Method for Synthesizing High-Entropy Alloy (HEA) Nanostructures

Abstract

A method for synthesizing high-entropy alloy (HEA) nanostructures, each having a HEA shell uniformly grown on a nanocore, is provided. The method comprises: mixing nanostructure seeds, a plurality of metal precursors, one or more reducing agents and a surfactant in a solvent to form a first mixture; subjecting the first mixture to ultrasonication under an ultrasonication temperature; degassing the first mixture upon heating at a degassing temperature under vacuum with magnetic stirring; purging the first mixture with an inert gas; and keeping the first mixture at a growth temperature for a growth time to form the HEA nanostructures. The provided method is a low-temperature, facile, general, wet-chemical, seeded epitaxial growth method which can synthesize a library of unconventional-phase HEA nanostructures, e.g., 4H-Au@HEA nanowires and 2H/fcc-Au@HEA nanosheets with 5-10 components by using 4H-Au NWs and 2H/fcc-Au NSs as seeds respectively.

Description

FIELD OF THE INVENTION

The present invention generally relates to synthesis of high-entropy alloy (HEA) nanostructures. More specifically the present invention relates synthesis of unconventional phase high-entropy alloy nanowires and nanosheets for highly efficient electrocatalysis.

BACKGROUND OF THE INVENTION

As an advanced route for clean and sustainable energy conversion, electrocatalysis provides a solution to the worldwide problems on energy crisis and environmental pollution. For instance, H₂ is an efficient clean energy carrier with high-energy density, and has been considered as the most promising clean energy to replace traditional fossil fuels which lead to the depletion of energy resources and severe environmental issues (e.g., global warming and air pollution). As clean and efficient technology, electrochemical water splitting shows great potential in replacing the traditional technology for H₂ production (e.g., steam reforming of natural gas or other light hydrocarbons) in the future world, owing to the advantages of high efficiency, excellent adaptability, and nearly zero carbon emissions. A typical reaction route of water electrolysis includes cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER), and different electrochemical reactions occur in different electrolytes.

The rational synthesis and development of advanced catalysts with high performance are of great importance to efficiently turn raw materials into useful products. To date, the well-established proton exchange membrane (PEM)-based water electrolysis, which operates under acidic conditions, possesses many advantages compared to alkaline water electrolysis, such as compact design, higher voltage efficiency and higher gas purity. However, PEM-based water electrolysis is hampered by the low efficiency, instability and high cost of anodic electrocatalysts for OER. To overcome these disadvantages, more highly active electrocatalysts for both HER and OER are required. As benchmark commercial materials, noble metal catalysts (Pt/C) and noble-metal oxide catalysts (IrO₂ and RuO₂) exhibit satisfying HER and OER performance, respectively. However, their high cost and rare reserves remain limiting the large-scale applications. Furthermore, noble-metal catalysts always suffer from operational instability (e.g., dissolution, agglomeration, and show poor tolerance for poisoning) during HER/OER at high-current densities. Therefore, it is desirable to develop low-cost, high-efficiency, earth-abundant, economical electrocatalysts to substitute the noble-metal-based catalysts.

Noble metal nanomaterials have been extensively used as highly efficient catalysts in diverse electrochemical reactions. Taking the electrochemical HER as an example, platinum-group noble metals, such as Pt, are considered as highly efficient catalysts for HER. However, commercial platinum-group noble metal catalysts, e.g., Pt/C, still suffer from the scarcity, high cost, and poor catalytic stability, hindering their widespread use in practical applications. It is thus imperative to design and develop advanced noble metal catalysts with superior catalytic activity and stability to further enhance the utilization efficiency of noble metal atoms. To date, to the best of the knowledge, most reported catalysts on the market are noble-metal-based nanostructures with relatively simple compositions (i.e., one to three elements). In comparison, as promising catalysts in heterogeneous catalysis, HEAs generally consist of five or more metals, which display structure-dependent synergistic effects and enhanced catalytic performance in terms of activity and durability. Moreover, HEAs can largely decrease the usage of noble metals and thus reduce the cost of catalysts, which have attracted increasing research interests. Therefore, with the development of nanotechnology, HEA catalysts have made great progress in the field of electrocatalysis.

Impressively, the crystal phase has emerged as an increasingly significant structural parameter in determining the physicochemical properties and functionalities of nanomaterials. In particular, unconventional phases could endow nanomaterials with intriguing properties and superior performance in electrocatalysis. Although some research groups reported the HEA nanostructures with thermodynamically stable fcc phase for HER or OER, there is still no commercial technology for the preparation of novel HEA nanocatalysts with unconventional phases.

To date, alkaline water electrolysis technologies have been well developed and even commercially available for industrial H₂ production. In contrast, the well-established PEM electrolysis with faster response, lower Ohmic losses, higher voltage efficiency and gas purity requires to be operated in acidic environment. However, the PEM-based water electrolysis is hampered by the low efficiency, instability and high cost of anodic electrocatalysts for OER. Many OER electrocatalysts are prone to dissolution and/or surface structure transformation under the oxidizing OER potentials in the harsh acidic environment, finally leading to a drastic decrease in catalytic performance.

Recently, HEAs containing five or more elements with similar atomic ratios have aroused extensive research interest in electrocatalysis for their enhanced catalytic activity and stability. Compared with nanomaterials with relatively simple compositions (i.e., one to three elements), HEAs exhibit unique and intriguing advantages, including adjustable compositions, distorted lattices, sluggish diffusion, and synergistic effect between various elements. To date, various advanced synthetic techniques/strategies, such as the thermal-shock-based method, moving bed pyrolysis, acute chemical reduction, sputtering, transient electrosynthesis, and wet-chemical method, etc. have been developed to prepare HEA nanomaterials. However, most of these existing synthetic approaches still suffer from harsh synthetic conditions and complicated synthetic protocols, and are only adopted to prepare HEA nanomaterials with specific components. In addition, although great efforts have been devoted to rational designing HEA nanostructures with tailored compositions, sizes, dimensions and morphologies, the research towards finely tuning the crystal phase of HEA nanomaterials is still lacking, i.e., the rational design and synthesis of HEA electrocatalysts with unconventional phases remain a great challenge.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, a method for synthesizing high-entropy alloy (HEA) nanostructures, each having a HEA shell uniformly grown on a nanocore, is provided. The method comprises: mixing nanostructure seeds, a plurality of metal precursors, one or more reducing agents and a surfactant in a solvent to form a first mixture; subjecting the first mixture to ultrasonication under an ultrasonication temperature; degassing the first mixture upon heating at a degassing temperature under vacuum with magnetic stirring; purging the first mixture with an inert gas; and keeping the first mixture at a growth temperature for a growth time to form the HEA nanostructures.

According to a second aspect of the present invention, a high-entropy alloy (HEA) nanostructure having a HEA shell uniformly grown on a nanocore is provided. The HEA nanostructure is fabricated by the method according to the first aspect of the present invention.

According to a third aspect of the present invention, a bifunctional catalyst comprising HEA nanostructures according to the second aspect of the present invention is provided.

According to a fourth aspect of the present invention, a proton exchange membrane-based electrolyzer comprising an anode and a cathode, both applied with bifunctional catalyst according to the third aspect of the present invention is provided.

In some embodiments of the present invention, the nanostructure seeds are 4H—Au nanowire seeds such that the nanocore is a nanowire formed of 4H—Au.

In some embodiments of the present invention, the nanostructure seeds are 2H/fcc-Au nanosheet seeds such that the nanocore is a nanosheet formed of 2H/fcc-Au.

In some embodiments of the present invention, the plurality of metal precursors includes Pt-based compound, Ir-based compound, Ni-based compound, Co-based compound, and Fe-based compound such that the HEA shell is formed of quinary 4H—IrPtNiCoFe.

In some embodiments of the present invention, the Pt-based compound, Ir-based compound, Ni-based compound, Co-based compound and Fe-based compound are Pt(acac)₂, Ir(acac)₃, Ni(acac)₂, Co(acac)₃ and Fe(acac)₃, respectively.

In some embodiments of the present invention, the 4H—Au nanowire seeds, the metal precursors Pt(acac)₂, Ir(acac)₃, Ni(acac)₂, Co(acac)₃, and Fe(acac)₃ have weight ratio of 5:5:2:3:3.

The method provided by the present invention is a low-temperature, facile, general, wet-chemical, seeded epitaxial growth method which can synthesize a library of unconventional-phase HEA nanostructures, e.g., 4H—Au@HEA nanowires (NWs) and 2H/fcc-Au@HEA nanosheets (NSs) with 5-10 components by using 4H—Au NWs and 2H/fcc-Au NSs as seeds respectively. As a result, a library of 4H—Au@HEA core-shell nanostructures composed of up to ten metallic elements (Ir, Pt, Ni, Fe, Co, Rh, Pd, Ru, Cu, and Mn) can be synthesized by simply tuning the synthetic parameters.

The present invention has the following advantages:

First, compared with traditional thermal-shock-based approaches with higher synthetic temperatures (e.g., 2000 K), the as-developed synthetic strategy is a relatively low-temperature and rapid wet-chemical synthetic method, which greatly simplifies the synthetic protocols, reduces the cost, and increases the reproducibility. This synthetic method can be readily scaled up by amplifying the amounts of reactants and the volume of reactors, showing great potential in the large-scale production of a series of HEA nanostructures with unconventional phases, e.g., 4H—Au@HEA NWs and 2H/fcc-Au@HEA NSs. To a great extent, the protocol simplifies the synthetic strategies for HEA catalysts.

Second, the as-developed synthetic strategy is a general and robust method for the controlled synthesis of a library of HEA nanostructures with different morphologies and dimensions. For example, the provided epitaxial growth strategy can prepare the one-dimensional (1D) HEA nanostructures, i.e., 4H—Au@HEA core-shell NWs with 5-10 components, including quinary 4H—IrPtNiCoFe, senary 4H—IrPtNiCoFeRh, septenary 4H—IrPtNiCoFeRhPd, octonary 4H—IrPtNiCoFeRhPdRu, novenary 4H—IrPtNiCoFeRhPdRuCu and denary 4H—IrPtNiCoFeRhPdRuCuMn HEA on 4H—Au NWs, as well as the two-dimensional (2D) HEA nanostructures, i.e., 2H/fcc-Au@HEA NSs, including quinary 2H/fcc-IrPtNiCoFe, senary 2H/fcc-IrPtNiCoFeRh, septenary 2H/fcc-IrPtNiCoFeRhPd, octonary 2H/fcc-IrPtNiCoFeRhPdRu, novenary 2H/fcc-IrPtNiCoFeRhPdRuCu and denary 2H/fcc-IrPtNiCoFeRhPdRuCuMn HEA on 2H/fcc-Au NSs. Noted that the combination of the metal elements (i.e., Ir, Pt, Ni, Co, Fe, Rh, Pd, Ru, Cu and Mn) in HEA nanostructures can be randomly tuned to achieve different uses. The invention enriches the library of HEA nanostructures with different components, morphologies, dimensions and crystal phases.

Third, the as-prepared HEA nanostructures possess unconventional phases, i.e., HEA NWs with an unconventional 4H phase and HEA NSs with an unconventional 2H/fcc heterophase. To date, to the best of the knowledge, almost all the reported HEA nanostructures possess the thermodynamically stable fcc phase. Moreover, a phase-dependent study of electrocatalytic performance has been systematically investigated. It is found that the bifunctional 4H—Au@IrPtNiCoFe NWs exhibit better HER and OER catalytic performance than that of fcc-IrPtNiCoFe NWs.

Fourth, the as-prepared 4H—Au@HEA core-shell NWs can be used as effective surface enhanced Raman scattering (SERS) platform to in situ detect the Raman signals of molecule absorption and desorption to unravel the real-time HER and OER mechanism. The 4H-HEA shell on 4H—Au is relatively thin (<2 nm), which can be used to “borrow” the electromagnetic field of the SERS-active core (i.e., 4H—Au) to enhance the Raman signals of molecules on or near the shell surface. Therefore, the reactions catalyzed by the 4H-HEA shell can be easily detected and monitored by the in situ SERS method.

Fifth, the 4H—Au@HEA NWs can be used as advanced electrocatalysts with high activity and good long-term stability towards diverse electrochemical reactions such as HER, OER, oxygen reduction reaction (ORR), alcohol oxidation reaction (AOR), and formic acid oxidation reaction (FAOR). Taking the quinary 4H—Au@IrPtNiCoFe HEA NW as an example, the bifunctional 4H—Au@IrPtNiCoFe HEA nanocatalysts exhibit the excellent HER and OER performance, among the best bifunctional multielement alloy catalysts for water splitting in the world. Impressively, the as-prepared 4H—Au@IrPtNiCoFe HEA catalysts exhibit superior catalytic activity and stability at a large current density, showing great potential in industrial-level water splitting. We also applied them as both the anode and cathode catalysts in a PEM electrolyzer to simulate the overall water splitting condition at the industrial level. And the bifunctional 4H—Au@IrPtNiCoFe HEA catalysts exhibit superior industrial-level catalytic activity and stability.

Sixth, the bifunctional 4H—Au@IrPtNiCoFe HEA is a pH-universal catalyst for overall water splitting, which can be used in both alkaline (1 M KOH) and acidic (0.5 M H₂SO₄) conditions. Noted that the 4H—Au core could stabilize the structure of the 4H-HEA shell during the large-current-density water electrolysis via an epitaxial growth manner. After 70 h overall water splitting in 0.5 M H₂SO₄, the structure of 4H—Au@IrPtNiCoFe HEA NWs still maintains, while the structure of fcc-IrPtNiCoFe HEA NWs is severely distorted and aggregated, severely decreasing the catalytic stability. It indicates that the bifunctional 4H—Au@IrPtNiCoFe HEA nanocatalysts can overcome the structural instability problem during electrolysis in the acidic condition, showing great potential in acidic PEM-based electrolyzer.

Therefore, due to the advantage of high-entropy effect (i.e., adjustable compositions, distorted lattice, sluggish diffusion, and synergistic effect between various elements) and unconventional phase, the bifunctional 4H—Au@IrPtNiCoFe HEA catalysts exhibit the world-best performance for HER and OER in both alkaline and acidic conditions. And the 4H—Au@IrPtNiCoFe HEA catalysts show great potential in the pH-universal overall water splitting at the industrial level.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are described in more details hereinafter with reference to the drawings, in which:

FIG. 1 depicts schematic illustration of the epitaxial growth of the quinary IrPtNiFeCo HEA on 4H—Au NWs to form the quinary 4H—Au@IrPtNiFeCo HEA NWs.

FIGS. 2A to 2D depict a low-magnification SEM image, TEM image, ABF-STEM image, and atomic-resolution ABF-STEM image of a representative 4H—Au NWs; indicating the typical atomic stacking sequence of 4H phase, i.e., ABCB stacking. The arrow in FIG. 2B indicates a by-product fcc-Au NP. FIGS. 2E to 2F depicts the corresponding FFT pattern, XRD pattern of the 4H—Au NWs.

FIGS. 3A and 3B depict SEM image and HAADF-STEM image of a representative 4H—Au@IrPtNiFeCo HEA NW; FIG. 3C depicts a corresponding FFT pattern recorded in the pink area in FIG. 3B.

FIG. 4 depicts XRD patterns of the quinary 4H—Au@IrPtNiFeCo HEA NWs and 4H—Au NWs.

FIG. 5A depicts EDS spectrum of the quinary 4H—Au@IrPtNiFeCo HEANWs. The additional element signals arise from the silicon substrate. FIGS. 5B and 5C depict the atomic ratio of different elements in the quinary 4H—Au@IrPtNiFeCo HEANWs obtained by EDS and inductively coupled plasma optical emission spectroscopy (ICP-OES), respectively.

FIG. 6 depicts atomic ratio of different elements of the quinary IrPtNiFeCo HEA on 4H—Au recorded by SEM-EDS.

FIG. 7 depicts STEM image and the corresponding EDS elemental mappings of a segment of the quinary 4H—Au@IrPtNiFeCo HEA NW.

FIG. 8 depicts atomic-resolution HAADF-STEM image showing the interface between the quinary 4H—IrPtNiFeCo HEA and 4H—Au NW.

FIG. 9 depicts the corresponding integrated pixel intensity profiles of the quinary 4H—IrPtNiFeCo HEA in four rectangles in FIG. 8, respectively.

FIGS. 10A to 10F depict XPS spectra of Au 4f, Pt 4f, Ir 4f, Ni 2p, Co 2p and Fe 2p of the quinary 4H—Au@IrPtNiFeCo HEA NWs, respectively.

FIGS. 11A to 11C depict schematic models and the corresponding HAADF-STEM images of the quinary 4H—Au@IrPtNiFeCo HEANWs at the reaction time of 5 min, 20 min and 60 min, respectively; FIGS. 11D to 11F depict the atomic ratio of different elements in the quinary 4H—Au@IrPtNiFeCo HEA NWs obtained by SEM-EDS at the reaction time of 5 min, 20 min and 60 min, respectively.

FIG. 12 depicts HAADF-STEM image, STEM image, EDS elemental mapping images, the corresponding FFT pattern, the recorded atomic-resolution HAADF-STEM image of a senary 4H—Au@IrPtNiFeCoRh HEA NW.

FIGS. 13A to 13C depicts low-magnification SEM image, HAADF-STEM image, and XRD pattern of the senary 4H—Au@IrPtNiFeCoRh HEA NWs, respectively.

FIG. 14A depicts EDS spectrum of the senary 4H—Au@IrPtNiFeCoRh HEA NWs. The additional element signals arise from the silicon substrate. FIG. 14B depicts the atomic ratio of different elements in the senary 4H—Au@IrPtNiFeCoRh HEA NWs obtained by SEM-EDS.

FIGS. 15A to 15G depict Au 4f, Pt 4f, Ir 4f, Ni 2p, Co 2p, Fe 2p and Rh 3d XPS spectra of the senary 4H—Au@IrPtNiFeCoRh HEA NWs, respectively.

FIG. 16 depicts HAADF-STEM image, STEM image, EDS elemental mapping images, the corresponding FFT pattern, the recorded atomic-resolution HAADF-STEM image of a septenary 4H—Au@IrPtNiFeCoRhPd HEA NW.

FIGS. 17A to 17C depicts low-magnification SEM image, HAADF-STEM image, and XRD pattern of the septenary 4H—Au@IrPtNiFeCoRhPd HEA NW, respectively.

FIG. 18A depicts EDS spectrum of the septenary 4H—Au@IrPtNiFeCoRhPd HEA NW. The additional element signals arise from the silicon substrate. FIG. 18B depicts the atomic ratio of different elements in the septenary 4H—Au@IrPtNiFeCoRhPd HEA NW obtained by SEM-EDS.

FIGS. 19A to 19H depict Au 4f, Pt 4f, Ir 4f, Ni 2p, Co 2p, Fe 2p, Rh 3d and Pd 3d XPS spectra of the septenary 4H—Au@IrPtNiFeCoRhPd HEANWs, respectively.

FIG. 20 depicts HAADF-STEM image, STEM image, EDS elemental mapping images, the corresponding FFT pattern, the recorded atomic-resolution HAADF-STEM image of an octonary 4H—Au@IrPtNiFeCoRhPdRu HEA NW.

FIGS. 21A to 21C depicts low-magnification SEM image, HAADF-STEM image, and XRD pattern of the octonary 4H—Au@IrPtNiFeCoRhPdRu HEA NW, respectively.

FIG. 22A depicts EDS spectrum of the octonary 4H—Au@IrPtNiFeCoRhPdRu HEA NW. The additional element signals arise from the silicon substrate. FIG. 22B depicts the atomic ratio of different elements in the octonary 4H—Au@IrPtNiFeCoRhPdRu HEA NW obtained by SEM-EDS.

FIGS. 23A to 23I depict Au 4f, Pt 4f, Ir 4f, Ni 2p, Co 2p, Fe 2p, Rh 3d, Pd 3d, and Ru 3d XPS spectra of the octonary 4H—Au@IrPtNiFeCoRhPdRu HEA NW, respectively.

FIG. 24 depicts HAADF-STEM image, STEM image, EDS elemental mapping images, the corresponding FFT pattern, the recorded atomic-resolution HAADF-STEM image of a novenary 4H—Au@IrPtNiFeCoRhPdRuCu HEA NW.

FIGS. 25A to 25C depict low-magnification SEM image, HAADF-STEM image, and XRD pattern of the novenary 4H—Au@IrPtNiFeCoRhPdRuCu HEA NW, respectively.

FIG. 26A depicts EDS spectrum of the novenary 4H—Au@IrPtNiFeCoRhPdRuCu HEA NW. The additional element signals arise from the silicon substrate. FIG. 26B depicts the atomic ratio of different elements in the novenary 4H—Au@IrPtNiFeCoRhPdRuCu HEA NW obtained by SEM-EDS.

FIGS. 27A to 27J depict Au 4f, Pt 4f, Ir 4f, Ni 2p, Co 2p, Fe 2p, Rh 3d, Pd 3d, Ru 3d and Cu 2p XPS spectra of the novenary 4H—Au@IrPtNiFeCoRhPdRuCu HEANWs, respectively.

FIG. 28 depicts HAADF-STEM image, STEM image, EDS elemental mapping images, the corresponding FFT pattern, the recorded atomic-resolution HAADF-STEM image of a denary 4H—Au@IrPtNiFeCoRhPdRuCuMn HEA NW.

FIGS. 29A to 29C depict low-magnification SEM image, HAADF-STEM image, and XRD pattern of the denary 4H—Au@IrPtNiFeCoRhPdRuCuMn HEA NW, respectively.

FIG. 30A depicts EDS spectrum of the denary 4H—Au@IrPtNiFeCoRhPdRuCuMn HEA NW. The additional element signals arise from the silicon substrate. FIG. 30B depicts the atomic ratio of different elements in the denary 4H—Au@IrPtNiFeCoRhPdRuCuMn HEA NW obtained by SEM-EDS.

FIGS. 31A to 31K depict Au 4f, Pt 4f, Ir 4f, Ni 2p, Co 2p, Fe 2p, Rh 3d, Pd 3d, Ru 3d, Cu 2p and Mn, 2p XPS spectra of the denary 4H—Au@IrPtNiFeCoRhPdRuCuMn HEA NWs, respectively.

FIG. 32A depicts a low-magnification SEM image of 2H/fcc-Au NSs; FIGS. 32B and 32C depicts TEM images of the 2H/fcc-Au NSs. FIG. 32D depicts atomic-resolution HAADF-STEM image of the 2H/fcc-Au NSs, indicating the 2H and fcc heterophase; FIG. 32E depicts the corresponding FFT pattern of FIG. 32D; and FIG. 32F depicts XRD pattern of the 2H/fcc-Au NSs.

FIG. 33A depicts HAADF-STEM image, STEM image, EDS elemental mapping images, the corresponding FFT pattern, the recorded atomic-resolution HAADF-STEM image of a senary 2H/fcc-Au@IrPtNiFeCoRh HEA NS; FIG. 33B depicts HAADF-STEM image, STEM image, EDS elemental mapping images, the corresponding FFT pattern, the recorded atomic-resolution HAADF-STEM image of a septenary 2H/fcc-Au@IrPtNiFeCoRhPd HEA NS; FIG. 33C depicts HAADF-STEM image, STEM image, EDS elemental mapping images, the corresponding FFT pattern, the recorded atomic-resolution HAADF-STEM image of an octonary 2H/fcc-Au@IrPtNiFeCoRhPdRu HEA NS; FIG. 33D depicts HAADF-STEM image, STEM image, EDS elemental mapping images, the corresponding FFT pattern, the recorded atomic-resolution HAADF-STEM image of a novenary 2H/fcc-Au@IrPtNiFeCoRhPdRuCu HEA NS; FIG. 33E depicts HAADF-STEM image, STEM image, EDS elemental mapping images, the corresponding FFT pattern, the recorded atomic-resolution HAADF-STEM image of a denary 2H/fcc-Au@IrPtNiFeCoRhPdRuCuMn HEA NS.

FIGS. 34A to 34E depict TEM image, HAADF-STEM image, atomic-resolution HAADF-STEM image, the corresponding FFT pattern, and XRD pattern of a quinary fcc-IrPtNiFeCo HEA NW, respectively.

FIG. 35A depicts STEM and the corresponding EDS elemental mappings of the quinary fcc-IrPtNiFeCo HEA NW; FIG. 35B depicts EDS spectrum; FIGS. 35C and 35D depict atomic ratios of different elements in the quinary fcc-IrPtNiFeCo HEA NW obtained by EDS and ICP-OES, respectively.

FIGS. 36A to 36E depict Pt 4f, Ir 4f, Ni 2p, Co 2p and Fe 2p XPS spectra of the quinary fcc-IrPtNiFeCo HEA NW, respectively.

FIGS. 37A and 37B depict TEM image and HRTEM image of the commercial Pt/C, respectively.

FIG. 38A depicts HER polarization curves of 4H—Au@IrPtNiFeCo HEA NWs, fcc-IrPtNiFeCo HEA NWs and commercial Pt/C recorded in N₂-saturated 0.5 M H₂SO₄ electrolyte at a scan rate of 5 mV s⁻¹; FIG. 38B depicts Tafel plots for HER obtained from the corresponding polarization curves in FIG. 38A; FIG. 38C depicts comparison of the overpotentials for different HER catalysts at the current density of 10 mA cm⁻² and 100 mA cm⁻²; FIG. 38D depicts comparison of the overpotentials for different OER catalysts at the current density of 10 mA cm⁻² and 100 mA cm⁻².

FIGS. 39A and 39B depict cyclic voltammograms recorded for the 4H—Au@IrPtNiFeCo HEA NWs and the fcc-IrPtNiFeCo HEA NWs in the approximate region of 0.81-0.91 V vs. RHE at various scan rates, respectively; FIG. 39C depicts capacitive currents at 0.86 V as a function of scan rate of the 4H—Au@IrPtNiFeCo HEA NWs and the fcc-IrPtNiFeCo HEANWs in 0.5 M H₂SO₄.

FIGS. 40A and 40B depict EIS Nyquist plots of the 4H—Au@IrPtNiFeCo HEA NWs and fcc-IrPtNiFeCo HEA NWs, respectively.

FIG. 41 depicts chronoamperometric test result for HER of the 4H—Au@IrPtNiFeCo HEA NWs at a current density of 10 mA cm⁻².

FIG. 42A depicts a TEM image of a quinary 4H—Au@IrPtNiFeCo HEA NWs loaded on carbon; FIGS. 42B to 42E depict TEM image, HRTEM image, the corresponding FFT pattern; STEM and the corresponding EDS elemental mappings (of a segment) of the quinary 4H—Au@IrPtNiFeCo HEA NW loaded on carbon after the accelerated durability test for HER, confirming the 4H phase maintains after the accelerated durability test for HER.

FIG. 43A depicts a TEM image of a quinary fcc-IrPtNiFeCo HEA NWs loaded on carbon; FIGS. 43B to 43E depict TEM image, HRTEM image, the corresponding FFT pattern; STEM and the corresponding EDS elemental mappings (of a segment) of the quinary fcc-IrPtNiFeCo HEA NW loaded on carbon after the accelerated durability test for HER, confirming the fcc phase maintains after the accelerated durability test for HER.

FIGS. 44A and 44B depict TEM image and HRTEM image of the commercial IrO₂, respectively.

FIG. 45A depicts OER polarization curves of 4H—Au@IrPtNiFeCo HEA NWs, fcc-IrPtNiFeCo HEA NWs and commercial IrO₂ recorded in N₂-saturated 0.5 M H₂SO₄ electrolyte at a scan rate of 5 mV s⁻¹; FIG. 45B depicts Tafel plots for OER obtained from the corresponding polarization curves in FIG. 45A; FIG. 45C depicts comparison of the overpotentials for different OER catalysts at the current density of 10 mA cm⁻² and 100 mA cm⁻²; FIG. 45D depicts comparison of overpotential at the current density of 10 mA cm⁻² for 4H—Au@IrPtNiFeCo HEA NWs and some previously reported multielement electrocatalysts for OER.

FIG. 46 depicts chronoamperometric test result for OER of the 4H—Au@IrPtNiFeCo HEA NWs at a current density of 10 mA cm⁻².

FIG. 47A depicts a TEM image of a quinary 4H—Au@IrPtNiFeCo HEA NWs loaded on carbon; FIGS. 47B to 47E depict TEM image, HRTEM image, the corresponding FFT pattern; STEM and the corresponding EDS elemental mappings (of a segment) of the quinary 4H—Au@IrPtNiFeCo HEA NW loaded on carbon after the accelerated durability test for OER, confirming the 4H phase maintains after the accelerated durability test for OER.

FIG. 48A depicts a TEM image of a quinary fcc-IrPtNiFeCo HEA NWs loaded on carbon; FIGS. 48B to 48E depict TEM image, HRTEM image, the corresponding FFT pattern; STEM and the corresponding EDS elemental mappings (of a segment) of the quinary fcc-IrPtNiFeCo HEA NW loaded on carbon after the accelerated durability test for OER, confirming the fcc phase maintains after the accelerated durability test for OER.

FIG. 49A depicts schematic diagrams of a PEM electrolyzer according to one embodiment of the present invention; FIG. 49B depicts bifunctional 4H—Au@IrPtNiFeCo HEA NWs for HER and OER.

FIG. 50A depicts polarization curves of 4H—Au@IrPtNiFeCo∥4H—Au@IrPtNiFeCo, fcc-IrPtNiFeCo∥fcc-IrPtNiFeCo and commercial Pt/C∥commercial IrO₂;

FIG. 50B depicts comparison of the overpotentials at the current density of 10 mA cm⁻², 100 mA cm⁻² and 500 mA cm⁻²; and FIG. 5C depicts chronopotentiometry tests at 100 mA of the 4H—Au@IrPtNiFeCo∥4H—Au@IrPtNiFeCo, fcc-IrPtNiFeCo∥fcc-IrPtNiFeCo and commercial Pt/C∥commercial IrO₂ in a PEM electrolyzer carried out at room temperature.

DETAILED DESCRIPTION

In the following description, embodiments of the present invention are set forth as preferred examples. It will be apparent to those skilled in the art that modifications, including additions and/or substitutions may be made without departing from the scope and spirit of the invention. Specific details may be omitted so as not to obscure the invention; however, the disclosure is written to enable one skilled in the art to practice the teachings herein without undue experimentation.

A facile and general wet-chemical synthetic method is developed to achieve the controlled synthesis of a series of HEA nanomaterials with unconventional phases, including 4H—Au@HEA NWs and 2H/fcc-Au@HEA NSs. All the starting chemicals including solvents, precursors and surfactants are available from commercial sources.

Exemplary chemicals used in the synthetic method include: Iridium (III) acetylacetonate (Ir(acac)₃, 97%), Platinum (II) acetylacetonate (Pt(acac)₂, 97%), nickel (II) acetylacetonate (Ni(acac)₂, 97%), cobalt (III) acetylacetonate (Co(acac)₃, 97%), iron (III) acetylacetonate (Fe(acac)₃, 97%), rhodium (III) acetylacetonate (Rh(acac)₃, 97%), palladium (II) acetylacetonate (Pd(acac)₂, 97%), ruthenium (III) acetylacetonate (Ru(acac)₃, 97%), copper (II) acetylacetonate (Cu(acac)₂, 97%), manganese (III) acetylacetonate (Mn(acac)₃, 97%), tungsten hexacarbonyl (W(CO)₆, 99%), glucose, oleylamine (OM, 70%), oleylamine (OM, ≥98%), (1-Hexadecyl) trimethylammonium chloride (CTAC, 99%), stearyl trimethyl ammonium bromide (STAB), gold (III) chloride hydrate (HAuCl₄·3H₂O, ˜49% Au basis), N-ethylcyclohexylamine (99%), 1,2-dichloropropane (99%), hexane (technical grade), toluene (technical grade, ≥99%) and ethanol (technical grade) were purchased from Sigma-Aldrich.

Briefly, the method used for the synthesis of 4H-HEA NWs comprises the usage of 4H—Au NWs as template, metal (Ir, Pt, Ni, Co, Fe, Rh, Pd, Ru, Cu and Mn) salts as precursors, oleylamine (OAm) as solvent, (1-Hexadecyl) trimethylammonium chloride (CTAC) as surfactant as well as W(CO)₆ and glucose as reducing agents. After sonicating for 1 h, the above-mentioned mixture was heated to a relatively low temperature and reacted for 1 h. Then a library of 4H—Au@HEA core-shell NWs with 5-10 components can be successfully synthesized, including quinary 4H—IrPtNiCoFe, senary 4H—IrPtNiCoFeRh, septenary 4H—IrPtNiCoFeRhPd, octonary 4H—IrPtNiCoFeRhPdRu, novenary 4H—IrPtNiCoFeRhPdRuCu and denary 4H—IrPtNiCoFeRhPdRuCuMn HEA on 4H—Au NWs via an epitaxial growth strategy.

The synthesis of 2H/fcc-Au@HEA NSs, including quinary 2H/fcc-IrPtNiCoFe, senary 2H/fcc-IrPtNiCoFeRh, septenary 2H/fcc-IrPtNiCoFeRhPd, octonary 2H/fcc-IrPtNiCoFeRhPdRu, novenary 2H/fcc-IrPtNiCoFeRhPdRuCu and denary 2H/fcc-IrPtNiCoFeRhPdRuCuMn HEA on 2H/fcc-Au NSs via an epitaxial growth strategy, is similar to that of 4H—Au@HEA NWs except using 2H/fcc-Au NSs as seeding templates.

Synthetic Protocol for 4H—Au NWs

In a typical synthetic protocol for 4H—Au NWs, 20 mg of HAuCl₄·3H₂O, 1.5 mL of oleylamine (98%), 400 μl of N-ethylcyclohexylamine, 20 mL of hexane and 200 μl of 1,2-dichloropropane were thoroughly mixed in a 40 mL glass vial. The vial was then sealed with PTFE tape and parafilm before being heated for 48 h in an oil bath pre-set at 57° C. The product was collected by centrifugation at 4000 rpm for 3 min, and washed with toluene for three times and then redispersed in 10 mL of toluene. The yield of 4H—Au NWs was about 2.5 mg.

Synthetic Protocol for 2H/fcc-Au NSs

In a typical synthetic protocol for 2H/fcc-Au NSs, 10 mg of HAuCl₄·3H₂O, 3 mL of 4-tert-butylpyridine, 4 mL of OAm (98%) and 5 mL of heptane were thoroughly mixed in a 20 mL glass vial. The vial was then sealed with PTFE tape and parafilm before being heated for 4 h in an oil bath pre-set at 80° C. The product was collected by centrifugation at 3000 rpm for 3 min, and washed with toluene for three times and then redispersed in 10 mL of toluene. The yield of 2H/fcc-Au NSs was about 1 mg.

Synthesis of 4H—Au@HEA Core-Shell NWs

As depicted in FIG. 1, by using the synthesized 4H—Au NWs (FIGS. 2A-2F) as seeds, a library of 4H-HEAs with up to ten metallic elements (Ir, Pt, Ni, Fe, Co, Rh, Pd, Ru, Cu, and Mn) can be epitaxially grown on the 4H—Au NWs to form the 4H—Au@HEA NWs via a facile and rapid wet-chemical route.

As shown in FIG. 2F, the XRD pattern displays five prominent peaks located at 36.2°, 37.4°, 38.2°, 40.9° and 64.8° which can be assigned to the (100)_4H, (101)_4H, (004)_4H, (102)_4H and (110)_4H planes of 4H—Au, respectively. The peak located at 44.3° is from a small amount of by-product, i.e., fcc-Au nanoparticles (NPs), obtained during the synthesis of 4H—Au NWs. Note that the peaks located at 38.2° and 64.8°, attributed to the (111)_f and (220)_f planes of the by-product fcc-Au NPs, are overlapped with the (004)_4H and (110)_4H planes of 4H—Au NWs, respectively.

In a typical synthesis of quinary 4H—Au@IrPtNiCoFe HEA core-shell NWs, 5 mg of as-prepared 4H—Au NWs, 5 mg of Pt(acac)₂, 5 mg of Ir(acac)₃, 2 mg of Ni(acac)₂, 3 mg of Co(acac)₃, 3 mg of Fe(acac)₃, 30 mg of W(CO)₆, 60 mg of glucose and 40 mg of CTAC were dissolved in 5 mL of oleylamine (OAm), followed by ultrasonication for 1 h at 30° C. The mixture was degassed upon heating at 50° C. under vacuum with vigorous magnetic stirring for 10 minutes. Then, the mixture was purged with Ar and heated to 250° C. for 1 h. The cooled product was collected by centrifugation at 4000 rpm for 3 min and washed three times with a toluene/ethanol (v/v=4:1) mixture. After that, the 4H—Au@IrPtNiCoFe HEA NWs were collected by centrifugation, and then redispersed in 10 mL toluene for storage.

The method can be extended to synthesize a library of 4H—Au@HEA core-shell NWs with 6-10 components, including senary 4H—Au@IrPtNiCoFeRh, septenary 4H—Au@ IrPtNiCoFeRhPd, octonary 4H—Au@IrPtNiCoFeRhPdRu, novenary 4H—Au@ IrPtNiCoFeRhPdRuCu and denary 4H—Au@IrPtNiCoFeRhPdRuCuMn HEA NWs, via changing the precursor ratio, reaction temperature, and time. The content of each metal salt was different in the precursor solutions, which were summarized in Table 1.

TABLE 1
Synthetic parameters of various HEA catalysts
	Synthetic parameters
	Amounts of metal precursors
	[Ir]/	[Pt]/	[Ni]/	[Fe]/	[Co]/	[Rh]/	[Pd]/	[Ru]/	[Cu]/	[Mn]/	Temp/	Time/
Catalysts	mg	mg	mg	mg	mg	mg	mg	mg	mg	mg	° C.	min
4H—Au@	5	5	2	3	3	/	/	/	/	/	250	60
IrPtNiFeCo
4H—Au@	5	5	2	3	3	6	/	/	/	/	250	60
IrPtNiFeCoRh
4H-Au@	5	5	2	3	3	6	5	/	/	/	280	60
IrPtNiFeCoRhPd
4H-Au@	5	5	2	3	3	6	5	8	/	/	280	90
IrPtNiFeCoRhPdRu
4H-Au@	5	5	2	3	3	6	5	8	4	/	280	90
IrPtNiFeCoRhPdRuCu
4H-Au@	5	5	2	3	3	6	5	8	4	5	280	90
IrPtNiFeCoRhPdRuCuMn
fcc-IrPtNiFeCo	5	5	4	7	7	/	/	/	/	/	210	300

It is worth mentioning that the synthetic conditions for the preparation of 4H—Au@ HEA NWs are tunable. The metal precursor can be chosen from the salt of acetylacetonate, chloride and nitrate (e.g., Nickel salts can be chosen from Ni(acac)₂, NiCl₂ and Ni(NO₃)₂). The reducing reagents can be chosen from W(CO)₆, Mo(CO)₆, formaldehyde and CO flow (CO molecule released from those reagents can facilitate the reducing reaction) as well as glucose. The surfactants can be chosen from CTAC and dodecyl trimethyl ammonium chloride (DTAC). Noted that only chloride surfactant is suitable, while bromide surfactant could induce the phase transition of Au NWs from 4H to fcc, and iodide surfactant could etch the Au NWs. The reaction temperature ranges from about 250° C. to about 280° C., while the reaction time can be in the range from 60 min to 90 min. Noted that some metal elements (e.g., Ru and Mn) are relatively difficult to be reduced, it is appropriate to increase the reaction temperature and prolong the reaction time. The synthetic process can be conducted under an inert atmosphere which can be either an Ar atmosphere or a N₂ atmosphere. The synthesis of 4H—Au@HEA NWs can be readily scaled up by proportionally increasing the amounts of reactants, including 4H—Au NWs, reducing reagents, solvent, metal precursors and surfactants.

Synthesis of 2H/fcc-Au@HEA Core-Shell NSs

By changing the 4H—Au NWs to 2H/fcc-Au NSs as seeding templates, the above synthetic method may be used to synthesize a library of 2H/fcc-Au@HEA core-shell NSs with 5-10 components, including quinary 2H/fcc-Au@IrPtNiCoFe, senary 2H/fcc-Au@IrPtNiCoFeRh, septenary 2H/fcc-Au@IrPtNiCoFeRhPd, octonary 2H/fcc-Au@IrPtNiCoFeRhPdRu, novenary 2H/fcc-Au@IrPtNiCoFeRhPdRuCu and denary 2H/fcc-Au@IrPtNiCoFeRhPdRuCuMn HEA NSs.

Synthesis of Fcc-IrPtNiFeCo HEA NWs

In a typical synthesis of quinary fcc-IrPtNiCoFe HEA NWs, 5 mg of Pt(acac)₂, 5 mg of Ir(acac)₃, 4 mg of Ni(acac)₂, 7 mg of Co(acac)₃, 7 mg of Fe(acac)₃, 10 mg of W(CO)₆, 60 mg of glucose, and 40 mg of stearyl trimethyl ammonium bromide (STAB) were dissolved in 5 mL of OAm, followed by ultrasonication for 1 h at 30° C. The mixture was degassed upon heating at 50° C. under vacuum with vigorous magnetic stirring for 10 minutes. Then, the mixture was purged with Ar and heated to 210° C. for 5 h. The cooled product was collected by centrifugation at 8000 rpm for 5 min and washed three times with a toluene/ethanol (v/v=1:2) mixture. After that, the fcc-IrPtNiCoFe HEA NWs were collected by centrifugation, and then redispersed in 10 mL toluene for storage.

Typical Characterization Methodology

Transmission electron microscopy (TEM) and dark-field scanning TEM (DF-STEM) images were obtained by using JEOL 2100F (Japan) operated at 200 kV. The high-angle annular DF-STEM (HAADF-STEM) images, annular bright-field STEM (ABF-STEM) images, and energy dispersive X-ray spectroscopy (EDS) results were obtained by JEOL ARM200F (JEOL, Tokyo, Japan) operated at 200 kV and equipped with double spherical aberration (Cs) correctors. Scanning electron microscope (SEM) images were recorded on a scanning electron microscope (Thermo Fisher Scientific, QUATTRO S). Rigaku SmartLab and Bruker D8 ADVANCE X-ray powder diffractometers with Cu Kα radiation sthece (λ=1.5406 Å) were used to record the X-ray diffraction (XRD) patterns of samples. The concentration of Au was measured by ICP-OES (PerkinElmer, Optima 8000DV). X-ray photoelectron spectroscopy (XPS) measurements were carried out on a VG ESCALAB 220I-XL system. C is peak (284.8 eV) was used to calibrate the binding energy of other elements. Raman measurements were conducted on a Renishaw inVia™ conformal Raman microscope with an excitation wavelength of 532 nm and a low power density of <5 mW/um⁻².

Characterization of 4H—Au@IrPtNiFeCo HEA Core-Shell NWs

As shown in the scanning SEM image (FIG. 3A), the as-prepared quinary 4H—Au@IrPtNiFeCo HEA maintains a wire-like morphology with a length of a few micrometers, similar to the original 4H—Au NWs (FIGS. 2A and 2B). The HAADF-STEM image of a typical 4H—Au@IrPtNiFeCo HEA NW (FIG. 3B) reveals that a thin HEA shell is uniformly grown on the 4H—Au core to form the core-shell structure. The HAADF-STEM image (FIG. 3A) and the corresponding fast Ftheier transform (FFT) pattern (FIG. 3C) confirm that the Au@IrPtNiFeCo HEA NW still maintains the 4H phase after the epitaxial growth of HEA shell, suggesting its good structural stability. The characteristic peaks located at 36.2°, 37.4°, 38.2°, 40.9° and 64.8° in the XRD pattern (FIG. 4) are ascribed to the (100)_4H, (101)_4H, (004)_4H, (102)_4H and (110)_4H planes of 4H phase, respectively. The peak located at 44.3° is from a small amount of by-product, i.e., fcc-Au nanoparticles (NPs) and fcc-IrPtNiFeCo HEA NWs, obtained during the synthesis of 4H—Au@IrPtNiFeCo HEA NWs (FIG. 2B). SEM-EDS shows that the atomic ratio of HEA shell/Au core in the quinary 4H—Au@IrPtNiFeCo HEA NWs is 37.95/62.05, which is close to and showing good agreement with that from ICP-OES measurement (40.45/59.55) (FIGS. 5A to 5C). And the atomic ratio of metal elements in the HEA shell on 4H—Au is with a Ir/Pt/Ni/Fe/Co ratio of 19.71/21.92/20.97/19.85/17.55 (FIG. 6). The EDS elemental mappings corroborate the uniform distribution of Ir, Pt, Ni, Fe and Co in the HEA shell on Au NW (FIG. 7). Furthermore, the atomic-resolution HAADF-STEM image shows that the distorted HEA shell in the prepared 4H—Au@IrPtNiFeCo HEA NW possesses the pure 4H phase, and the thickness of the thin HEA shell is ˜1-2 nm (FIG. 8). The integrated pixel intensities of two repeating units with the stacking sequence of “ABCB” along the [001]_4H direction in the 4H—IrPtNiFeCo HEA shell from the selected areas (FIG. 8) are shown in FIG. 9, where the average lattice spacing varies from 2.40 Å (g₃) to 2.41 Å (g₂) to 2.43 Å (g₄) and to 2.44 Å (g₁), illustrating lattice distortions in the as-prepared quinary 4H—IrPtNiFeCo HEAs on 4H—Au NWs.

The XPS was used to characterize the chemical valence of metal elements in the quinary 4H—Au@IrPtNiFeCo HEA NWs (FIGS. 10A to 10F). As shown in FIG. 10B, Pt 4f_7/2 and Pt 4f_5/2 peaks are located at 71.1 and 74.5 eV, respectively, which are assigned to zero valences. The Ir 4f spectra show the Ir mainly exists in the form of zero valences (60.5 eV) and a small amount of 4⁺ valence (61.2 eV) (FIG. 10C). The Ni 2p spectra display the coexistence of Ni⁰ (852.6 eV) and Ni²⁺ (855.7 eV) and a satellite peak located at 861.4 eV (FIG. 10D). The two Co 2p peaks can be assigned to Co 2p_3/2 (781.0 eV) and Co 2p_1/2 (797.2 eV) (FIG. 10E). The Fe 2p spectra in FIG. 10F shows two peaks at 711.6 eV and 724.8 eV, which can be attributed to Fe 2p_3/2 and Fe 2p_1/2, respectively. The XPS results indicate that the quinary 4H—Au@IrPtNiFeCo HEA NWs show metallic states.

To investigate the formation process of 4H—IrPtNiFeCo HEAs on 4H—Au NWs, time-dependent experiments were conducted at the reaction time of 5 min, 20 min and 60 min, and the corresponding products were characterized by HAADF-STEM and SEM-EDS (FIGS. 11A to 11F). As shown in FIGS. 11A and 11D, an atomically thin layer mainly consisted of transition metals (i.e., Ni and Fe) and Pt first appeared on the surface of 4H—Au NW at the reaction time of 5 min. As the reaction time increases to 20 min, abundant multielement alloy domains are formed on the 4H—Au NW via an island growth due to the large mismatch between the transition-metal elements in multielement alloy and Au, which could suppress the continuous layer-by-layer growth of multielement alloy on the surface of 4H—Au (FIGS. 11B and 11E). Then, with the increase of Ir and Co during the reduction reaction, the multielement alloy gradually changed into HEA with a balanced metal-element atomic ratio. Meanwhile, the mismatch between transition-metal elements in multielement alloy and Au gradually decreased with the increase of the noble metal atomic ratio. Finally, the multielement alloy domains merged seamlessly to form a uniform 4H—IrPtNiFeCo HEA shell on 4H—Au NW via an epitaxy manner at the reaction time of 60 min (FIGS. 11C and 11F).

Notably, the as-developed synthetic strategy is a general and robust method for the controlled synthesis of a library of 4H—Au@HEA core-shell NWs with an unconventional 4H phase, including senary 4H—Au@IrPtNiFeCoRh (FIGS. 12, 13A to 13C, 14A and 14B, 15A to 15G), septenary 4H—Au@IrPtNiFeCoRhPd (FIGS. 16, 17A to 17C, 18A and 18B, 19A to 19H), octonary 4H—Au@IrPtNiFeCoRhPdRu (FIGS. 20, 21A to 21C, 22A and 22B, 23A to 23I), novenary 4H—Au@IrPtNiFeCoRhPdRuCu (FIGS. 24, 25A to 25C, 26A and 26B, 27A to 27J) and denary 4H—Au@IrPtNiFeCoRhPdRuCuMn (FIGS. 28, 29A to 29C, 30A and 30B, 31A to 31K) HEA core-shell NWs, by simply adjusting the content of each salt in the metal precursors, reaction temperature and time. Impressively, all of the above-mentioned 4H—Au@HEA core-shell NWs possess the pure 4H phase, characterized by the HAADF-STEM, the corresponding FFT and XRD. Moreover, STEM-EDS elemental mappings and SEM-EDS spectra indicate that the 6-10 metal elements are evenly distributed on the 4H—Au NWs with a balanced atomic ratio. The XPS results demonstrate that all of the prepared 4H—Au@HEA core-shell NWs show metallic states.

In addition, a library of 2D HEA nanostructures, i.e., 2H/fcc-Au@HEA core-shell NSs, including quinary 2H/fcc-IrPtNiCoFe, senary 2H/fcc-IrPtNiCoFeRh, septenary 2H/fcc-IrPtNiCoFeRhPd, octonary 2H/fcc-IrPtNiCoFeRhPdRu, novenary 2H/fcc-IrPtNiCoFeRhPdRuCu and denary 2H/fcc-IrPtNiCoFeRhPdRuCuMn HEA on 2H/fcc-Au NSs (FIGS. 32A to 32F), via a similar epitaxial growth strategy (FIGS. 33A to 33E).

Electrocatalytic Performances of 4H—Au@IrPtNiFeCo HEA Core-Shell NWs

As known, HEA nanomaterials with tailored compositions, sizes, dimensions and morphologies are promising catalysts for various electrochemical reactions, including HER, OER, ORR, AOR and CO₂RR. However, to date, the crystal-phase-dependent study of HEA nanomaterials in electrocatalysis has been hardly reported.

As a proof-of-concept application, the HER performance of 4H—Au@IrPtNiFeCo HEA NWs with an Ir/Pt/Ni/Fe/Co ratio of ˜1.12/1.19/1.24/1.13/1 was investigated by a three-electrode system in acidic media. The three-electrode system includes a carbon paper electrode coated with a catalyst, the graphite rod, and the Ag/AgCl (saturated KCl) was employed as the working electrode, counter electrode, and reference electrode, respectively. The Ag/AgCl electrode was calibrated with respect to a reversible hydrogen electrode (RHE). All electrochemical measurements were conducted in N₂-saturated 0.5 M H₂SO₄ aqueous solution, or performed on a CHI750e electrochemical workstation (Shanghai Chenhua Instrument Corporation, China) at room temperature.

A catalyst ink was prepared. Firstly, the as-prepared catalysts were subsequently stirred for 3 days at room temperature. The catalyst containing 1 mg of 4H—Au@IrPtNiFeCo HEA NWs was re-dispersed in a mixed solution of isopropanol (700 μl) and Milli-Q water (280 μl). Subsequently, 20 μl of Nafion solution (5%) was added into the mixture which was then sonicated for 30 min to obtain the catalyst ink.

The working electrode was prepared by drop-casting 250 μg of the prepared catalyst ink (containing 4H—Au@IrPtNiFeCo HEA NWs) on a 0.5×0.5 cm carbon paper electrode (the mass loading of the catalyst is 1 mg cm⁻²). The obtained working electrode was dried under ambient conditions until the solvent was completely evaporated.

A Hg/HgO electrode may be used as the reference electrode, and Pt mesh (1.0×1.0 cm) as the counter electrode. Before the test, highly pure O₂ (99.999%) was bubbled into the electrolyte for 15 min. linear sweep voltammetry was conducted at a scan rate of 5 mV s⁻¹ between 1.2 and 1.8 V versus RHE.

Before electrocatalytic measurements, cyclic voltammetry (CV) was employed at a potential range of 0.05-0.8 V (vs. RHE) to remove the capping agents on the surface of catalysts. All the polarization curves were measured at room temperature with a scan rate of 5 mV s⁻¹ between −0.4 and 0.05 V versus RHE. For the accelerated durability test of 4H—Au@IrPtNiFeCo HEA NWs catalyst in N₂-saturated 0.5 M H₂SO₄ electrolyte, chronopotentiometry was measured under a constant current density of 10 mA cm⁻² at room temperature. Electrochemical impedance spectroscopy (EIS) measurements were carried out in the frequency range of 0.1 Hz-100 kHz with 10 mV amplitude to obtain the solution resistance (Rs). All the polarization curves were corrected by 90% iRs compensation. Current densities were normalized by the geometric area of the electrode.

To investigate the crystal-phase-dependent performance of HEA nanomaterials, fcc-IrPtNiFeCo HEA NWs (FIGS. 34A to 34E, 35A to 35D, 36A to 36E) synthesized by using a previously reported synthetic method with slight modification were also used as a catalyst for HER. As a comparison, the HER performances of commercial Pt/C (FIGS. 37A and 37B) were also tested under the same conditions.

The HER polarization curves of these catalysts were recorded in N₂-saturated 0.5 M H₂SO₄ at a scan rate of 5 mV s⁻¹ (FIG. 38A). Impressively, the 4H—Au@IrPtNiFeCo HEA delivers a low overpotential of only 7 mV and 88 mV to achieve the current densities of 10 and 100 mA cm⁻², respectively, which are lower than those of fcc-IrPtNiFeCo HEA (12 and 98 mV) and commercial Pt/C (16 and 207 mV) (FIG. 38C).

It is worth mentioning that the fcc-IrPtNiFeCo HEA NWs, which possess similar composition, dimension and morphology with the 4H—Au@IrPtNiFeCo HEA NWs, require a higher overpotential of 5 mV to achieve the current density of 10 mA cm⁻², indicating the significant role of unconventional 4H phase in boosting the HER performance of HEA catalysts. Such low overpotential (7 mV) of the 4H—Au@IrPtNiFeCo HEA catalyst to drive the current density of 10 mA cm⁻² toward HER is among the best of the reported multielement-alloy-based HER catalysts under acidic conditions (FIG. 38D and Table 2).

TABLE 2
Summary of electrocatalytic HER performances of Multielement-
metal-based catalysts in acidic media in the present invention
and some previous representative works
		Over-
		potential	Tafel
		(mV) at	slope
Catalyst	Electrolyte	10 mA cm−2	(mV dec−1)
4H—Au@IrPtNiFeCo NWs	0.5M H2SO4	7	22.6
fcc-IrPtNiFeCo NWs	0.5M H2SO4	12	39.9
Commercial Pt/C	0.5M H2SO4	16	27.5
fcc-IrPdPtRhRu HEA NPs	1M HClO4	6	N.A.
fcc-2H-fcc-Pd45@Ir55 NPs	0.1M HClO4	11	26
fcc-PtRuRhCoNi HEA NWs	0.5M H2SO4	13	23.8
fcc-PdMoGaInNi HEA NSs	0.5M H2SO4	13	179.8
fcc-Co—RuIr NPs	0.1M HClO4	14	31.1
fcc-PdCu/Ir NPs	0.1M HClO4	20	N.A.
fcc-IrCo-PHNC NPs	0.5M H2SO4	21	26.6
fcc-PtIr/PtOx NWs	0.5M H2SO4	23	38
fcc-NiCoFePtRh HEA NPs	0.5M H2SO4	27	30.1
fcc-Au@AuIr2 NPs	0.5M H2SO4	29	15.6

Moreover, FIG. 38B shows the analysis of Tafel slopes which was conducted to evaluate the kinetic behaviors of the catalysts in HER. The Tafel slope of 4H—Au@IrPtNiFeCo HEA (27.1 mV dec⁻¹) is lower than those of fcc-IrPtNiFeCo HEA (31.0 mV dec⁻¹) and commercial Pt/C (32.6 mV dec⁻¹), suggesting the faster reaction kinetics of 4H—Au@IrPtNiFeCo HEA catalyst during the HER process.

In addition, the double-layer capacitance (C_dl) was obtained by recording CV curves at different scanning rates from 20 to 100 mV s⁻¹ (FIGS. 39A to 39C), eventually determining the electrochemically active surface area (ECSA) of the catalysts. The C_dl of 4H—Au@IrPtNiFeCo HEA is 5.3 mF cm⁻², larger than that of fcc-IrPtNiFeCo HEA (3.7 mF cm⁻²), evidencing the significant active-site exposure of 4H—Au@IrPtNiFeCo HEA catalysts.

The EIS curves shown in (FIGS. 40A and 40B) display a smaller charge transfer resistance (R_ct) of 4H—Au@IrPtNiFeCo HEA (1.2Ω) than that of fcc-IrPtNiFeCo HEA (2.0Ω), suggesting the facilitated electron transfer and faster electrocatalytic kinetics for HER.

Finally, the long-term electrochemical stability of 4H—Au@IrPtNiFeCo HEA catalyst in 0.5 M H₂SO₄ was also investigated by the chronoamperometric test. The current density in the chronoamperometric curve shows a negligible variation under a constant current density of 10 mA cm⁻² during the HER test for 80 h, indicating the good catalytic stability of the 4H—Au@IrPtNiFeCo HEA for HER under acidic conditions (FIG. 41).

Moreover, TEM images and FFT patterns of 4H—Au@IrPtNiFeCo HEA after the durability test show that the wire-like morphology and the 4H phase still maintain (FIGS. 42A to 42D). The EDS elemental mapping confirms the uniform distribution of metal elements on 4H—Au NWs after the durability test (FIG. 42E).

In comparison, the fcc-IrPtNiFeCo HEA NWs are severely distorted and aggregated, resulting in relatively poor HER stability (FIGS. 43A to 43E).

Impressively, the 4H—Au@IrPtNiFeCo HEA NWs can also exhibit crystal-phase-dependent OER performances in acidic media. For comparison, the fcc-IrPtNiFeCo HEA NWs and commercial IrO₂ (FIGS. 44A and 44B) were also measured in N₂-saturated 0.5 M H₂SO₄.

As shown in the polarization curves in FIG. 45A, the 4H—Au@IrPtNiFeCo HEA delivers a low overpotential of only 225 mV and 294 mV to achieve the current densities of 10 and 100 mA cm⁻², respectively, which are lower than those of fcc-IrPtNiFeCo HEA (248 and 344 mV) and IrO₂ (308 and 509 mV) (FIG. 45C). Notably, Such a low overpotential (225 mV) of the 4H—Au@IrPtNiFeCo HEA catalyst to drive the current density of 10 mA cm⁻² toward OER is among the lowest values of the reported multielement-alloy-based OER catalysts under acidic conditions (FIG. 45D and Table 3).

TABLE 3
Summary of electrocatalytic OER performances of Multielement-
metal-based catalysts in acidic media in this work
and some previous representative works
		Overpotential	Tafel
		(mV) at	slope
Catalyst	Electrolyte	10 mA cm−2	(mV dec−1)
4H—Au@IrPtNiFeCo NWs	0.5M H2SO4	225	46.8
fcc-IrPtNiFeCo NWs	0.5M H2SO4	248	53.1
Commercial IrO2	0.5M H2SO4	308	63.0
fcc-np-AlNiCoIrMo HEAs	0.5M H2SO4	233	55.2
fcc-Co—RuIr NPs	0.1M HClO4	235	66.9
fcc-Au@AuIr2 NPs	0.5M H2SO4	261	N.A.
fcc-Pd@Ir3L nanocubes	0.1M HClO4	263	59.3
fcc-IrCuNi NPs	0.5M H2SO4	273	41
fcc-PdCu/Ir NPs	0.1M HClO4	283	59.6
fcc-PtIr/PtOx NWs	0.5M H2SO4	285	N.A.
fcc-Rh22Ir78 NPs	0.5M H2SO4	292	N.A.
fcc-Pt-doped IrNi NPs	0.1M HClO4	308	47.5
fcc-IrCo-PHNC NPs	0.5M H2SO4	309	53.8

Furthermore, the Tafel slope of 4H—Au@IrPtNiFeCo HEA is as low as 46.8 mV dec⁻¹, which is lower than those of fcc-IrPtNiFeCo HEA (53.1 mV dec⁻¹) and commercial IrO₂ (63.0 mV dec⁻¹), implying the fastest reaction kinetics of 4H—Au@IrPtNiFeCo HEA toward acidic OER (FIG. 45B).

Finally, the stability of the 4H—Au@IrPtNiFeCo HEA catalyst was evaluated via a long-term durability test in 0.5 M H₂SO₄. The current density in the chronoamperometric curve shows a negligible variation under a constant current density of 10 mA cm⁻² during the OER test for 70 h, implying the good catalytic stability of the 4H—Au@IrPtNiFeCo HEA for OER under acidic condition (FIG. 46).

Moreover, TEM images and FFT patterns of 4H—Au@IrPtNiFeCo HEA after the durability test show that the wire-like morphology and the 4H phase still maintain (FIGS. 47A to 47D). The EDS elemental mapping confirms the uniform distribution of metal elements on 4H—Au NWs after the durability test (FIG. 47E).

In comparison, the fcc-IrPtNiFeCo HEA NWs are severely distorted and aggregated, resulting in relatively poor OER stability (FIGS. 48A to 48E).

The superior HER and OER performances of the 4H—Au@IrPtNiFeCo HEA catalyst could be explained as follows.

First, compared with the thermodynamically stable fcc phase, the unconventional 4H phase in the HEA catalyst could contribute to the enhancement of its catalytic activity. As evidenced by the previous studies, the unconventional 4H phase in metal nanomaterials can serve as undercoordinated active sites for electrocatalysis owing to the optimized electronic structure deriving from the unique atomic arrangements. For example, the 4H—Au nanoribbons display advantages in both activity and selectivity over fcc-Au nanorods in electrocatalytic CO₂RR.

Second, the 4H—IrPtNiFeCo HEA nanostructure exhibits unique intrinsic advantages, including adjustable compositions, distorted lattice, sluggish diffusion, and synergistic effect between various elements, resulting in enhanced electrocatalytic performance.

Third, the electronic structures of core-shell heterostructures can also be modified by the internal strain arising from the lattice mismatch between the 4H—Au core and 4H—IrPtNiFeCo HEA shell with lattice distortion, leading to the optimized adsorption free energy of hydrogen on the catalyst surface.

Fourth, the 4H—Au core in the 4H—Au@IrPtNiFeCo HEA catalyst can serve as a solid framework, which supports the 4H—IrPtNiFeCo HEA nanostructure without distortion and aggregation during the long-term electrocatalysis, resulting in the outstanding catalytic stability of the 4H—Au@IrPtNiFeCo HEA catalyst.

Electrochemical Measurements in PEM Electrolyzer

To further evaluate the overall water splitting performance of the bifunctional 4H—Au@IrPtNiFeCo HEA catalyst in industrial operating systems, a prototype PEM electrolyzer with 4H—Au@IrPtNiFeCo HEA NWs as both the anode and cathode catalysts (FIGS. 49A and 49B) is fabricated. The fcc-IrPtNiFeCo∥fcc-IrPtNiFeCo and the benchmark Pt/C∥IrO₂ electrode couple were used as comparisons.

First, the Nafion 117 membranes (DuPont) were sequentially treated with 3 wt % H₂O₂, 0.5 M H₂SO₄, and Milli-Q water at 80° C. for 1 h. After cooling to room temperature, the treated N117 membranes were washed and preserved in Milli-Q water. The 4H—Au@IrPtNiFeCo HEA catalysts were used as both the cathode and anode materials for the PEM electrolyzer. The as-prepared catalyst ink was then dropped on the carbon cloth (FuelCellStore, USA) and dried in the vacuum overnight. After the membrane electrode assembly (MEA) was constructed with the 4H—Au@IrPtNiFeCo HEA NWs or Pt/C-coated carbon cloth (cathode), treated Nafion 117 membrane (separator), and the 4H—Au@IrPtNiFeCo HEA NWs or commercial IrO₂/carbon paper electrode (anode), it was applied in the PEM electrolyzer.

The PEM electrolyzer was operated at room temperature using 0.5 M H₂SO₄ as the electrolyte solution under the flowing rate of 8 mL min⁻¹. LSV curves were measured at a scan rate of 5 mV s⁻¹. The stability of the PEM electrolyzer using 4H—Au@IrPtNiFeCo HEA NWs as both cathode and anode catalyst, denoted as 4H—Au@IrPtNiFeCo∥4H—Au@IrPtNiFeCo electrode couple, was evaluated by measuring chronopotentiometry at 100 mA cm⁻² for 70 h.

The fcc-IrPtNiFeCo∥fcc-IrPtNiFeCo and the benchmark Pt/C∥IrO₂ electrode couples were used as comparisons. The polarization curves indicate that the cell voltages of 4H—Au@IrPtNiFeCo∥4H—Au@IrPtNiFeCo electrode couple required only 1.43 V and 1.63 V to reach a current of 10 mA and 100 mA, respectively, which are lower than those of the fcc-IrPtNiFeCo∥fcc-IrPtNiFeCo electrode couple (1.52 V and 1.67 V) and the benchmark Pt/C∥IrO₂ electrode couple (1.57 V and 1.75 V) (FIGS. 50A and 50B). Upon applying a constant current of 100 mA, no substantial increase of 4H—IrPtNiFeCo∥4H—Au@IrPtNiFeCo electrode couple in cell voltage was observed after 70 h of overall water electrolysis, while the cell voltages of fcc-IrPtNiFeCo∥fcc-IrPtNiFeCo and Pt/C∥IrO₂ electrode couples significantly increased after 40 h and 50 h, respectively (FIG. 50C). A detailed comparison of the PEM performance of the work with previously reported multielement-alloy-based catalysts is provided in Table 4. These results strongly demonstrate the great potential of the bifunctional 4H—Au@IrPtNiFeCo HEA catalysts for practical green H₂ production in the PEM electrolyzer.

TABLE 4
Summary of overall water splitting performances of electrocatalysts in acidic
media in the present invention and some previous representative works
		Current	Voltage	Stability
Catalyst	Electrolyte	density	(V)	(h)
4H—Au@IrPtNiFeCo∥4H—Au@IrPtNiFeCo	0.5M H2SO4	10	1.43
		100	1.63	72
fcc-IrPtNiFeCo∥fcc-IrPtNiFeCo	0.5M H2SO4	10	1.52
		100	1.67	50
Commercial Pt/C∥Commercial IrO2	0.5M H2SO4	10	1.57
		100	1.75	40
Ir-NSG∥Ir-NSG	0.1M HClO4	10	1.42	24
Li—IrSe2∥Li—IrSe2	0.5M H2SO4	10	1.44	24
RuIrOx∥RuIrOx	0.5M H2SO4	10	1.45	24
IrOx/GDY∥IrOx/GDY	0.5M H2SO4	10	1.49	30
RuCu NSs ∥RuCu NSs	0.5M H2SO4	10	1.49	15
SS Pt—RuO2 HNSs∥SS Pt—RuO2 HNSs	0.1M HClO4	10	1.49	100
Ru/Co—N—C∥Ru/Co—N—C	0.5M H2SO4	10	1.49	20
Ru0.85Zn0.15O2-δ∥Ru0.85Zn0.15O2-δ	0.5M H2SO4	10	1.50	50
d-ZnIr(OH)6 NSs∥d-ZnIr(OH)6 NSs	0.5M H2SO4	10	1.508	10
Ir-SA@Fe@NCNT∥Ir-SA@Fe@NCNT	0.5M H2SO4	10	1.51	12
Co—RuIr∥Co—RuIr	0.1M HClO4	10	1.52	25
RuTe2 PNRs∥RuTe2 PNRs	0.5M H2SO4	10	1.52	24
Ru NSs∥Ru NSs	0.5M H2SO4	10	1.53	~2
RuB2∥RuB2	0.5M H2SO4	10	1.53	~9
Ir@N-G-750∥Ir@N-G-750	0.5M H2SO4	10	1.54	40

The embodiments may be chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications that are suited to the particular use contemplated. While the methods disclosed herein have been described with reference to particular operations performed in a particular order, it will be understood that these operations may be combined, sub-divided, or re-ordered to form an equivalent method without departing from the teachings of the present disclosure. Accordingly, unless specifically indicated herein, the order and grouping of the operations are not limitations. While the apparatuses disclosed herein have been described with reference to particular structures, shapes, materials, composition of matter and relationships . . . etc., these descriptions and illustrations are not limiting. Modifications may be made to adapt a particular situation to the objective, spirit and scope of the present disclosure. All such modifications are intended to be within the scope of the claims appended hereto.

Claims

1. A method for synthesizing high-entropy alloy (HEA) nanostructures, each having a HEA shell uniformly grown on a nanocore, the method comprising: mixing nanostructure seeds, a plurality of metal precursors, one or more reducing agents and a surfactant in a solvent to form a first mixture;subjecting the first mixture to ultrasonication under an ultrasonication temperature;degassing the first mixture upon heating at a degassing temperature under vacuum with magnetic stirring;purging the first mixture with an inert gas; andkeeping the first mixture at a growth temperature for a growth time to form the HEA nanostructures.

2. The method of claim 1, wherein the nanostructure seeds are 4H—Au nanowire seeds such that the nanocore is a nanowire formed of 4H—Au.

3. The method of claim 2, wherein the plurality of metal precursors includes Pt-based compound, Ir-based compound, Ni-based compound, Co-based compound, and Fe-based compound such that the HEA shell is formed of quinary 4H—IrPtNiCoFe.

4. The method of claim 3, wherein the Pt-based compound, Ir-based compound, Ni-based compound, Co-based compound and Fe-based compound are Pt(acac)2, Ir(acac)3, Ni(acac)2, Co(acac)3 and Fe(acac)3, respectively.

5. The method of claim 4, wherein the 4H—Au nanowire seeds, the metal precursors Pt(acac)2, Ir(acac)3, Ni(acac)2, Co(acac)3, and Fe(acac)3 have weight ratio of 5:5:2:3:3.

6. The method of claim 1, wherein the nanostructure seeds are 2H/fcc-Au nanosheet seeds such that the nanocore is a nanosheet formed of 2H/fcc-Au.

7. The method of claim 6, wherein the plurality of metal precursors includes Pt-based compound, Ir-based compound, Ni-based compound, Co-based compound, and Fe-based compound such that the HEA shell is formed of quinary 2H/fcc-IrPtNiCoFe.

8. The method of claim 7, wherein the Pt-based compound, Ir-based compound, Ni-based compound, Co-based compound and Fe-based compound are Pt(acac)2, Ir(acac)3, Ni(acac)2, Co(acac)3 and Fe(acac)3, respectively.

9. The method of claim 8, wherein the 4H—Au nanowire seeds, the metal precursors Pt(acac)2, Ir(acac)3, Ni(acac)2, Co(acac)3, and Fe(acac)3 have weight ratio of 5:5:2:3:3.

10. A high-entropy alloy (HEA) nanostructure having a HEA shell uniformly grown on a nanocore, wherein the HEA nanostructure is synthesized by the method of claim 1.

11. The high-entropy alloy (HEA) nanostructure of claim 10, wherein the nanostructure seeds are 4H—Au nanowire seeds such that the nanocore is a nanowire formed of 4H—Au.

12. The high-entropy alloy (HEA) nanostructure of claim 11, wherein the plurality of metal precursors includes Pt-based compound, Ir-based compound, Ni-based compound, Co-based compound, and Fe-based compound such that the HEA shell is formed of quinary 4H—IrPtNiCoFe.

13. The high-entropy alloy (HEA) nanostructure of claim 12, wherein the Pt-based compound, Ir-based compound, Ni-based compound, Co-based compound and Fe-based compound are Pt(acac)2, Ir(acac)3, Ni(acac)2, Co(acac)3 and Fe(acac)3, respectively.

14. The high-entropy alloy (HEA) nanostructure of claim 13, wherein the 4H—Au nanowire seeds, the metal precursors Pt(acac)2, Ir(acac)3, Ni(acac)2, Co(acac)3, and Fe(acac)3 have weight ratio of 5:5:2:3:3.

15. The high-entropy alloy (HEA) nanostructure of claim 14, wherein the nanostructure seeds are 2H/fcc-Au nanosheet seeds such that the nanocore is a nanosheet formed of 2H/fcc-Au.

16. The high-entropy alloy (HEA) nanostructure of claim 15, wherein the plurality of metal precursors includes Pt-based compound, Ir-based compound, Ni-based compound, Co-based compound, and Fe-based compound such that the HEA shell is formed of quinary 2H/fcc-IrPtNiCoFe.

17. The high-entropy alloy (HEA) nanostructure of claim 16, wherein the Pt-based compound, Ir-based compound, Ni-based compound, Co-based compound and Fe-based compound are Pt(acac)2, Ir(acac)3, Ni(acac)2, Co(acac)3 and Fe(acac)3, respectively.

18. The high-entropy alloy (HEA) nanostructure of claim 17, wherein the 4H—Au nanowire seeds, the metal precursors Pt(acac)2, Ir(acac)3, Ni(acac)2, Co(acac)3, and Fe(acac)3 have weight ratio of 5:5:2:3:3.

19. A bifunctional catalyst comprising the high-entropy alloy (HEA) nanostructures of claim 10.

20. A proton exchange membrane-based electrolyzer, comprising an anode and a cathode, both applied with bifunctional catalyst of claim 19.