species help balance and stabilize the overall coordination en- vironment and electronic structure, thereby enhancing the long- term stability of HE(Ru,Mo)-MOFs for OER (Figure 7i). The formation of surface layers plays a critical role in enhanc- ing the structural and electrochemical stability of HEMs dur- ing water splitting. The surface layers enriched with specific el- ements can act as protective barriers against corrosion and dis- solution under harsh electrochemical conditions. Stability stud- ies on the FeCoNiRu HEA reveal that its exceptional durability is primarily attributed to the formation of a spinel oxide surface layer during the electrochemical reaction.[107] This spinel struc- ture effectively preserves the active sites derived from the intrin- sic architecture of the HEA, thereby maintaining high catalytic performance and contributing to the material’s remarkable sta- bility (Figure 7j). Moreover, the oxidized P and S species in high- entropy phosphorus sulfide (HEPS) form an anionic protective layer that shields the catalytically active metal sites from aggrega- tion and dissolution.[197] Simultaneously, the in situ formation of V2Ox species serves as a stabilizer to against additional oxidation of HEPS, further enhancing the durability for OER. As a result, maintains stable performance for over 1200 h in a practical elec- trolyzer with negligible activity loss. 4. High Entropy Materials for Water Splitting Water splitting is a promising method to produce green hydro- gen. However, efficient water splitting requires catalysts that can sustain high catalytic activity and stability under extreme opera- tional conditions (acidic and alkaline media). HEMs, composed of multiple principal elements with altered electronic structures, surface chemistries, adsorption properties, and stability, can sig- nificantly boost the catalytic performance for HER and OER. Moreover, HEMs offer the flexibility to incorporate more abun- dant and cost-effective elements into the precious metals (Pt, Ir)- based electrocatalysts to reduce the cost with no compromise of the catalytic performance. This section will focus on HEMs for water splitting, highlighting their unique properties and the spe- cific roles of each element (Table 1 and 2). 4.1. Hydrogen Evolution Reaction The primary strategy for generating hydrogen through electro- catalysis is HER, a key half-reaction in water splitting. HER in- volves two-electron transfer withmultiple reaction steps occurred at the cathode–electrolyte interface. The proton source for HER depends on the electrolyte pH, that is, H2 produced via the re- duction of H3O + in acidic conditions and the reduction of H2O in alkaline media. There are two steps for HER in acidic condi- tions, including the Volmer reaction with the adsorption of hy- drogen atoms on the catalyst surface (forming H*), and the Hey- rovsky or the Tafel reaction when H2 is produced. In the Hey- rovsky pathway, a hydrogen molecule is formed by the combi- nation of an adsorbed hydrogen atom (H*) and the proton (H+) from the electrolyte with one electron from the electrode. In con- trast, the Tafel reaction involves the direct combination of two ad- jacent adsorbed hydrogen atoms (H*) to produce molecular hy- drogen (H2). Similarly, HER in alkaline conditions also proceeds via either the Volmer–Heyrovsky or Volmer–Tafel mechanisms. However, an additional water dissociation step is required to gen- erate H* and OH− (Volmer step). Subsequently, H* reacts with another watermolecule and an electron to formH2, a step known as the Heyrovsky step in alkaline conditions. Alternatively, the Tafel step, involving the combination of two H* species to form Adv. Mater. 2025, 2506117 2506117 (12 of 39) © 2025 The Author(s). Advanced Materials published by Wiley-VCH GmbH 15214095, 0, D ow nloaded from https://advanced.onlinelibrary.w iley.com /doi/10.1002/adm a.202506117 by A nsto, W iley O nline L ibrary on [17/07/2025]. See the T erm s and C onditions (https://onlinelibrary.w iley.com /term s-and-conditions) on W iley O nline L ibrary for rules of use; O A articles are governed by the applicable C reative C om m ons L icense http://www.advancedsciencenews.com http://www.advmat.de www.advancedsciencenews.com www.advmat.de Table 1. Summary of HEMs for the HER. Strategies Catalysts Electrolyte Overpotential [mV] for 10 mA cm−2 Tafel slope [mV dec−1] Refs. 0D HEMs PdPtRuRhAu HEAs 0.5 m H2SO4 70.07 30.3 [163] Pt-Pd-Ni-Co-Mn 1 m KOH 22.6 77.5 [201] Pt18Ni26Fe15Co14Cu27/C 1 m KOH 11 30 [44] PtCoNiRuIr/C 0.5 m H2SO4 18 34.2 [87] PtPdIrRhAuAgCu- rEGO 1 m KOH 11.3 59.9 [95] NiCoMoPtRu HEANCs 1 m KOH 9.5 29.8 [52] Pt4FeCoCuNi 1 m KOH 20 31 [51] Ru– PtFeNiCuW/CNTs 1 m KOH 16 27 [50] PtRuPdCoNi HEA 0.5 m H2SO4 16 27 [84] PtRuMoFeCoNi 0.5 m H2SO4 11 28.7 [86] PdFeCoNiCu/C 1 m KOH 18 39 [146] PtPdCoNiMn 1 m KOH 48.7 20.7 [96] PtPdCoNiMn 1 m KOH 13 29.6 [91] PtFeCoNiIr/C 0.5 m H2SO4 20.3 12.9 [99] NiCoFeMoMn@6h 1 m KOH 14 29 [208] FeCoNiCuMn HEA 1 m KOH 281 (100) 53 [110] FeCoNiCuMo 6 m KOH 68 100 [97] NiCoFeMnCrP 1 m KOH 253 94.5 [98] 1D HEMs Al−Ni−Co−Ru−Mo 1 m KOH 24.5 30.3 [204] PtNiCoFeMo 0.5 m H2SO4 42(100) 21.2 [203] CuNiCoRuIr HEA NT 1 m KOH 22 69 [202] CoZnCdCuMnS@CF 1 m KOH 173 93.4 [114] FeCoNiCuMnN/CC- 400 1 m KOH 184 113 [111] 2D HEMs PdMoGaInNi 0.5 m H2SO4 13 127.6 [205] Pt/HE-LDH 0.5 m H2SO4 42 42 [112] Co0.6(VMnNiZn)0.4PS3 1 m KOH 65.9 65.5 [88] CrFeCoNiZn HES 0.5 m H2SO4 15 105 [92] (MoWNbTaV)S2 0.5 m H2SO4 84 90 [93] FeNiCoMnVOx 1 m KOH 81 88 [209] (FeMnMoNi)Se2 1 m KOH 30.5 49.6 [210] FeCoNiCuP 1 m KOH 113 52.4 [34] 3D HEMs PtPdRuMoNi-HEA 0.1 m KOH 25 38 [175] PtFeCoNi@HCS 1 m KOH 29 136.42 [190] (Continued) Adv. Mater. 2025, 2506117 2506117 (13 of 39) © 2025 The Author(s). Advanced Materials published by Wiley-VCH GmbH 15214095, 0, D ow nloaded from https://advanced.onlinelibrary.w iley.com /doi/10.1002/adm a.202506117 by A nsto, W iley O nline L ibrary on [17/07/2025]. See the T erm s and C onditions (https://onlinelibrary.w iley.com /term s-and-conditions) on W iley O nline L ibrary for rules of use; O A articles are governed by the applicable C reative C om m ons L icense http://www.advancedsciencenews.com http://www.advmat.de www.advancedsciencenews.com www.advmat.de Table 1. (Continued) Strategies Catalysts Electrolyte Overpotential [mV] for 10 mA cm−2 Tafel slope [mV dec−1] Refs. Pt-Pd-Ni-Co-Mn HEA 1 m KOH 43.7 75.5 [184] FeCoNiRu-450 1 m KOH 40 84 [201] PtMoPdRhNi 1 m KOH 9.7 25.9 [107] PtFeCoNiCu HEA 0.5 m H2SO4 30.7 28.1 [166] Pt34Fe5Ni20Cu31Mo9Ru 0.5 m H2SO4 9 27 [185] Pt26Ir7Fe13Co22Ni32 1 m KOH 29 44.5 [113] NiCoMoZnCu HEANFA 1 m KOH 242.9(500) 61.4 [161] V1.0CuCoNiFeMn 1 m KOH 50(50) 148 [212] NNM-HEA@CF 1 m KOH 85.2(100) 86.3 [168] H-FeCoNiCuMo 1 m KOH 21 18.5 [173] Mil53 MOF 1 m KOH 206 118.7 [211] CoFeNiCrMnP/NF 1 m KOH 51(100) 48 [36] HEOC 0.5 m H2SO4 57 34.6 [33] Defect (MoWVNbTa)C 0.5 m H2SO4 156 78 [100] (FeCoNiB0.75)97Pt3 1 m KOH 27 30.9 [216] IrRuRhMoW HEA 0.1 m KOH 28 51 [217] Pt(Co/Ni)MoPdRh HEAs 1 m KOH 16.5 26.8 [58] HEA-PdPtRhIrCu 1 m KOH 15 37 [142] CoNiCuZnFeP 1 m KOH 318 121.3 [189] PdFeCoNiCu- pHENs 1 m KOH 38.4 29 [215] HESAs PtIrCuNiCo 1 m KOH 22 / [213] PtRuRhPdRe-MoSe2 1 m KOH 35 90 [106] Ordered HEMs Pt4FeCoCuNi PCPAF-HEA/C 1 m KOH 0.5m H2SO4 20 24 31 29 [51, 214] FeCoNiAlTi 1 m KOH 88.2 40.1 [43] NiCoFeMoMn 1 m KOH 14 29 [208] FeCoNiMnRu/CNFs 1 m KOH / 67.4 [121] HEI_800/C 1 m KOH 128 21 [122] (FeCoNi)(RuPt) HEI 1 m KOH / 47.1 [127] (CoNiRhIrRu)Sb3 0.5m H2SO4 200 / [128] PtRuFeCoNi 0.5m H2SO4 41.3 / [123] FeCoNiMnMoP 1 m KOH 55 65.2 [129] Reconstructive HEMs PtRu2.9Fe0.15Co1.5Ni1.3 FeCoNiRu-450 NiFeCrVTi HEA 1 m KOH 1 m KOH 3.5% NaCl 11.8 40 37.9 26.3 84 36.2 [107, 181, 218] AlCoCrFeNi 1 m KOH 92.89 [133] CNFMPO 1 m KOH 43 33.5 [134] NiCoMoZnCu/CFC 1 m KOH / 61.4 [212] (Continued) Adv. Mater. 2025, 2506117 2506117 (14 of 39) © 2025 The Author(s). Advanced Materials published by Wiley-VCH GmbH 15214095, 0, D ow nloaded from https://advanced.onlinelibrary.w iley.com /doi/10.1002/adm a.202506117 by A nsto, W iley O nline L ibrary on [17/07/2025]. See the T erm s and C onditions (https://onlinelibrary.w iley.com /term s-and-conditions) on W iley O nline L ibrary for rules of use; O A articles are governed by the applicable C reative C om m ons L icense http://www.advancedsciencenews.com http://www.advmat.de www.advancedsciencenews.com www.advmat.de Table 1. (Continued) Strategies Catalysts Electrolyte Overpotential [mV] for 10 mA cm−2 Tafel slope [mV dec−1] Refs. FeCoNiVCrZn HEA 1 m KOH 249 50 [135] FeCoNiCuPd HEA 1 m KOH 29.7 47.2 [109] Functionalized HEMs HEA@Pt Pt/HE-LDH 1 m KOH 0.5 m H2SO4 13.7 42 30.6 42 [112, 225] HEPi/C 0.5 m H2SO4 40 36 [119] Pt/(FeCoNiCrAl)3O4 1 m KOH 22 25.9 [228] Pt- (LaCeSmYErGdYb)O 0.5 m H2SO4 12 10 [219] PtNiCuMnMo HEA 1 m KOH 44 74 [231] PtCoNiMoRh@Rh 0.5 m H2SO4 9.1 8.74 [232] MoS2@HEP 1 m KOH 71 58 [233] HEA/MoS2/MoP 1 m KOH 148 71.98 [270] PtRhCoNiCu/CC 1 m KOH 19 26.9 [222] V-Co2P@HE 1 m KOH 33 47.44 [271] NiMoCoMnLa@Ni 1 m KOH 146 79 [226] HEA/CNT-10 1 m KOH 30.7 71 [191] MoS2@HEP 1 m KOH 71 58 [233] FeCoNiWCuOOH@Cu 1 m KOH 200 24 [230] AC-HEA- CuAgAuPtPd 0.5 m H2SO4 9.5 31 [220] NiFeCoZn/NiZn- Ni/NF 1 m NaOH / 46.58 [229] FeCoNiMnZn/N- CNT 1 m KOH 184 112 [221] CoNiCuMgZn- 40@C 1 m KOH 158 36.1 [227] Fe20Co20Ni20Mo20Al20 1 m KOH 223 39.8 [192] (WNiCoMoRu)POx/C 0.5 m H2SO4 40 36 [119] H2, remains unchanged from that in acidicmedia. The additional water dissociation step introduces an extra energy barrier and leads to a higher overpotential for HER in alkaline conditions. Therefore, highly efficient electrocatalysts are essential for reduc- ing the overpotential and improving the catalytic performance of HER. Among various catalysts, HEMs have emerged as promis- ing candidates for HER due to their unique properties, includ- ing the high-entropy effect, lattice distortion effect, and cocktail effect.[198–200] 4.1.1. Pristine Noble Metal-Based HEMs Precious metal-based materials, such as Pt, Ru, Pd, Rh, and Au, have long been considered the benchmark for HER, due to their outstanding intrinsic activity, favorable electronic structures, and robust stability. Alloying these noble metals together to form HEMs can lead to the development of unique structural and elec- tronic properties. The synergistic interactions among multiple metal elements in HEMs may significantly enhance their cat- alytic performance for HER. PdPtRuRhAu HEAs with a parti- cle size of 3.14 nm have demonstrated exceptional HER perfor- mance, with negligible activity loss over 100 h at high current densities of 500 and 1000 mA cm−2. The Pd-Au bridge sites have been identified as the active sites for HER.[44] Moreover, alloying these noblemetals with transitionmetals is also an effective strat- egy to enhance HER performance while simultaneously reduc- ing noble metal usage. The types and compositions of alloying transition elements are highly tunable, allowing for broad flexi- bility in catalyst design. A novel PtPdNiCoMnHEA has been pre- pared by room-temperature electrodeposition method.[201] Theo- retical calculations revealed that the noble metals, surface Pt, and Pd sites exhibited optimized hydrogen adsorption capabilities, serving as the primary active sites. The Ni and Co species with their high density of states near the Fermi level facilitated the electron transfer and accelerate the HER kinetics (Figure 8a,b). Adv. Mater. 2025, 2506117 2506117 (15 of 39) © 2025 The Author(s). Advanced Materials published by Wiley-VCH GmbH 15214095, 0, D ow nloaded from https://advanced.onlinelibrary.w iley.com /doi/10.1002/adm a.202506117 by A nsto, W iley O nline L ibrary on [17/07/2025]. See the T erm s and C onditions (https://onlinelibrary.w iley.com /term s-and-conditions) on W iley O nline L ibrary for rules of use; O A articles are governed by the applicable C reative C om m ons L icense http://www.advancedsciencenews.com http://www.advmat.de www.advancedsciencenews.com www.advmat.de Table 2. Summary of HEMs for the OER. Strategies Catalysts Electrolyte Overpotential [mV] for 10 mA cm−2 Tafel slope [mV dec−1] Refs. 0D HEMs RuMnFeMoCo 0.5 m H2SO4 170 49.7 [20] IrFeCoNiCu-HEA 0.1 m HClO4 302 58 [164] (RuIrFeCoNi)O2 0.5 m H2SO4 261 56.35 [239] FeNiCuWRu 1 m KOH 267 32.4 [234] PtFeCoNiMnGa HEA 1 m KOH 243 40.2 [183] FeCoNiMnRuLa/CNT 1 m KOH 281 47.5 [235] IrRuNiMoCo 0.5 m H2SO4 243 56.2 [53] IrPdCuFeNiCoMo 0.5 m H2SO4 235 51 [27] FeCoNiPdW 1 m KOH 227 33 [182] (FeNiCoCrMnV) HEO 1 m KOH 247 45 [170] CoFeNiCrMn HEO 1 m KOH 307 34.7 [25] La(CrMnFeCo2Ni)O3 1 m KOH 325 51.2 [151] (CrMnFeCoNi)Sx 1 m KOH 295(100) 66 [29] FeCoNiMoW HEA 1 m KOH 233 36.7 [54] HEOH(FeNiCoCrMn)Cl 1 m KOH 250 / [236] FeCoNiCuMn 1 m KOH 383(0.5) / [110] CoFeNiMnMoPi 1 m KOH 277.5 74 [237] FeCoNiCuMo-O 1 m KOH 272 41 [170] 1D HEMs AlNiCoRuMo 1 m KOH 250 54.5 [204] AlNiCoIrMo np-HEA 0.5 m H2SO4 233 55.2 [160] FeCoNiCuPd 1 m KOH 194 39.8 [109] CrMnFeCoNi HESOs 1 m KOH 360 41 [245] 2D HEMs (FeCoNiMoRu)3O4 1 m KOH 199 40 [241] CoCuFeAgRu 1 m KOH 280 70.7 [26] FeCoCuMnRuB 1 m KOH 233 61 [108] CoCuFeMoOOH@Cu 1 m KOH 199 48.8 [41] CoFeMnCuZn 1 m KOH 267 45 [89] CoFeNiMoWTe 0.5 m H2SO4 373 40.6 [187] FeCoMoW 1 m KOH 332 63.6 [90] (CrFeCoNiMo)3O4 1 m KOH 255.3 37 [32] FeCoNiWCuOOH@Cu 1 m KOH 200 23 [230] FeCuCoNiZn 1 m KOH 236 43 [240] 3D HEMs M-RuIrFeCoNiO2 0.5 m H2SO4 189 49 [85] (RuSnSbReF)Ox 0.5 m H2SO4 156 23.87 [177] (Continued) Adv. Mater. 2025, 2506117 2506117 (16 of 39) © 2025 The Author(s). Advanced Materials published by Wiley-VCH GmbH 15214095, 0, D ow nloaded from https://advanced.onlinelibrary.w iley.com /doi/10.1002/adm a.202506117 by A nsto, W iley O nline L ibrary on [17/07/2025]. See the T erm s and C onditions (https://onlinelibrary.w iley.com /term s-and-conditions) on W iley O nline L ibrary for rules of use; O A articles are governed by the applicable C reative C om m ons L icense http://www.advancedsciencenews.com http://www.advmat.de www.advancedsciencenews.com www.advmat.de Table 2. (Continued) Strategies Catalysts Electrolyte Overpotential [mV] for 10 mA cm−2 Tafel slope [mV dec−1] Refs. PtPdFeCoNi/HOPNC 1 m KOH 310 88.7 [65] ZnNiCoIrMn 0.5 m H2SO4 237 46 [69] FeCoNiRu 1 m KOH 306 45 [107] MnCoNiCuZn 1 m NaOH 300 57 [246] NiFeCoMnOOH 1 m KOH 194 67.96 [171] FeCoNiMnMo 1 m KOH 279 56.1 [195] FeNiCo CrMnS2 1 m KOH 199 39.1 [141] FeCoNiMoCrOOH 1 m KOH 172 35.53 [64] MnFeCoNiVPS 1 m KOH 245 63.43 [197] ZnCoNiFeV 1 m KOH 253 49 [68] FeCoNiCuYP/C 1 m KOH 259 64 [67] HE-MHOFs 1 m NaOH 410(100) 57 [246] FeNiCoCrMn 1 m KOH 229 40 [71] Ni-HEA 1 m KOH 217 46.3 [243] FeCoNiMoMn 1 m KOH 218 53 [244] Defect CoFeMnCuZn 1 m KOH 267 45 [245] (CrMnCoNiFe)0.2BOx 1 m KOH 253 64 [180] FeCoNiCuMoB 1 m KOH 201 41.3 [116] CrMnFeCoNi 0.1 m KOH 265 37.9 [251] HEOs-Ov 1 m KOH 284 53 [143] FeCoNiMnW@CCC 1 m KOH 253 41 [188] FeCoNiCrMn 1 m KOH 282 64.3 [249] HESAs FeCoNiRu-HESAC 1 m KOH 280 / [248] (FeCoNiCrCuAl)S@La 1 m KOH 253 51.75 [247] Ordered HEMs MCPS Al0.5NiCoCrMo0.5 0.1 m KOH 1 m KOH 288 327 27.7 168.91 [55, 130] Reconstructive HEMs FeNiCoCrXS2 (CoNiFeCuCr)Sex FeCoNiRu-450 1 m KOH 1 m KOH 1 m KOH 199 / 243 39.1 46.78 45 [107, 136, 141] FeNiCoCrMnV 1 m KOH 220 / [170] NiFeCoMnAl 1 m KOH 190 47.62 [120] AlCoCrFeNi 1 m KOH / 39.7 [133] FeCoNiCuPd/CFC 1 m KOH 194 39.8 [109] NiCoZnCuMg) Fe2O4 1 m KOH 286 136 [258] CNFMPO 1 m KOH 252 44.3 [134] a-NiFeCoVMo 1 m KOH 172 / [137] (FeCoMnZnMg)3O4 1 m KOH 240 59 [138] FeCoNiCrMn 1 m KOH 263 50.9 [139] FeCoNiCrVB 1 m KOH 237 24.2 [259] (VFeNiCoCu)3O4 HESO 1 m KOH 181 48.49 [260] HES/NF 1 m KOH / 47.6 [261] FeCoNiZnOOH/NF 1 m KOH / 49.2 [252] FeCoNiMnBPOx 1 m KOH 248 42.3 [253] FeCoNiCuMoOOH/NF 1 m KOH 201 39.4 [254] Ru5CoNiCuMn-BDC 1 m KOH 240 33 [257] (Continued) Adv. Mater. 2025, 2506117 2506117 (17 of 39) © 2025 The Author(s). Advanced Materials published by Wiley-VCH GmbH 15214095, 0, D ow nloaded from https://advanced.onlinelibrary.w iley.com /doi/10.1002/adm a.202506117 by A nsto, W iley O nline L ibrary on [17/07/2025]. See the T erm s and C onditions (https://onlinelibrary.w iley.com /term s-and-conditions) on W iley O nline L ibrary for rules of use; O A articles are governed by the applicable C reative C om m ons L icense http://www.advancedsciencenews.com http://www.advmat.de www.advancedsciencenews.com www.advmat.de Table 2. (Continued) Strategies Catalysts Electrolyte Overpotential [mV] for 10 mA cm−2 Tafel slope [mV dec−1] Refs. FeCoNiMnBOx 1 m KOH 266 64.5 [262] HEO-P-1 1 m KOH 254 47.4 [35] FCNMWO 1 m KOH 313 40.95 [255] (FeCoNiCuRu)S2 1 m KOH 193 46 [256] Ny@ZrFeCoNiAlOSO4 1 m KOH 257 37.8 [273] FeCoNi2F4(OH)4 1 m KOH 298 56 [10] Functionalized HEMs CrZr-HEA-Rgo AuSA-MnFeCoNiCu HE(Ru,Mo)-MOF 0 m KOH 1 m KOH 1 m KOH / 213 267 75.33 27.5 36.3 [24, 182, 196] (FeCoNiCrCuAl)S@La 1 m KOH 246 51.75 [247] np-HEO/Pt 1 m KOH 260 50.5 [268] Ag@CoCuFeAgMoOOH 1 m KOH 218 35.3 [147] FeCoNiMnCr 1 m KOH 261 42.2 [149] HEO-NPs@C@HE- MOF 1 m KOH / 43.9 [274] (FeNiCuCoZn) 90-xV20P10 1 m KOH 228 23.6 [269] FeCoNiMnCr@MoS2- C 1 m KOH 210 40.3 [275] HEA/MoS2/MoP 1 m KOH 230 63.54 [270] (CrFeCoNi)97O3 1 m KOH 196 29 [66] IrRuCoNiCu/CC 1 m KOH 166 69.1 [222] V-Co2P@HE 1 m KOH 227 53.77 [271] MnCr2O4@O-HEA 1 m KOH 268 51.6 [272] NiCo(FeCrCoNiAl 0.1)Ox 1 m KOH 381 60.9 [263] FeCoNiMnCuPx/C 1 m KOH 239 72.5 [264] FeCoNiMo HEA-MoC 1 m KOH 232 61.47 [265] FeCoNiMnCr HEA-HEO 1 m KOH 255 37.3 [266] R-SNCFCA4.5 1 m KOH 228 80.52 [267] HEO/Ti3C2Tx-0.5 1 m KOH 360 99 [193] The PtPdNiCoMn HEA demonstrated an overpotential of only 22.6 mV at a current density of 10 mA cm−2 for alkaline HER. Except for noble metals as catalytic sites, the transition metal can also be the adsorption sites for HER. For instance, Pt combined with transition metals formed ultrasmall and uniformly sized Pt18Ni26Fe15Co14Cu27 HEAs (Figure 8c), in which Pt atoms acted as electron reservoirs, modulating the electronic structure of the surrounding transitionmetals to boost the alkaline HER.[44] Dur- ing the HER process, the water molecules initially adsorbed onto Fe sites, while the resulting *OH species are stabilized by adja- cent hollow sites. H* adsorption preferentially occurred at hollow sites near Ni and Co (Figure 8d), and this synergistic interaction among the elements collaboratively enhanced the HER perfor- mance, achieving an ultra-low overpotential (𝜂10) of only 11 mV for alkaline HER. Although low-dimensional nanoparticles offer a high sur- face area and numerous active sites, they are susceptible to aggregation, which can compromise long-term stability and catalytic efficiency. In contrast, HER electrocatalysts based on 1D, 2D, or 3D HEMs further expand their properties and kinetics. For instance, 1D HEM structures (e.g., nanowires or nanotubes), can not only facilitate rapid electron transport along their longitudinal axis, but also enhance reaction ki- netics and structural stability.[202,203] The etched AlNiCoRuMo HEAs exhibited a 1D nanowire-like morphology with diameters ranging from 20 to 100 nm, featuring ultrafine nanopores on the surface.[204] The distinctive 1D nanowire structure pro- vided efficient pathways for electron transport (Figure 8e), significantly enhancing the structural stability. Remarkably, the AlNiCoRuMo nanowires maintained 95.2% of their initial Adv. Mater. 2025, 2506117 2506117 (18 of 39) © 2025 The Author(s). Advanced Materials published by Wiley-VCH GmbH 15214095, 0, D ow nloaded from https://advanced.onlinelibrary.w iley.com /doi/10.1002/adm a.202506117 by A nsto, W iley O nline L ibrary on [17/07/2025]. See the T erm s and C onditions (https://onlinelibrary.w iley.com /term s-and-conditions) on W iley O nline L ibrary for rules of use; O A articles are governed by the applicable C reative C om m ons L icense http://www.advancedsciencenews.com http://www.advmat.de www.advancedsciencenews.com www.advmat.de Figure 8. a) PDOS (states/eV-atom) for HEA with and without hydrogen coverage. b) H-1s states are weakly coupled with TM-d states. a,b) Reproduced with permission.[201] Copyright 2024, Wiley-VCH. c,d) The elemental mapping and the binding energy mapping of Pt18Ni26Fe15Co14Cu27 nanoparticles (scale bar, 5 nm). c,d) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license.[44] Copyright 2020, Springer Nature. e,f) HER polarization curves and long-term durability test of AlNiCoRuMo. e,f) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license.[204] Copyright 2020, American Chemical Society. g,h) TEM and HAADF-STEM images of PdMoGaInNi nanosheets. g,h) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license.[205] Copyright 2022, American Chemical Society. i) HER polarization curves for nanoporous NiCoFeMoMn. j–k) The colored ΔGH* and ΔEH2O comparisons of SA and un-SA; the pure red area means this site is not easy to be adsorbed. l) The d-PDOS plots of H2O and Mo, Mn, Ni and Fe adsorbed by H2O directly on un-SA surface. i–l) Reproduced with permission.[208] Copyright 2022, Elsevier. activity after 100 h of continuousHER testing, demonstrating the positive role of their structural stability in ensuring HER durabil- ity (Figure 8f). 2D HEMs with layered structures offer a balance between active site exposure and robust stability. The unique 2D nanosheets can also facilitate charge transfer and optimize ΔGH*, thereby accelerating HER kinetics. 2D PdMoGaInNi HEA nanosheets have been synthesized using a solution-phase method, which exhibited a graphene-like, wrinkled, single-phase face-centered cubic structure and an approximate thickness of 1.6 nm (Figure 8g,h).[205] These nanosheets demonstrated an exceptionally low overpotential of merely 13 mV at a current density of 10 mA cm−2, along with excellent long-term stability for over 200 h at 100 mA cm−2 in a proton exchange membrane water electrolyzer. This significant durability and performance improvement can be attributed to the introduction of Pd to protect this unique 2D layered structure, which is conducive to rapid mass transport during the reaction. Moreover, 3D porous frameworks further enhance mass transport and active site accessibility.[161] Their interconnected porous networks improve gas diffusion and electrolyte penetration, enabling efficient catal- ysis at industrial-scale current densities with low overpotentials. For instance, Pt (Co/Ni) MoPdRh HEA nanoflowers, assembled from ultrathin nanosheets (≈1.68 nm), exhibited remarkable advantages in alkaline HER.[205] Furthermore, the mesoporous 3D structures of PtPdRuMoNi nanospheres achieved by low- temperature triblock copolymer-assisted wet-chemical method featured a mesopore size of 10–12 nm and an inter-pore distance of ≈16 nm.[175] The mesoporous structure highly exposed active sites and achieves a mass activity of 167 A g−1 at an overpotential of 30 mV, far exceeding that of commercial Pt/C (34 A g−1). The high entropy properties enable electron transfer among the multi-elements, optimizing the d-band center of Pt, Pd, and Adv. Mater. 2025, 2506117 2506117 (19 of 39) © 2025 The Author(s). Advanced Materials published by Wiley-VCH GmbH 15214095, 0, D ow nloaded from https://advanced.onlinelibrary.w iley.com /doi/10.1002/adm a.202506117 by A nsto, W iley O nline L ibrary on [17/07/2025]. See the T erm s and C onditions (https://onlinelibrary.w iley.com /term s-and-conditions) on W iley O nline L ibrary for rules of use; O A articles are governed by the applicable C reative C om m ons L icense http://www.advancedsciencenews.com http://www.advmat.de www.advancedsciencenews.com www.advmat.de Ru, and effectively regulating the adsorption energy of HER intermediates. 4.1.2. Pristine Non-Noble Metal-Based HEMs While precious metal-based HEMs offer excellent HER cat- alytic performance, their high cost and scarcity have driven the search for alternative materials. Non-precious metal cata- lysts have emerged as promising candidates due to their natu- ral abundance, cost-effectiveness, and tunable electronic struc- tures. Non-precious metal-based HEMs for HER primarily in- clude transition metals such as Ni, Co, Fe, Mo, W, V, Mn, and Cr. Different elements are generally suited to different types of HEMs and contribute distinct properties that influence HER performance.[206,207] For instance, Ni, Co, and Fe-based alloys have been widely used as alkaline HER electrocatalysts due to their excellent electrical conductivity and moderate hydrogen binding energy. Mo and W-based sulfides or carbides can form Pt-like active sites and exhibit excellent HER activity. V, Ti, Cr, etc. contribute to electronic modulation and structural stabil- ity in HEMs. Integrating transition metals with highly matched atomic radii into HEMs can generate homogeneous solid solu- tions, leveraging cocktail effects to boost catalytic performance. For instance, the NiCoFeMoMn HEA nanoparticles achieved a remarkably low overpotential of ≈14 mV at a current density of 10 mA cm−2, significantly outperforming the commercial Pt/C electrode (32 mV) (Figure 8i).[208] Among the multiple elements, Ni, Co, and Fe were the main active centers for HER, promoting hydrogen adsorption (ΔGH* =−0.05,−0.03, and 0.01 eV) and the adsorption energy of H2O (ΔGH2O = −0.15, −0.19, and −0.3 eV). (Figure 8j–l) Mo contributed by enhancing water adsorption and dissociation through strong orbital interactions between its d- orbital spin density and the O 2p orbital of H2O, thereby fur- ther accelerating HER kinetics. In addition to the active elements (Fe, Co, and Ni) serving as HER active sites, the synergistic ef- fects within HEMs can transform traditionally inactive elements (e.g., Cu) into highly competitive active sites for HER. A highly active FeCoNiCuMn HEA nanoparticle was developed through a polymer fiber nanoreactor strategy.[110] The strong local elec- tronic interactions among the multiple metal sites within the Fe- CoNiCuMn HEA enabled Cu to serve as the primary active site for alkaline HER. This is attributed to its lowest H2O dissocia- tion barrier (0.54 eV) and the closest H* adsorption-free energy (−0.085 eV) to the ideal value of 0 eV. The FeCoNiCuMn HEA demonstrated an overpotential of 281 mV at a high current den- sity of 100 mA cm−2, surpassing the performance of commercial Pt/C catalysts (302 mV). Similar to noble metal-based HEMs, transitioning from nanoparticles to higher-dimensional structures (1D, 2D, and 3D) further accelerates the reaction kinetics of HEMs for HER.[33,209–211] 1D nanowire high-entropy CoZnCdCuMnS ex- hibited a needle-like morphology.[114] This unique structure not only shortens the diffusion pathways of reactants and products but also increases the exposure of active sites. Specifically, Co served as the primary active sites, and the other elements effi- ciently modulate the electronic distribution of the active sites, thereby optimizing the H* adsorption energy and achieving high HER performance. In 1 m KOH solution, CoZnCdCuMnS demonstrated excellent electrocatalytic performance for HER, achieving a current density of 10 mA cm−2 at an overpoten- tial of 173 mV. 2D materials offer several unique advantages due to their ultrathin, atomically layered structures. For in- stance, Co0.6(VMnNiZn)0.4PS3 high entropy 2D nanosheets with a thickness of ≈2.8 nm were synthesized by combining con- ventional solid-phase reaction and ultrasonic-assisted technology (Figure 9a).[88] The nanosheet structure enabled it to expose abundant active sites, achieving a low overpotential of 65.9 mV and a Tafel slope of 65.5 mV dec−1 for alkaline HER. Moreover, mechanistic investigations suggest that Mn sites lowered the en- ergy barrier for water dissociation, thereby facilitating the Volmer step, while the edge S sites and substrate P sites served as active sites for hydrogen adsorption, collaboratively enhancing the alka- line HER performance (Figure 9b). The 3D hierarchical architec- tures with interconnected conductive networks can efficiently ex- pose active sites while maintainingmechanical integrity. Benefit- ing from the advantage of a 3D unique structure, NiCoMoZnCu HEA nanoflower array (HEANFA) exhibited outstanding HER activity, requiring only 242.9 and 307.5 mV to achieve industrial- level current densities of 500 and 1000 mA cm−2, considerably outperforming the commercial Pt/C catalyst (365 and 437 mV) (Figure 9c–e).[212] DFT calculations were performed to determine the water dissociation energy barriers by investigating each ele- ment in NiCoMoZnCuHEANFA as the active site. The results in- dicate that theH2Odissociation energy barriers at Co,Mo, Cu,Ni, and Zn sites are 0.51, 0.43, 0.76, 0.61, and 0.39 eV, respectively, all significantly lower than that of pure Pt (0.95 eV). This suggests that the HEANFA surface facilitated the H2O dissociation into H* during the Volmer step. Additionally, the free energy of the Heyrovsky step at various HEANFA surface metal sites (−0.12, 0.11, −0.05, and −0.23 eV) is also markedly lower than that on Pt (0.23 eV), optimizingΔGH* closer to zero and thereby enhancing HER activity. 4.1.3. High Entropy Single Atoms Catalysts High entropy single-atom catalysts (HESACs) maximize the uti- lization of metal atoms and optimize the geometric structure of active metals for HER. The metal cations adjacent to the active metal act as promoters to regulate the electronic structure, orbital configuration, and catalytic activity of the active metal through non-bonding and interaction. Due to the high energy of isolated metal species, the HESACs require substrates to anchor and sta- bilize thesemultiple isolatedmetal atoms, including carbon (e.g., carbon black, carbon quantum dots, and graphene.) and non- carbon substrates (transition metal sulfides, oxides, etc.). For in- stance, a one-step laser implantation strategy has been used to anchor Pt, Ir, Cu, Ni, and Co metal atoms on defective carbon black (Figure 9f).[213] The defects on carbon black acted as an- choring sites to load 42 wt.% of multiple atoms of high qual- ity. The cocktail effect results in a mass activity of PtIrCuNiCo HESAC with low noble metal loading 11 times higher than that of commercial Pt/C. Combining theoretical calculations and ex- perimental results, it is found that the distribution of each atom ratio in PtIrCuNiCo HESACs is closer to the activity distribution of the corresponding metal in the HER volcano diagram, thus achieving excellent catalytic activity (Figure 9g). Furthermore, Adv. Mater. 2025, 2506117 2506117 (20 of 39) © 2025 The Author(s). Advanced Materials published by Wiley-VCH GmbH 15214095, 0, D ow nloaded from https://advanced.onlinelibrary.w iley.com /doi/10.1002/adm a.202506117 by A nsto, W iley O nline L ibrary on [17/07/2025]. See the T erm s and C onditions (https://onlinelibrary.w iley.com /term s-and-conditions) on W iley O nline L ibrary for rules of use; O A articles are governed by the applicable C reative C om m ons L icense http://www.advancedsciencenews.com http://www.advmat.de www.advancedsciencenews.com www.advmat.de Figure 9. a) The CoVMnNiZnPS3 crystallizes in a monoclinic structure (space group C2/m), with top-view illustrations demonstrating the structural evolution from pristine CoPS3 to CoVMnNiZnPS3. b) HER free-energy diagram of different sites in CoVMnNiZnPS3. a,b) Reproduced with permission.[88] Copyright 2022, American Chemical Society. c) SEM image and d) HAADF-STEM image and EDS mapping of NiCoMoZnCu HEA nanoflower. e) LSV curves of HEANFA and other comparative catalysts in 1.0 M KOH solution. c–e) Reproduced with permission.[212] Copyright 2024, Wiley-VCH. f) HAADF STEM images of Pt SAs in CB. g) Calculated distribution plots for PtIrCuNiCo HESACs. f,g) Reproduced with permission.[213] Copyright 2023, American Chemical Society. h) Polarization curves of PtRuRhPdRe-MoSe2 and other comparative catalysts in acidic and electrolytes. h) Reproduced with permission.[106] Copyright 2024, Wiley-VCH. the chalcogens (S, Se) in 2D transition metal dichalcogenides (TMDs) have lone pairs of electrons and certain electronegativ- ity, which can interact with metal atoms to achieve a stable effect. In addition, the abundant vacancies on the surface of TMDs can act as anchoring sites to stabilize the metal atoms. Through the substrate-mediated strategy to control the reversible redox reac- tion at the interface of TMDs and transition metal ions, high- atom-density PtRuRhPdRe-MoSe2 (HESAs-01) was obtained. [106] Chemically synthesized MoSe2 contained electron-donating Se vacancies that endowed redox capability, whereas the Mo vacan- cies were generated upon the introduction of Pt, Ru, Rh, Pd, or Re ions. The incorporated metal cations are reduced by MoSe2 and spontaneously migrate to occupy the Mo vacancies, coordi- nating with Se without metallic bond formation (Figure 9h). In both acidic and alkalinemedia, HESAs-01 showed excellent HER activity (𝜂10,acid = 32 mV, 𝜂10,alkaline = 35 mV), which is better than the state-of-the-art single-atommaterials. The multi-metal single atoms enhanced the metal–support interaction and further ad- just the electronic state of MoSe2, stimulating high HER activity and stability. 4.1.4. High Entropy Intermetallic Compared to the traditional disordered HEMs, HEI possess higher structural stability, more precise electronic regulation, Adv. Mater. 2025, 2506117 2506117 (21 of 39) © 2025 The Author(s). Advanced Materials published by Wiley-VCH GmbH 15214095, 0, D ow nloaded from https://advanced.onlinelibrary.w iley.com /doi/10.1002/adm a.202506117 by A nsto, W iley O nline L ibrary on [17/07/2025]. See the T erm s and C onditions (https://onlinelibrary.w iley.com /term s-and-conditions) on W iley O nline L ibrary for rules of use; O A articles are governed by the applicable C reative C om m ons L icense http://www.advancedsciencenews.com http://www.advmat.de www.advancedsciencenews.com www.advmat.de more effective site isolation effect, and multifunctionality. There- fore, HEI inherits the advantages of HEMs and intermetallic compounds to be promising candidates for HER. When the dis- orderedHEMs transform into an orderedHEI structure, it is usu- ally accompanied by a change in the electronic structure, for ex- ample, adjusting the d-band center. For example, changing the annealing temperature and time can effectively adjust the or- der degree of Pt4FeCoCuNi nanocrystals to achieve highly or- dered, partially ordered, and fully disordered samples.[51] Com- pared with partially ordered and fully disordered Pt4FeCoCuNi, in the highly ordered sample, the d-band center was positioned closest to the Fermi level, which significantly improved the bind- ing affinity of HER intermediates and boosted catalytic effi- ciency. Moreover, the alternating stacking of Pt in the highly ordered Pt4FeCoCuNi crystal structure promoted H* coupling and Fe/Co/Cu/Ni species favor water dissociation, synergisti- cally enhancing the HER activity. As a result, the highly ordered Pt4FeCoCuNi only took 20 mV to reach j of 10 mA cm−2, which is much lower than the other two counterparts (32 and 47 mV, respectively). In addition to the annealing condition affecting the order degree of HEMs, the metal species also promote the trans- formation from disordered to ordered HEI structure. For exam- ple, the introduction of Fe induced the change of PtCuPdAg from the L11 phase (PCPAF-HEA/C) to the L10 intermetallic phase (PCPAF-HEI/C).[214] In 0.5 m H2SO4 solution, PCPAF-HEI/C showed high specific activity (SA = 34.9 mA cm−2) and low Tafel slope (29 mV dec−1), which is significantly better than that of PCPAF-HEA/C (SA= 26.16mA cm−2, Tafel slope= 38mVdec−1) and Pt/C (SA = 1.681 mA cm−2, Tafel slope = 35 mV dec−1). The excellent alkalineHER activity of PCPAF-HEI/C can be attributed to the stronger d–d interaction and high entropy stabilization ef- fect in its ordered structure comparedwith PCPAF-HEA/C. Com- pared with pure Pt (−0.587 eV), the H* of the Pt site in PCPAF- HEI/C was optimized with a ΔGH* of −0.434 eV, which further alleviates the excessive binding of hydrogen on the surface and accelerates the desorption of H2. 4.1.5. Defective HEMs Defects play a crucial role in modifying electronic structures, in- creasing active sites, and improving reaction kinetics. Common types of defects include vacancies, dislocations, stacking faults, and voids, all of which play a significant role in boosting the HER catalytic performance.[215] Wire electrical dischargemachin- ing (WEDM) technique produces ultra-high temperatures fol- lowed by rapid quenching, enabling the transformation of bulk (MoWVNbTa)C into sub-10 nm nanoparticles.[100] Interestingly, high-entropy carbide nanoparticles exhibit a high density of de- fects, including steps, vacancies, and stacking faults, resulting from the ultrafast quenching process (Figure 10a,b). DFT calcu- lations were conducted using vacancy defects as a representative model. Sites containing Mo or W species show low ΔGH*, sug- gesting they serve as active sites for acidic HER. The DFT cal- culation took the vacancy defect as a representative. Sites con- taining Mo or W species show low ΔGH*, suggesting they served as active sites for acidic HER (Figure 10c). Furthermore, when surrounded by vacancy defects, these sites exhibited more favor- able ΔGH* values compared to those in the intact crystal struc- ture, due to a moderated d-band center value and the shielding effect of the vacancy for electron transfer. The unique defect-rich microstructure and high configurational entropy synergistically boosted the acidic HER activity, exhibiting a low overpotential of 156 mV at 10 mA cm−2, outperforming monometallic carbides (238–495 mV) and bulk HECs (402 mV). These findings further reinforce that defect-rich structures, particularly those incorpo- rating vacancies and lattice distortions, play a crucial role in en- hancing HER activity. Defects engineering high-entropy metallic glass (HEMG) sur- faces can also dramatically enhance HER efficiency. Significant lattice distortions and stacking faults have been introduced to the nanoporous (FeCoNiB0.75)97Pt3 HEMGwith a nanocrystalline surface structure, which optimized atomic configurations and modulated electronic interactions.[216] Moreover, such lattice dis- tortion significantly enhances the adsorption energy of H2O at Fe and Co sites in the stacking faults structure. During the water ad- sorption step, the H2O adsorption energies at Fe and Co sites in the stacking faults structure reached −0.70 and −0.44 eV, respec- tively, substantially higher than those on the Pt (111) surface. This indicates that defects facilitate the optimization of atomic config- urations and promotewater adsorption. In theHERprocess, sites such as Pt top, Pt-Co bridge, and Pt-Ni-Ni hollow exhibit favorable ΔGH*values, serving as critical active sites. Notably, the Pt sites coordinated with B atoms demonstrateΔGH* values close to zero (0.0074 and 0.0312 eV), contributing significantly to the enhance- ment ofHER performance. The resulting defect-rich architecture enabled ultralow overpotentials of 104 mV at 1000 mA cm−2 for HER under alkaline conditions, with exceptional long-term sta- bility exceeding 200 h at 100 mA cm−2. The lattice distortions can also result in unsaturated oxygen sites to enhance the sur- face oxophilicity. Due to the high entropy effect, the introduction of Mo and W caused local tensile and compressive microstrain in IrRuRhMoWHEA sub-nanoparticles (Figure 10d–f).[217] These strain defect