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• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
  • B01J37/16—Reducing
  • B01J37/18—Reducing with gases containing free hydrogen
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
  • B01J23/70—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of the iron group metals or copper
  • B01J23/74—Iron group metals
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
  • B01J23/70—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of the iron group metals or copper
  • B01J23/74—Iron group metals
  • B01J23/745—Iron
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
  • B01J23/70—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of the iron group metals or copper
  • B01J23/74—Iron group metals
  • B01J23/755—Nickel
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  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J25/00—Catalysts of the Raney type
  • B01J25/02—Raney nickel
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  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
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  • B01J35/00—Catalysts, in general, characterised by their form or physical properties
  • B01J35/30—Catalysts, in general, characterised by their form or physical properties characterised by their physical properties
  • B01J35/33—Electric or magnetic properties
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  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J35/00—Catalysts, in general, characterised by their form or physical properties
  • B01J35/50—Catalysts, in general, characterised by their form or physical properties characterised by their shape or configuration
  • B01J35/56—Foraminous structures having flow-through passages or channels, e.g. grids or three-dimensional monoliths
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J35/00—Catalysts, in general, characterised by their form or physical properties
  • B01J35/60—Catalysts, in general, characterised by their form or physical properties characterised by their surface properties or porosity
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J35/00—Catalysts, in general, characterised by their form or physical properties
  • B01J35/80—Catalysts, in general, characterised by their form or physical properties characterised by their amorphous structures
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
  • B01J37/02—Impregnation, coating or precipitation
  • B01J37/0215—Coating
  • B01J37/0225—Coating of metal substrates
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B1/00—Electrolytic production of inorganic compounds or non-metals
  • C25B1/01—Products
  • C25B1/02—Hydrogen or oxygen
  • C25B1/04—Hydrogen or oxygen by electrolysis of water
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
  • C25B11/051—Electrodes formed of electrocatalysts on a substrate or carrier
  • C25B11/073—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
  • C25B11/051—Electrodes formed of electrocatalysts on a substrate or carrier
  • C25B11/073—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material
  • C25B11/075—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of a single catalytic element or catalytic compound
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
  • C25B11/051—Electrodes formed of electrocatalysts on a substrate or carrier
  • C25B11/073—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material
  • C25B11/091—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of at least one catalytic element and at least one catalytic compound; consisting of two or more catalytic elements or catalytic compounds
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
  • Y02E60/30—Hydrogen technology
  • Y02E60/36—Hydrogen production from non-carbon containing sources, e.g. by water electrolysis

Definitions

• the present invention relates to a method for improving the catalytic activity of an oxygen evolution reaction (OER) catalyst comprising a substrate with a catalytic metallic composite coating. It also relates to OER electrodes comprising the improved OER catalysts.
• OER oxygen evolution reaction
• Electrochemical and solar water splitting have been considered as one of the important alternatives to produce hydrogen fuels in a large scale on the cathode while oxygen evolution reaction (OER) is taking place on the anode.
• OER oxygen evolution reaction
• HER hydrogen evolution reaction
• OER catalysts To increase OER kinetics, a huge amount of work to synthesize efficient catalysts has been done. Whilst iridium dioxide (Ir0 2 ) and ruthenium dioxide (Ru0 2 ) are the most active OER catalysts currently known, their use is often not commercially viable due to their high cost and non-sustainable sources. The use of alternative OER catalysts on the basis of first-row transition metals and their complexes have been investigated. For example, non-precious-metal catalysts such as nickel-based compounds have been described as OER catalysts in recent years.
• Ni/Fe -based materials as OER catalysts which can afford a high water photolysis efficiency (e.g. 12.3%) via perovskite photovoltaics have been reported.
• the electrocatalytic activity in terms of overpotential to deliver an acceptable high current density, e.g. 100 mA.cm " , is unsatisfactory.
• catalytic materials need to overcome the slow reaction kinetics of the oxygen evolution reaction (OER), which generally requires a large amount of overpotential ( ⁇ ) to generate H 2 at an acceptable rate.
• OER oxygen evolution reaction
• At least preferred embodiments of the present invention were to provide a facile and general method to improve the catalytic activity of an OER electrode. It would also be advantageous if at least preferred embodiments of the present invention were to enhance the efficiency of a Ni/Fe -based OER electrode, for example by lowering the overpotential with high current density thereby reducing the input energy cost in water splitting.
• the present inventors have undertaken considerable research and have for the first time demonstrated that by treating an OER catalyst comprising a catalytic metallic composite coating supported on a substrate with a reducing agent, the catalytic activity of the metallic composite coating, and thus the catalytic activity of the OER catalyst, is significantly improved.
• the method of the invention improves the catalytic activity of the OER catalyst without requiring the use of expensive precious metals, and is achieved through inexpensive processing techniques with readily available equipment.
• the improved OER catalyst may in turn be used as an OER electrode having similarly improved qualities.
• a method for improving the catalytic activity of an OER catalyst comprising a substrate with a catalytic metallic composite coating comprising:
• the metallic composite coating may be a metallic composite thin film coating.
• the metallic composite coating may comprise a bimetallic composite.
• the bimetallic composite is typically a bimetallic oxide composite, a bimetallic hydroxide composite or a mixture thereof.
• the bimetallic composite is selected from the group consisting of a nickel- iron composite, a nickel-cobalt composite, a manganese-iron composite, a manganese- cobalt composite, or a manganese-zinc composite.
• the bimetallic composite is a nickel-iron composite, such as a nickel-iron composite comprising a nickel-iron oxide, a nickel-iron hydroxide, or a mixture thereof (e.g. a nickel-iron oxyhydroxide).
• the nickel-iron composite has a formula of Ni2 X Fe 3y (OH)2x+3y, wherein x is a number between about 0.1 and about 2 and y is a number between about 0.1 and about 2.
• x and y may, independently of each other, be a number between 0.1 and 1.8, 0.1 and 1.5, 0.1 and 1.0, 0.1 and 0.5, 0.2 and 1.8, 0.2 and 1.5, 0.2 and 1.0, 0.2 and 0.5, 0.3 and 1.8, 0.3 and 1.5, 0.3 and 1.0, 0.3 and 0.5, 0.5 and 1.8, 0.5 and 1.5, 0.5 and 1.0, 0.5 and 0.8, 1.0 and 1.8, or 1.0 and 1.5 (such as 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9 or 2.0).
• the metallic composite may be porous.
• the metallic composite is amorphous.
• the amorphous metallic composite coating may comprise nanosheets, nanoflakes, or a combination thereof.
• the metallic composite is crystalline.
• the reducing agent is selected from the group consisting of sodium borohydride (NaBH 4 ), hydrazine or hydrogen gas.
• the reducing agent is NaBH 4 .
• the reducing agent is preferably exposed to the metallic composite coating as a solution (i.e. the reducing agent is dissolved in a suitable solvent and contacts the metallic composite coating while in solution), and may be exposed for a period of between 30 sec to 100 min, for example between 10 min to 30 min, for example between 15 min to 25 min. Still further, in this embodiment, the solution may have a temperature of between 10 °C and 50 °C, for example between 15 °C and 30 °C.
• time and/or temperature may be varied depending on various parameters (such as, for example, the concentration of the reducing agent, the overall volume of the solvent, the size of the coated object, the surface area of the coating, the metal loading within the coating, the composition of the coating, the thickness of the coating, and the activity of the reducing agent) and will be able to determine
• the reducing agent is exposed to the metallic composite coating as a gas (i.e. the reducing agent is in a gaseous state when it contacts the coating), and may be exposed for a period of between 30 sec to 100 min, for example, between 10 min to 30 min. Still further, in this embodiment, the gas may have a temperature of between 200 °C and 800 °C, for example, between 300 °C and 500 °C. In some embodiments, the reducing agent is a substantially pure gas. In other embodiments, the gaseous reducing agent is mixed with other gases (for example, nitrogen, argon or other inert gases) in an amount of from about 1 %v/v to about 99 %v/v (e.g. 10% to 99%, 50% to 95%, 75% to
• time and/or temperature may be varied depending on various parameters (such as, for example, the concentration (%v/v) of the gaseous reducing agent, the overall volume of the reaction vessel, the size of the coated object, the surface area of the coating, the metal loading within the coating, the composition of the coating, the thickness of the coating, and the activity of the reducing agent) and will be able to determine appropriate times and/or temperatures and/or concentrations etc. for exposing the metallic composite coating to the reducing agent in order to improve the catalytic activity of the coating.
• the substrate is an electrically conductive substrate which is preferably porous, for example, a nickel foam (NF).
• NF nickel foam
• an OER electrode comprising a substrate with a nickel-iron composite coating, wherein the nickel-iron composite has been exposed to a reducing agent, to thereby increase oxygen vacancy density in the nickel-iron composite coating.
• the reducing agent is typically selected from the group consisting of sodium borohydride (NaBH 4 ), hydrazine and hydrogen gas.
• the reducing agent is NaBH 4 .
• the substrate is an electrically conductive substrate, which is preferably porous. More preferably still, the porous electrically conductive substrate is a nickel foam.
• a method for improving the catalytic activity of an OER electrode comprising a substrate with a catalytic nickel-iron (NiFe) composite coating, the method comprising:
• Figures 1 (a) and (b) show an SEM image of NiFe/NF before NaBH 4 treatment at various magnifications.
• Figure 2 shows synthesized reduced NiFe/NF (denoted R-NiFe/NF) electrode after NaBH 4 treatment using SEM (a,b) and TEM (c,d).
• Figure 3 show the XRD pattern of pristine (NiFe/NF) and reduced (R-NiFe/NF) electrode.
• Figure 4 shows XPS spectra of the NiFe composites before and after NaBH 4 treatment, a) XPS survey, b) High resolution XPS spectra of Ni 2p, c) High resolution XPS spectra of Ni 3p and Fe 3p, d) High resolution XPS spectra of Fe 2p, and e) High resolution XPS spectra of O Is of NiFe composite before NaBH 4 treatment.
• Figure 5 shows XPS spectra of the NiFe composites before and after NaBH 4 treatment, a) High resolution XPS spectra of Fe 2p, and b) High resolution XPS spectra of O Is, of NiFe composite before NaBH 4 treatment.
• Figure 6 shows at a) high resolution O Is XPS spectrum of NiFe-OOH after reduction treatment, at b-f) further characterization of NiFe-OOH before and after NaBH 4 treatment; b) Raman Spectra, c) PL spectra, d) EPR spectra, e) band-gap energy determination via UV-VIS spectroscopy and f) EIS response.
• Figure 7 shows Raman spectra at high wave numbers of Ni-Fe hydroxide films.
• Figure 8 shows UV-VIS diffuse reflectance spectra of pristine and reduced NiFe-OOH thin films.
• Figure 9 shows at a,b) OER polarization curves of oxygen electrode, before and after NaBH 4 treatment in 1 M KOH at scan rate of 5 mV.S "1 with 95% iR-compensation of; a) NiFe/NF electrode, b) NiFe/GC electrode, c) multi-step current process obtained for the R-NiFe/NF electrode in 1M KOH. The current density started at 100 mA.cm " and finished at 550 mA.cm " , d) chronopotentiometry of reduced NiFe/NF electrode in 0.1 and 1 M KOH.
• Figure 10 shows linear sweep voltammograms of NiFe/NF electrodes in 0.1 M KOH at scan rate of 5 mV.S "1 and 95% iR compensation.
• Figure 11 shows Tafel slopes of NiFe/NF electrodes in a) 1 M and b) 0.1 M KOH
• Figure 12 shows double-layer capacitance measurements for determining ECSA of reduced NiFe/NF electrode in 1 M KOH, a) cyclic voltammetry in a non-Faradaic region of the voltammogram at scan rates of 0.005, 0.01, 0.02, 0.025, 0.05, 0.1, 0.2, 0.4, and 0.8 V/S. b) cathodic and anodic currents at -0.15 V vs. Ag/AgCl versus scan rate.
• Figure 13 shows experimental ring and disk currents for the pristine and R-NiFe/GC electrode on RRDE at 1600 rpm in Ar-saturated 1 M KOH.
• Figure 14 shows linear sweep voltammograms at scan rate of 5 mV.S "1 and 95% iR compensation of a) Ni/NF and b) Fe/NF in 1 M KOH.
• Figure 15 shows linear sweep voltammograms at scan rate of 5 mV.S "1 of a) pristine NiFe/NF and b) reduced NiFe/NF in 1 M KOH.
• Figure 16 shows cyclic voltammetric curves obtained with the R-NiFe/NF electrode in 1 M KOH at scan rate of 5 mV.S "1 with 95% iR- compensation.
• Figure 17 shows SEM images of R-NiFe/NF electrode after an extended period utilised in OER condition.
• Figure 18 shows a) OER polarization curves of Ni-Fe Layered Double Hydroxide/CNT and b)NiFe/NF synthesized by hydrothermal method before and after NaBH 4 treatment in 1 M KOH at scan rate of 5 mV.S " with 95% iR-compensation.
• metal composite a composite material comprising a metal and at least one other element, where the at least one other element may or may not be a metal.
• metal oxide composite a metallic composite material comprising at least one metal oxide.
• metal hydroxide composite a metallic composite material comprising at least one metal hydroxide.
• oxygenoxide a mixed oxide and hydroxide (i.e. a material comprising at least one metal oxide and at least one metal hydroxide).
• nanosheet a sheet- like structure having a substantially planar type three dimensional0 structure having a substantially constant width of less than about 100 nm (e.g. less than about 80, 50, 40, 30, 20, 10, 5 nm) in one dimension, and extending from several nanometers to several hundred nanometers in each other dimension.
• nanooflake a flake-like three dimensional structure, extending from several nanometers (e.g. 2, 5, 10, 20 nm) to several hundred nanometers (e.g. 200, 300, 400,5 500, 600, 700 nm) in each dimension.
• the inventors have found a method that can be used to improve the catalytic activity of an OER catalyst.
• the improved OER catalyst prepared by the method of the invention o may be used as an OER electrode having improved efficiency towards OER.
• the present invention provides a method for improving the catalytic activity of an OER catalyst.
• the OER catalyst comprises a substrate with a catalytic metallic composite coating.
• the method comprises a step of exposing the metallic
• metallic composite coating is increased relative to the metallic composite coating which has not been exposed to a reducing agent.
• the catalytic metallic composite is a composite material having catalytic activity in the o OER and comprising a metal and at least one other element, where the at least one other element may or may not be a metal.
• the composite is also capable of having an increased oxygen vacancy density as a result of exposure to the reducing agent.
• the oxygen vacancy density is the number of oxygen vacancies per unit volume.
• the increase in oxygen vacancy density may be from a zero or non-zero oxygen vacancy density in the material prior to the exposure to the reducing agent. In other words, the material prior to exposure to the reducing agent may have no oxygen vacancy density or may have some oxygen vacancy density.
• an oxygen vacancy is generated by an oxygen atom being removed from the material.
• an oxygen vacancy may be generated or formed from metallic composite materials such as metallic oxide composite materials, metallic hydroxide composite materials, metallic oxyhydroxide composite materials (i.e. materials comprising a mixture of metallic oxide and metallic hydroxide composite materials) and mixtures thereof.
• metallic composite material may be any metallic composite material which, at least prior to exposure to the reducing agent, comprises oxygen atoms.
• the oxygen atoms may be present in the material in any form (e.g as a metal hydroxide or a metal oxide).
• the oxygen atoms may be completely or partially removed in forming or introducing the oxygen vacancy (e.g.
• exposing the metallic composite material to the reducing agent may remove all of the oxygen atoms previously present in the metallic composite material, or may remove only a portion of the number of oxygen atoms from the metallic composite material, leaving some oxygen atoms remaining).
• the metallic composite material may already contain oxygen vacancies.
• exposing the metallic composite material (containing oxygen vacancies and oxygen atoms) to the reducing agent will increase the number of oxygen vacancies, thereby increasing the overall oxygen vacancy density of the material.
• exposing the metallic composite material (containing oxygen atoms and no oxygen vacancies) to the reducing agent will form oxygen vacancies, thereby increasing the number of oxygen vacancies and the oxygen vacancy density of the material.
• the method of the invention relates to a method for improving the catalytic activity of an OER catalyst.
• OER refers to the oxygen evolution reaction; an anodic reaction that accompanies, in aqueous electrolytes, cathodic processes such as metal electrowinning and hydrogen production via water electrolysis. As mentioned previously, for the latter process, the anodic overpotential is recognised as a major factor in limiting operational efficiency.
• the improved OER catalyst is particularly suited for use under alkaline electrolyte
• Water oxidation is one of the half reactions of water splitting.
• a nickel-iron catalyst under alkaline electrolyte conditions, the following reactions are o relevant:
• the oxidation step is typically the most demanding because it requires the coupling of 4 electron and proton transfers and the formation of an oxygen- o oxygen bond. Since hydrogen can be used as an alternative clean burning fuel, there exists a need to split water efficiently.
• Oxygen Evolution Reaction OER; i.e.
• An OER catalyst is one which catalyses the OER, i.e. exhibits
• OER electrode refers to an electrode which exhibits high activity for the OER.
• the OER catalyst is comprised of a substrate with a catalytic metallic composite
• any substrate which is capable of supporting a catalytic metallic composite coating may be used.
• the catalytic metallic composite coatings have a certain affinity to the supporting substrates, thereby avoiding the usage of chemical binders.
• the method of the present invention may also be applied to improve the catalytic performance of catalysts which employ chemical binders
• the catalytic metallic composite is in the form of a coating.
• the catalytic metallic composite "coats" the substrate. By this is meant that a surface of the substrate is in contact with a surface of the catalytic metallic composite. As described herein, the composite may be directly in contact with the substrate or may be in contact with the substrate in an indirect manner such as, for example, by way of a binder.
• the coating is typically a layer on the surface of the substrate and may be of any thickness that is capable of performing in the method of the invention.
• the coating may be a complete or a partial coating. In other words, the coating may completely coat the substrate (i.e. completely coat or cover the substrate), or may be a partial coating (i.e. coat or cover a portion of the substrate). Typically, the coating will cover at least a portion, preferably a substantial portion (or the entire portion), of the substrate that is or will be exposed to the electrolyte/solution when the catalyst is used.
• a metal foam e.g. nickel foam
• a metal foam has a cellular structure consisting of a solid metal with gas-filled pores (voids) comprising a portion of the volume.
• the pores can be sealed (closed-cell foam) or interconnected (open-cell foam).
• the nickel foam is an open-cell foam.
• a defining characteristic of metal foams is a high porosity: typically only 5-25% of the volume is the base metal, making these materials ultralight with a high surface area.
• Metal foams, including nickel foams can be purchased from commercial suppliers having various properties (e.g. various porosities, pore volumes, thickness, alloy compositions or densities).
• the method of the present invention is particularly suitable for improving the catalytic activity of OER catalysts having substrates as described in International Patent Application No. PCT/AU2015/000478, i.e. metal foams.
• metal foams such as, for example, a nickel foam as described in PCT/AU2015/000478, is that such foams may be an electrically conductive porous material which is relatively inert and does not significantly deteriorate in aqueous solution.
• various metal foams are commercially available and may be relatively inexpensive.
• a further advantage of metal foams is that they are robust, and where weight considerations are a factor for the final use of the catalytic assembly, they provide excellent weight efficiency.
• substrates described herein include nickel foam (NF) and carbon nanotubes (CNT).
• a metallic composite coating has a certain catalytic activity towards OER, the efficiency of which is improved using the method of the invention.
• the catalytic metallic composite coatings are deposited using either the electrodeposition process described in International Patent Application No.
• the method comprises a step of exposing the metallic composite coating to a reducing agent.
• a reducing agent is an element or compound that donates an electron to another chemical species in a redox chemical reaction.
• the reducing agent described in the examples is sodium borohydride (NaBH 4 ). NaBH 4 is a common and readily available reducing agent.
• other reducing agents e.g., sodium borohydride (NaBH 4 ).
• NaCNBH 3 , NaBH(OAc) 3 , LiAlH 4 , LiBH 4 , LiEt 3 BH, diisobutylaluminium hydride, borane and borane adducts such as BH 3 » THF) may also be used in the method of the present invention.
• the metallic composite coating is exposed to the reducing agent. That is, the reducing agent is introduced to the metallic composite coating in such a manner so as to allow it to come into contact with the metallic composite coating (e.g. submerging, or at least partially immersing, the metallic composite into a solution comprising the reducing agent or pouring or spraying a solution comprising the reducing agent onto the metallic composite).
• the metallic composite is in the form of a coating on the substrate when exposed to the reducing agent. In other embodiments, the metallic composite is exposed to the reducing agent and later formed into a metallic coating on the substrate. Upon coming into contact (i.e. upon exposure), the metallic composite coating is subsequently reduced by the reducing agent.
• the oxygen vacancy density in the metallic composite coating is increased upon exposure to the reducing agent.
• reduction of the metallic composite coating by the reducing agent increases oxygen vacancy density in the metallic composite coating.
• Oxygen vacancy refers to a defect in which an oxygen atom is removed from the lattice, leaving a vacancy behind with two electrons. It should be noted that oxygen vacancy
• oxygen vacancies are introduced, which may be confined on the surface or in the interior of the metallic composite coating.
• the increase in oxygen vacancy density enhances the electrical conductivity as well as charge transportation of the metallic composite coating. In this way, the catalytic activity of an OER catalyst is improved, and, in preferred embodiments, is significantly improved.
• the metallic composite coating may be a metallic composite thin film coating.
• thin film as used herein is taken to mean a film (i.e. a material in a planar/sheet-like form) having an average thickness of less than approximately 1 micron (e.g. ⁇ 0.9 ⁇ , ⁇ 0.75 ⁇ , ⁇ 0.5 ⁇ , ⁇ 0.25 ⁇ , ⁇ 0.2 ⁇ or ⁇ 0.1 ⁇ ).
• the metallic composite coating may comprise a bimetallic composite, such as, for example, is a bimetallic oxide composite, a bimetallic hydroxide composite or a mixture thereof (an oxyhydroxide).
• the metallic composite is provided as a coating on a substrate, with or without a binder therebetween.
• the composite may be a bimetallic composite.
• the bimetallic composite may be selected from the group consisting of a nickel-iron composite, a nickel-cobalt composite, a manganese-iron composite, a manganese-cobalt composite, or a manganese-zinc composite.
• the method of the present invention may be used on various catalytic metallic oxide systems wherein the metal ions can be reduced to a lower oxidation state upon exposure to a suitable reducing agent, thereby creating oxygen vacancies.
• the inventors have found that when a bimetallic composite coating is used, exposure of the bimetallic composite coating to the reducing agent increases oxygen vacancy density in the bimetallic composite coating significantly more than when each of the individual metallic coatings are exposed to the reducing agent under identical conditions. Without wishing to be bound by theory, the inventors believe that there is a synergetic effect of the metals (in the bimetallic composite) on the catalyst structure which provides particularly good performance in the OER catalyst after the reduction treatment (i.e. after exposing the metallic composite to the reducing agent). According to experiments conducted by the inventors, although Ni/NF and Fe/NF after reduction show improved OER performance, the reduced NiFe/NF is more efficient than the reduced individual metal hydroxides.
• the bimetallic composite is a NiFe oxyhydroxide composite which is electrodepo sited on a NF substrate according to the method described in International Patent Application No. PCT/AU2015/000478.
• NiFe is known to exhibit catalytic activity towards OER.
• the NiFe oxyhydroxide composite is typically provided as an amorphous porous coating comprised of nanosheets. That is, in one embodiment, the metallic composite is amorphous which means that it is a solid that does not have an ordered structure such as the ordered structure of a crystalline material.
• the NiFe/NF has a hierarchical porous structure as which is advantageous to its use as an OER catalyst for the reasons discussed in International Patent Application No.
• the reducing agent is provided as a solution (i.e. a solution comprising a reducing agent and a suitable solvent).
• the reducing agent may also be provided in a gaseous form. This may have particular advantages for particular applications, for example, when the reducing agent is not readily soluble, or when the pore size of a porous metallic composite coating and/or substrate is particularly small. It is envisaged that, in this scenario, exposing the reducing agent as a gas would allow higher diffusivity of the reducing agent into the metallic composite coating.
• the metallic composite coating will preferably be exposed to the reducing agent for an optimal period of time to produce an optimal increase in oxygen vacancy density.
• the optimal time period may be, for example, between 30 seconds to 100 minutes, for example between 10 min to 30 min, for example between 15 minutes and 25 minutes.
• the temperature of the solution may need to be optimised.
• the optimal temperature of the solution may be between 10 C and 50 C, for example between 15 C and 30 C.
• the optimal time period may be, for example, between 30 seconds to 100 minutes, for example between 10 minutes and 30 minutes.
• the temperature of the gas may need to be optimised.
• the 5 optimal temperature of the gas may be between 200 °C and 800 °C, preferably between 300 °C and 500 °C.
• the gas may also be "diluted" by including a further suitable gas (e.g. a mixture of the gaseous reducing agent and a suitable, preferably inert, gas such as nitrogen or argon).
• a further suitable gas e.g. a mixture of the gaseous reducing agent and a suitable, preferably inert, gas such as nitrogen or argon.
• the substrate may be an electrically conducting substrate.
• the electrically conducting substrate may be porous, such as, for example NF.
• the method of the invention is particularly suitable to improving the catalytic activity of, and thus the effectiveness of, bimetallic OER catalytic assemblies, such as those o described in International Patent Application No. PCT/AU2015/000478.
• the method of the invention is particularly suitable to improve the catalytic activity of catalytic assemblies described in International Patent Application No. PCT/AU2015/000478 where the porous metallic composite is an electrodepo sited NiFe composite on a NF substrate.
• the present invention provides an OER electrode comprising a substrate with a nickel-iron composite coating, wherein the nickel-iron composite has been exposed to a reducing agent, to thereby increase oxygen vacancy density in the nickel-iron composite coating.
• the present invention provides method for improving the catalytic activity of an OER electrode comprising a substrate with a catalytic nickel-iron composite coating, the method comprising:
• the reduced NiFe/NF (denoted R-NiFe/NF) nanosheets with rich oxygen deficiencies were prepared via a two-step process.
• a thin NiFe hydroxide sheet with yellow color covered the nickel foam was firstly electrodepo sited onto the nickel foam (NF) by the eleetrodeposition process recently described in our earlier International Patent
• Nickel foam (purchased from Goodfellow, UK, 95% purity and 95% porosity) was sonicated in 5 M HC1 for 30 minutes to remove nickel oxide layer and then rinsed with water and ethanol several times and left to dry in air.
• Ni-Fe eleetrodeposition was carried out by eleetrodeposition at 10 °C. Co-deposition of Ni and Fe was done with one electrolyte containing both metallic sources. Nitrate salt of Ni and Fe was used to make the eleetrodeposition electrolyte. To achieve a Ni-Fe alloy, 3 mM Ni(N0 3 )2 and 3 mM Fe(N0 3 ) 3 were dissolved in water without any additive.
• Ni/NF and Fe/NF was fabricated by electrodeposition of each metal from solution 6 mM of individual metal source.
• nitrate ion reacts with water and produces hydroxide ions.
• the generated hydroxide ions then reacts with Ni and Fe ions in the electrolyte (Equation 5) and bimetallic hydroxide forms on the surface of electrodes.
• the electrochemical surface area (ECSA) of each electrocatalyst is determined by double layer capacitance (C DL ) according to Equation 7:
• ECSA C DL / C S (7)
• C DL is calculated from absolute average of slopes of lines in the plot of currents versus scan rates. In order to measure the currents (/), open circuit potential (OCP) was measured in the solution and then CV in a window potential of OCP + 0.05 V at different scan rates was recorded. The anodic and cathodic currents in Figure 12a were used to plot Figure 12b. Electrochemical Evaluation
• the electrochemical experiments were performed under normal bench-top laboratory condition with a CH760 Electrochemical Workstation (CH Instrument, Texas, USA) using a three-electrode cell arrangement. Ag/AgCl electrode with 1 M KC1 solution and 5 Pt wire were used as the reference and counter electrode. CVs and LSVs measurement were performed with the scan rate of 5 mV.s -1 . Tafel slope determination was measured with the scan rate of 0.1 mV.s -1 . The electrochemical impedance spectroscopy (EIS) test was performed by B.A.S. potentiostat in a frequency range of 100 kHz to 0.01 Hz. All current densities in this specification were calculated by using the geometric surface l o area of the working electrode.
• EIS electrochemical impedance spectroscopy
• Ni-Fe Layered Double Hydroxide (NiFe-LDH) with carbon nanotube (CNT) support was synthesized by sonication of mildly oxidized multi-wall CNT and DMF and the mixing with Ni(N0 3 )2 and Fe(N0 3 ) 3 at 85 °C for 4 hours. Then more water and DMF was added to solution and the obtained solution was autoclaved for 12 hours at 120 °C followed by 2 more hours at 160 °C. Afterwards, the product was collected by filter. Hydrothermally synthesized NiFe/NF also was made using the autoclave for 12 hours at 120 °C and then a 6 hour drying step at 80 °C.
• a nickel foam was inserted into the Teflon tube of the autoclave with a solution containing Ni(N0 3 ) 2 , Fe(N0 3 ) 3 and urea.
• X-ray diffraction (XRD) patterns of NiFe/NF and R-NiFe/NF electrodes show no other peaks apart from metallic nickel, suggesting that the materials deposited and after NaBH 4 reduction are both amorphous ( Figure 3). This was further confirmed by high- resolution transmission electron micros-copy (HRTEM) where no typical lattice fringes corresponding to Ni, Fe or NiFe composites were detected in Figure 2c and Figure 2d. Oxygen vacancy (OV) has been reported to play an important role in both
• the oxygen status in the NiFe (oxy)hydroxide before and after reduction was examined in the Ols core level spectra.
• the OV density formed can be estimated by taking the area of the peak ratio of oxygen loss to lattice oxygen. From Figure 6a and Figure 5b, the OV density in the R-NiFe/NF electrode is 7.0, which is almost doubled compared with that of 3.2 in the pristine electrode.
• E g for pristine NiFe composite deposit is 2.9 eV while it is 2.2 eV for reduced one (see Figure 6e).
• the decreased band gap energy for treated Ni-Fe oxyhydroxide leads to higher conductivity due to more narrow electronic bands.
• reduction treatment of the electrode by NaBH 4 leads to less electrical resistance values which are also confirmed by electrical impedance spectroscopy (EIS).
• Figure 6f demonstrates the Nyquist plots for pristine and reduced OER electrodes for assessing the charge transfer process.
• the semi-circle curve of the obtained data reveals that the charge transfer resistance (R ct ) is decreased from 79 to 36 ohm after the reduction treatment, indicating a faster charge transport of the reduced electrode.
• the pristine and reduced electrodes were directly used as OER working electrode and tested in an alkaline media using a standard three electrodes electrochemical cell set-up with scan rate of 5 mV.s "1 .
• OER performances of reduced and pristine NiFe/NF electrodes in 1 and 0.1 M KOH are shown in Figure 9a and 10, respectively.
• the oxidation peak seen after 1.3 V (vs RHE) belongs to transformation between Ni(OH) 2 to NiOOH. Scanning to a higher anodic potential, a steady increase of oxygen evolution was accompanied by a significantly increased oxidation current for R-NiFe/NF electrode.
• Tafel slopes for the electrodes have also been evaluated.
• the Tafel slope of pristine and reduced electrode is, respectively, 47 and 40 mV.dec -1 in 1 M KOH, and 60 and 51 mV.dec -1 in 0.1 M KOH, which are less than for Ir0 2 , Ru0 2 .
• This small Tafel slope for reduced electrode further demonstrates the more efficient kinetics of water oxidation with less polarization loss.
• the electrochemical surface area (ECS A) of R-NiFe/NF has been calculated and compared with the pristine electrode.
• the ECSA of each electrode is determined by double layer capacitance in 1 M KOH solution ( Figure 12).
• a roughness factor of 50 is determined for both pristine and reduced OER electrodes.
• Figure 14 illustrates that dipping the electrodes into the NaBH 4 solution improved the Ni/NF and Fe/NF electrodes conductivity and, accordingly, their OER performance. However, this increase is not as much as for NiFe/NF.
• a bimetallic composite e.g. NiFe/NF
• a unimetallic material e.g. Ni/NF and Fe/NF.
• the inventors believe there is a synergetic effect in bimetallic composite materials, giving rise to a catalyst structure that is capable of achieving excellent OER performance (after exposure to the reducing agent).
• Ni-Fe was first electrodeposited on glass carbon (GC) and then treated with NaBH 4 .
• the three consequent OER polarization curves investigated for the pristine and reduced NiFe electrodeposited onto the planar GC electrode in 1M KOH solution are shown in Figure 15. It can be seen that the second polarization curve for both electrodes are significantly influenced by the gas bubbles generated on the first scan and consequently a significant decrease in the current occurs. However, when the bubbles attached on NiFe/GC electrode are removed, the catalytic activity of the NiFe/GC electrodes is recovered (third scan).
• Figure 9b shows the same effectiveness of NaBH 4 treatment for NiFe/GC electrode.
• Figure 9c exhibits a multi-step chronopotentiometric curve for R-NiFe/NF in 1M KOH.
• the current is increased from 100 to 550 mA.cm "2 in increments of 50 mA cm "2 every 500 s, and the corresponding changes of potential are recorded.
• the potential reaches 1.5 V gradually.
• the potential blooms at 1.55 V and remains constant for the remaining 500s. Similar behavior is seen for all current densities up to 550 mA.cm "2 during the test.
• the NaBH 4 reduction process was found useful for other OER catalysts made from different materials and methods.
• the effect of the reduction treatment on NiFe-LDH with carbon nano tube (CNT) support and on NiFe/NF synthesized by hydrothermal method has been investigated and significant enhancement of OER activity observed (Figure 18).
• exposing the metallic composite coating to a reducing agent can be a fast and simple method to increase the OER electrocatalytic activity of metal hydroxide based catalysts by reduction of species on the surface of catalyst.
• the treatment creates some defects, in particular, oxygen vacancies, in the metal hydroxide catalyst structure and accordingly narrows the band gap energy, resulting in electrical conductivity enhancement of the materials.
• the inventors have found that introducing oxygen vacancy (OV) in R-NiFe/NF nanosheets improves its donor density, active sites and even decrease the energy required for H 2 0 adsorption, thus enhancing the OER performance of R-NiFe/NF nanosheets.
• OV oxygen vacancy
• the direct chemical treatment of the Ni-Fe film on nickel foam as a 3D substrate not only provide it large surface area, fast charge transport pathways and improved contact resistance, but it also produces binder-free electrode for water- splitting or advanced metal-air battery devices.
• Using the reduced electrode, in accordance with the present invention as an anode, a surprisingly high OER activity (on R-NiFe/NF), which outperforms all the Ni-Fe based materials in alkaline previously reported, was observed.

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