Recent progress in high-entropy intermetallics for advanced catalysis

Jingchun Guo *^a, Huiling Zheng ^a, Xucheng Fu ^a and Mingming Fan *^b
aDepartment of Experimental and Practical Teaching Management, West Anhui University, Lu'an 237012, China. E-mail: 43000013@wxc.edu.cn
^bCollege of Physics and Optoelectronic Engineering, Taiyuan University of Technology, Taiyuan 030024, China. E-mail: fanmingming08@163.com

First published on 22nd July 2025

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Jingchun Guo

Jingchun Guo is now working in West Anhui University. He received his bachelor's degree from Jilin University in 2010 and his PhD degree from Hokkaido University in 2017. After graduation, he joined Shenzhen University as a postdoctoral fellow in 2018. His research interests include the controllable synthesis of metallic nanocrystals, solar-blind UV photodetection, and electrochemical and photoelectrochemical catalysis.

Huiling Zheng

Huiling Zheng is now working at West Anhui university. She received her bachelor's degree from Suzhou University in 2014 and PhD degree from the University of Science and Technology of China in 2020. Her interests include the synthesis of graphene oxide-based nanomaterials, and their application in pollution management.

Xucheng Fu

Xucheng Fu is now working in West Anhui University. He received his bachelor's degree from Anhui Normal University in 2000 and his PhD degree from Hefei Institutes of Physical Science, Chinese Academy of Science in 2011. His research interests include functional nanomaterials and photoelectrochemical analysis.

Mingming Fan

Mingming Fan received his bachelor's degree from Jilin University in 2010 and his PhD degree from Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences in 2015. He is now working in Taiyuan University of Technology. His research interests include solar-blind UV photodetection based on wide-band-gap oxide semiconductors and photoelectrochemical catalysis.

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1. Introduction

Over the past two decades, a brand-new alloy system referred to as high-entropy alloys (HEAs), which is also known as multi-component alloys, compositionally complex alloys or multi-principal-element alloys, has garnered significant research attention.^1–20 HEAs normally contain five or more principal elements in a single phase with equimolar ratios.^1,2 HEAs have greatly expanded the alloy compositional space to unexplored regions and brought many unique properties such as exceptional strength, ductility and toughness,³ high thermoelectricity,⁴ high-performance rechargeable battery materials,⁵ outstanding hydrogen storage,⁶ and superparamagnetism.⁷ The bulk HEAs were mostly prepared by arc melting, mechanical milling or melt spinning techniques in the early stage, which hindered their electrocatalytic applications because of limited exposed active sites.^1–7 Notably, HEAs have been extended to the nanoscale with the development of synthesis technology, expanding their applications in the field of electrocatalysis.^8–20

In fact, alloying has been proved to be an effective strategy for tailoring the binding energy of active sites and reactants/intermediates.^21–23 For instance, alloying Pt with transition metals (e.g., Fe, Co, or Ni) has been shown to downshift the d-band center of Pt, weakening the binding affinity between the Pt surface and oxygenated species, thereby enhancing oxygen reduction reaction (ORR) activities.²⁴ Alloying of two or more metal elements yields two distinct types of alloys based on the crystal structure order: intermetallics (IMs) and random alloys (RAs).^25–31 The constituent elements occupy crystallographic sites randomly without atomic ordering in RAs, with the crystal structure dominated by one of the constituent elements. In contrast, the constituent elements occupy a definite crystallographic site with a defined stoichiometry and atomic ordering arising from long-range atomic ordering in IMs.

To date, IMs have been demonstrated to exhibit distinct advantages as electrocatalysts attributed to their ordered atomic arrangement and thermodynamically stable state compared to RA counterparts even with the identical chemical composition.^25–31 For instance, the Pt–Cu, Pt–Fe, and Pr–Zr IMs exhibited substantially reduced leaching of nonnoble metals (Cu, Fe, and Zr) than disordered Pt–Cu, Pt–Fe, and Pr–Zr RA counterparts after a long-term electrochemical stability test.^32–34 In addition, the regular structure of IMs would ensure the homogeneous active site distributions, rendering IMs ideal platforms for investigating the structure-to-property relationships.^30,31

Notably, HEAs^3–20 and IMs^25–31 have gained increasing attention in electrocatalytic applications. Nonetheless, high-entropy intermetallics (HEIMs), featuring ordered atomic arrangements and exceptional physicochemical properties, remain scarcely explored so far.^35–37 In this review, we will summarize the preparation of HEIMs by alloying and dealloying, co-impregnation and annealing, disorder-to-order transition, and chemical co-reduction methods. After highlighting their applications in electrocatalysis, acetylene semihydrogenation and propane dehydrogenation, we address the current bottlenecks in HEIM synthesis and conclude with forward-looking perspectives.

2. Synthetic strategies

2.1 Alloying and dealloying

Numerous HEAs have been synthesized via high-temperature melting of constituent metallic elements (alloying).^1,2 Ding et al. reported the synthesis of FeCoNiCrNb HEIMs through a combined alloying and dealloying strategy.³⁸ Eutectic high entropy alloy (EHEA) bulk samples were firstly fabricated via arc melting (Co 22.2 at%, Cr 22.2 at%, Fe 22.2 at%, Ni 22.2 at% and Nb 11.2 at%) and subsequent thermal annealing (1200 °C for 6 h). The EHEA contains a face centered cubic (fcc) solid solution and Laves phase. EHEA ribbon samples were then obtained via melt spinning. An FeCoNiCrNb intermetallic–oxide core–shell porous nanostructure was finally obtained by selective dealloying of the EHEA ribbon samples in dilute chloriazotic acid at 50 °C for 5 h. The fcc phase was etched away and only the Laves phase was left due to the excellent corrosion resistance. Fig. 1a shows the scanning electron microscopy (SEM) image of the as-obtained Fe–Co–Ni–Cr–Nb intermetallic–oxide core–shell porous nanostructure. The high-resolution transmission electron microscopy (HRTEM) image in Fig. 1b and energy dispersive X-ray spectroscopy (EDX) elemental mapping analysis in Fig. 1c show an obvious 10 nm thick compact oxide film composed of O and Fe–Co–Ni–Cr–Nb on the surface of the Laves phase. The fast Fourier transformation (FFT)-filtered HRTEM in Fig. 1d reveals the alternating large (yellow spot) and small atom (blue spot) layers in the underlying Laves phase and the inset FFT image shows a typical ordered structure. By comparison, the surface oxide layer as shown in Fig. 1e is completely amorphous.

Jia and co-workers also obtained FeCoNiAlTi HEIMs by an alloying and dealloying process (1.0 M HCl solution for 15 h).³⁹ Before dealloying, the as-spun FeCoNiAlTi HEIMs have an ultra-smooth surface and presents dual-phase microstructure composed of a Heusler and L1₂ intermetallic phase. The FeCoNiAlTi HEIMs present a uniform, dendritic-like porous morphology after dealloying as shown in Fig. 1f. In addition, the FeCoNiAlTi HEIMs after dealloying completely evolve to the L1₂ intermetallic phase, indicating that the obtained dendritic-like porous morphology is on account of the Heusler phase dissolution. The high-angle annular dark-field scanning TEM (HAADF-STEM) image with the corresponding selected area electron diffraction (SAED) pattern of the dendritic like FeCoNiAlTi HEIMs in Fig. 2g further confirms their periodically ordered L1₂-type atomic configuration. HAADF imaging and elemental maps of Fe, Co, Ni, Al, and Ti in Fig. 2h highlight the specific sublattice occupations in the FeCoNiAlTi HEIMs. The Al and Ti atoms occupy the vertices and the Fe, Co, Ni atoms occupy the face centers and regulate the atomic configuration in the L1₂-type intermetallic structure.

Shi et al. prepared Mo(NiFeCo)₄ IMs (the Mo(NiFeCo)₄ IMs are formed by Fe and Co atoms randomly and partially substituting the Ni sublattice in the intermetallic MoNi₄ matrix) seamlessly integrated on a hierarchical nanoporous nickel (Ni) skeleton (Mo(NiFeCo)₄/Ni HEIMs) electrode by facile and scalable alloying/dealloying procedures.⁴⁰ Ni₁₂Fe₂Co₂Mo₄Al₈₀ alloy ribbons are first produced by arc-melting and subsequent melt-spinning to form a multiphase nanostructure that is composed of α-Al and intermetallic Al₃Ni phases. The Ni₁₂Fe₂Co₂Mo₄Al₈₀ alloy ribbons were then immersed into N₂-purged 6 M KOH solution at 70 °C until there were no more bubbles to get the Mo(NiFeCo)₄/Ni HEIMs. The SEM image of the Mo(NiFeCo)₄/Ni electrode in Fig. 1i shows a hierarchical nanoporous architecture consisting of interpenetrative large channels and small nanopores in interconnected Ni ligaments. Fig. 1j shows a representative atomic resolution HAADF-STEM image of Mo(NiFeCo)₄/Ni, where the constituent Mo(NiFeCo)₄ IMs and the Ni substrate viewed along their 〈113〉 and 〈011〉 zone axes is identified by the corresponding FFT patterns of selected areas (Fig. 1k and l), respectively. Fig. 1m shows the STEM-EDX elemental mappings of nanoporous Mo(NiFeCo)₄/Ni electrode, where the Ni, Fe, Co, and Mo atoms distribute along the Ni skeleton. The residual Al atoms prefer to locate in the interior of Ni ligaments because of the selective etching of surface Al during the chemical dealloying procedure. Therefore, only Ni, Fe, Co and Mo atoms are present on the surface of the nanoporous Mo(NiFeCo)₄/Ni electrode.

2.2 Co-impregnation and annealing

The co-impregnation of metal precursors followed by a high-temperature annealing method was frequently employed to prepare HEAs.^12,20 Furukawa and co-workers successively reported the preparation of (NiFeCu)(GaGe) HEIMs (around 4.9 nm) loaded on silica (NiFeCuGaGe/SiO₂),⁴¹ (PtCoCu)(GeGaSn) HEIMs (around 2.2 nm) supported on Ca-modified amorphous silica ((PtCoCu)(GeGaSn)/Ca–SiO₂),⁴² and (PtCoNi)(SnInGa) HEIMs (around 4 nm) loaded on CeO₂ ((PtCoNi)(SnInGa)/CeO₂),⁴³ respectively. (NiFeCu)(GaGe)/SiO₂ was prepared by mixing metal precursors (atomic ratio Ni:Fe:Cu:Ga:Ge = 1:1:1:1:1) and added dropwise to SiO₂. The resulting mixture was calcined at 400 °C for 1 h, followed by reduction at 700 °C in flowing H₂. As shown in Fig. 2a, the (NiFeCu)(GaGe) HEIMs were obtained by partially replacing Ni and Ga atoms in NiGa IMs by Fe (Cu) and Ge atoms, respectively.⁴¹ Fig. 2b shows the HAADF-STEM image of (NiFeCu)(GaGe)/SiO₂ and the corresponding elemental maps acquired by EDX. They performed the same method to prepare (PtCoCu)(GeGaSn) and (PtCoNi)(SnInGa) HEIMs by partially replacing Pt and Ge atoms in PtGe IMs by Co(Ni) and Ga(Sn) as shown in Fig. 2c and d,⁴² and Pt and Sn atoms in PtSn IMs by Co(Ni) and In(Ga) as shown in Fig. 2e and f.⁴³

The high temperature annealing process would inevitably accelerate metal nanoparticle sintering and aggregation that leads to larger crystallites and agglomeration.^26,27 To solve this problem, Yang et al. prepared 5 quinary Pt-based (Pt₄FeCoNiCu, Pt₄FeCoNiMn, Pt₄FeCoCuMn, Pt₄FeNiCuMn, Pt₄CoNiCuMn), and 1 senary (Pt₅FeCoNiCuMn) HEIMs (around 5 nm) by wet-impregnation of an aqueous solution containing H₂PtCl₆ and other metal salts and subsequent high-temperature thermal reduction in 5% H₂/Ar on porous S-doped carbon (S–C) supports.⁴⁴ The Pt-based HEIMs were loaded on porous S–C supports to introduce strong chemical interaction between Pt and S atoms (Pt–S bond), which largely suppressed nanoparticle sintering. They named this synthetic approach a sulfur-anchoring synthesis strategy. The atomic-resolution HAADF-STEM image in Fig. 3a shows the ordered tetragonal structure and alternating stacking of Pt and non-Pt metal (Fe, Co, Ni, Cu, Mn) columns of Pt₅FeCoNiCuMn HEIMs. The EDX elemental mappings in Fig. 3b further confirmed the homogeneous distribution of Pt and other metallic elements. Wang et al. used the same synthesis strategy to prepare Pt₄FeCoCuNi HEIMs with an average particle size of 5.1 nm on S–C supports.⁴⁵ A very similar synthesis strategy, named a hydrogenated borophene (HB)-triggered synthesis method, was used by Zeng et al. to prepare Pt₄FeCoNiCu HEIMs with sub-4 nm size on B/C supports due to the formation of Pt–B bonds.⁴⁶ The Pt–C and Pt–B bonds are verified to play key roles in suppressing the sintering of Pt-based HEIMs even up to 1000 °C.^44–46 Hu et al. utilized a hollow-carbon confinement method to obtain Pt_0.45Fe_0.18Co_0.12Ni_0.15Mn_0.10 HEIMs with a uniform size of 5.9 nm.⁴⁷ Polystyrene (PS) nanospheres were added during the co-impregnation process and finally decomposed after high temperature annealing to form hollow-carbon confined Pt_0.45Fe_0.18Co_0.12Ni_0.15Mn_0.10 HEIMs.

IMs and RAs are commonly synthesized by using different annealing temperatures.^25–27 Notably, Zhu et al. found that the annealing atmospheres also exert critical influences on the formation of HEIMs and HEAs.⁴⁸ Fig. 3c–e show novel 2D nitrogen-rich mesoporous carbon (mNC) nanosheet supported Co_0.31Fe_0.27Ni_0.19Cu_0.09Pd_0.14 HEIMs (around 10 nm) by co-impregnation of metal salt precursors and subsequent NH₃ annealing at 750 °C.⁴⁸ The atom-resolved HAADF-STEM images in Fig. 3d and e display two distinct atomic column types, which are clearly distinguishable attributed to their disparate average atomic numbers (Pd and Fe, Co, Ni, Cu). The Co_0.31Fe_0.27Ni_0.19Cu_0.09Pd_0.14 HEIMs exhibit L1₂ composition-ordered structure by the atom-resolved HAADF-STEM images and further FFT investigation. Interestingly, mNC supported HEAs with a disordered fcc phase were obtained under N₂ annealing at 750 °C. The theoretical calculation results demonstrated that the ordered Co_0.31Fe_0.27Ni_0.19Cu_0.09Pd_0.14 HEIMs are more stabilized in an NH₃ atmosphere because NH₃ is more favorable to be absorbed on the ordered HEIMs while N₂ prefers binding on the disordered fcc HEAs.

Conventionally, the (001) facet in metallic nanocatalysts has been considered as a low-activity facet relative to the (111) facet. However, Feng et al. reported the preparation of PtIrFeCoCu (Pt:Ir:Fe:Co:Cu = 34.8:12.3:21.7:20.0:11.2) HEIMs (around 6 nm) with higher ORR activity in the (001) facet than (111) facet as shown in Fig. 3f–h.⁴⁹ The well-defined Pt/Ir atoms (brighter intensity) are in one sublattice, while the Co, Fe, and Cu atoms (darker intensity) are uniformly distributed in the other sublattice, both with the L1₀ structure. PtIrFeCoCu HEIMs and HEAs were obtained by co-impregnation and high temperature annealing at 850 °C and 450 °C, respectively. Surprisingly, the ORR activities follow the order of PtIrFeCoCu (001) > PtIrFeCoCu (111) > PtFe (111) > PtFe (001) > Pt (111) > Pt (001), a trend that contradicts conventional wisdom. This finding highlights that PtIrFeCoCu HEIMs can transform the traditionally low-activity (001) facet into an ultrahigh-activity catalytic surface.

It was reported that metallic nanocatalysts with 2–3 nm size have the highest mass activity (MA) and specific activity (SA) values in ORR performances.⁵⁰ However, the HEIMs toward the ORR with 2–3 nm size have rarely been reported. Chen et al. successfully synthesized PtFeCoNiCuZn (Pt:Fe:Co:Ni:Cu:Zn = 48.16:10.12:9.97:9.12:13.12:9.51) HEIMs with an innovative ultrasmall diameter of 2 nm via a synergistic approach involving an innovative ultrasmall size of only 2 nm by co-impregnation of Pt, Fe, Co, Ni, and Cu precursors and Zn-based ZIF-8 with dodecahedral porous carbon nitride (Zn–DPCN) and subsequent 800 °C treatment as shown in Fig. 3i.⁵¹ This achievement is attributed to the spatial confinement imposed by the small pore size (less than 3 nm) of the Zn–DPCN support. In contrast, PtFeCoNiCuZn HEAs were obtained under identical conditions but with thermal treatment at 600 °C.

2.3 Disorder-to-order transition

The traditional synthesis of IMs needs long furnace annealing durations due to the slow heating/cooling rates and induced agglomeration and phase separation.^26,27 Cui et al. reported the fabrication of nanoscale HEIMs with up to eight different elements by a multi-elemental disorder-to-order (HEA to HEIM) phase transition strategy as shown in Fig. 4a within only a few minutes.⁵² Specifically, the metal salt precursors were rapidly Joule-heated at ∼1100 K for 55 ms to produce disordered HEAs. The produced disordered HEAs were then rapidly Joule-heated at ∼1100 K for 5 min to obtain HEIMs due to the atomic rearrangement (disorder-to-order transition), followed by rapid cooling. The obtained Pt(Fe_0.7Co_0.1Ni_0.1Cu_0.1) HEIMs in Fig. 4b show alternating brighter columns of Pt atoms and darker columns of Fe, Co, Ni, and Cu atoms. Atomic-resolution EDX mapping indicates the L1₀ intermetallic structure of Pt(Fe_0.7Co_0.1Ni_0.1Cu_0.1) HEIMs. Specifically, octonary (Pt_0.8Pd_0.1Au_0.1)(Fe_0.6Co_0.1Ni_0.1Cu_0.1Sn_0.1) HEIMs with the L1₀ phase were also prepared by this disorder-to-order phase transition strategy as shown in Fig. 4c.

Wang et al. reported the preparation of multi-walled carbon nanotube (MWCNT) supported PtRhFeNiCu HEIMs by a disorder-to-order phase transition strategy as shown in Fig. 4d.⁵³ The disordered PtRhFeNiCu HEAs/MWCNT was firstly prepared by co-impregnation of metal precursors followed by 2 h of 400 °C annealing treatment. The obtained disordered PtRhFeNiCu HEAs/MWCNT was reheated at 700 °C for 2 h to form ordered PtRhFeNiCu HEIMs/MWCNT as shown in Fig. 4e–g. The STEM-EDX elemental mapping in Fig. 4h demonstrates the uniform dispersion of all elements.

2.4 Chemical co-reduction

Chemical co-reduction synthesis in the liquid phase presents great potential to produce HEIMs with good dispersion and less agglomeration in comparison to high temperature annealing because of the relatively low reaction temperature (typically, <350 °C).^30,31 Chen et al. synthesized 2D PtRhBiSnSb HEIMs by co-reduction of Pt, Rh, Bi, Sn, and Sb salt precursors at 220 °C for 1 h.⁵⁴ Fig. 5a shows the TEM images of well-dispersed 2D PtRhBiSnSb HEIMs with an average edge length of 6.2 nm. The HAADF-STEM image in Fig. 5b with well-defined stacking sequences of atomic columns reveals the ordered atomic arrangements of 2D PtRhBiSnSb HEIMs. The lattice spacing of the (100) planes at the particle edges was measured to be 0.361 nm, intermediate between the (100) spacings of PtBi (0.374 nm), PtSn (0.355 nm), and PtSb (0.358 nm) IMs. A consistent lattice spacing of 0.204 nm indicated a uniform hexagonal close-packed (hcp) crystal structure. EDX mapping (Fig. 5c) shows enriched Rh and Bi concentrations in the particle cores, whereas Pt, Sn, and Sb are preferentially distributed near the edges. 2D PtBiSnSb IMs were also synthesized without the addition of the Rh salt precursor under identical experimental conditions, serving as a control experiment.

Zhan et al. reported the synthesis of 2D PtBiPbNiCo HEIMs with an average diameter of 26.2 nm and thickness of 8.4 nm by co-reduction of Pt, Bi, Pb, Ni, and Co salt precursors at 180 °C for 5 h as shown by the TEM images in Fig. 5d and e.⁵⁵ EDX demonstrates that the metal composition is Pt:Pb:Bi:Ni:Co = 41.7:33.4:10.8:6.3:7.8. HRTEM analysis exhibited a lattice spacing of 0.215 nm, corresponding to the (110) facet of PtPb IMs (Fig. 5f). The ion scattering spectrum (inset of Fig. 5f) reveals that the 2D PtBiPbNiCo HEIMs show distinct core–shell structures characterized by abrupt changes in elemental signals at the interface. Furthermore, aberration-corrected HAADF-STEM-EDX elemental mappings in Fig. 4g and h demonstrate the homogeneous distribution of all constituent elements. 2D PtPb IMs were also synthesized without the addition of Ni, Co, and Bi salt precursors under identical experimental conditions, serving as a control experiment.

3. Electrocatalytic applications

3.1 Water splitting

Water splitting is a fundamental chemical reaction of decomposing water (H₂O) into hydrogen (H₂) and oxygen (O₂) with immense significance for clean energy production and sustainable technology.^56–59 Water splitting consists of two half-reactions: the hydrogen evolution reaction (HER) at the cathode: 2H₂O + 2e⁻ → H₂ + 2OH⁻ (in alkaline media) or 2H⁺ + 2e⁻ → H₂ (in acidic media); and the oxygen evolution reaction (OER) at the anode: 4OH⁻ → O₂ + 2H₂O + 4e⁻ (in alkaline media) or 2H₂O → O₂ + 4H⁺ + 4e⁻ (in acidic media). A standard reaction potential of 1.23 V vs. reversible hydrogen electrode (RHE) is required to process electrocatalytic water splitting. In practice, overpotentials much larger than 1.23 V (>1.4 V) are necessary to split H₂O efficiently due to the sluggish kinetics of the OER and HER (energy losses due to kinetic barriers).⁵⁹ Thus, developing electrocatalysts with faster OER and HER kinetics and good electrocatalytic stability is urgent. The OER performance of porous FeCoNiCrNb HEIMs (Fig. 1a–e) prepared by the alloying and dealloying method was investigated.³⁸ Fig. 6a shows the OER polarization curves of FeCoNiCrNb HEIMs and the benchmarked catalyst RuO₂ in 0.1 M KOH aqueous electrolyte. FeCoNiCrNb HEIMs exhibit a small overpotential (0.288 V @ 10 mA cm⁻²) for the OER, which is much lower than that of RuO₂ (0.384 V @ 10 mA cm⁻²). Moreover, FeCoNiCrNb HEIMs display a smaller Tafel slope (27.7 mV dec⁻¹) compared to RuO₂ (106 mV dec⁻¹), demonstrating the ultrafast reaction kinetics. In addition, the overpotential has almost no increase and there is no structural degradation or damage after 30 h of electrochemical stability testing, indicating the excellent durability. Fig. 6b depicts the schematic graph of the porous IM supported multicomponent oxide film hierarchy. After the alloying and top-down dealloying process, the prepared FeCoNiCrNb HEIMs have an intermetallic–oxide core–shell structure in a mechanical self-supporting manner. As a result, the intermetallic substrate provides robust mechanical and chemical support while the multicomponent oxide/hydroxide layer provides numerous active sites for the OER.

The HER performance of dendritic-like porous FeCoNiAlTi HEIMs (Fig. 1f–h) prepared by the alloying and dealloying method was investigated.³⁹ Fig. 6c shows the HER polarization curves of various FeCoNiAlTi HEIMs (as-spun FeCoNiAlTi HEIMs and the FeCoNiAlTi HEIMs with different dealloying times of 2 h, 5 h, 10 h, 15 h, and 20 h) and the benchmarked Pt sheet in 1 M KOH aqueous electrolyte. The D15 h (dealloying time of 15 h) FeCoNiAlTi HEIMs achieved an overpotential of 88.2 mV @ 10 mA cm⁻² and Tafel slope of 40.1 mV dec⁻¹, which are much better than these of Pt sheet (overpotential of 145 mV @ 10 mA cm⁻² and Tafel slope of 79.3 mV dec⁻¹). Respective chronoamperometry measurements at static current densities of 20 and 100 mA cm⁻² for both 40 h were conducted with respect to the HER stability. The overpotentials have no significant amplification and the surface morphology and chemical states are still comparable with the original states of the reused D15 h HEIMs. A Tafel slope of 40.1 mV dec⁻¹ suggests that the hydrogen production process is proceeded by the Volmer–Heyrovsky mechanism, where a H₂O molecule requires extra energy to be initially split into an adsorbed hydrogen atom (H_ad) and a hydroxyl ion (OH⁻) on the catalytic interface, and then H_ad is detached from the interface to react with a hydrogen proton (H*) in water to complete the hydrogen production. The density functional theory (DFT) results suggest that the H₂O molecules prefer to adsorb on the top of each single metal atom in the FeCoNiAlTi HEIMs and the Ti atoms possess preferable adsorption energy (E_ad) compared with other elements (Fe, Co, Ni, Al). Notably, all metal sites (Fe, Co, Ni, Al, Ti) have higher E_ad compared to Pt. As illustrated in Fig. 6d, the Ti atoms with strong E_ad initially adsorb H₂O molecules to perform the Volmer step and the isolated Al atoms coordinating with Fe, Co, and Ni atoms to provide a beneficial Gibbs free energy (ΔG_H*) value for the Heyrovsky step with H_ad adsorption and desorption. A detached H_ad from the FeCoNiAlTi HEIM surface is then combined with an H* in water to complete the hydrogen evolution process. The mixing of five metal elements with unique atomic configurations elicits a robust synergistic effect, enabling effective modulation of the electronic structure. Furthermore, compared to conventional HEAs with random solid solutions, the FeCoNiAlTi HEIMs with homogeneously distributed atoms in an ordered L1₂ lattice exhibit distinguished site-isolation, which further optimizes the electronic structure. Therefore, the porous dendritic surface morphology and strong synergistic and structural site-isolation effects account for the superior electrocatalytic HER performance of dendritic-like porous FeCoNiCrNb HEIMs.

The nanoporous Mo(NiFeCo)₄/Ni HEIMs (Fig. 1i–m) were developed as versatile electrocatalysts for highly efficient electrochemical overall water splitting.⁴⁰ Fig. 6e shows the HER polarization curves for nanoporous Mo(NiFeCo)₄/Ni, MoNi₄/Ni, NiFeCo/Ni, bare Ni electrodes, and commercial Pt/C nanocatalysts supported on a nanoporous Ni skeleton (Pt/C/Ni) in 1 M KOH aqueous electrolyte. The nanoporous Mo(NiFeCo)₄/Ni electrode exhibited an overpotential of 47 mV @ 100 mA cm⁻² and Tafel slope of 35 mV dec⁻¹, which are much better than those of MoNi₄/Ni (overpotential of 64 mV @ 100 mA cm⁻² and Tafel slope of 62 mV dec⁻¹), NiFeCo/Ni (overpotential of 146 mV @ 100 mA cm⁻² and Tafel slope of 77 mV dec⁻¹), bare Ni (overpotential of 219 mV @ 100 mA cm⁻² and Tafel slope of 174 mV dec⁻¹), and Pt/C/Ni (overpotential of 68 mV @ 100 mA cm⁻² and Tafel slope of 43 mV dec⁻¹). The chronoamperometry measurements at a static current density of 430 mA cm⁻² for more than 500 h were conducted with respect to the HER stability. The overpotential has no evident fluctuation and the surface morphology still keeps the initial hierarchical nanoporous structure. Surprisingly, the nanoporous EON-Mo(NiFeCo)₄/Ni (Mo(NiFeCo)₄/Ni after being electrooxidized in alkaline media and nitrified in an NH₃/Ar atmosphere) electrode exhibits a small overpotential (0.35 V @ 960 mA cm⁻²) and a Tafel slope of 34 mV dec⁻¹ for the OER. As a result, nanoporous Mo(NiFeCo)₄/Ni and EON-Mo(NiFeCo)₄/Ni are assembled as cathode and anode materials to demonstrate their practical applications in water electrolysis as shown in the inset of Fig. 6f. The nanoporous Mo(NiFeCo)₄/Ni-based alkaline water electrolyzer (AWE) can deliver a current density of 100 mA cm⁻² at a voltage of 1.579 V, which is much lower than the value of a nanoporous Ni-supported commercial Pt/C and RuO₂/C based AWE (1.766 V). Moreover, the nanoporous Mo(NiFeCo)₄/Ni-based AWE maintains outstanding stability in current density of 130 mA cm⁻² at 1.6 V for more than 600 h. The charge transfer from NiFeCo to Mo in Mo(NiFeCo)₄ takes place due to the electronegativity difference between Mo and Ni, Fe, and Co. This not only results in the distinct adsorption of hydrogen intermediates and hydroxyl on Mo and NiFeCo components to promote water dissociation but also enables the Mo(NiFeCo)₄ to have a near-optimal adsorption energy to be conducive to the adsorption and combination of hydrogen intermediates. The hierarchical nanoporous Ni skeleton can facilitate electron transfer and H₂O/HO⁻ species transport to abundant Mo(NiFeCo)₄ electroactive sites. As a consequence, the nanoporous Mo(NiFeCo)₄/Ni-based AWE exhibits superior electrochemical water splitting properties.

3.2 Oxygen reduction reaction

Recently, environment-friendly and sustainable energy technologies such as fuel cells (FCs)^22,23 have attracted tremendous attention because of numerous environmental problems induced by the excessive consumption of traditional fossil fuels.^60–62 The FCs can convert the chemically stored energy into electrical energy by the cathodic ORR and anodic electrooxidation of chemical fuels (hydrogen, methanol, ethanol, formic acid, etc.) with high-energy conversion efficiency and relatively low working temperatures.²² In particular, proton exchange membrane FCs (PEMFCs) have water as the only byproduct, making them the most promising future solution. However, the commercialization of FCs is limited by the sluggish kinetics of the cathodic ORR due to the problematic activation/cleavage of the strong OO bond involving four coupled protons and electrons.²³ The PtIrFeCoCu HEIMs (Fig. 3f–h) show extremely superior performance for the ORR and practical performance for PEMFCs.⁴⁹ As shown in Fig. 7a, the PtIrFeCoCu HEIMs/C achieves an ultrahigh MA of 7.14 A mg_{noble metals}⁻¹ at 0.85 V versus RHE and substantially enhanced stability for 60000 potential cycles in 0.1 M HClO₄ solution. The MA is 20.4 and 2.4 times higher than those of the commercial Pt/C (0.35 A mg_Pt⁻¹) and PtIrFeCoCu HEAs/C (3.03 A mg_{noble metal}⁻¹), respectively. PtIrFeCoCu HEIMs/C can maintain 100% of the initial current density after 60 h of chronopotentiometry testing, which is much better than that of Pt/C (only 64.4% left after a 10 h test). Especially, the practical PEMFC test also demonstrated that the PtIrFeCoCu HEIMs afford a current density of 4.25 A cm⁻² at 0.4 V, much higher than that of commercial Pt/C (3.67 A cm⁻²). DFT calculations were performed to get further insight into the ultrahigh intrinsic activity of PtIrFeCoCu HEIMs/C for the ORR and FCs. Fig. 7b–e show the relative reaction activities between PtIrFeCoCu HEIMs (001), Pt (001), PtIrFeCoCu HEIMs (111), and Pt (111) surfaces at potentials of 0 V, 0.75 V and 1.23 V. The free energy diagrams of PtIrFeCoCu HEIMs (001) and Pt (001) as shown in Fig. 7b and c at U = 0.75 V for O* hydrogenation reactions indicate that the PtIrFeCoCu HEIMs (001) (ΔG = 0.24 eV) are more thermodynamically favorable for the ORR than the Pt (001) (ΔG = 0.43 eV). Also Fig. 7d and e at U = 0.75 V for OH* hydrogenation reactions indicate that the PtIrFeCoCu HEIMs (111) is more thermodynamically favorable (ΔG = 0.26 eV) for the ORR than the Pt (111) (ΔG = 0.30 eV). Therefore, the free energy results confirmed that both the (001) and (111) surfaces in the PtIrFeCoCu HEIMs catalyst possess a higher ORR activity than the corresponding surface in pure Pt. Additionally, the rate-limiting steps are OH* + H⁺ + e⁻ → H₂O on the PtIrFeCoCu HEIMs (001) and Pt (001) surfaces, and O* + H⁺ + e⁻ → OH* on PtIrFeCoCu HEIMs (111) and Pt (111) surfaces, respectively. As shown in Fig. 7f, the activation barrier for the rate-limiting step over PtIrFeCoCu HEIMs (111) (0.604 eV) is lower than that over the Pt (111) surface (0.626 eV), indicating a faster dynamic process on PtIrFeCoCu HEIMs (111). Most surprisingly, the activation barrier for the rate-limiting step over the PtIrFeCoCu HEIMs (001) surface is only 0.486 eV, much lower than those over the PtIrFeCoCu HEIMs (111) surface and Pt (111) surface, significantly demonstrating that the PtIrFeCoCu HEIMs (001) facet is an ultrahigh-activity facet for the ORR, mainly contributing to the superior ORR activity of PtIrFeCoCu HEIMs. It was reported that the ORR activity at 0.9 V reaches the maximum when the d-band center position downshifts about 0.32 eV relative to pure Pt (111).^63,64 The electronic density of states (DOSs) of the d-band for Pt atoms on the surfaces of PtIrFeCoCu HEIMs (001), PtIrFeCoCu HEIMs (111), PtFe (001), PtFe (111), Pt (111), and Pt (001) was compared as shown in Fig. 7g. The d-band center downshifts for PtIrFeCoCu HEIMs (001), PtIrFeCoCu HEIMs (111), PtFe (001), and PtFe (111) are 0.31 eV, 0.28 eV, 0.22 eV, and 0.40 eV relative to pure Pt (111), respectively. Therefore, the ORR activities follow the order of PtIrFeCoCu (001) > PtIrFeCoCu (111) > PtFe (111) > PtFe (001) > Pt (111) > Pt (001). In addition, the normalized X-ray absorption near edge structure (XANES) spectra and Bader charge analysis identify the electron density transfer from Fe/Co/Cu atoms to Pt/Ir atoms in both PtIrFeCoCu HEIMs and HEAs. However, the ordered PtIrFeCoCu HEIMs possesses a more efficient regulation of electron structure than PtIrFeCoCu HEAs. Furthermore, the d-band center downshifts for PtIrFeCoCu HEAs (001) and PtIrFeCoCu HEAs (111) are 0.17 eV and 0.37 eV relative to pure Pt (111), which are clearly inferior to those of PtIrFeCoCu HEIMs (001) (0.31 eV) and PtIrFeCoCu HEIMs (111) (0.28 eV), endowing ordered PtIrFeCoCu HEIMs with enhanced selectivity and activity toward the ORR relative to their PtIrFeCoCu HEAs counterparts.

The ultrasmall 2 nm-size PtFeCoNiCuZn HEIMs (Fig. 3i) delivers MAs of 2.403 and 8.22 A mg_Pt⁻¹ at 0.90 V and 0.85 V vs. RHE, which are respectively 19 and 18.7 times higher than those of the commercial Pt/C catalyst (0.126 A mg_Pt⁻¹ and 0.44 A mg_Pt⁻¹ at 0.90 V and 0.85 V vs. RHE) as shown in Fig. 7h. Furthermore, the ultrasmall 2 nm-size PtFeCoNiCuZn HEIMs also exhibit much better stability than the commercial Pt/C catalyst. DFT calculations were performed to get further insight into the ultrahigh intrinsic ORR activity of PtFeCoNiCuZn HEIMs. Fig. 7i indicates that the electrons are generally transferred from non-noble metals to Pt caused by the ligand effect. In particular, Zn atoms are more conducive to bonding with oxygen due to the appearance of empty orbitals. The projected partial densities of states (PDOSs) in Fig. 7j reveal the detailed electronic structures of PtFeCoNiCuZn HEIMs, indicating strong bonds between different metals and Pt serve as electron-transfer medium (Pt 5d almost overlaps the electron orbitals of all the other elements). The d-band center of Pt in PtFeCoNiCuZn HEIMs downshifts by 0.44 eV compared to that of Pt (111) as shown in Fig. 7k. Additionally, the calculated Gibbs free energies of the intermediates of Pt28, Fe8, and Zn3 sites are compared with Pt (111). Pt (111) shows a ΔG value of 1.006 eV for its rate-limiting step (*O₂ → *OOH intermediate) at an equilibrium potential of 1.23 V vs. RHE equilibrium potential. As shown in Fig. 7l, the Pt28 site on the PtFeCoNiCuZn HEIMs surface has a ΔG value of 0.875 eV corresponding to the *O₂ hydrogenation step, which is lower than that of the Pt (111) surface. Notably, the ΔG values of the rate-limiting steps of Fe8 and Zn3 are only 0.959 (Fig. 7m) and 0.909 eV (Fig. 7n), respectively, indicating that these non-noble-metal sites have been modulated into highly active sites for the ORR.

3.3 Fuel oxidation reactions

Hydrogen is predominantly utilized as an anodic fuel attributing to its high power density and zero-carbon emission.²² However, hydrogen FCs confront substantial challenges in storage, transportation, refueling infrastructure, and safety protocols.^65,66 In contrast, liquid fuels such as methanal, ethanal, and formic acid offer superior ease in terms of storage, transportation, and refueling. However, the electrochemical kinetics of the methanol oxidation reaction (MOR), ethanol oxidation reaction (EOR), and anodic formic acid oxidation reaction (FAOR) are notably slower than those of the hydrogen oxidation reaction (HOR).⁶⁶ In particular, the EOR involves a complex process with multiple steps, including ethanol adsorption, C–C bond scission, dehydrogenation, electrooxidation of adsorbed intermediates (e.g., CO), and formation of soluble species (e.g., acetaldehyde and acetic acid), all of which substantially reduce the reaction rate and conversion efficiency. The MOR performances of the as-obtained 2D PtRhBiSnSb HEIMs (Fig. 5a–c) are shown in Fig. 8a–d.⁵⁴ The 2D PtRhBiSnSb HEIMs exhibit an unprecedented MA of 19.529 A mg_Pt+Rh⁻¹ for the MOR, which is 8.6 times higher than that of the Pt/C electrocatalyst as shown in Fig. 8a. DFT calculations were performed to compare the adsorption energies of CH₃OH, CO₂, and CO on 2D PtRhBiSnSb HEIMs and PtBiSnSb IMs to elucidate the origin of this enhancement as shown in Fig. 8b. The results reveal strengthened adsorption of both CH₃OH and CO₂ on the 2D PtRhBiSnSb HEIMs surface, promoting efficient methanol activation. Conversely, CO adsorption is significantly weakened on the 2D PtRhBiSnSb HEIMs surface, conferring exceptional resistance to CO poisoning during the MOR. Mechanistic insights into the preferential reaction pathway were obtained by examining the energy profiles shown in Fig. 8c (CO₂ pathway) and Fig. 8d (CO pathway). For the CO₂ pathway (Fig. 8c), the 2D PtRhBiSnSb HEIMs exhibit a pronounced kinetic advantage with a minimal energy barrier of 0.41 eV for the rate-determining step (CHOH* → CHO*). Furthermore, the reaction energy for CO₂ formation of the 2D PtRhBiSnSb HEIMs is notably more exothermic compared to PtBiSnSb IMs, indicating enhanced thermodynamic favorability. In contrast, the CO pathway on PtRhBiSnSb HEIMs presents a substantially higher energy barrier of 0.77 eV, suppressing the generation of toxic CO intermediates and directing the reaction toward complete oxidation. This dual effect (accelerated CO₂ production and inhibited CO formation) underscores the superior MOR performances of the 2D PtRhBiSnSb HEIMs.

The EOR performances of the as-obtained ordered PtRhFeNiCu HEIMs/MWCNT (Fig. 4e–g) are shown in Fig. 8e–h.⁵³ The PtRhFeNiCu HEIMs exhibit enhanced C–C bond cleavage and CO tolerance during the EOR process with MA and SA of 914 mA mg_Pt⁻¹ and 1.40 mA cm_Pt⁻² towards the EOR, respectively, which are much higher than those of PtRhFeNiCu HEAs (578 mA mg_Pt⁻¹ and 1.32 mA cm_Pt⁻²) and Pt/C (202 mA mg_Pt⁻¹ and 0.36 mA cm_Pt⁻²) as shown in Fig. 8e. The calculated local density of states (LDOS) in Fig. 8f indicates that the d-band center shifts from −2.13 eV in PtRhFeNiCu HEAs to −2.28 eV in PtRhFeNiCu HEIMs, which could weaken the CO intermediate adsorption and inhibit CO poisoning. This indicates that the PtRhFeNiCu HEIMs exhibit enhanced selectivity toward the CO₂ pathway relative to their PtRhFeNiCu HEAs counterparts. The reaction process of the EOR in Fig. 8g demonstrates that PtRhFeNiCu HEIMs display a higher catalytic activity with a smaller reaction free energy value of 0.30 eV for the potential limiting step compared with the 0.43 eV observed for PtRhFeNiCu HEAs for the electrochemical reaction. Significantly, C–C bond cleavage on PtRhFeNiCu HEIMs required a lower energy barrier of 0.29 eV, in stark contrast to the substantially higher barrier of 0.49 eV observed on PtRhFeNiCu HEAs. As a result, the PtRhFeNiCu HEIMs demonstrate enhanced selectivity toward the C–C bond cleavage compared with PtRhFeNiCu HEAs. Fig. 8h depicts different configurations of the transition states (TS) of C–C bond breaking. On PtRhFeNiCu HEIMs, CO adsorbs at the top site of an adjacent Pt atom, a position that coincides with the most stable adsorption site for Conversely, on PtRhFeNiCu HEAs, the strong adsorption affinity of neighboring Rh atoms drives CO to occupy the bridge site between Rh and Pt, deviating from the more stable Rh top site preferred in This structural disparity results in a relatively lower energy TS on PtRhFeNiCu HEIMs compared to PtRhFeNiCu HEAs. The underlying mechanism can be attributed to the ordered atomic arrangement in HEIMs, where each Rh atom is surrounded by Pt atoms, whereas HEAs allow for adjacent Rh–Rh pairs. Consequently, compared to PtRhFeNiCu HEAs, PtRhFeNiCu HEIMs exhibit enhanced selectivity toward the C–C bond cleavage in the CO₂ pathway, along with superior EOR performance in both electrochemical and chemical reaction steps, which are attributed to the ordered atomic structures, optimal electronic configurations and reduced activation energy barrier.

The FAOR performances of the as-obtained 2D PtBiPbNiCo HEIMs (Fig. 5d–h) are shown in Fig. 8i–l.⁵⁵ Fig. 8i presents the MAs and SAs of 2D PtBiPbNiCo HEIMs/C, 2D PtPb IMs/C, and Pt/C for the FAOR. The 2D PtBiPbNiCo HEIMs exhibit the highest MA (7.1 A mg_Pt⁻¹) and SA (27.2 mA cm⁻²), achieving a 3-fold and 56-fold enhancement in MAs, compared to PtPb IMs/C (2.6 A mg_Pt⁻¹, 9.0 mA cm⁻²) and commercial Pt/C (0.127 A mg_Pt⁻¹, 0.205 mA cm⁻²). DFT calculations were performed to investigate the thermochemical energetics of the FAOR via dehydrogenation (HCOOH → CO₂ + H₂ + 2e⁻) and dehydration (HCOOH → CO + H₂O) pathways on 2D PtBiPbNiCo HEIMs/C, PtPb IMs/C, and Pt/C substrates. The direct dehydrogenation pathway is kinetically preferable to the dehydration route, as the latter generates toxic CO intermediates that strongly adsorb onto catalyst active sites, thereby inhibiting electrocatalytic performance. Fig. 8j illustrates the corresponding intermediates (HCOO*, COOH* and CO*) and free energy profiles. The Gibbs free energy diagram of Pt/C reveals that the dehydration pathway is energetically favorable due to the stronger surface binding of CO* than CO₂, which leads to suppressed FAOR activity. Furthermore, the first dehydrogenation step (HCOOH to HCOO* or COOH*) is the rate-determining step for PtPb IMs/C, whereas no rate-determining step exists on PtBiPbNiCo HEIMs/C due to both dehydrogenation steps (HCOOH to HCOO* and COOH*) being exothermic reactions. Specifically, the two dehydrogenation steps on PtBiPbNiCo HEIMs/C are downhill (0.05 and 0.12 eV), indicating kinetically favorable reaction pathways and superior FAOR performance. Consequently, the exceptional FAOR activity of PtBiPbNiCo HEIMs arises from the suppression of the CO-producing dehydration pathway and the thermodynamic favorability of consecutive dehydrogenation steps.

4. Acetylene semihydrogenation and propane dehydrogenation

Acetylene semihydrogenation stands as a pivotal process in the production of polymer-grade ethylene from crude ethylene streams, playing a critical role in the petrochemical industry's value chain.^67,68 Palladium (Pd)-based catalysts are commonly used as selective semihydrogenation catalysts.^69,70 However, the difficulty in preventing the overhydrogenation of ethylene at near-complete acetylene conversions, limited content and high cost of Pd hinder the development. The NiFeCuGaGe HEIMs/SiO₂ catalysts as shown in Fig. 2a and b exhibit exceptionally superior activity, selectivity, and long-term stability in the acetylene semihydrogenation reaction.⁴¹ Fig. 9a presents the temperature dependence of C₂H₂ conversion and C₂H₄ selectivity of NiGa/SiO₂ (black triangle) and NiFeCuGaGe HEIMs/SiO₂ (red ball) in the absence (left figure) and presence (right figure) of excess ethylene. NiGa/SiO₂ exhibited a sharp decrease in ethylene selectivity (an approximately 60% drop in the left figure and even negative selectivity was obtained in the right figure) after reaching 100% conversion (170 °C). In contrast, the NiFeCuGaGe HEIMs/SiO₂ showed no loss of selectivity, even after reaching 100% conversion at higher temperatures in the absence and presence of excess ethylene. The ethylene purification performance (right figure) demonstrated that semihydrogenated acetylene and co-fed ethylene are hydrogenated to ethane for NiGa/SiO₂. On the other hand, the NiFeCuGaGe HEIMs/SiO₂ exhibited nearly 100% C₂H₂ conversion and C₂H₄ selectivity with negligible activity toward ethylene hydrogenation, demonstrating it as a good catalyst for ethylene purification. DFT calculation suggests the multi-metallization induced geometric effects result in the significantly enhanced catalytic performance of NiFeCuGaGe HEIMs. The multi-metallization induced surface relaxation significantly reduced the surface energy and adsorption energy of ethylene on Ni sites of NiFeCuGaGe HEIMs. Moreover, the NiFeCuGaGe HEIMs exhibit a lower activation energy barrier for acetylene semihydrogenation compared to NiGa. Further investigations of the ethylene desorption and overhydrogenation in Fig. 9b reveal that the NiFeCuGaGe HEIMs allow the fast desorption of ethylene but do not facilitate overhydrogenation, thereby allowing the highly selective semihydrogenation of acetylene without overhydrogenation.

Global demand for propylene, a pivotal fundamental raw material in the petrochemical industry, has faced growing scarcity following the shale gas revolution.⁷¹ Consequently, the development of highly efficient technologies to enhance propylene yield from shale gas has become a pressing priority.⁷² Propane dehydrogenation (PDH) has emerged as the most promising technology for propylene synthesis in recent years.⁷³ However, the endothermic nature of PDH necessitates elevated operating temperatures (>600 °C) to achieve sufficient propylene yields, a condition that inevitably induces severe catalyst deactivation because of sintering.⁷⁴ The (PtCoCu)(GeGaSn) HEIMs/Ca–SiO₂ catalysts as shown in Fig. 2c and d exhibit exceptionally outstanding catalytic stability for PDH even at 600 °C.⁴² The PDH catalytic performances of (PtCoCu)(GeGaSn) HEIMs/Ca–SiO₂, PtFeCoCuGa HEAs/Ca–SiO₂, PtGe/Ca–SiO₂, and Cu–Pt single-atom alloy (SAA)/Ca–SiO₂ catalysts at 600 °C without co-feeding H₂ are shown in Fig. 9c. The Cu–Pt SAAs/Ca–SiO₂ catalyst and PtGe/Ca–SiO₂ catalysts were rapidly deactivated within 20 h due to the aggregation of nanoparticles and coke accumulation. The PtFeCoCuGa HEAs/Ca–SiO₂ exhibited gradual deactivation primarily owing to coking during the reaction. Conversely, (PtCoCu)(GeGaSn) HEIMs/Ca–SiO₂ was not deactivated within 100 h. In fact, (PtCoCu)(GeGaSn) HEIMs/Ca–SiO₂ could retain >30% conversion and 99% propylene selectivity up to 260 h at 600 °C without sintering. Interestingly, (PtCoCu)(GeGaSn) HEIMs/Ca–SiO₂ showed a short induction period in PDH at the initial stage of the reaction (<10 h). However, the XRD results confirmed that there is no change in the bulk structure of (PtCoCu)(GeGaSn) HEIMs. A plausible explanation is that Pt, Co, and Cu atoms gradually diffuse within the subsurface region at 600 °C. This diffusion process enables the continuous optimization of the surface Pt/Co/Cu atomic ratio through interaction with hydrocarbons. Therefore, although the bulk structure of HEIMs is well-defined, their surface structures are highly dynamic and responsive to the reaction environment, resulting in reconstructions that are distinct from the bulk. However, this requires more evidence to confirm. The propylene selectivity and catalyst stability are fundamentally governed by the difference in activation energy barriers between the third C–H scission and propylene desorption, quantified as ΔE = E_A3 − E_d = E_A3 + E_ad (where E_A1, E_A2, and E_A3 denote the activation energy for the first, second, and third C–H bond cleavage, E_d represents the propylene desorption energy, and E_ad is the adsorption energy of propylene). The calculated ΔE values shown in Fig. 9d were in the following order: Pt (−0.11 eV) ≪ Pt₁@Cu (0.55 eV) < Pt₃Sn (0.60 eV) < Pt₁@PtGa (0.74 eV) < PtGe (020) (1.05 eV) < (PtCoCu)(GeGaSn) HEIMs (040) (1.04–1.54 eV), demonstrating the exceptional selectivity and stability of (PtCoCu)(GeGaSn) HEIMs. Further investigations demonstrated that the enhanced PDH selectivity and stability of (PtCoCu)(GeGaSn) HEIMs are in fact determined by the isolated Pt sites of (PtCoCu)(GeGaSn) HEIMs. In common HEAs, there remain a number of Pt–Pt sites due to the random distribution of constituent atoms, which are known to induce side reactions such as hydrogenolysis, cracking, and coking.^75–77 Conversely, the site-specific multi-metallization allows the isolation of Pt by Co and Cu (PtCoCu)(GeGaSn) in HEIMs. Moreover, inert metals (Ga and Sn) further improve the thermal stability. As a result, the single-atom-like Pt sites (isolated Pt atoms surrounded by other inert constituent metals) act as a highly selective and stable catalyst for PDH even at elevated reaction temperatures.

The harsh reaction condition of high temperature for PDH introduces side reactions (such as overdehydrogenation and C–C cracking) and inevitable sintering of catalysts.⁷⁸ The oxidative dehydrogenation of propane with CO₂ as an oxidant (CO₂-ODP) represents a highly attractive strategy for high-propylene-yield production coupled with simultaneous CO₂ valorization. The (PtCoNi)(SnInGa) HEIMs/CeO₂ as shown in Fig. 2e and f exhibited exceptional CO₂-ODP catalytic activity, C₃H₆ selectivity, coke resistance, thermal stability, and CO₂ utilization stability.⁴³ As shown in Fig. 9e and f, the (PtCoNi)(SnInGa) HEIMs/CeO₂ exhibit the highest C₃H₈ conversion and C₃H₆ selectivity compared with Pt/CeO₂, Pt–Co–In/CeO₂, and PtSn/CeO₂ catalysts even after 50 h of CO₂-ODP testing at 600 °C. Fig. 9g shows the DFT calculated activation energies of propane dehydrogenation and CO₂ activation of (PtCoNi)(SnInGa) HEIMs, Pt–Co–In, and PtSn surfaces, which are consistent with the experimental data as shown in Fig. 9e and f. Fig. 9h shows the density of states projected on the d orbitals (d-DOS) of the surface transition metals on (PtCoNi)(SnInGa) HEIMs and PtSn. PtSn exhibits a low d-DOS near the Fermi level due to alloying with Sn, which may potentially contribute to elevated activation energies of propane dehydrogenation and CO₂ activation. In contrast, the (PtCoNi)(SnInGa) HEIMs exhibit an intense upshifted peak near the Fermi level compared with PtSn due to the doping of Ni and Co, resulting in enhanced activity in propane dehydrogenation and CO₂ reduction.

5. Summary and perspectives

In this review, we summarized the synthesis of HEIMs by alloying and dealloying, co-impregnation and annealing, disorder-to-order transition, and chemical co-reduction methods. The merits of ordered crystalline architectures and high-entropy characteristics afford broadly tunable compositional landscapes and site-specific active site engineering of HEIMs. As a result, the HEIMs exhibit enhanced catalytic activity and robust durability towards the HER, OER, ORR, MOR, EOR, FAOR, acetylene semihydrogenation and propane dehydrogenation. Therefore, HEIMs are potential candidates for catalytic applications in energy conversion and storage technologies. Nonetheless, numerous challenges persist in the context of synthesizing well-defined and well-supported HEIMs with more rational design strategies, broader applications, cost reduction, and process scalability.

5.1 Theory-guided synthesis of HEIMs

Although various HEIMs have been reported, there is a lack of systematic theoretical guidance. Ren et al. developed a random forest machine learning model (AS-RF) with high accuracy (94.6%) to successfully realize the prediction of single-phase solid solutions in quinary HEAs.⁷⁹ The AS-RF conducted a systematic prediction of 224 quinary HEA phase diagrams (210 previously unreported) in the chemical space containing Cr, Co, Fe, Ni, Mn, Al, and Cu elements. Cao et al. proposed a computationally efficient and physically interpretable descriptor to quantitatively assess the local reactivity of noble-metal HEA surfaces toward the ORR.⁸⁰ This descriptor integrates the intrinsic d-band filling of the active site and the neighbourhood electronegativity and the predicted results are highly consistent with the experimental reactivity trends of noble-metal HEAs. However, such theory-guided results toward HEIMs are rarely reported. Therefore, there is an urgent requirement to develop theory-guided synthesis of HEIMs.

5.2 Controllable synthesis of HEIMs

As reported by Feng et al., the ORR activities follow the order of PtIrFeCoCu (001) > PtIrFeCoCu (111) > PtFe (111) > PtFe (001) > Pt (111) > Pt (001).⁴⁹ Therefore, the exposed crystal facet significantly affects the catalytic performance. However, the high temperature annealing treatment usually results in particles with irregular morphologies (normally spherical-like shape) and inhomogeneous size. Wet chemistry synthesis including one-pot synthesis and seed-mediated synthesis in the liquid phase proved to be an effective strategy to synthesize intermetallic nanoparticles with specific morphologies.^30,31 However, HEIMs with specific morphologies are rarely reported.^54,55 Therefore, there exists an imperative need to explore novel synthetic strategies for the precise regulation of both the structural architecture and morphological features of HEIMs.

5.3 Synthesis of HEIMs with suitable supports

The catalyst supports exert a fundamental impact on the catalytic performance, governing multiple aspects critical for selectivity, activity, and stability.^81–84 In particular, the encapsulation layers over metal nanocatalysts have garnered significant research interest due to its diverse and intriguing physicochemical phenomena. For instance, Zhang et al. reported that the TiO₂ encapsulation overlayer on Ir nanoparticles significantly increased the activation energy for CO₂ hydrogenation toward CH₄ while decreasing it for CO, thereby enabling the adjustment of CO₂ hydrogenation selectivity from CH₄ to CO in Ir/TiO₂ catalysts.⁸⁵ Xu et al. reported that the TiO₂ encapsulation overlayer on Ni nanoparticles increases the electron density of interfacial Ni active sites, leading to a remarkable improvement in catalytic performance toward the water–gas shift reaction compared with the conventional 15% Ni/TiO₂ catalyst.⁸⁶ Li et al. revealed that both the stability and activity towards the methanol steam reforming reaction are improved by constructing the ZnO encapsulation overlayer on Cu nanoparticles.⁸⁷ However, most HEIMs were loaded on carbon-based supports.^{43–49,51–55} Therefore, it is necessary to develop the synthesis of HEIMs with suitable supports such as TiO₂,^85,86 ZnO,⁸⁷ zeolites,⁸⁸ metal–organic frameworks,⁸⁹ MXenes,⁹⁰ and so on.

5.4 Broader applications of HEIMs

Despite the water splitting, ORR and fuel oxidation reactions, the HEIMs should be further explored for their applications in other electrocatalytic domains due to their remarkable physicochemical properties. For example, the carbon dioxide reduction reaction, a complex multiproton-coupled multielectron transfer process, warrants further investigation with HEIM electrocatalysts.⁹¹ The nitrogen reduction reaction is challenging in terms of kinetics and thermodynamics compared with the HER owing to its six electron transfer process.⁹² Beyond electrocatalysis, HEIMs hold promise for expanding into emerging frontiers such as photothermal catalysis, photoelectrocatalysis, and energy storage applications.

5.5 More in-depth mechanistic study of HEIMs

Owing to the harsh electrochemical reaction conditions (e.g., strong acids, strong bases, elevated temperatures, and high pressures), most of the catalysts undergo surface reconstruction during the electrocatalytic process, which means the genuine active sites are the in situ reconstructed species rather than the as-prepared one.^93–96 For example, during the electrocatalytic CO₂ reduction reaction, the defect-rich ultrathin Pd nanosheets with a dominant (111) facet were transformed into crumpled sheet-like structures prevalent in (100) sites.⁹⁷ V leaching induces the surface reconstruction of inactive CoFeV spinel oxide to highly active Co_xFe_1−xOOH under OER conditions.⁹⁸ However, for HEIMs, current research that identifies the authentic active sites through surface reconstruction remains scarce. The surface of (PtCoCu)(GeGaSn) HEIMs maybe reconstructed through interaction with hydrocarbons during PDH at 600 °C.⁴² However, there is no incontrovertible evidence. Therefore, operando and in situ characterization techniques (SEM, TEM, XRD, Raman, XPS, FTIR, and differential electrochemical mass spectrometry) are urgently needed to elucidate the surface reconstruction and genuine active sites of HEIMs during catalytic reactions. In addition, advanced in situ techniques enable effective monitoring of the growth processes of HEIMs, thereby facilitating a deeper understanding of their uncharted growth mechanisms.

5.6 Cost reduction of HEIMs

The cost of catalysts constitutes a pivotal factor in determining their practical applicability for industrial processes. Therefore, for the widespread implementation of HEIMs in industrial settings, the critical challenge lies in achieving substantial substitution of noble metals (e.g., Pt, Ir, Ru) with non-noble metal (e.g., Fe, Co, Ni) components while maintaining comparable electrocatalytic performance. In this context, an effective strategy to mitigate costs involves the rational design of HEIMs featuring low noble metal loadings.

5.7 Large-scale practical production of HEIMs

To date, the commercial-scale synthesis of HEIMs remains a substantial challenge that must be addressed to enable their future industrial applications. The majority of currently prepared HEIMs have been evaluated under controlled laboratory-scale conditions, where precise control over synthesis parameters (e.g., temperature, pressure, reactant stoichiometry) allows for the fabrication of materials with tailored nanostructures and properties, yet maintaining catalytic activity and stability during large-scale practical production poses significant hurdles.

In this comprehensive review, we systematically elaborate on the synthetic strategies of HEIMs, encompassing alloying and dealloying, co-impregnation and annealing, disorder-to-order transition, and chemical co-reduction methods. After highlighting their applications in water splitting, ORR, fuel oxidation reactions, acetylene semihydrogenation and propane dehydrogenation, we address the current bottlenecks associated with the synthesis of HEIMs and conclude the article with perspectives. We anticipate that this review will serve as a valuable guideline for fostering future research and unlocking novel applications of HEIMs in diverse catalytic domains.

Data availability

Data availability is not applicable to this article as no new data were created or analyzed in this study.

Author contributions

Jingchun Guo: conceptualization, writing the original draft, review and editing. Huiling Zheng, Xucheng Fu, and Mingming Fan: review and editing. All authors contributed to the discussion and commented on the article.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by the Anhui Provincial Natural Science Foundation (2308085MB51), the Excellent Research and Innovation Team Project of Anhui Province (2023AH010077), and the Fundamental Research Program of Shanxi Province (Grant No. 202203021211135).

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