A method of forming a high-entropy oxide nanostructure

Abstract

The present disclosure broadly relates to a method of preparing a supported high-entropy oxide nanostructure The method may comprise the step of: irradiating, with a laser, a substrate coated with a hydrogel to form the high-entropy oxide nanostructure, wherein the hydrogel comprises at least five metal salts, a cross-linking agent, a carbonaceous substance and water to form a high-entropy oxide nanostructure. There is also disclosed herein a high- entropy oxide nanostructure produced by the method as well as the use of the high-entropy oxide nanostructure for forming hydrogen.

Description

[0001] A Method of Forming a High-Entropy Oxide Nanostructure

[0002] References to Related Application

[0003] This application claims priority to Singapore Application No. 10202402296R filed with the Intellectual Property Office of Singapore on 1 August 2024, the contents of which are incorporated herein by reference.

[0004] Technical Field

[0005] The present invention generally relates to methods of preparing a high-entropy oxide nanostructure. The present invention also relates to a high-entropy oxide nanostructure produced by the method and use of the high-entropy oxide nanostructure for forming hydrogen, where the high-entropy oxide nanostructure acts as a catalyst in electrolysis of water.

[0006] Background Art

[0007] One of the most fundamentally and technologically important electrochemical reactions is the electrolysis of water which can generate eco-friendly and renewable clean energy (in particular, hydrogen gas) for metal -air batteries, fuel cells, and other green energy conversion systems. Although the design of such energy conversion systems is complicated, the main principles are fundamentally based on a two-electrode unit, where an oxygen evolution reaction (OER) occurs at the anode and a hydrogen evolution reaction (HER) or oxygen reduction reaction (ORR) occurs at the cathode. However, to ensure efficient water electrolysis, catalysts must be used to overcome the high thermodynamic barrier in electron transfer processes. Alkaline water electrolysis (AWE) and proton exchange membrane water electrolysis (PEMWE) are two technologies which may produce hydrogen gas with high purity. Noble metals have been recognised as excellent electrocatalysts, for example. Pt, RuOz, and IrOj. However, their rarity, poor stability and high cost can hamper their large- scale fabrication in practice. Consequently, new catalysts that can lower the loading of precious metals while retaining effective properties are in demand. Moreover, the highly alkaline or acidic conditions required in many systems can typically shorten the lifetime of traditional metal -based catalysts.

[0008] There is a need for rapid and efficient methods of synthesising catalysts with high stability. High entropy alloy (HEA) is a class of alloys that are generated by five or more metals, wherein the metals are in equal mole amounts. HEA is attractive due to its outstanding mechanical performance, corrosion resistance, and adjustable properties.

[0009] A variation of HEAs are high-entropy oxides (HEOs), which correspondingly comprise five or more different metal cations arranged in a single-phase crystal lattice structure. HEOs share certain properties with HEAs owing to their lattice distortions and high-entropy configurations. They also share what is referred to as a “cocktail effect”, wherein synergistic properties may' exceed what is predicted from the individual components of the HEA / HEO. HEOs and HEAs can also share properties such as improved stability, mechanical robustness, ionic / electronic conduction, dielectric behaviour, and catalytic activity. Nanoparticle-sized HEAs / HEOs, in particular, have the ability to facilitate distinctive catalytic reactions for nitrogen reduction reaction (NRR), CO? reduction, methanol and ethanol oxidations, as well as oxygen and hydrogen evolution reactions. The composition of a HEA / HEO catalyst significantly influences the catalytic performance. It has been reported that the combination of Co, Ni, Fe and Cu may enhance the stability of the OER.

[0010] Moreover, the differences in atomic radii between various metals may cause lattice strain in HEA / HEO nanoparticles, which can improve their surface properties and promote catalytic activity. It has been widely^ reported that the defects on electrocatalyst surface optimize the electronic structure, charge distribution and local environment around active sites, improving the adsorption behaviour during electrocatalysis. Although defect engineering has been well developed in various kinds of catalysts, such as metal oxide, lay ered double hydroxide, and two-dimensional materials, it is rare to induce defects in HEA / HEO catalysts to enhance their intrinsic activities.

[0011] Accordingly, there is a need for new methods of generating HEOs comprising suitable defect sites for electrocatalytic activity.

[0012] Accordingly, there is a need to provide a method for preparing HEO catalysts, and uses thereof that overcomes, or at least ameliorates, one or more of the disadvantages described above, or to provide a useful alternative.

[0013] Summary

[0014] According to a first aspect, there is provided a method for preparing a high-entropy oxide, the method comprising the step of: irradiating, with a laser, a substrate coated with a hydrogel to form the high-entropy oxide nanostructure, wherein the hydrogel comprises at least five metal salts, a cross -linking agent, a carbonaceous substance and water.

[0015] Advantageously, the method described herein may provide a relatively fast means of producing the high-entropy oxide nanostructure, with relatively lower material requirements and lower operating costs. The high-entropy oxide nanostructure may be formed when coated onto the substrate, forming a high-entropy oxide nanostructure -coated substrate.

[0016] According to a second aspect, there is provided a high-entropy oxide nanostructure prepared by the method as described herein.

[0017] Advantageously, the high-entropy oxide nanostructure described herein may provide properties suitable for catalysing water electrolysis.

[0018] According to a third aspect, there is provided an electrode, membrane electrode assembly or electrochemical cell comprising the high-entropy oxide nanostructure as described herein.

[0019] According to a fourth aspect, there is provided use of the electrode, the membrane electrode assembly or electrochemical cell as described herein to produce hydrogen.

[0020] Advantageously, the electrode or membrane electrode assembly as described herein may provide effective means of producing hydrogen as a fuel source. Definitions

[0021] The following words and terms used herein shall have the meaning indicated:

[0022] As used herein, the singular forms “a”, ‘"an”, and ‘"the” designate both the singular and the plural, unless expressly stated to designate the singular only.

[0023] A “membrane’’ is a layer that can be used to separate electrodes in an electrochemical reaction while allowing for ion transfer between them.

[0024] An “anion exchange membrane” (or “AEM”) is a semipermeable membrane intended to allow anion transfer but prevents the movement of other substances, such as gases. A “proton exchange membrane” (or “PEM”) is a selectively permeable membrane that allows H1ions to pass while preventing the passage of other substances such as hydrogen or oxygen gas. A “membrane electrode assembly” (or “MEA”) is an arrangement within a cell comprising a membrane (such as a PEM or AEM) sandwiched between the electrodes. An MEA may facilitate the cell’s electrochemical reactions by allowing ion exchange between the electrodes while electrically insulating them.

[0025] The “oxygen evolution reaction” (or “OER”) is a chemical reaction that produces oxygen gas from water. In an electrochemical cell, the OER occurs at the surface of the anode.

[0026] The “hydrogen evolution reaction” (or “HER”) is a chemical reaction that produces hydrogen gas from water. The “oxygen reduction reaction” (or “ORR”) is a chemical reaction that reduces oxygen gas to water or hydroxide ions. In an electrochemical cell, the HEW and ORR occur at the surface of the cathode.

[0027] A “high-entropy alloy” (or “HEA”) is an alloy system comprising at least five principal metal elements in near-equal proportions. A “high-entropy oxide” (or “HEO”) is a single-phase oxide system comprising multiple metal cation species, wherein the metal elements are in near-equal proportions. HEAs and HEOs have high configurational entropy which can stabilise simple crystal structures, which may lead to enhanced mechanical, thermal, and chemical properties such as high strength, ductility, corrosion resistance and thermal stability.

[0028] A “xerogel” is a porous, solid material that is formed by drying a hydrogel. The xerogel may be described as a form of hydrogel, or as a dried hydrogel. In a xerogel, much of the water content of the hydrogel has been removed, although the 3D network formed by the hydrogel components is partly or substantially retained in the xerogel. A xerogel may be formed by heating a hydrogel to partly or substantially remove the hydrogel water content through evaporation. A xerogel and a dried hydrogel may be referred to as a hydrogel.

[0029] The word “substantially” does not exclude “completely” e.g. a composition which is “substantially free” from Y may be completely free from Y. Where necessary, the word “substantially” may be omitted from the definition of the invention.

[0030] Unless specified otherwise, the terms “comprising" and “comprise”, and grammatical variants thereof, are intended to represent “open” or “inclusive” language such that they include recited elements but also permit inclusion of additional, unrecited elements. As used herein, the term “about”, in the context of concentrations of components of the formulations, typically means + / - 5% of the stated value, more typically + / - 4% of the stated value, more typically + / - 3% of the stated value, more typically, + / - 2% of the stated value, even more typically + / - 1% of the stated value, and even more typically + / - 0.5% of the stated value. Moreover, “about” may be understood by persons of ordinary skill in the art to allow for small or non-substantial variations reflecting the appropriate level of precision according to the context in which it is used.

[0031] Throughout this disclosure, certain embodiments may be disclosed in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the disclosed ranges. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub-ranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range.

[0032] As used herein, the term “about” and the use of ranges in general, whether or not qualified by the term about, means that the number comprehended is not limited to the exact number set forth herein, and is intended to refer to values substantially within the quoted range while not departing from the scope of the invention.

[0033] Certain embodiments may also be described broadly and generically herein. Each of the narrower species and subgeneric groupings falling within the generic disclosure also form part of the disclosure. This includes the generic description of the embodiments with a proviso or negative limitation removing any subject matter from the genus, regardless of whether or not the excised material is specifically recited herein.

[0034] Detailed Disclosure of Embodiments

[0035] Exemplary', non-limiting embodiments of a method for preparing a high-entropy oxide nanostructure will now' be disclosed.

[0036] The method for preparing a high-entropy oxide nanostructure comprises the step of: irradiating, with a laser, a substrate coated with a hydrogel to form the high-entropy oxide nanostructure, wherein the hydrogel comprises at least five metal salts, a cross-linking agent, a carbonaceous substance and water.

[0037] The metal salts may be selected from a ruthenium salt, a cobalt salt, a nickel salt, a zinc salt, a manganese salt, an iridium salt, a magnesium salt, a copper salt, an iron salt, a chromium salt, a bismuth salt, an aluminium salt, a gallium salt, a cerium salt, a gadolinium salt, a lanthanum salt, a neodymium salt, a praseodymium salt, a selenium salt, a platinum salt, a gold salt, a rhodium salt, a palladium salt, a molybdenum salt, or a yttrium salt. The metal salts may be five metal salts.

[0038] The metal salts may be metal halides, such as metal chlorides. Where the metal salts are chlorides of ruthenium, manganese, cobalt, nickel, zinc, iridium, palladium or rhodium, metal chlorides may then be RuCh 3H2O, MnCh 4H2O, CoCl26H2O, NiCl26H2O, ZnCl22H2O, IrCl3 3H2O, PdCl2 2H2O, or RhCT nH2O. wherein n is from 0 to 3. Corresponding chlorides to the other metal salts mentioned in the preceding paragraph can be used accordingly.

[0039] The metal salts may be metal nitrates. The metal nitrates may be Mn(NO34l RO. Co(NO3)26H2O, NI(N03)2-6H2(X Zn(NO3)26H2O, Pd(NO3)2, Rh(NO3)3, Ir(NO3)3, or RU(NO)(NO3)3. Corresponding nitrates to the other metal salts mentioned in the preceding paragraph can be used accordingly.

[0040] The metal salts may be metal acids. The metal acids may be chloroplatinic acid, chloroauric acid, or phosphomolybdic acid.

[0041] Each metal of the at least five metal salts may have a mole ratio that differs by not more than about 5%, or about 4%, or about 3%, or about 2%, or about 1%, or about 0.5%, or about 0.3%, or about 0.2%, or about 0.1%, between each metal. This mole ratio may be considered as an equimolar ratio. The metals of the at least five metal salts may have an equimolar ratio of the metals within the five metal salts.

[0042] The carbonaceous substance may be selected from graphite, graphene, carbon nanofibers, carbon nanospheres, carbon black, hard carbon, activated carbon or a combination thereof. The carbonaceous substance may be graphite.

[0043] The crosslinking agent may be selected from polyvinyl alcohol, alginate, chitosan, gelatin, carrageenan, PEG, PEGDA, polyacrylamide, polyacrylate, polyaspartic acid, nanocellulose or a combination thereof. The crosslinking agent may be polyvinyl alcohol. The crosslinking agent may help to confine the metal precursors onto the substrate and may facilitate the in- situ growth of the HEO nanostructure upon laser irradiation.

[0044] The crosslinking agent may undergo carbonisation. The carbonaceous substance may serve as a photothermal agent to generate localised temperatures of over 2000 K within milliseconds when the laser is applied to the hydrogel. During this irradiation, the crosslinking agent, such as polyvinyl alcohol, may undergo carbonisation, resulting in the formation of a carbonaceous skeleton. The irradiation may occur in a low vacuum environment. The carbonaceous skeleton may help to increase the surface area of the electrode and / or act as a protective matrix to enhance the stability of the resulting high-entropy oxide nanostructure in harsh electrochemical environments.

[0045] The hydrogel may further comprise ethylene glycol, propylene glycol or glycerol. The relatively high water content of the hydrogel (which then may be further dried to form a dried hydrogel, for example, in the form of a xerogel) may allow for the effective dissolution and homogeneous dispersion of the various precursors. This uniform distribution may be useful for ensuring an atomic -level mixing of multiple metal elements, which directly contributes to the formation of homogeneous, single -phase high-entropy oxide crystals during the laser irradiation process.

[0046] The method may further comprise, before the irradiating step, a step of coating the hydrogel onto the substrate. The coating step may comprise dip coating, drop casting, spin coating or spray coating.

[0047] The method may further comprise, before the coating step, a step of forming the hydrogel from a hydrogel solution. The hydrogel solution, which may also be referred to herein as the precursor solution, may be heated to form the hydrogel. The hydrogel solution may be formed by dissolving the metal salts in a solvent, followed by adding the crosslinking agent and the carbonaceous substance. The solvent may be selected from water, ethylene glycol solution, propylene glycol solution or glycerol solution. The resulting solution may be made homogeneous by stirring and / or heating to form the hydrogel solution. The hydrogel solution may then be heated to a suitable temperature under suitable stirring conditions and duration to dissolve the crosslinking agent and thereby form the hydrogel. The heating temperature may be about 60 °C to about 100 °C, about 60 °C to about 80 °C, about 80 °C to about 100 °C, about 70 °C to about 90 °C, or about 75 °C to about 85 °C. The stirring conditions may be magnetic stirring at around 350 rpm to 450 rpm and for a duration of about 15 minutes to about 45 minutes.

[0048] The method may further comprise, afterthe coating step, a step of drying the coated substrate. The drying step may convert the hydrogel into a dried hydrogel. The dried form of the hydrogel may be a xerogel. The drying step may reduce the water content of the hydrogel by more than about 10%, about 20%, about 30%, about 40%, about 50%, about 60%, about 70%, about 80%, about 90%, about 95%, about 98%, or about 99%. The drying step may comprise drying the coated substrate at a temperature of between ambient temperature to about 150 °C, or between about 40 °C to about 120 °C, or between about 60 °C to about 100 °C or between about 70 °C to about 90 °C. The drying may take place in ambient air or under inert gas.

[0049] The wavelength of the laser may be between about 800 nm to about 1300 nm, or between about 900 nm to about 1200 nm, or between about 1000 nm to about 1100 nm, or between about 1040 nm to about 1080 nm. The laser spot may have a diameter of about 1 pm to about 100 pm, or about 5 pm to about 70 pm, or about 10 pm to about 50 pm, or about 20 pm to about 40 pm. The output power of the laser may be between about 5 to about 200 W, or about 10 to about 100 W, or about 30 to about 50 W. The scanning speed of the laser may be between about 50 to about 2000 mm / s, or about 100 to about 1000 mm / s, or about 300 to about 500 mm / s. The pulse duration of the laser may be between about 0.1 ns to about 20 ns, or about 0.3 ns to about 10 ns, about 1 ns to about 3 ns. The line scanning range of the laser may be between about 5 to about 100 pm, about 10 to about 60 pm, or about 20 to about 40 pm.

[0050] The “laser-assisted pyroly sis synthesis” (or “LAPS”) is a protocol w hich uses a hydrogel, for example, a dried hydrogel such as in the form of a xerogel, and laser-induced carbothermal technology to rapidly coat a catalytical high-entropy oxide nanostructure onto a substrate. During LAPS, the soluble metal precursors may be uniformly dissolved and fixed in the hydrogel. With the assistance of laser irradiation, the hydrogel may be heated to thousands of degrees evaporating the non-metal parts and oxidising the metal precursors to form the high-entropy oxides (for example Co, Ni, Mn, Zn, Ru, Ir) in several seconds. This may provide a fast-coating protocol which can effectively “print” the high-entropy oxides as a nanostructure layer onto a substrate. The LAPS protocol may produce high-entropy oxide nanostructures largely and / or densely. In comparison to the traditional synthesis methods, the method described herein may yield small and / or uniform high entropy structure. This may help to ensure that more active sites of high-entropy oxide catalysts can be exposed to electrolytes, which enables the acceleration of the catalytic reaction. The LAPS protocol as described herein where photothermal agents such as the carbonaceous substances are used allows the generation of localised temperatures to over 2000 K within milliseconds, evaporating many of the non-metal-based components, and helping to facilitate the formation of compositionally uniform high-entropy oxide in a relatively short duration.

[0051] The substrate may be selected from titanium, tungsten, platinum, or an alloy thereof. The high-entropy oxide nanostructure when coated onto the substrate, may collectively with the substrate, be termed as a high-entropy oxide nanostructure-coated substrate.

[0052] The titanium may be titanium foil or titanium foam. The substrate may be a tungsten crucible. The method may further include a step of removing the formed high-entropy oxide from the tungsten crucible to obtain a high-entropy oxide nanostructure powder. The high-entropy oxide nanostructure powder may be mixed with Nafion solution, which may be sprayed on the sides of a Nafion membrane and dried. One of more of the high entropy oxide nanostructure -coated titanium foam substrates may be pressed with one or more of the high-entropy oxide nanostructure-coated Nafion membranes to form a membrane electrode assembly (MEA).

[0053] The high-entropy oxide may be RuMnCoNiZn or RuIrCoNiZn.

[0054] Exemplary', non-limiting embodiments of the high entropy oxide nanostructure -coated substrate and high entropy oxide nanostructure powder as described herein will now be disclosed.

[0055] The high-entropy oxide nanostructure may be polycrystalline. The high-entropy oxide nanostructures may consist of nanoparticles with an average particle diameter of between about 2 to about 20 nm, or about 3 to about 15 nm, or about 4 to about 10 nm, or about 5 to about 8 nm, or about 6 to about 7 nm.

[0056] Exemplary’, non-limiting embodiments of the electrode, membrane electrode assembly or electrochemical cell comprising the high-entropy oxide nanostructure -coated substrate, and uses thereof, will now be disclosed.

[0057] The electrode or membrane electrode assembly may use an acidic electrolyte. The acidic electrolyte may be a H2SO4 solution. The working temperature of the electrode or membrane electrode assembly may be in the range of about 40 to about 100 °C, or about 60 to about 80 °C

[0058] The use of the electrode, membrane electrode assembly or electrochemical cell as described herein is to produce hydrogen from water. The hydrogen produced may be used immediately or stored.

[0059] Brief Description of Drawings

[0060] The accompanying drawings illustrate a disclosed embodiment and serve to explain the principles of the disclosed embodiment. It is to be understood, however, that the drawings are designed for purposes of illustration only, and not as a definition of the limits of the invention.

[0061] Fig. 1

[0062] Fig. 1 is a schematic diagram of a method for preparing high-entropy oxide on a substrate using a laser-assisted pyrolysis synthesis (LAPS) protocol. Fig. 2a

[0063] Fig. 2a is a low resolution field emission scanning electron microscopy (FESEM) image at magnification of 800x showing a titanium foam before coating with hydrogel.

[0064] Fig. 2b

[0065] Fig. 2b is a high resolution FESEM image at magnification of 20,000x showing a titanium foam after coating with hydrogel .

[0066] Fig. 2c

[0067] Fig. 2c is a high resolution FESEM image at magnification of 50,000x showing a high- entropy oxide synthesised using a water hydrogel on titanium foam.

[0068] Fig. 2d

[0069] Fig. 2d is a high resolution FESEM image at magnification of 20,000x showing a high- entropy oxide synthesised using an ethylene glycol hydrogel on titanium foam.

[0070] Fig. 3a

[0071] Fig. 3a is an XRD spectrum of a high-entropy oxide on titanium foam, synthesised using a water hydrogel.

[0072] Fig. 3b

[0073] Fig. 3b is a XRD spectrum of a high-entropy oxide on titanium foam, synthesised using an ethylene glycol hydrogel.

[0074] Fig. 4a

[0075] Fig. 4a is a high-resolution transmission electron microscopy (HRTEM) image at magnification of 800,000* showing a high-entropy oxide synthesised using a water hydrogel.

[0076] Fig. 4b

[0077] Fig. 4b is a HRTEM image at magnification of 6,000,000* showing a high-entropy oxide synthesised using a water hydrogel.

[0078] Fig. 4c

[0079] Fig. 4c is a HRTEM image at magnification of 50,000* showing a high-entropy oxide synthesised using an ethylene glycol hydrogel.

[0080] Fig. 4d

[0081] Fig. 4d is a HRTEM image at magnification of 500,000* showing a high-entropy oxide synthesised using an ethylene glycol hydrogel. Fig. 5

[0082] Fig. 5 is a schematic diagram showing a membrane electrode assembly (MEA) comprising two layers of the high-entropy oxide-coated titanium foam and a high-entropy oxide-coated Nafion membrane in between.

[0083] Fig. 6

[0084] Fig. 6 is a graph showing a linear sweep voltammetry (LSV) curve for a RuMnCoNiZn high- entropy oxide catalyst and a RuIrCoNiZn high-entropy oxide catalyst at a scan rate of 5 mV / s for the overall water splitting in 0.5M H2SO4.

[0085] Detailed Description of Figures

[0086] Referring to Fig. 1, there is shown a method 1 for preparing a high-entropy oxide nanostructure, where at least five metal salts 2 are added to a solvent 3 and dissolved to form a solution 4. A crosslinking agent 5 and carbonaceous substance 6 are added to the solution 4 and the resulting mixture is heated and stirred to form a hydrogel 7. The hydrogel 7 is drop cast onto a substrate 8 and dried to form a dried hydrogel -coated substrate 9. A laser beam 10 is used to irradiate the coated substrate 9 to give the high -entropy oxide nanostructure 1 1 .

[0087] Referring to Fig. 5, there is shown a membrane electrode assembly (MEA) 12 comprising a high-entropy oxide nanostructure powder-coated Nafion 115 membrane 13 functioning as a proton exchange membrane (PEM) positioned in between a high-entropy oxide-coated titanium foam 14 functioning as the anode, and a high-entropy oxide-coated titanium foam 15 functioning as the cathode.

[0088] Examples

[0089] Non-limiting examples of the invention will be further described in greater detail by reference to specific Examples, which should not be construed as in any way limiting the scope of the invention.

[0090] Example 1: RuMnCoNiZn hydrogel comprising ethylene glycol

[0091] As shown in the method 1 of Fig. 1 , metal salts 2 in the form of metal chlorides RuCF ■ 3H2O (>99.0%), MnCl2-4H2O (>98.0%), CoCl2-6H2O (>98.0%), NiC12'6H2O (>98.0%) and ZnCl22H2O were dissolved in a solvent 3 in the form of 20% ethylene glycol solution to form a solution 4. The mole ratio of each metal ion in the solution was 1 : 1 : 1 : 1 : 1 , and the total concentration of metal ions was 0.8 M. A crosslinking agent 5 in the form of polyvinyl alcohol (PVA) solution (100 g / L) was added into the solution 4. A carbonaceous substance 6 in the form of graphite was then dispersed into the resulting solution in an amount such that the graphite concentration was 3.3 g / L in the dispersion. The resulting dispersion was heated at 80 °C with 400 ipm magnetic stirring for 30 minutes to dissolve the PVA and form the hydrogel 7 in the form of RuMnCoNiZn hydrogel comprising ethylene glycol.

[0092] Example 2: RuMnCoNiZn hydrogel comprising water

[0093] As shown in the method 1 of Fig. 1 , metal salts 2 in the form of metal chlorides RuCL • 3H2O (>99.0%), MnCl2'4H2O (>98.0%), CoCl2-6H2O (>98.0%), NiCl2-6H2O (>98.0%) and ZnCl2- 2H2O were dissolved in a solvent 3 in the form of water to form a solution 4. The mole ratio of each metal ion in the solution was 1 : 1 : 1 : 1 : 1 , and the total concentration of metal ions was 0.8 M. A crosslinking agent 5 in the form of polyvinyl alcohol (PVA) solution (100 g / L) was added into the solution 4. A carbonaceous substance 6 in the form of graphite was then dispersed into the resulting solution in an amount such that the graphite concentration was 3.3 g / L in the dispersion. The resulting dispersion was heated at 80 °C with 400 rpm magnetic stirring for 30 minutes to dissolve the PVA and form the hydrogel 7 in the form of RuMnCoNiZn hydrogel comprising water.

[0094] Example 3: RulrCoNiZn hydrogel comprising ethylene glycol

[0095] As shown in the method 1 of Fig. 1, metal salts 2 in the form of metal chlorides RUC13- 3H2O (>99.0%), IrCl33H2O (>98.0%), CoCl2-6H20 (>98.0%), NIC12- 6H2O (>98.0%) and ZnCl22H2O were dissolved in a solvent 3 in the form of 20% ethylene glycol solution to form a solution 4. The mole ratio of each metal ion in the solution was 1 : 1 : 1 : 1 : 1 , and the total concentration of metal ions was 0.8 M. A crosslinking agent 5 in the form of polyvinyl alcohol (PVA) solution (100 g / L) was added into the solution 4. A carbonaceous substance 6 in the form of graphite was then dispersed into the resulting solution in an amount such that the graphite concentration was 3.3 g / L in the dispersion. The resulting dispersion was heated at 80 °C with 400 ipm magnetic stirring for 30 minutes to dissolve the PVA and form the hydrogel 7 in the form of RulrCoNiZn hydrogel comprising ethylene glycol.

[0096] Example 4: RulrCoNiZn hydrogel comprising water

[0097] As shown in the method 1 of Fig. 1, metal salts 2 in the form of metal chlorides RuCL 3H2O (>99.0%), IrCl33H2O (>98.0%), CoCl2-6H20 (>98.0%), NiCl2-6H2O (>98.0%) and ZnCl2'2H2O were dissolved in a solvent 3 in the form of water to form a solution 4. The mole ratio of each metal ion in the solution w as 1 : 1 : 1 : 1 : 1 , and the total concentration of metal ions was 0.8 M. A crosslinking agent 5 in the form of polyvinyl alcohol (PVA) solution (100 g / L) was added into the solution 4. A carbonaceous substance 6 in the form of graphite was then dispersed into the resulting solution in an amount such that the graphite concentration was 3.3 g / L in the dispersion. The resulting dispersion was heated at 80 °C with 400 rpm magnetic stirring for 30 minutes to dissolve the PVA and form the hydrogel 7 in the form of RulrCoNiZn hydrogel comprising water.

[0098] Examples 5 to 8: HEO-coated titanium foam

[0099] As shown in the method 1 of Fig. 1, to a substrate 8 in the form of a titanium foam substrate (as shown in Fig. 2a) was applied the hydrogel of any one of Examples 1 to 4 by drop casting, followed by drying at 80 °C in ambient air to convert the hydrogel into a dried hydrogel in the form of a xerogel to give a coated substrate 9 in the form of a coated titanium foam substrate (as shown in Fig. 2b). The dried hydrogel-coated substrate was irradiated with a laser beam 10 in the form of a laser beam having a wavelength of 1064 nm, laser spot diameter of 30 pm, output power of between 30.0 to 50.0 W, scanning speed of between 300 to 500 mm / s, pulse duration of 2 ns, and a line scanning range of between 20 to 40 pm. The dried hydrogel w as evaporated by the laser to form a high-entropy oxide nanostructure 11 on the substrate in the form of a high-entropy oxide-coated titanium foam.

[0100] Using Example 1 yielded Example 5: RuMnCoNiZn high-entropy oxide -coated titanium foam (synthesised with ethylene glycol hydrogel - as shown in Fig. 2d, Fig. 3b, Fig. 4c and Fig. 4d) Using Example 2 yielded Example 6: RuMnCoNiZn high-entropy oxide-coated titanium foam (synthesised with water hydrogel - as shown in Fig. 2c, Fig. 3a, Fig. 4c and Fig. 4d)

[0101] Using Example 3 y ielded Example 7: RuTrCoNiZn high-entropy oxide-coated titanium foam (synthesised with ethylene glycol hydrogel)

[0102] Using Example 4 yielded Example 8: RuIrCoNiZn high-entropy oxide-coated titanium foam (synthesised with water hydrogel)

[0103] Examples 9 to 12: High-entropy oxide nanostructure powder

[0104] As shown in the method 1 of Fig. 1, a hydrogel 7 in form of the hydrogel of any one of Examples 1 to 4 was placed into a substrate 8 in the form of a tungsten crucible, follow cd by drying at 80 °C in ambient air to convert the hydrogel into a dried hydrogel in the form of a xerogel. The dried hydrogel-coated substrate 9 in the form of a dried hydrogel-coated tungsten crucible was irradiated with a laser beam 10 in the form of a laser beam having a wavelength of 1064 nm, laser spot diameter of 30 pm, output power of between 30.0 to 50.0 W, scanning speed of between 300 to 500 mm / s, pulse duration of 2 ns, and a line scanning range of between 20 to 40 pm. The dried hydrogel was evaporated by the laser to form a high-entropy oxide layer on the substrate to give a high-entropy oxide nanostructure 11 in the form of a high-entropy oxide-coating on the tungsten crucible. The high-entropy oxide nanostructure was then removed from the tungsten crucible as a powder.

[0105] Using Example 1 yielded Example 9: RuMnCoNiZn high-entropy oxide nanostructure powder (synthesised with ethylene glycol hydrogel)

[0106] Using Example 2 yielded Example 10: RuMnCoNiZn high-entropy oxide nanostructure powder (synthesised with water hydrogel)

[0107] Using Example 3 yielded Example 11: RuIrCoNiZn high-entropy oxide nanostructure powder (synthesised with ethylene glycol hydrogel)

[0108] Using Example 4 yielded Example 12: RuIrCoNiZn high-entropy oxide nanostructure powder (synthesised with water hydrogel)

[0109] Examples 13 and 14: Membrane electrode assembly (MEA), proton exchange assembly (PEM) electrochemical cell, for hydrogen generation

[0110] The high-entropy oxide nanostructure powders of Examples 9 and 11 were mixed with Nafion solution (~5 wt%) (obtained from Fuel Cell Store, Bryan, Texas USA) to form two types of precursor powder solutions, respectively. The two precursor powder solutions were then sprayed on the two sides of a Nafion 115 membrane (obtained from Chemours, Wilmington, Delaware USA) and dried.

[0111] Using Example 9 yielded Example 13: RuMnCoNiZn high-entropy oxide nanostructure - coated Nafion 115 membrane (wherein the nanostructure is synthesised with ethylene glycol hydrogel).

[0112] Using Example 11 yielded Example 14: RuIrCoNiZn high-entropy oxide nanostructure - coated Nafion 115 membrane (wherein the nanostructure is synthesised with ethylene glycol hydrogel). Two layers of the RuIrCoNiZn high-entropy oxide-coated titanium foam of Example 7 , with the coated Nafion 115 membrane of Example 14 located in between, were pressed together at 80 °C for 5 minutes under a pressure of 2 T to form the sandwich structure of the membrane electrode assembly (MEA), as shown in the schematic diagram of Fig. 5. This MEA was installed into a titanium proton exchange assembly (PEM) electrochemical cell.

[0113] The RuIrCoNiZn high-entropy oxide-coated titanium foam of Example 7 functioned as both the anode and the cathode. A 0.5 M H2SO4 solution or 1 M KOH solution were used as the electrolytes for three -electrode system water electrolysis. The thrcc-clcctrodc water electrolysis was tested at room temperature.

[0114] The RuIrCoNiZn high-entropy oxide-coated titanium foam of Example 7 functioned as both the anode and the cathode. Pure water (with 18.2 MQ cm) was used as the feeding water. The feeding water was heated to 80 °C and pumped into the PEM electrochemical cell using a peristaltic pump with a flow rate of 40 mL min- 1. A thermocouple and a temperature sensor were installed in the PEM electrochemical cell to maintain a working temperature in the range of 60 to 80 °C.

[0115] The PEM electrochemical cell provided a current density of 1.0 to 1.1 A / cm2when a bias potential difference of 1.7 to 1.8 V was applied to the MEA, as shown by the linear sweep voltammetry (LSV) curve in Fig. 6.

[0116] Equipment

[0117] In the Examples disclosed herein, a fiber laser was used and selected for a wavelength of 1064 nm, and 2 ns pulse width; a JEOL JSM-7600f was used for scanning electron microscopy; a Shimadzu XRD-6000 was used for X-ray diffraction analysis; a JEM-21 OOF was used for transmission electron microscopy; and a Kratos AXIS Supra spectrometer was used for X-ray photoelectron spectroscopy .

[0118] Industrial Applicability

[0119] The high-entropy oxide nanostructure as described herein may be used in hydrogen generation industries to produce hydrogen from the electroly sis of w ater.

[0120] It will be apparent that various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims.

Claims

Claims1. A method for preparing a high-entropy oxide nanostructure, the method comprising the step of: irradiating, with a laser, a substrate coated with a hydrogel to form the high- entropy oxide nanostructure, wherein the hydrogel comprises at least five metal salts, a cross-linking agent, a carbonaceous substance and water.

2. The method of claim 1, wherein the metal salts arc selected from a ruthenium salt, a cobalt salt, a nickel salt, a zinc salt, a manganese salt or an iridium salt.

3. The method of claim 1 or 2, wherein one or more of the at least five metal salts are metal halides.

4. The method of any one of claims 1 to 3, wherein each metal of the at least five metal salts has a mole ratio that differs by not more than about 5% between each metal.

5. The method of claim 4, wherein the mole ratio is an equimolar ratio.

6. The method of any one of claims 1 to 5, wherein the crosslinking agent is selected from polyvinyl alcohol, alginate, chitosan, gelatin, carrageenan, PEG, PEGDA, polyacrylamide, polyacrylate, polyaspartic acid, nanocellulose or a combination thereof.

7. The method of claim 6, wherein the crosslinking agent is polyvinyl alcohol.

8. The method of any one of claims 1 to 7, wherein the carbonaceous substance is selected from graphite, graphene, carbon nanofibers, carbon nanospheres, carbon black, hard carbon, activated carbon or a combination thereof.

9. The method of claim 8, wherein the carbonaceous substance is graphite.

10. The method of any one of claims 1 to 9, wherein the hydrogel further comprises ethylene glycol, propylene glycol or glycerol.

11. The method of any one of claims 1 to 10, wherein the method further comprises, before the irradiating step, a step of coating the hydrogel onto the substrate.

12. The method of claim 11, wherein the coating step comprises dip coating, drop casting, spin coating or spray coating.

13. The method of claim 11 or 12, wherein the method further comprises, before the coating step, a step of forming the hydrogel from a hydrogel solution.

14. The method of any one of claims 11 to 13, wherein the method further comprises, after the coating step, a step of drying the coated substrate.

15. The method of claim 14, wherein the drying step converts the hydrogel into a dried hydrogel in the form of a xerogel.

16. The method of claim 14 or 15, wherein the drying step comprises dry ing the coated substrate at a temperature of between about 60 °C to about 100 °C.

17. The method of any one of claims 1 to 16, wherein the wavelength of the laser is between about 800 nm to about 1300 nm.

18. The method of any one of claims 1 to 17, wherein the substrate is selected from titanium, tungsten, platinum, or an alloy thereof.

19. The method of claim 18, wherein the titanium is titanium foil ortitanium foam.

20. A high-entropy' oxide nanostructure prepared by the method of any one of claims 1 to 19.21 . An electrode, membrane electrode assembly or electrochemical cell comprising the high-entropy oxide nanostructure of claim 20.

22. Use of the electrode, the membrane electrode assembly or the electrochemical cell of claim 21 to produce hydrogen.

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