High-entropy alloys, methods of synthesis thereof, and electrocatalytic applications thereof

Abstract

Disclosed is a high-entropy alloy catalyst, a composition of which includes at least five metals of Au, Pd, Fe, Co, Ni, Cu, Mn, Cr, Ag, Pt, or Mo, wherein the high-entropy alloy catalyst is effective to catalyze an electrochemical reaction.

Description

103440-054PCT (UML 2024-020) HIGH-ENTROPY ALLOYS, METHODS OF SYNTHESIS THEREOF, AND ELECTROCATALYTIC APPLICATIONS THEREOF CROSS-REFERENCE TO RELATED APPLICATION This application claims priority to U.S. Provisional Application No.63 / 655,225, filed on June 03, 2024, in the U.S. Patent and Trademark Office, and all the benefits accruing therefrom under 35 U.S.C. §119, the content of which is incorporated by reference herein in its entirety. STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH & DEVELOPMENT

[0001] This invention was made with government support under #N00014-22-1-2654 awarded by the Office of Naval Research, and #N00014-25-1-2038 awarded by the Office of Naval Research. The government has certain rights in the invention. BACKGROUND

[0002] Nanoscale electrocatalysts are critical materials for efficient electrochemical water splitting that can provide carbon-free generation of fuels and energy. Presently, platinum-group metals (PGMs) exhibit the highest efficiency in water splitting. Nonetheless, their commercialization and growth are impeded by cost and durability concerns. Therefore, to establish cost-effective electrolysis systems, there is a need to develop highly active electrocatalysts with reduced noble metal loading, and leverage Earth's abundant resources. BRIEF SUMMARY

[0003] Disclosed herein are high-entropy alloy catalysts, methods of synthesis thereof, and electrocatalytic applications thereof.

[0004] Disclosed is a high-entropy alloy catalyst, a composition of which includes at least five metals of Au, Pd, Fe, Co, Ni, Cu, Mn, Cr, Ag, Pt, or Mo, wherein the high-entropy alloy catalyst is effective to catalyze an electrochemical reaction.

[0005] Disclosed too is a method of synthesizing a high-entropy alloy catalyst via wet-chemical synthesis, the method including: providing at least five metal-chloride precursors in a stoichiometric ratio, wherein each of the at least five metal-halide precursors independently include a transition metal; contacting the at least five metal-chloride precursors with a polymer additive and a polar solvent to provide a reaction mixture; heating the reaction mixture at a temperature greater than a combined melting point of the at least five metal-chloride precursors to form resulting particles; washing the resulting particles with the polar solvent to remove remaining precursors or the polymer additive; and separating the resulting particles by centrifugation to synthesize the high-entropy alloy catalyst.103440-054PCT (UML 2024-020) BRIEF DESCRIPTION OF THE DRAWINGS

[0006] FIG.1A is a representative high resolution-TEM (HR-TEM) image of AuPdFeNiCo.

[0007] FIG.1B1 is a zoomed view of the AuPdFeNiCo HR-TEM image of FIG.1A.

[0008] FIGS.1B2 and 1B3 show the fast Fourier transformation (FFT) results of the square region of AuPdFeNiCo of FIG.1B2.

[0009] FIG.1C is a STEM image of the corresponding energy dispersive spectroscopy (EDS) elemental mapping of (FIG.1D) Au, (FIG.1E) Pd, (FIG.1F) Fe, (FIG.1G) Ni, and (FIG.1H) Co.

[0010] FIG.2A is X-ray diffraction (XRD) patterns of Au NPs, Pd NPs and AuPdFeNiCo high-entropy alloys (HEAs) with JCPDS data (Au: 04-0748, Pd: 46-1043).

[0011] FIG.2B is XRD patterns of (111) fcc peak of Au NPs, Pd NPs and AuPdFeNiCo high-entropy alloys (HEAs).

[0012] FIG.3A is an XPS survey spectrum of the synthesized AuPdFeNiCo.

[0013] FIGS.3B to 3F show high-resolution XPS spectra of (3B) Au 4f, (3C) Pd 3d, (3D) Fe 2p, (3E) Co 2p, and (3F) Ni 2p.

[0014] FIGS.4A to 4F show characteristics of Au, Pd, Fe, Ni, Co, AuPdFeNiCo, and Pt / C: HER polarization curve of Au, Pd, Fe, Ni, Co, AuPdFeNiCo, and Pt / C with (4A) 0.5M H2SO4, (4C) 1M PBS, (4E) 1M KOH, corresponding radar chart comparison of the potential characteristics (4B) 0.5M H2SO4, (4D) 1M PBS, and (4F) 1M KOH.

[0015] FIG.5 is a schematic drawing illustrating the fabrication of AuPdFeNiCo based HEAs.

[0016] FIG.6 is an XRD data for CuNiCoMnCr.

[0017] FIG.7 is a STEM-EDS data for MoNiCoMnCr.

[0018] FIG.8A is a representative HR-TEM image of AuPdFeNiCo HEA NPs; (FIG.8B is a particle size distribution histogram of AuPdFeNiCo HEA NPs, FIGS.8C to 8E are each HR-TEM characterization of AuPdFeNiCo HEAs with different magnification.

[0019] FIGS.9A1 to 9C are HR-TEM characterization of AuPdFeNiCo HEA NPs (9A1-9A3) FFT images of AuPdFeNiCo HEAs corresponding high resolution-TEM (HR-TEM) image of the area enclosed by the insert box. (upper right: Fe3O4, lower right: AuPdFeNiCo HEAs) (9B) The intensity line profile along the lattice in the red box. (9C) The intensity line profile along the lattice in the blue box.

[0020] FIGS.10A to 10H are STEM-EDS characterization of AuPdFeNiCo HEA NPs (10A) Representative scanning transmission electron microscopy (STEM) image and (10B) zoomed view of STEM image. (10C) crossline profile for all five elements of the corresponding STEM image and energy dispersive spectroscopy (EDS) elemental mapping of (10D) Au, (10E) Pd, (10F) Fe, (10G) Ni, and (10H) Co.

[0021] FIG.11 represents synchrotron wide-angle X-ray scattering (WAXS) of AuPdFeNiCo HEA NPs.

[0022] FIGS.12A to 12C are Summary of HER activity of electrocatalysts in different electrolytes.103440-054PCT (UML 2024-020)

[0023] FIGS.13A to 13C are corresponding Tafel slope of Au, Pd, Fe, Ni, Co, AuPdFeNiCo, and Pt / C (13A) 0.5M H2SO4, (13B) 1M PBS, (13C) 1M KOH.

[0024] FIGS. 14A to 14F are CV curves at different scan rates from 10 to 200 mV s ¹ in thecorresponding electrolytes for AuPdFeNiCo, and fitted Cdlbased on CV, (14A), (14D) 0.5M H2SO4, (14B), (14E) 1M PBS, and (14C), (14F) 1M KOH.

[0025] FIG.15 is an equivalent circuit model for the EIS plots of AuPdFeNiCo HEAs.

[0026] FIGS.16A to 16C are the results of HER-stability test of AuPdFeNiCo HEAs with chronopotentiometry curve, (16A) 0.5M H2SO4, (16B) 1M PBS, and (16C) 1M KOH.

[0027] FIG.17 shows X-ray diffraction (XRD) characterization of AuPdFeNiCo HEA NPs before and after 12 hr of continuous HER operation in different pH-electrolytes.

[0028] FIG.18A is a schematic diagram illustrating the transformation in crystal structure from bcc to fcc with increasing Au contents; and FIGS.18B and 18C are synchrotron-based wide-angle X-ray scattering (WAXS) spectra of AuPdFeCoNi HEAs. The fitting for HEAs with 9-24 at.% Au (three lines in the middle) and 31 at.% Au (top line) is shown. FIG.18C is an enlarged view of the WAXS profiles around the (111) and (110) reflections of FIG.18B.

[0029] FIG.19 is an image showing bcc-to-fcc phase control in AuPdFeCoNi HEAs: HR-TEM, lattice- and atomic- resolved HR-TEM, fast-Fourier-transform (FFT) images of AuPdFeCoNi HEAs during bcc- to-fcc transformation are shown, the first three columns (labeled 9 at. %, 15 at. %, 24 at. %) indicate bcc- rich regions, and the fourth column (labeled 31 at. %) indicate fcc phase regions (scale bar 5 nm).

[0030] FIGS.20A1 to 20E are local structure characterization and simulation of AuPdFeCoNi HEAs (bottom three lines for 9-24 at.% Au, and top line for 31 at.% Au): FIGS.20A1 to 20A3 shows comparison of the observed PDF data for HEAs with different Au contents, with enlarged views of thePDF profiles around the peaks at r ~5.0 (FIG. 2A2) and ~9.0 (FIG. 2A3), highlighting certainnearest-neighbor pairs and corresponding theoretical partial pair correlation functions. The experimental PDF and simulated PDF data (FIG.20D) 24 at.% Au, Rwp

[0031] FIGS.21A to 21F show evaluation of electrochemical HER activity in controlled phase HEAs. (FIG.21A) HER polarization curves of the as-synthesized catalysts. (FIG.21B) corresponding Tafel slope. (FIG.21C) comparison of the reduction-oxidation values of AuPdFeCoNi HEAs in the potential range of -0.1V to 1.5V. (FIG.21D) Long-term stability of HEAs (15 at% Au) and Pt / C (10%) for 240hr at 10mA / cm2(FIG.21E) mass activity polarization of the as-synthesized catalysts and (FIG.21F) mass activity at 150 mV.

[0032] FIG. 22 shows an XRD pattern of AuPdFeCoNi HEAs of 2 =20-80 .

[0033] FIG.23 shows an DF-STEM image of the HEAs with different Au content and the corresponding EDS map showing each element : Au (2ndcolumn), Pd (3rdcolumn), Fe (4thcolumn), Co (5thcolumn), and Ni (6thcolumn).103440-054PCT (UML 2024-020)

[0034] FIG.24 is a pair distribution function (PDF) data for the support baseline using Kapton tube.

[0035] FIGS.25A to 25C are local structure characterization: (FIG.25A) Comparison of the observed PDFgui simulation model data at a scattering range of 2.8 to 20 . (FIG.25B) Enlarged views of the PDF profiles around the peaks at r 5.0 , highlighting certain nearest-neighbor pairs. (FIG.25C) Enlarged views of the PDF profiles around the peaks at r 9.0 , highlighting certain nearest-neighbor pairs.

[0036] FIGS.26A to 26C are local structure characterization of Au-based bimetallic-ternary-HEAs systems. (FIG.26A) Comparison of the observed PDF experiment data at a scattering range of 2.8 to 20 . (FIG.26B) Enlarged views of the PDF profiles around the peaks at r 5.0 , highlighting certain nearest-neighbor pairs. (FIG.26C) Enlarged views of the PDF profiles around the peaks at r 8.6 , highlighting certain nearest-neighbor pairs.

[0037] FIG.27 shows an ICP-OES data for Au content (at.%) (left y-axis; circle) and the Rwp values from PDF analysis (right y-axis; square).

[0038] FIGS.28A1 to 28D5 shows high-resolution XPS spectra of AuPdFeCoNi HEAs. (FIGS.28A1 to A5) Au 31 at.%, (FIGS.28B1 to B5) Au 24 at.%, (FIGS.28C1 to C5) Au 15 at.%, and (FIGS.28DA1 to D5) Au 9 at.%.

[0039] FIG.29 shows HER polarization curves of the as-synthesized HEAs with 15 at.% Au after stability testing: Initial (solid line), 24hr (sphere), 140hr (pentagon), and 240hr (triangle).

[0040] FIG.30 shows long-term stability of AuPdFeCoNi HEAs for 24hr at 10mA / cm2.

[0041] FIG.31 shows XRD patterns of as-synthesized HEAs with 15 at.% Au after HER stability test.

[0042] FIGS.32A to 32C show electrochemical HER activity (left y-axis; square) and the ratio of Au 4f, Pd 3d, and Fe 2p oxidation state (right y-axis; circle) versus Au contents.

[0043] FIGS.33A to 33B show electrochemical HER activity (left y-axis; square) and the ratio of Ni 2p and Co 2p oxidation state (right y-axis; circle) versus Au contents.

[0044] FIGS.34A to 34C show HER-stability test of AuPdFeNiCo HEAs (at%: Au 22.1, Pd 35.4, Fe 15.5, Ni 9.9, Co 17.1) with chronopotentiometry curve, (34A) 0.5M H2SO4, (34B) 1M PBS, and (34C) 1M KOH.

[0045] FIGS.35A to 35B show HER polarization curves depending on different metal compositions with (35A) 0.5M H2SO4, and (35B) 1M KOH.

[0046] FIG.36 show HER polarization curves depending on different metal compositions in 1M KOH.

[0047] FIG.37 shows electrochemical durability under pH-switching conditions, demonstrating a scalable hydrogen evolution electrode based on a high-performance catalyst. DETAILED DESCRIPTION

[0048] A detailed description of one or more embodiments of the disclosed composition and method are presented herein by way of exemplification and not limitation with reference to the Figures.103440-054PCT (UML 2024-020)

[0049] The creation of electrocatalysts with reduced concentrations of platinum-group metals (PGMs, e.g., platinum (Pt), iridium (Ir), palladium (Pd), and ruthenium (Ru)) remains a critical challenge for electrochemical reactions, such as electrochemical hydrogen production. High-entropy alloys (HEAs) are compositionally complex solid solutions that contain five or more elements, and provide an attractive approach to controlling electrocatalyst composition. In particular HEAs offer a distinct type of catalyst with tunable compositions and engineered surface activity, significantly enhancing the hydrogen evolution reaction (HER). Specifically, the compositional complexity of HEAs, resulting from the random distribution of atoms with different radii and its irregular surface which closely mimics an almost continuous distribution, differentiates it from binary / ternary alloys. As such, HEAs provide a wide diversity of electronic and physical structures for electrochemical reactions. Furthermore, adding more components to HEAs, compared to binary alloy catalysts, significantly reduces the use of noble metals by dilution. In principle, these unique attributes of HEAs have an opportunity to enhance the efficacy of electrochemical hydrogen evolution reaction (HER) over a broad pH range due to their durability and versatile morphologies.

[0050] In the realm of water splitting, optimizing the interplay between electrocatalysts and electrolytes remains pivotal to enhancing overall catalytic activity. While significant breakthroughs have propelled innovations in acidic and alkaline electrolytes, understanding of the water-splitting in neutral to near- neutral electrolytes remains comparatively nascent. It also remains challenging to identify electrocatalysts that will operate well over a range of pHs.

[0051] The use of Au in hydrogen electrocatalysis research is limited in part due to its low intrinsic hydrogen evolution activity. However, Au generally has high quality electron transport and is oxidatively and structurally stable, making it an attractive component for pH-versatile electrocatalysts. Several studies have reported the use of Au as an electrocatalyst in universal-pH electrolytes, encompassing not only well-known acidic and alkaline conditions but also neutral electrolytes. These distinct attributes have driven scientific exploration in this field, resulting in reported research on various compositions involving Au-Pd, Au-Ni, Au-Cu and others. However, the extent of modification in the electronic properties is confined by the range of compositional tunability, limiting the performance.

[0052] Accordingly, disclosed herein are HEA catalysts with precisely controlled compositions as well as crystalline and local structures. A facile wet-chemical method is used to synthesize the HEAs, enabling fine tuning of the composition and structure, which provides unexpectedly improved electrocatalytic activity and stability.

[0053] Disclosed herein is a high-entropy alloy catalyst, a composition of which includes at least five metals of Au, Pd, Fe, Co, Ni, Cu, Mn, Cr, Ag, Pt, or Mo. The high-entropy alloy catalyst is effective to catalyze an electrochemical reaction. The electrochemical reaction may comprise at least one of hydrogen evolution reaction, oxygen reduction reaction, carbon dioxide reduction reaction, or nitrogen reduction reaction.103440-054PCT (UML 2024-020)

[0054] In an aspect, the composition comprises: Au, Pd, Fe, Co, and Ni (herein referred to as “AuPdFeNiCo”); Cu, Ni, Co, Mn, and Cr (herein referred to as “CuNiCoMnCr”); Mo, Ni, Co, Mn, and Cr (herein referred to as “MoNiCoMnCr”); or Au, Ag, Pt, Pd, and Co (herein referred to as “AuAgPtPdCo”).

[0055] In an aspect, when the composition comprises Au, Pd, Fe, Co, and Ni, a content of the Au is 5 to 31 atomic percent, 9 to 28 atomic percent, or 15 to 25 atomic percent, a content of the Pd is 10 to 40 atomic percent, 13 to 38 atomic percent, or 16 to 35 atomic percent, a content of the Fe is 13 to 52 atomic percent, 20 to 45 atomic percent, or 30 to 40 atomic percent, a content of the Co is 7 to 18 atomic percent, 8 to 15 atomic percent, or 9 to 14 atomic percent, a content of the Ni is 5 to 14 atomic percent, 6 to 12 atomic percent, or 7 to 10 atomic percent, wherein each content is based on a total atomic content of the metals in the high-entropy alloy catalyst. In an aspect, the metals may be expanded to other transition metals. The present disclosure is not limited to the above content ranges and compositions; however, according to high-entropy alloy theory, the content of each metal preferably falls within the range of 5 to 35 atomic percent. For example, for the CuNiCoMnCr catalyst, a content of the Cu may be 5 to 35 atomic percent, 9 to 28 atomic percent, or 15 to 25 atomic percent, a content of the Ni may be 5 to 35 atomic percent, 9 to 28 atomic percent, or 15 to 25 atomic percent, a content of the Co may be 5 to 35 atomic percent, 9 to 28 atomic percent, or 15 to 25 atomic percent, a content of the Mn may be 5 to 35 atomic percent, 9 to 28 atomic percent, or 15 to 25 atomic percent, a content of the Cr may be 5 to 35 atomic percent, 9 to 28 atomic percent, or 15 to 25 atomic percent, wherein each content is based on a total atomic content of the metals in the high-entropy alloy catalyst. For the MoNiCoMnCr catalyst, or for the AuAgPtPdCo catalyst, a content of each metal may be similar to the aforementioned CuNiCoMnCr catalyst.

[0056] When the composition comprises Au, Pd, Fe, Co, and Ni, the high-entropy alloy catalyst may have diffraction peaks at diffraction angles of 38.3±0.5°, 44.5±0.5°, and 64.9±0.5°, when analyzed by X-ray diffraction using CuK radiation. Optionally, the high-entropy alloy catalyst adopts a bcc phase, andmay have a diffraction peak at a diffraction angle of 40.1±0.5°. A diffraction peak may have a full width at half maximum (FWHM) of 1° or less.

[0057] When the composition comprises Au, Pd, Fe, Co, and Ni, the high-entropy alloy catalyst may have a crystal structure comprising a body-centered cubic (bcc), a face-centered cubic (fcc), or a mixed bcc / fcc phase. Preferably, the high-entropy alloy catalyst may have the mixed bcc / fcc phase, which may contribute to the improved catalytic activity, and / or stability, which will be discussed further.

[0058] The high-entropy alloy catalyst may be effective to catalyze the electrochemical reaction in an electrolyte that is acidic, neutral, or basic (alkaline), having a pH of pH 0 to 14, pH 2 to 12, or pH 4 to 10. The electrolyte may be any suitable electrolyte available in the field: any suitable acidic electrolyte (e.g., H2SO4, HCl, HNO3, H3PO4, or a combination thereof) any suitable neutral electrolyte (e.g., phosphate-buffered saline (PBS), Na SO , KNO , or a combination thereof), or any suitable alkaline electrolyte103440-054PCT (UML 2024-020) (e.g., KOH, NaOH, LiOH, or a combination thereof), each in any suitable concentrations.

[0059] An average size of the high-entropy alloy catalyst may be 2 to 45 nanometers (nm), 5 to 40 nm, or 10 to 30 nm. The average size of the high-entropy alloy catalyst herein is based on a mean of a longest dimension of each particle of the catalyst.

[0060] In an aspect, the composition of the high-entropy alloy catalyst comprises a total noble metal content of 10 wt% or less, 7 wt% or less, or 5 wt% or less, based on a total weight of the metals of the high-entropy alloy catalyst, wherein the noble metal comprises Au, Pd, or Pt. In an aspect, the total noble metal content is targeted 5 wt% or less, however, to control the fcc phase in the mixed fcc-bcc structure, the Au content may be temporarily increased, allowing the total noble metal content to extend up to 10 wt% or less. When the noble metal content is in the aforementioned ranges, there is a notable cost benefit by using less noble metals. AuPdFeNiCo as an example, a more effective approach is performed concerning the synthesis strategy of AuPdFeNiCo HEAs incorporating Fe, Ni, and Co to achieve cost savings in catalyst design and enhance HER efficiency.

[0061] In an aspect, the high-entropy alloy catalyst is further disposed on a support material. Non- limiting examples of the support material comprises carbon materials (vulcan carbon, graphene, graphene oxide, reduced graphene oxide, or carbon nanotube), transition metal dichalcogenides (TMDs) such as MoS2, MoSe2, or PtS2, or metal oxides such as TiO2, ZnO, or CeO2. The support material may comprise a combination of the aforementioned materials. The use of support materials can help improve electrical conductivity, mechanical strength, and durability in high-temperature and extreme environments. The high-entropy alloy catalyst may be disposed on the support material at a weight percent of 5 to 40%, 10 to 30%, or 15 to 25%.

[0062] In an aspect, the high-entropy alloy catalyst is more stable than a Pt catalyst (e.g., a commercial Pt / C catalyst or a state of the art Pt / C catalyst; 10 wt% Pt / C catalyst). The high-entropy alloy catalyst may be disposed on the support material, e.g., 25 wt% high-entropy alloy catalyst on carbon (25% HEA / C catalyst). The high-entropy alloy catalyst may be more stable than a Pt catalyst, after 10 hours or longer, 50 hours or longer, or 100 hours or longer, under an identical condition when used for catalyzing hydrogen evolution reaction. A comparison may be made by normalizing to a mass loading of noble metals in each of the high-entropy alloy catalyst and the Pt catalyst. For example, in 0.5 M H2SO4electrolyte at 10 mA / cm2, and at the same temperature, the high-entropy alloy catalyst may be more stable than the Pt catalyst indicated by less change in an overpotential over the aforementioned reaction time. Further details of the stability are discussed elsewhere herein. In an aspect, a mass activity of the high-entropy alloy catalyst (or the high-entropy alloy catalyst on the support material) normalized to a mass loading of noble metals of the high-entropy alloy catalyst is greater than a mass activity of the Pt catalyst normalized to a mass loading of Pt, under an identical condition when used for catalyzing hydrogen evolution reaction.

[0063] In an aspect, the high-entropy alloy catalyst (or the high-entropy alloy catalyst on the support103440-054PCT (UML 2024-020) material) maintains its catalytic activity for at least 10 hours, 50 hours, 100 hours, 200 hours, or 240 hours. Maintaining catalytic activity is determined when the change in an overpotential is within 10%, preferably within 5%, compared to an initial overpotential, at a constant current density. The current density may be 10 to 100 mA / cm2. For example, in 0.5 M H2SO4electrolyte at 10 mA / cm2, at room temperature, the high-entropy alloy catalyst may have a potential drop of less than 30 mV, less than 20 mV, or less than 10 mV. Further details of the stability are discussed elsewhere herein.

[0064] As mentioned, a facile wet-chemical synthesis enables fine control of the desired high-entropy alloy catalyst. Disclosed herein is a design of multi-element materials of at least five metals, that generate positive stimulation in the HER activity within a universally applicable pH electrolyte. Wet-chemical synthesis strategies and the meticulous control over constituent forms and structures are disclosed herein, in particular for Au-containing high-entropy alloy catalyst, while the method can be extended to other metal-containing compositions.

[0065] Disclosed is a method of synthesizing a high-entropy alloy catalyst via wet-chemical synthesis, the method comprising: providing at least five metal-halide precursors in a stoichiometric ratio, wherein each of the at least five metal-halide precursors independently comprise a transition metal; contacting the at least five metal-halide precursors with a polymer additive and a polar solvent to provide a reaction mixture; heating the reaction mixture at a temperature greater than a combined melting point of the at least five metal-halide precursors to form resulting particles.

[0066] In an aspect, the heating may be performed at 200°C to 300°C, or about 230°C, for a sufficient time e.g., 30 minutes to 2 hours, or 1 hour to 1.5 hours, to allow particle formation. During the reaction, the solution color may change to black, indicating particle formation. The reaction may be conducted under ambient or inert atmosphere conditions depending on the specific implementation.

[0067] The method further comprises, after forming the resulting particles, washing the resulting particles with the polar solvent to remove remaining precursors or the polymer additive, and separating the resulting particles by centrifugation to synthesize the high-entropy alloy catalyst.

[0068] The transition metal may comprise any suitable transition metal, such as elements in Groups 3 to 12 of the periodic table, comprising scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), yttrium (Y), zirconium (Zr), niobium (Nb), molybdenum (Mo), ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), hafnium (Hf), tantalum (Ta), tungsten (W), iridium (Ir), platinum (Pt), gold (Au), or a combination thereof. In an aspect, the transition metal may comprise Au, Pd, Fe, Co, Ni, Cu, Mn, Cr, Ag, Pt, Mo, or a combination thereof. In an aspect, the transition metal may comprise Au, Pd, Fe, Co, and Ni. In an aspect, the transition metal may comprise Cu, Ni, Co, Mn, and Cr. In an aspect, the transition metal may comprise Mo, Ni, Co, Mn, and Cr. In an aspect, the transition metal may comprise Au, Ag, Pt, Pd, and Co.

[0069] In an aspect, non-limiting examples of the polar solvent may include water (e.g., deionized water), ethanol, acetone, methanol, dimethyl sulfoxide, or a combination thereof. In an aspect, non-103440-054PCT (UML 2024-020) limiting examples of the polymer additive may include poly(N-vinyl-2-pyrrolidone), polyvinyl alcohol, polyethylene glycol, polyethyleneimine, sodium borohydride, or a combination thereof. In an aspect, the metal halide precursors may comprise a metal chloride, a metal bromide, a metal iodide, a metal fluoride, or a combination thereof.

[0070] In an aspect, the polar solvent is water, optionally deionized water, ethanol, or a combination thereof, and when the water and the ethanol is used, a ratio of the water to the ethanol may be 1:10 to 1:1, or 1:3. In an aspect, the polymer additive is poly(N-vinyl-2-pyrrolidone).

[0071] A ratio of the at least five metal halide precursors may be in equimolar amounts.

[0072] When the transition metal comprises Au, Pd, Fe, Co, and Ni, the metal halide precursor may comprises a metal chloride precursor, and a molar ratio of the metal halide precursors of Au:Pd:Fe:Co:Ni may be 0.2:1:1:1:1 to 10:1:1:1:1, 0.5:1:1:1:1 to 5:1:1:1:1, or 1:1:1:1:1 to 3:1:1:1:1.

[0073] Disclosed is A high-entropy alloy catalyst, a composition of which comprises at least five metals of Au, Pd, Fe, Co, Ni, Cu, Mn, Cr, Ag, Pt, or Mo, wherein the high-entropy alloy catalyst is effective to catalyze an electrochemical reaction; or a method of synthesizing a high-entropy alloy catalyst via wet- chemical synthesis, the method comprising: providing at least five metal-halide precursors in a stoichiometric ratio, wherein each of the at least five metal-halide precursors independently comprise a transition metal; contacting the at least five metal-halide precursors with a polymer additive and a polar solvent to provide a reaction mixture; heating the reaction mixture at a temperature greater than a combined melting point of the at least five metal-halide precursors to form resulting particles; washing the resulting particles with the polar solvent to remove remaining precursors or the polymer additive; and separating the resulting particles by centrifugation to synthesize the high-entropy alloy catalyst, wherein the composition comprises: Au, Pd, Fe, Co, and Ni; Cu, Ni, Co, Mn, and Cr; Mo, Ni, Co, Mn, and Cr; or Au, Ag, Pt, Pd, and Co; wherein when the composition comprises Au, Pd, Fe, Co, and Ni, a content of the Au is 5 to 31 atomic percent, a content of the Pd is 10 to 40 atomic percent, a content of the Fe is 13 to 52 atomic percent, a content of the Co is 7 to 18 atomic percent, a content of the Ni is 5 to 14 atomic percent, wherein each content is based on a total atomic content of the metals in the high- entropy alloy catalyst; wherein the electrochemical reaction comprises hydrogen evolution reaction, oxygen reduction reaction, carbon dioxide reduction reaction, or nitrogen reduction reaction; wherein the high-entropy alloy catalyst is effective to catalyze the electrochemical reaction in an electrolyte of pH 0 to 14; wherein the high-entropy alloy catalyst is more stable than a Pt catalyst, wherein the high-entropy alloy catalyst is more stable than a Pt catalyst, after 10 hours or longer under an identical condition when used for catalyzing hydrogen evolution reaction, when a comparison is made by normalizing to a mass loading of noble metals in each of the high-entropy alloy catalyst and the Pt catalyst, or wherein a mass activity of the high-entropy alloy catalyst normalized to a mass loading of noble metals of the high- entropy alloy catalyst is greater than a mass activity of the Pt catalyst normalized to a mass loading of Pt, under an identical condition when used for catalyzing hydrogen evolution reaction, or wherein the high-103440-054PCT (UML 2024-020) entropy alloy catalyst maintains its catalytic activity for at least 10 hours, optionally at least 240 hours, as determined by a change in an overpotential of 10 % or less, compared to an initial overpotential; wherein an average size of the high-entropy alloy catalyst is 2 nanometers to 45 nanometers; wherein when the composition comprises Au, Pd, Fe, Co, and Ni, wherein the high-entropy alloy catalyst has diffraction peaks at diffraction angles of 38.3±0.5°, 44.5±0.5°, and 64.9±0.5°, and optionally 40.1±0.5°, whenanalyzed by X-ray diffraction using CuK radiation, or wherein the high-entropy alloy catalyst has acrystal structure comprising a body-centered cubic (bcc), a face-centered cubic (fcc), or a mixed bcc / fcc phase; wherein the composition comprises a total noble metal content of less than 10 weight percent, based on a total weight of the metals in the high-entropy alloy catalyst, wherein the noble metal comprises Au, Pd, or Pt; wherein the high-entropy alloy catalyst is further disposed on a support material, wherein the support material comprises carbon materials comprising vulcan carbon, graphene, graphene oxide, reduced graphene oxide, or carbon nanotube, transition metal dichalcogenides comprising MoS2, MoSe2, PtS2, or metal oxides comprising TiO2, ZnO, or CeO2, or a combination thereof; wherein the metal halide precursor comprises a metal chloride precursor, and the at least five metal halide precursors are in equimolar amounts; wherein when the transition metal comprises Au, Pd, Fe, Co, and Ni, the metal halide precursor comprises a metal chloride precursor, and a molar ratio of the metal halide precursors of Au:Pd:Fe:Co:Ni is 0.2:1:1:1:1 to 10:1:1:1:1; wherein the polar solvent is water, optionally deionized water, ethanol, or a combination thereof, and when the water and the ethanol is used, a ratio of the water to the ethanol is 1:3; or wherein the polymer additive is poly(N-vinyl-2-pyrrolidone).

[0074] The AuPdFeNiCo HEAs synthesized by the disclosed method exhibit highest HER electrocatalytic activity compared with each single metal in a wide range of pH electrolytes, including acidic, alkaline, and neutral conditions. Specifically, under acidic medium, the overpotential of 40 mV (at a current density of 10 mA / cm2) is observed for HER, which surpasses that of Pt (52 mV, at a current density of 10mA / cm2) single foil. These findings contribute to the efficiency of HEAs for utility in water electrolysis by controlling nanoscale composition.

[0075] Also, the disclosed HEAs may be useful for electrochemical systems in various global regions with water having different pH or salinity (brackish, seawater, fresh, etc.), as well as ballast water treatment and purification that require extended catalyst lifetimes. Consequently, its potential as an innovative material in various applications beyond hydrogen evolution is anticipated, owing to its exceptional stability in diverse environments. Also, HEAs possess excellent conductivity and chemical stability, making them capable of enhancing the performance of energy storage devices such as batteries and fuel cells. Further a wet chemical synthesis method is utilized to synthesize HEAs in colloidal liquid form, providing an advantage for versatile support material combinations.

[0076] The present inventive concept will be described in greater detail through the following examples. However, it will be understood that the examples are provided only to illustrate the present inventive concept and not to be construed as limiting the scope of the present inventive concept.103440-054PCT (UML 2024-020) EXAMPLES Characterization of materials

[0077] The general characterization of morphology and structure was obtained using a high-resolution, dark-field scanning transmission electron microscope (HR-TEM / DF-STEM, JEOL, JEM-2100Plus, 200 kV) and energy dispersive X-ray spectroscopy (EDS) with elemental mapping feature. The crystal phase and structure were characterized by X-ray diffraction (XRD, Rigaku Miniflex X-ray diffractometer,CuK , ( = 1.5406 Å) and synchrotron wide-angle X-ray scattering (WAXS, NSLS- , = 0.1665 Å)measurements. X-ray photoelectron spectroscopy (XPS) was performed on a PHI Versaprobe II using anAl K X-ray radiation, and XPS peaks were calibrated to the C 1s peak at 284.8 eV. The quantificationanalyses for HEAs by Inductively coupled plasma with an optical emission spectrometer (ICP-OES) were measured using an Agilent 5110 and analyzed data using Agilent ICP Expert software. Synchrotron X-ray Analytical Techniques

[0078] Synchrotron X-ray pair distribution function (PDF) and wide-angle X-ray scattering (WAXS) were measured using a wavelength of 0.24152 Å on beamline 28-ID-1 at the National Synchrotron Light Source (NSLS-II), Brookhaven National Laboratory. The measurements were conducted at an X-ray energy of 74.4 keV, and the data were processed and reduced using the standard data reduction protocols employed, including pyFAI for azimuthal integration and PDFgetX3 for creation of the PDFs. All samples were dropcast onto Kapton and dried before measurement. To minimize interference from the PDF peaks of the Kapton tube support at 1.38 Å and 2.42 Å, background removal was performed using the PDFgetx3gui software with diffraction data collected from the empty Kapton, DI water, and ethanol. The parameters used for this process were Qmax = 19.7, Qmaxinst= 21.60, and rpoly = 0.42. After that, the PDF data was fitted and analyzed to assess the functionality of local structures, interfaces, chemical bonds, and component-based phases using PDFgui. Aand beamline instrument parametrization were performed by fitting the NIST LaB6standard at the BNL NSLS-II to ensure high reliability in the simulations. Additionally, the lattice constants for the HEAs used in the modeling were calculated based on the XRD results: Au 9 at.% (4.07 Å), Au 15 at.% (4.07 Å), Au 31 at.% (4.077 Å), and Au 27 at.% (4.078 Å), respectively. The experimental PDF was calculated atomic pair distribution function, denotedG(r), and is the truncated Fourier transform of the total scattering structure function S(Q) using,where Q is the magnitude of the scattering impulse, S(Q) is extracted from the Bragg and diffuse components of X-ray, neutron, or electron powder diffraction intensity. Additionally, for macroscopicscatterers, G(r) can be derived from the known structural model as follows,103440-054PCT (UML 2024-020) (r) is the atomic pair density and is the atomic number density, which represents mean weighted density of neighbor atoms at distance r from an atom at the origin. is the scattering factor of atom (i), is the average scattering factoris the separation between atoms i and j. Details of the experimental PDF determination have been reported elsewhere. Electrochemical measurements

[0079] A potentiostat (Pine research, Wavedriver 100) with a three-electrode system was used to perform electrochemical measurements of working electrode activity at ambient temperature (~25 °C) in acidic electrolytes (0.5M H2SO4). The working electrode was prepared by sonicating a mixture solution containing 1 mg of HEAs, 3 mg of mixed of supporting materials (Vulcanized Carbon), 50 L Nafion, and 300 L IPA to make the electrocatalyst ink. Subsequently, 10 L of ink solution was drop-cast onto an L-type glassy carbon with an active area 0.07 cm2. All working electrodes were prepared in the same method. All the potentials vs. Ag / AgCl reference values were calibrated to a reversible hydrogenelectrode (RHE) using the Nernst equation as below: / 0.0591 0.197 V (4)where ERHEis the converted potential of the vs. the reference electrode, EAg / AgClis the experimentally measured potential and 0.197 is the standard potential of Ag / AgCl. All electrochemical measurements were collected using iR compensation. The HER activity of AuPdFeCoNi HEAs were tested by linear sweep voltammetry (LSV) in the range of 0 V to -1.5 V vs. RHE at a rate of 5 mV / s with collected data up to ~100 mA / cm2. To determine the Tafel slope and mass activity, polarization curves were analyzed from LSV plot, and the mass values were obtained by ICP-OES. The oxidation-reduction potential was evaluated by cyclic voltammetry (CV) at 200 mV / s. For durability comparison, the HEAs were tested using chronopotentiometry (CP) with the electrolyte replaced every 48 hours to minimize pH effects, as hydrogen evolution can lead to a gradual increase in pH due to the consumption of hydrogen ions. Example 1 Nanoparticle Synthesis

[0080] AuPdFeNiCo HEA NPs were synthesized from Au, Pd, Fe, Ni, and Co precursor salts via a wet- chemical process. The precursors were prepared by dissolving 0.5 mmol of AuCl3(99.99%, Alfa Aesar), 0.5 mmol of PdCl2(99.99%, Acros Organics), 5 mmol of FeCl2H8O4(99%, Acros Organics), 5 mmol of NiCl2(99.995%, Alfa Aesar), and 5 mmol of CoCl2H12O6(99.998%, Alfa Aesar) in 40 mL ultrapure water. Then, 0.1g of poly(N-vinyl-2-pyrrolidone) (PVP, M.W.40,000, Alfa Aesar) powder was added to the metal solution to prevent aggregation. The temperature of the mixture solution was maintained at 230 °C under reflux for 1hr. After that, the temperature was allowed to cool to room temperature. The black powder product was purified by centrifugation and resuspension in a mixture of ethanol and water (equal ratio). Finally, the AuPdFeNiCo powder was collected after centrifugation and dried under vacuum. Electrochemical hydrogen evolution reaction measurements103440-054PCT (UML 2024-020)

[0081] All electrochemical measurements were performed at ambient temperature (~25 °C) using an electrochemical potentiostat (Pine research, Wavedriver 100) connected to a commercial three-electrode system in various electrolytes 0.5M H2SO4, 1M PBS, and 1M KOH. A Pt mesh and silver / silver chloride electrode (Ag / AgCl) were used as counter and reference electrodes, respectively. For preparing the working electrode, an electrocatalyst ink was prepared by sonicating a mixture solution containing 1 mg of AuPdFeNiCo nanoparticles, 3 mg of vulcanized carbon (Carbon Black, XC-72, Fuel Cell Earth), 50 L Nafion (5 wt.% of dispersion, Thermo Scientific), and 300 L isopropyl alcohol (IPA, Ricca). Then, 10 L of ink solution was drop cast onto a glassy carbon electrode (active area 0.07 cm2). To compare the HER activity of the prepared working electrode, the commercial Pt / C (10%) and various bulk single metal foils such as Au (25 m, 99.95%, Sigma-Aldrich), Pd (25 m, 99.9%, Thermo Scientific), Fe (100 m, 99.995%, Thermo Scientific), Ni (100 m, 99.994%, Thermo Scientific), and Co (100 m, 99.995%, Alfa Aesar) were respectively prepared for individual evaluation. All the potentials vs Ag / AgCl referencevalues were converted to the reversible hydrogen electrode (RHE) using the Nernst equation as below:where ERHEis the converted potential of the vs. the reference electrode, EAg / AgClis the experimentally measured potential and / isstandard potential of Ag / AgCl (0.197V @ 25 °C). The HER activity of AuPdFeNiCo was quantified by linear sweep voltammetry (LSV), Tafel slope analysis, cycle voltammetry (CV), double layer capacitance (Cdl), and electrochemical impedance spectroscopy (EIS). LSV test was obtained in the range of 0 V to -2.0 V vs. RHE at a rate of 5 mV / s and converted to the Tafel slope using raw data after LSV testing. CV was measured in each potential range based on electrolytes at varied scan rates (10-200 mV / s). The Cdlof the catalysts was calculated from CV using theequationwhere jaand jcare different current densities between anodic and cathodic, v is the scan rate in mV / s from CV. EIS measurements were performed using a Nyquist plot in the range of 100 kHz to 0.1 Hz at 10 mA / cm2. The experimental data were fitted with the Pine-view software and Origin software using appropriate equivalent circuits. Results

[0082] The AuPdFeNiCo HEA nanoparticles (NPs) were synthesized via a wet-chemical reaction, where five metal chloride precursors and poly(N-vinyl-2-pyrrolidone) (PVP, Mw 40,000) were mixed in ultrapure water at 230 °C into an equimolar ratio (approximately 3.125 at.% each of Au and Pd, and 31.25 at.% each of Fe, Ni, and Co) (FIG.5). Without wishing to be bound by theory, PVP may facilitate the formation of metal alloys by augmenting stability and preventing aggregation.

[0083] Morphological, structural, and compositional analysis of AuPdFeNiCo HEA NPs was conducted using high-resolution transmission electron microscopy (HR-TEM), scanning transmission electron103440-054PCT (UML 2024-020) microscopy (STEM) with energy dispersive spectroscopy (EDS) mapping, and powder X-ray diffraction (XRD). Examination of STEM and HR-TEM images revealed an average particle size of ~13.45 nm (Fig. 8B). Higher magnification imaging reveals a lattice spacing of 2.40 Å (Figs.1A-1H and Figs.8A-8E). This spacing corresponds to the (111) plane , as corroborated by fast Fourier transformation (FFT) (Fig. 1B) and XRD patterns (Figs.2A-2B) associated with HEA NPs. Additionally, the crystalline structure of AuPdFeNiCo HEA NPs exhibits distinct diffraction peaks corresponding to the (111), (200), and (220) facets. Fig.1C- Fig.1H and Fig.10A to 10H show the EDS mapping and line scan profiles of the AuPdFeNiCo HEA NPs, demonstrating a uniform dispersion of the five elements . A noteworthy aspect is the formation of a seemingly Fe-rich shell (FIGS.11 and 12). This was verified through the edge FFT and the distribution pattern observed in the low-magnification EDS Mapping. Fig.9A1 to 9C shows the HR-TEM image of AuPdFeNiCo HEA NPs and Fe NPs with an interplanar spacing of 2.4 Å and 2.57 Å, including different FFT images in the inserted images, respectively. These images appear to depict iron or iron oxide nanoparticles formed either as side products or as nuclei during the reaction. Additionally, to mitigate potential nanoparticle damage from prolonged exposure to the electron beam during mapping characterization, the mapping duration was restricted to less than 10 min.

[0084] XRD patterns show that the AuPdFeNiCo HEA NPs exhibit a fcc structure, with a lattice constant of a = 4.15 (Fig 2). For comparison, FIGS.2A-2B shows the XRD patterns of Au and Pd NPs, exhibiting clearly defined crystalline phases of the (111), (200), and (220) planes. In contrast, the diffraction pattern of AuPdFeNiCo HEA NPs is positioned between that of Au NPs and Pd NPs, indicating crystalline phases at 38.3°, 44.5°, and 64.9°, which corresponds to the (111), (200), and (220) planes, respectively. Moreover, the long-range order of the AuPdFeNiCo HEA NPs using high-resolution synchrotron wide-angle X-ray scattering (WAXS) was validated, which showcases analogous results to XRD but with additional high quality diffraction orders (Fig.11).

[0085] To understand the electronic structure and chemical state of the AuPdFeNiCo HEA NPs, X-ray photoelectron spectroscopy (XPS) was performed. Fig.3A shows XPS survey spectra for AuPdFeNiCo HEA NPs in the range of 0–1,100 eV, revealing the presence of each constituent element state. A high resolution scan of the Au 4f region (Fig.3B) shows well-resolved peaks with binding energies (BEs) of 83.8 eV and 87.4 eV, which can be assigned to metallic gold (Au04f7 / 2and Au04f5 / 2). Similarly, the corresponding Pd 3d peaks were located at 335.2 eV and 340.5 eV, which can be indicated with metallic palladium (Pd03d5 / 2and Pd03d3 / 2) (Fig.3C). Fig.3D shows the resolved peaks in the Fe 2p region with BEs of 707.3 eV, 710.6 eV, and 713.2 eV, which correspond with metallic iron, Fe2+, and Fe3+. In the Co 2p region (Fig.3E), the spectrum has a broad feature that can be deconvoluted into peaks at 780.7 eV (Coo) and 784.0 eV (Co2+). For the Ni 2p region (Fig.3F), the presence of metallic nickel and nickel oxides as Ni0and Ni2+are attributed to BEs at 854.5 eV and 856.7 eV. Interestingly, the BEs of each metal exhibited noticeable shifts in the AuPdFeNiCo HEA NPs compared to the bulk (Table 1). Specifically, there are lower shifts observed in the BEs of metallic gold and palladium. Conversely, the core levels of metallic iron, cobalt, and nickel showed a higher shift compared to their respective bulk103440-054PCT (UML 2024-020) metal values. These core level shifts of each element indicate the electron charge transfer between elements within the HEA. This phenomenon confirms that electron redistribution reduces atom size disparity, suggesting local lattice distortion, and indicating diverse chemical environments for each element. Taken together, HR-TEM / STEM, XRD, and XPS results strongly demonstrate the morphology, crystalline, electronic structure, and chemical BEs from Au, Pd, Fe, Co, and Ni core levels in HEAs that reflect the formation of HEAs. Table 1 shows the binding energy of the metallic core level peaks for each

[0086] Based on the above electrochemical hydrogen evolution reaction (HER) performance study, the as-prepared AuPdFeNiCo HEAs on glassy carbon (GC) were used as working electrodes in various pH conditions of electrolytes via a typical three-electrode system. Compared with the HER activity of bulk single metals as Au, Pd, Fe, Ni, and Co, the linear sweep voltammetry (LSV) polarization curve ofAuPdFeNiCo HEAs shows its superior activity toward HER with an overpotential ( ) of 45 mV, 279mV, and 43 mV at the current density of 10 mA / cm2in 0.5M H2SO4, 1M PBS, and 1M KOH,respectively. Specifically, the extensively recognized commercial Pt / C ( =53 mV, 43 mV), hailed as apremier catalyst in both acid and alkaline electrolytes, displayed certain drawbacks in comparison to AuPdFeNiCo, particularly evident at the onset and overpotential (@10mA / cm2), as shown in Fig.4A, 4C and Fig.4E. Moreover, in Fig.12A-12C, illustrated are the potential values corresponding to each current density. Furthermore, derived were the linear region of Tafel slopes of each catalyst from the LSV polarization curve (Fig.13A-13C). The resulting values for AuPdFeNiCo in different pH electrolytes were 32, 132, and 55 mV per decade, respectively.

[0087] Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) was conducted to determine the electrical double-layer capacitance (Cdl) and resistance (ohm). Fig.14A to 14F shows the CV and Cdl values for AuPdFeNiCo were 25.6 mF / cm-2, 25.5 mF / cm-2, and 55.81 mF / cm-2based on different scan rates as 10-200 mV / s and different pH electrolytes. As shown in Fig.15, the Nyquist plotwas utilized to assess the charge transfer resistance of AuPdFeNiCo, indicating value of 1.65 ohm ( ) in0.5M H2SO4, 2.4 in 1M PBS, and 2.4 in 1M KOH at the overpotential (@10 mA / cm2). FIGS.16A to 16C show the results of HER-stability test of AuPdFeNiCo HEAs with chronopotentiometry curve, (16A) 0.5M H2SO4, (16B) 1M PBS, and (16C) 1M KOH. FIG.17 shows X-ray diffraction (XRD) characterization of AuPdFeNiCo HEA NPs before and after 12 hr of continuous HER operation in different pH-electrolytes.103440-054PCT (UML 2024-020)

[0088] All experimental results regarding the electrochemical HER activity were summarized to compare the key functional performance parameters by radar chart, as illustrated in Fig.4B, Fig.4D, and Fig.4F. Above all, reducing the quantity of precious metal catalysts is crucial to producing cost-effective and efficient HER catalysts for practical use. In this context, the gold and palladium content determined by inductively coupled plasma – optical emission spectrometry (ICP-OES) was merely 1.5 wt% and 2.4 wt%, respectively (Table 2), significantly lower than that of commercial Pt / C (10 wt%). Even more intriguingly, despite the generally recognized low HER activity of Au, to our knowledge, HEAs designs based on Au have exhibited exceptional electrocatalytic performance compared with currently publishing data for HER activity, suggesting the potential for achieving commercially viable electrocatalyst performance levels. The remarkable HER performance showcased by the designed HEA in this work was attainable due to a progressive and adaptable approach such as component selection, control of morphology and structure, and friendly synthesis strategy. Table 2 shows composition of AuPdFeNiCo HEAs determined by ICP-OES. Table 2Example 2. Control of the Structure in AuPdFeNiCo High-Entropy Alloys

[0089] The influence of crystal structures and their interrelationships in nanoscale materials and energy systems is extensive. Especially in multi-component HEAs, controlling and understanding crystal structures can profoundly impact the materials and energy industries. Thus, investigated were the control and comprehension of BCC and FCC crystal structures based on HEAs comprising AuPdFeNiCo, with varying gold compositions. As a result, increasing the gold content facilitated the formation of FCC structures in the HEAs. Chemicals

[0090] The metal precursors of AuCl3(99.99%), CoCl2H12O6(99.998%), NiCl2(99.995%), and poly(N- vinyl-2-pyrrolidone) (PVP, M.W.40,000) were purchased from Alfa Aesar (USA). PdCl2(99.99%), FeCl2H8O4(99%), ethanol, Nafion (5 wt.%), and Pt / C (10 wt.%) were purchased from Thermo Scientific (USA). The isopropyl alcohol (IPA) and vulcanized carbon (Carbon Black, XC-72) were purchased from Ricca and Fuel Cell Earth, respectively. Sulfuric acid (ACS reagent, 98%) purchased from Sigma- Aldrich was used to prepare the 0.5M H2SO4electrolyte. All experiments used ultrapure water (18.2M ). For the standard calibration curve used in the ICP-OES analysis, multi-element solution 3 (for Auand Pd, 10 μg / mL) and multi-element calibration standard 2A (for Fe, Co, and Ni, 10 μg / mL) from Agilent were used. Synthesis of AuPdFeCoNi HEAs

[0091] First, a mixture of equal molar amounts (0.5 mmol) of four metal precursors, excluding gold, and a controlled molar amount of gold (0.1, 0.5, 1, and 5 mmol) were completely dissolved in mixture of103440-054PCT (UML 2024-020) ethanol and DI water (3:1, total volume: 40 mL). To achieve homogeneous mixing, each metal precursor was sequentially added to the solution at 30-minute intervals, starting with gold and followed by the other metals. Then, the mixed solution was heated to 230°C in a vertical coiled condenser system to control the wetting process. Next, poly(N-vinyl-2-pyrrolidone) (PVP, M.W. 40,000) was added to the preheated mixed metal solution. After 1 hour, when the solution turned black, it was allowed to cool to room temperature (RT, 20°C). Next, the obtained precipitate was washed five times with ethanol and DI water using centrifugation at 14,000 rpm for 15 min to remove residual PVP. Finally, the black powder was thoroughly dried under vacuum. All sample preparation procedures are carried out in the same method. The phase from bcc-to-fcc can be controlled as a function of Au-content in the HEA (FIG.18A).

[0092] The phase control and evolution of the various AuPdFeCoNi HEAs was investigated by synchrotron-based wide-angle X-ray scattering (WAXS). As a first approximation of phase content, the (111) [for fcc at 2.67 Å -1] and (110) [for bcc 2.80 Å -1] Bragg reflections were analyzed (FIGS.18B- 18C). The initially synthesized AuPdFeCoNi HEAs displayed a mixed fcc-bcc phase. However, it was found that increasing Au-content can stabilize a single fcc phase, the AuPdFeCoNi HEA in fcc phase. This is confirmed by the change in the intensities of the (111) and (110) reflections. In all cases, the (110) bcc reflection decreased with increasing Au content, eventually disappearing (low-Au content, mixed fcc / bcc, high-Au content fcc). These are confirmed using powder X-ray diffraction (XRD), FIG.22. Both diffraction techniques revealed identical phase control characteristics, suggesting that the phase control and atomic arrangements of HEAs can be controlled by manipulating the relative Au composition. The elemental composition of the HEAs was quantified by inductively coupled plasma optical emission spectroscopy (ICP-OES), revealing that the HEAs contained Au, Pd, Fe, Co, and Ni in the range of 5-35 atomic % (Table 3). Table 3Characterization of AuPdFeCoNi HEAs

[0093] To better understand the nanoscale structure and crystallite morphology, high resolution electron microscopy was used. Dark-field scanning transmission electron microscopy (DF-STEM) and energy- dispersive X-ray spectroscopy (EDS) elemental mapping images of the synthesized HEAs (FIG.23) shows a uniform distribution of individual metals of Au, Pd, Fe, Co, and Ni across the nanoparticles. High-resolution TEM and fast Fourier transform (FFT) analysis show the prototypical atomic arrangement of the bcc phase (ABAB planar lattice stacking arrangements) and the fcc phase (ABCABC stacking arrangements) in the synthesized AuPdFeCoNi HEAs (FIG.19). As reported previously, Fe prominently favors the formation of the bcc-phase alloy structures. However, in local environments103440-054PCT (UML 2024-020) enriched with Au atoms, a collective tendency for the structure to stabilize fcc, as compared to bcc, is observed. This behavior is dependent on the specific metal concentration, which govern the stabilized phases, as theoretically predicted previously. Correspondingly, for lower Au contents (9-24 at.%) both bcc and fcc phases are observed (FIG.19). In contrast, at relatively higher Au content (31 at.%), only the fcc phase is observed, indicating that the Au content dictates the phase, with higher Au concentrations preferentially stabilizing the fcc structure.

[0094] Understanding the atomic arrangements in HEA nanostructures desires multiple characterization approaches to understand the integrated structural and electronic properties. Specifically, synchrotron X- ray total scattering and X-ray photoelectron spectroscopy (XPS) were used. Synchrotron pair distribution function (PDF) analysis was used to systematically characterize the local ordering behavior and chemical bonding motifs in fcc / bcc mixed phase AuPdFeCoNi HEA nanoparticles. Unlike conventional X-ray diffraction tools that primarily asses long-range order, PDF analysis provides both long-range atomic arrangements (over 10 nm) and detailed short-range structural information, including bond lengths and local defects. These are typically understood using small-box modeling of atomic interactions within a specific structural region.

[0095] Bimetallic, ternary, and quaternary alloy models centered around Au were modeled to reveal the partial pair-correlation models of HEAs through PDF analysis (FIG.20A1-20A3 and FIGS.24-26C) within the 2.8-20 Å range. The partial pair correlation functions for specific local nearest-neighbor pairs (e.g., Au-Fe, Au-Ni, Au-Co) were analyzed using PDFgui. In the PDF, it is observed that peak positions near < 5 Å corresponds to Au-Pd and Au-Fe correlations, while peaks greater than 5 Å correspond to Au- Co and Au-Ni correlations. FIGS.20A1-A3 and FIGS.25A-26C show that the Au-Au pair distance decreases from 5.1 Å to 5.0 Å, while the distances for pairs involving Au, such as Au-Pd, Au-Fe, Au-Ni, and Au-Co pair distance changes from 8.58 Å 8.64 Å 8.66 Å 8.68 Å, as the Au content increases during HEA synthesis. These changes suggest that the reduction in Au-Au distance results from local lattice distortion and compression of Au-Au bonds, while the increase in other pair distances reflects structural rearrangement and atomic expansion due to the larger atomic radius of Au compared to Fe, Ni, and Co. Specifically, the lower electron density of Au, coupled with the higher electron affinity of Pd and other metals, leads to distinct atomic interactions that influence the contraction or expansion of atomic distances within the alloy.

[0096] To ensure high reliability in the PDF models, a Qdamp and beamline instrument parametrization were established through fitting the NIST LaB6standard at the BNL NSLS-II. Furthermore, the disclosed results (Rwp= 12-21%, FIGS..20B-20E) indicate that simulations optimized using the PDFgui model for cubic structure yield fitting values with high reliability for the synthesized HEAs in the 0.98 to 30 Å range. This approach provides predictive scattering patterns corresponding to the structural or domain inputs, specifically regarding the ensemble structural ordering of atomic arrangements up to 30 Å in the sample. Interestingly, increasing the Au content led to a gradual decrease in the Rwpvalue (reflecting the103440-054PCT (UML 2024-020) fcc model error) from 21% to 12%, indicating the presence of a changing crystal structure mixture from bcc+fcc (higher Rwp) to pure fcc (lower Rwp) (FIG.27).

[0097] To understand the structure better, a series of systematic binary, ternary, quaternary, and quinary PDF models were built, starting with AuPd (FIGS.20A1-20A3 and FIGS.26A-26C). This sequential approach reveals clear structural trends: local distortions in structure emerge near ~4.5 Å, changes in coordination environments are visible, and broadening effects are visible (e.g.4.75-5.2 Å) which is indicate of the configurational disorder in HEAs. Interestingly, the first peak (2.86-2.88 Å) of the synthesized AuPdFeCoNi sample shows strong agreement with the simulated PDF of most combinations, including AuPd and AuPdFeCoNi, indicating that the highest correlated scattering is associated with Au and Pd interactions. Additionally, changes in coordination environments are associated with Pd-Fe interactions, leading to a structural distortion around 8.1 Å due to the Pd-Fe atomic arrangement. Furthermore, Ni-Ni and Ni-Pd interactions at ~4.88 Å contribute to further structural distortions observed between 4.56 and 4.9 Å. The observed trends not only refine our structural interpretation but also highlight the general applicability of PDF modeling in elucidating the atomic-scale complexity of HEAs.

[0098] To complement the physical structural insights from PDF and HR-TEM, XPS was employed to quantify the composition-dependent electronic changes in HEA nanoparticles. The Au 4f, Pd 3d, and 2p orbitals of Fe, Ni, and Co were characterized by XPS to determine the metallic state and surface composition in HEA systems (FIGS.28A1-28D5). With increasing Au incorporation, a positive shift of Au 4f7 / 2binding energy (BE) from 83.6 to 84 eV was measured. Positive shifts were also observed in Pd 3d, and Fe 2p spectra, which shifted by 0.2 - 0.5 eV towards higher binding energies in the high-Au content alloys compared to the low-Au content alloys (FIGS.28A1-28D5 and Table 4). However, for Co 2p and Ni 2p, a negative shift in the binding energies was observed, with Co 2p shifting from 780.6 to 780.5 eV and Ni 2p shifting from 854.5 to 854.0 eV with increasing Au content. Specifically, the broadening of Co 2p, accompanied by a strong satellite feature, indicates oxidation. As the Au content increased, Co 2p showed an increased fraction of higher oxidation states, reflective of partial reduction ofCo while retaining some Co² state. In contrast, Ni 2p exhibited a decrease in both binding energy andsatellite peak intensity, indicative of charge redistribution and a lower oxidation state. These results demonstrate that the oxidation states of Ni and Co are modified differently compared to Au and Pd in the HEA system, likely due to their distinct local chemical environments. The observed shifts in the binding energies of the Au 4f, Pd 3d, Fe 2p, Ni 2p, and Co 2p orbitals are consistent with previously reported binary, ternary, and more complex multi-metallic systems and have been attributed to the formation of Au-based HEAs at the atomic level. Table 4 shows chemical shift phenomenon for Au, Pd, Fe, Co, and Ni chemical states.103440-054PCT (UML 2024-020) Table 4Electrocatalytic results

[0099] Tests were performed to observe how the AuPdFeCoNi HEAs impact their electrocatalytic properties toward the hydrogen evolution reaction (HER). Considering the unique structure and catalytic durability of systematically controlled HEAs, the HER capabilities of these HEA nanoparticles with tunable phases were quantified. HER activities of four HEA electrocatalysts were measured and compared with commercial Pt / C (10 wt.%). With the exception of the HEA (Au 31 at.%) electrocatalyst, AuPdFeCoNi HEAs exhibit HER catalytic activities similar to commercial Pt / C, with lower onset potentials and overpotentials along with remarkable durability in extended testing (FIGS.21A-21F).

[0100] The electrochemical HER for the phase-controlled HEA electrocatalysts required varying overpotentials to achieve a current density of 10 mA / cm². Specifically, the overpotentials were Au 9 at.% (17 mV), Au 15 at.% (32 mV), Au 24 at.% (78 mV), and Au 31 at.% (160 mV), as shown in FIG.21A. Tafel slopes were determined for AuPdFeCoNi HEAs and Pt / C to assess the kinetic properties of the electrocatalysts, with HEA (Au 9 at.%, 32.4 mV dec-1), HEA (Au 15 at.%, 33.5 mV dec-1), HEA (Au 24 at.%, 35.9 mV dec-1), HEA (Au 31 at.%, 121.2 mV dec-1), and Pt / C (27.3 mV dec-1) reflecting desirable kinetic electrochemical activity (FIG.21B). At higher Au content (31 at.%), the HER activity decreased substantially, likely due to the poor electrocatalytic properties of Au hindering proton adsorption.

[0101] Building on the Tafel slope analysis, the electrochemical redox behavior of HEAs with different compositions was investigated using cyclic voltammetry (CV). FIG.21C shows that the CV of the Au-enriched HEAs exhibits a significant shift to a more positive potential in the negative scan direction. This is attributed to the intrinsic electronic properties of Au, which has a higher reduction potential compared to the other constituent metals (Pd, Fe, Co, and Ni). This behavior indicates that the phase character and electronic interactions of other metals in the HEA modulate the electrochemical activity at the catalyst-substrate interface. Specifically, as the Au content increases, a monotonic shift in the first reduction peak from 1.0 V to 1.05 V is observed. This correlates with a reduced catalytic activity for the HER, as evidenced by an increase in overpotential from 75 mV to 160 mV at 10 mA / cm2. Ultimately, this observation confirms the presence of metallic binding effects on the surface of the HEAs, which influence HER activity through the combined effects of electronic structure modifications, surface active sites, and atomic-scale interactions unique to the HEA composition.

[0102] The durability of HEAs was investigated using chronopotentiometry in 0.5 M H2SO4electrolyte. The potential of HEAs remained stable over 10 days (FIG.21D and FIGS.29-30), compared103440-054PCT (UML 2024-020) to 65 mV to 100 mV (vs RHE) drop in potential over 13 hr for Pt / C. This is noteworthy as the robust chemical and structural bonding of HEAs not only enhances their catalytic activity but also ensures excellent stability in acidic solutions. Furthermore, XRD of the HEAs after durability evaluation remains unchanged, further underscoring the exceptional structural stability of HEAs (FIG.31).

[0103] Notably, the mass activity normalized to the loading of noble metals for HEA (Au 15 at.%) at 2 A / mgPGMis 111 mV, is lower than that of commercial Pt / C (160 mV) (FIGS.21E-21F). These results indicate that the economic cost of HEAs ($ 18.36 / g) is lower than that of commercial Pt / C cathodes ($ 59.30 / g, Sigma-Aldrich Chemical Co., Ltd), suggesting their great potential in electrochemical hydrogen production applications.

[0104] When taken together, changes in structure from mixed fcc / bcc phases to an fcc phase as Au content increases have pronounced impacts on the catalytic activity that can be understood through electronic and structural changes. Synchrotron-PDF modeling shows the most pronounced local Au-Pd interactions occur with increasing percentage Au content. Associated significant atomic rearrangements are also observed, with Pd-Fe and Pd-Ni interactions contributing to longer range few-A distortions. Associated changes in the electronic environment are observed by XPS binding energy shifts, indicating internal charge redistribution and altered electronic interactions. The observed shift in the first CV reduction peak aligns with these electronic modifications, further linking the phase mixture to improved HER activity. Notably, the highest HER activities were observed at Au 9 at% and 15 at%, where a mixed bcc-fcc structure was present. This suggests that the mixed structure, observed in the FFT (FIG.19) and synchrotron PDF modeling (FIG.20 and FIG.27), significantly influences the electronic properties, as seen in FIG.32A-33B. For Au, Pd, and Fe, increasing active metal oxidation state ratios, such asAu1+ / Au0, Pd² / Pd, and Fe³ / Fe² , correlated with higher overpotentials, which is consistent with thegeneral trend that higher oxidation states of these metals tend to reduce electron transfer efficiency forthe HER process. In contrast, for Ni and Co, higher ratios of satellite Ni / Ni² and satellite Co / Co³ wereassociated with lower overpotentials, indicating that these oxidation states facilitate electron transfer and enhance catalytic activity. This behavior demonstrates how electrochemical behavior can be determined from the interplay present in a structure-electronic relationship.

[0105] FIGS.34A to 34C show HER-stability test of AuPdFeNiCo HEAs with chronopotentiometry curve, (34A) 0.5M H2SO4, (34B) 1M PBS, and (34C) 1M KOH. FIGS.35A to 35B show HER polarization curves depending on different metal compositions with (35A) 0.5M H2SO4, and (35B) 1M KOH.

[0106] FIG.37 shows electrochemical durability under pH-switching conditions, demonstrating a scalable hydrogen evolution electrode based on a high-performance catalyst. Electrode size was 18 cm2with carbon paper substrate, control electrode was commercial Pt / C 10%. Chronoamperometry experimental process was carried out starting from 1M KOH to 1M PBS to 0.5M H2SO4to 1M PBS to 1M KOH, where reaction time for each step was 30 min. Regarding the electrode stability efficiency,103440-054PCT (UML 2024-020) AuPdFeNiCo (HEA) / C was 89.83 %, Pt / C was 56.26%. Stability efficiency was calculated by: Stability efficiency = (Final current / Initial current) * 100.

[0107] These findings also pave the way for the synthesis of HEAs via a wet-chemical method, allowing for precise control of their phase structure and compositions. Moreover, AuPdFeCoNi HEAs exhibit exceptional electrocatalytic activity with remarkable durability for the HER in acidic electrolyte, maintaining stable performance for 240 hours. Example 3 Representative HEAs containing other metal elements

[0108] CuNiCoMnCr and MoNiCoMnCr were synthesized to replace Au and Pd using the same synthesis method described above. The metal precursors were used in equimolar amounts, each at a concentration of 10 mM. XRD analysis (as shown in FIG.6) confirmed the crystal structure of the HEA, and STEM-EDS analysis (as shown in FIG.7) revealed a uniformly distributed elemental composition. Additionally, Multiple compositions of AuAgPtPdCo HEAs were synthesized using the same synthesis method described above as indicated in FIG.36. FIG.36 shows HER polarization curves depending on different HEA compositions in 0.5M H2SO4. Therefore, it is expected that various compositions of HEAs can be synthesized using the existing method.

[0109] While AuPdFeNiCo, CuNiCoMnCr, MoNiCoMnCr, and AuAgPtPdCo are mentioned as non- limiting examples, based on this universal synthesis strategy, the composition can be expanded to encompass additional combinations with various other metal elements to enhance energy production.

[0110] This disclosure further encompasses the following aspects.

[0111] Aspect 1. A high-entropy alloy catalyst, a composition of which comprises at least five metals of Au, Pd, Fe, Co, Ni, Cu, Mn, Cr, Ag, Pt, or Mo, wherein the high-entropy alloy catalyst is effective to catalyze an electrochemical reaction.

[0112] Aspect 2. A method of synthesizing a high-entropy alloy catalyst via wet-chemical synthesis, the method comprising: providing at least five metal-halide precursors in a stoichiometric ratio, wherein each of the at least five metal-halide precursors independently comprise a transition metal; contacting the at least five metal-halide precursors with a polymer additive and a polar solvent to provide a reaction mixture; heating the reaction mixture at a temperature greater than a combined melting point of the at least five metal-halide precursors to form resulting particles; washing the resulting particles with the polar solvent to remove remaining precursors or the polymer additive; and separating the resulting particles by centrifugation to synthesize the high-entropy alloy catalyst.

[0113] Aspect 3. The high-entropy alloy catalyst of any of the foregoing aspects, wherein the composition comprises: Au, Pd, Fe, Co, and Ni; Cu, Ni, Co, Mn, and Cr; Mo, Ni, Co, Mn, and Cr; or Au, Ag, Pt, Pd, and Co.

[0114] Aspect 4. The high-entropy alloy catalyst any of the foregoing aspects, wherein when the composition comprises Au, Pd, Fe, Co, and Ni, a content of the Au is 5 to 31 atomic percent, a content of103440-054PCT (UML 2024-020) the Pd is 10 to 40 atomic percent, a content of the Fe is 13 to 52 atomic percent, a content of the Co is 7 to 18 atomic percent, a content of the Ni is 5 to 14 atomic percent, wherein each content is based on a total atomic content of the metals in the high-entropy alloy catalyst.

[0115] Aspect 5. The high-entropy alloy catalyst of any of the foregoing aspects, wherein the electrochemical reaction comprises hydrogen evolution reaction, oxygen reduction reaction, carbon dioxide reduction reaction, or nitrogen reduction reaction.

[0116] Aspect 6. The high-entropy alloy catalyst of any of the foregoing aspects, wherein the high- entropy alloy catalyst is effective to catalyze the electrochemical reaction in an electrolyte of pH 0 to 14.

[0117] Aspect 7. The high-entropy alloy catalyst of any of the foregoing aspects, wherein the high- entropy alloy catalyst is more stable than a Pt catalyst, after 10 hours or longer under an identical condition when used for catalyzing hydrogen evolution reaction, when a comparison is made by normalizing to a mass loading of noble metals in each of the high-entropy alloy catalyst and the Pt catalyst, or wherein a mass activity of the high-entropy alloy catalyst normalized to a mass loading of noble metals of the high-entropy alloy catalyst is greater than a mass activity of the Pt catalyst normalized to a mass loading of Pt, under an identical condition when used for catalyzing hydrogen evolution reaction or wherein the high-entropy alloy catalyst maintains its catalytic activity for at least 10 hours, optionally at least 240 hours, as determined by a change in an overpotential of 10 % or less, compared to an initial overpotential.

[0118] Aspect 8. The high-entropy alloy catalyst of any of the foregoing aspects, wherein an average size of the high-entropy alloy catalyst is 2 nanometers to 45 nanometers.

[0119] Aspect 9. The high-entropy alloy catalyst of any of the foregoing aspects, when the composition comprises Au, Pd, Fe, Co, and Ni, wherein the high-entropy alloy catalyst has diffraction peaks at diffraction angles of 38.3±0.5°, 44.5±0.5°, and 64.9±0.5°, and optionally 40.1±0.5°, when analyzed byX-ray diffraction using CuK radiation, or wherein the high-entropy alloy catalyst has a crystal structurecomprising a body-centered cubic (bcc), a face-centered cubic (fcc), or a mixed bcc / fcc phase.

[0120] Aspect 10. The high-entropy alloy catalyst of any of the foregoing aspects, wherein the composition comprises a total noble metal content of less than 10 weight percent, based on a total weight of the metals in the high-entropy alloy catalyst, wherein the noble metal comprises Au, Pd, or Pt.

[0121] Aspect 11. The high-entropy alloy catalyst of any of the foregoing aspects, wherein the high- entropy alloy catalyst is further disposed on a support material, wherein the support material comprises carbon materials comprising vulcan carbon, graphene, graphene oxide, reduced graphene oxide, or carbon nanotube, transition metal dichalcogenides comprising MoS2, MoSe2, PtS2, or metal oxides comprising TiO2, ZnO, or CeO2, or a combination thereof.

[0122] Aspect 12. The method of any of the foregoing aspects, wherein the metal halide precursor comprises a metal chloride precursor, and the at least five metal halide precursors are in equimolar amounts.103440-054PCT (UML 2024-020)

[0123] Aspect 13. The method of any of the foregoing aspects, when the transition metal comprises Au, Pd, Fe, Co, and Ni, the metal halide precursor comprises a metal chloride precursor, and a molar ratio of the metal halide precursors of Au:Pd:Fe:Co:Ni is 0.2:1:1:1:1 to 10:1:1:1:1.

[0124] Aspect 14. The method of any of the foregoing aspects, wherein the polar solvent is water, optionally deionized water, ethanol, or a combination thereof, and when the water and the ethanol is used, a ratio of the water to the ethanol is 1:3.

[0125] Aspect 15. The method of any of the foregoing aspects, wherein the polymer additive is poly(N- vinyl-2-pyrrolidone).

[0126] The term “about” is intended to include the degree of error associated with measurement of the particular quantity based upon the equipment available at the time of filing the application. All ranges disclosed herein are inclusive of the endpoints, and the endpoints are independently combinable with each other.

[0127] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and / or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, element components, and / or groups thereof.

[0128] “Or” means “and / or” unless clearly stated otherwise. It is to be understood that the described elements may be combined in any suitable manner in the various embodiments and / or aspects. A “combination thereof” is open and includes any combination comprising at least one of the listed components or properties optionally together with a like or equivalent component or property not listed.

[0129] Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of skill in the art to which this application belongs.

[0130] While the present disclosure has been described with reference to an exemplary embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the present disclosure. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the present disclosure without departing from the essential scope thereof. Therefore, it is intended that the present disclosure not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this present disclosure, but that the present disclosure will include all embodiments falling within the scope of the claims.

Claims

103440-054PCT (UML 2024-020) CLAIMS 1. A high-entropy alloy catalyst, a composition of which comprises at least five metals of Au, Pd, Fe, Co, Ni, Cu, Mn, Cr, Ag, Pt, or Mo, wherein the high-entropy alloy catalyst is effective to catalyze an electrochemical reaction.

2. The high-entropy alloy catalyst of claim 1, wherein the composition comprises: Au, Pd, Fe, Co, and Ni; Cu, Ni, Co, Mn, and Cr; Mo, Ni, Co, Mn, and Cr; or Au, Ag, Pt, Pd, and Co.

3. The high-entropy alloy catalyst of claim 2, wherein when the composition comprises Au, Pd, Fe, Co, and Ni, a content of the Au is 5 to 31 atomic percent, a content of the Pd is 10 to 40 atomic percent, a content of the Fe is 13 to 52 atomic percent, a content of the Co is 7 to 18 atomic percent, a content of the Ni is 5 to 14 atomic percent, wherein each content is based on a total atomic content of the metals in the high-entropy alloy catalyst.

4. The high-entropy alloy catalyst of claim 1, wherein the electrochemical reaction comprises hydrogen evolution reaction, oxygen reduction reaction, carbon dioxide reduction reaction, or nitrogen reduction reaction.

5. The high-entropy alloy catalyst of claim 1, wherein the high-entropy alloy catalyst is effective to catalyze the electrochemical reaction in an electrolyte of pH 0 to 14.

6. The high-entropy alloy catalyst of claim 1, wherein the high-entropy alloy catalyst is more stable than a Pt catalyst, after 10 hours or longer under an identical condition when used for catalyzing hydrogen evolution reaction, when a comparison is made by normalizing to a mass loading of noble metals in each of the high-entropy alloy catalyst and the Pt catalyst, or wherein a mass activity of the high-entropy alloy catalyst normalized to a mass loading of noble metals of the high-entropy alloy catalyst is greater than a mass activity of the Pt catalyst normalized to a mass loading of Pt, under an identical condition when used for catalyzing hydrogen evolution reaction, or wherein the high-entropy alloy catalyst maintains its catalytic activity for at least 10 hours, optionally at least 240 hours, as determined by a change in an overpotential of 10 % or less, compared to an initial overpotential.

7. The high-entropy alloy catalyst of claim 1, wherein an average size of the high-entropy alloy catalyst is 2 nanometers to 45 nanometers.

8. The high-entropy alloy catalyst of claim 2, when the composition comprises Au, Pd, Fe, Co, and Ni, wherein the high-entropy alloy catalyst has diffraction peaks at diffraction angles of 38.3±0.5°,44.5±0.5°, 64.9±0.5°, and optionally 40.1±0.5°, when analyzed by X-ray diffraction using CuK radiation,or wherein the high-entropy alloy catalyst has a crystal structure comprising a body-centered cubic (bcc), a face-centered cubic (fcc), or a mixed bcc / fcc phase.103440-054PCT (UML 2024-020) 9. The high-entropy alloy catalyst of claim 1, wherein the composition comprises a total noble metal content of 10 wt% or less, based on a total weight of the metals in the high-entropy alloy catalyst, wherein the noble metal comprises Au, Pd, or Pt.

10. The high-entropy alloy catalyst of claim 1, wherein the high-entropy alloy catalyst is further disposed on a support material, wherein the support material comprises carbon materials comprising vulcan carbon, graphene, graphene oxide, reduced graphene oxide, or carbon nanotube, transition metal dichalcogenides comprising MoS2, MoSe2, PtS2, or metal oxides comprising TiO2, ZnO, or CeO2, or a combination thereof.

11. A method of synthesizing a high-entropy alloy catalyst via wet-chemical synthesis, the method comprising: providing at least five metal-halide precursors in a stoichiometric ratio, wherein each of the at least five metal-halide precursors independently comprise a transition metal; contacting the at least five metal-halide precursors with a polymer additive and a polar solvent to provide a reaction mixture; heating the reaction mixture at a temperature greater than a combined melting point of the at least five metal-halide precursors to form resulting particles; washing the resulting particles with the polar solvent to remove remaining precursors or the polymer additive; and separating the resulting particles by centrifugation to synthesize the high-entropy alloy catalyst.

12. The method of claim 11, wherein the transition metal comprises Au, Pd, Fe, Co, Ni, Cu, Mn, Cr, Ag, Pt, Mo, or a combination thereof.

13. The method of claim 11, wherein the transition metal comprises: Au, Pd, Fe, Co, and Ni; Cu, Ni, Co, Mn, and Cr; Mo, Ni, Co, Mn, and Cr; or Au, Ag, Pt, Pd, and Co.

14. The method of claim 11, wherein the metal halide precursor comprises a metal chloride precursor, and the at least five metal halide precursors are in equimolar amounts.

15. The method of claim 11, when the transition metal comprises Au, Pd, Fe, Co, and Ni, the metal halide precursor comprises a metal chloride precursor, and a molar ratio of the metal halide precursors of Au:Pd:Fe:Co:Ni is 0.2:1:1:1:1 to 10:1:1:1:

1.

16. The method of claim 11, wherein the polar solvent is water, optionally deionized water, ethanol, or a combination thereof, and when the water and the ethanol is used, a ratio of the water to the ethanol is 1:

3.

17. The method of claim 11, wherein the polymer additive is poly(N-vinyl-2-pyrrolidone).

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