Methods of producing catalytic coatings on substrates

Abstract

Disclosed herein is a method of producing a thin film of a metal oxide catalyst on a substrate. The method comprises providing a viscous precursor formed from a source of metal cations and a chelating agent; coating one or more surfaces of the substrate with the viscous precursor, whereby a thin film of the viscous precursor is retained on the substrate; and heat- treating the coated substrate, whereby the metal oxide catalyst is formed.

Description

METHODS OF PRODUCING CATALYTIC COATINGS ON SUBSTRATESTechnical Field

[0001] The present invention relates to producing thin films of metal oxide catalysts on substrates. In some forms, the substrates are electrodes and membranes for use in electrochemical cells including those found in electrolysers, fuel cells and redox flow batteries. In some forms the substrates are for use in catalytic reactors.Background Art

[0002] Electrochemical cells are integral components of devices that either generate electrical energy from chemical reactions or cause chemical reactions to occur via the application of electrical energy. Electrochemical cells are used in devices such as electrolysers, fuel cells and redox flow batteries, for example.

[0003] Catalysts are routinely used in electrochemical cells in order to facilitate the necessary chemical reactions. For example, catalysts used with electrodes facilitate the chemical reactions that occur at the electrodes’ surfaces by lowering the activation energy required for the reactions. For example, platinum-based catalysts are often employed at electrodes in electrolysers in order to reduce the overpotential required for water-splitting reactions. Alternatively, in proton exchange membrane fuel cells, catalysts facilitate the electrochemical reactions that produce electricity.

[0004] Catalyst coated membranes (CCM) and catalyst coated substrates (CCS) are commonly used in electrochemical devices. In the production of CCS, for example, the catalyst is deposited directly onto the electrode, which is typically a metal mesh or porous substrate of metals like titanium, nickel or stainless steel. The CCS production method uses a wide range of catalysts and substrates, including particles of noble metals such as platinum, ruthenium or iridium supported on carbon substrates in the form of carbon black, amorphous carbon powder or carbon nanotubes. However, CCS electrodes may experience problems with achieving uniform catalyst distribution and degradation under some conditions. Noble metals are also relatively expensive.

[0005] In other production methods, nanoparticles may be used to form metallic layers using methods such as plating (e.g. electroplating, electroless plating), Physical Vapour Deposition (PVD) techniques (e.g. sputtering), or Chemical Vapour Deposition (CVD) methods to deposit the catalyst in thin layers on the substrate. In these methods, the catalysts deposited are in puremetallic form. These methods are expensive and often slow, and generally not suitable for mass production.

[0006] It would be advantageous to provide substrates such as electrodes and membranes for use in electrochemical cells that have a catalyst coating, but which are cheaper to produce whilst maintaining or improving efficiency.Summary of Invention

[0007] The inventors of the invention the subject of the present application have discovered that it is possible to fabricate a catalyst coated substrate for use in an electrochemical cell or a substrate for use in a catalytic reactor, in a manner that improves its utility, primarily in terms of the quantity of catalytic metal required (and hence its cost of production) but also in terms of its operational efficiency. The inventors believe that their discovery has the potential to result in electrodes and / or membranes for use in electrolysers and other electrochemical devices such as fuel cells, as well as in catalytic reactors involving gas phase or liquid phase chemical reactions, that have a reduced catalyst loading than standard electrodes, without any loss of functionality (e.g. producing a similar current density).

[0008] In a first aspect there is provided a method of producing a thin film of a metal oxide catalyst on a substrate. The method comprises: providing a viscous precursor formed from a source of metal cations and a chelating agent; coating one or more surfaces of the substrate with the viscous precursor, whereby a thin film of the viscous precursor is retained on the substrate; and heat-treating the coated substrate whereby the metal oxide catalyst is formed.

[0009] The present invention enables the relatively straightforward production of thin layers of a metal oxide catalyst on a substrate’s surface. In the present invention, the catalyst is formed in situ on the surface of the substrate, which the inventors have discovered unexpectedly provides a significantly higher catalytic function compared to conventionally formed substrates such as those described above. Without wishing to be bound by theory, the inventors believe that this enhanced catalytic activity is probably due to the strained surface structure of the formed thin film.

[0010] The inventors have found that use of such a thin film allows for a significantly lower (an order of magnitude lower in the examples set out below) loading of catalyst as compared to prior art processes in which pre-formed particulate catalysts are applied to electrodes, without anyappreciable loss in functionality. This can result in significant cost savings, and particularly when the catalysts include platinum group metal catalysts.

[0011] In the present invention, precursors to the oxide thin films are deposited by dipping the substrate into the viscous precursor made by chelating metal cations and, optionally, a plasticizer such as ethylene glycol. Instead of using traditional catalyst coating methods like spraying or screen printing, the proposed method is more efficient and cost-effective, allowing for a near 100% efficiency in transferring the catalyst from its synthesis to the substrate. In stark contrast, prior art methods use expensive and hard to maintain equipment such as ultrasonic spray coating machines.

[0012] Advantageously, methods according to embodiments of the present invention may allow for a lower loading of catalysts as compared to particulate catalyst-coated substrates of the prior art, thus reducing cost of production. In addition, methods according to the present invention may provide more efficient and cost-effective methods of fabricating an electrode compared to traditional catalyst coating methods, where the pre-formed catalyst particles are inefficiently coated or deposited onto the electrode.

[0013] In some embodiments, the viscous precursor can be formed by dissolving the source of metal cations in water, adding the chelating agent and then heating the resultant mixture.

[0014] In some embodiments, a plasticiser (e.g. ethylene glycol or urea) may be added to the water before heating, which can help assist in producing continuous thin films.

[0015] In some embodiments, the viscosity of the viscous precursor may be adjustable in order to produce a thin film having a predetermined thickness (e.g. when dip coating). Viscosity may, for example, be adjusted by adding more or less plasticiser and / or chelating agent, reducing the solvent levels, changing the polymerisation time or temperature, or any suitable combination thereof.

[0016] In some embodiments, the chelating agent may be selected from one or more of the following: citric acid, glycolic acid, lactic acid, gluconic acid, aminopolycarboxylic acids, glycine, gelatin, ketones such as (3-diketones, acetylacetone and acetone, amines such as monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), triethylamine (TeA), ethylenediaminetetraacetic acid (EDTA), as well as L-ascorbic acid and D-sorbitol.

[0017] In some embodiments, the metal cation may be selected from one or more of the following: cobalt, iron, silver, molybdenum, copper, chromium, nickel, manganese, vanadium, platinum, iridium, ruthenium, rhodium palladium and osmium. The resultant thin film will include the corresponding metal oxide catalyst.

[0018] In some embodiments, the substrate may be coated with the precursor by dip coating, spray coating or painting.

[0019] In some embodiments, the substrate may be an electrode or a membrane for use in an electrochemical cell. In other embodiments the substrate may be for use in a traditional catalytic process (e.g. for use in a catalytic reactor involving gas or liquid phase chemical reactions).

[0020] In embodiments where the electrode is a metal electrode, such may, for example, be selected from a metal plate, a metal mesh or a perforated plate. The electrode may, for example, be formed of nickel, cobalt, iron, silver, titanium, stainless steel or alloys of any of these metals.

[0021] Alternatively, the electrode may be a ceramic electrode or a composite of a metal and ceramic electrode.

[0022] In embodiments where the substrate is a membrane, such may, for example, be an ion conducting polymer membrane, a ceramic membrane or a composite of polymer and ceramic membrane.

[0023] In some embodiments, the metal oxide catalyst may be a nanocrystalline or microcrystalline metal oxide catalyst.

[0024] In some embodiments, the method may further comprise a pre-drying step of the coated substrate before the heat-treatment. The pre-drying step may, for example, comprise heating the coated substrate to a temperature of about 100 °C for 5 to 20 min. The pre-drying step may, for example, comprise heating the coated substrate in a vacuum oven, freeze drier, supercritical fluid drier or air drier.

[0025] In some embodiments, the method may further comprise coating one or more of the surfaces of the coated substrate with a second viscous precursor formed from a source of second metal cations a chelating agent and optionally a plasticizer, wherein the second metal is different to the first metal, whereby a substrate having a plurality of thin films of metal oxide catalysts is produced.

[0026] In some embodiments, the method may further comprise applying an ionomer to the thin film of metal oxide catalyst formed on the coated substrate and curing the ionomer to form a coating on the substrate.

[0027] In some embodiments, the substrate may comprise a first thin-film layer comprising cobalt oxide coated on the substrate and a second thin-film layer comprising ruthenium oxide coated on the cobalt oxide thin-film catalyst layer.

[0028] In a second aspect, there is provided a substrate in the form of an electrode, membrane or a substrate for use in a catalytic process, produced according to the method of the first aspect of the present invention.

[0029] In a third aspect, there is provided a substrate in the form of an electrode, membrane or a substrate for use in a catalytic process, comprising a plurality of thin films of layered metal oxide catalysts on the substrate.

[0030] In some embodiments, the substrate may comprise two thin-film layers. In some embodiments, a first of the two thin-film layers may comprise a continuous thin film layer and a second of the two thin layers may comprise island nanostructures. In some embodiments, the two thin-film layers may comprise a cobalt oxide thin-film layer and a ruthenium oxide thin-film layer.

[0031] In some embodiments, the substrate may further comprise an ionomer layer applied to the one or more thin-film layers.

[0032] In a fourth aspect, there is provided a use of the substrate according to the second or third aspect of the invention in an electrochemical cell.

[0033] In a fifth aspect, there is provided a use of the substrate according to the second or third aspect of the invention in a catalytic process.

[0034] Additional features and advantages of the various aspects of the present invention will be described below in the context of specific embodiments. It is to be appreciated, however, that such additional features may have a more general applicability in the present invention than that described in the context of these specific embodiments.Brief Description of the Drawings

[0035] Embodiments of the present invention will be described in further detail below with reference to the following drawings, in which:

[0036] Figure 1 shows a process flow chart of a method of producing a coated substrate in the form of an electrode according to an embodiment of the invention.

[0037] Figure 2 shows a schematic drawing of a method of dip coating an electrode according to an embodiment of the invention.

[0038] Figure 3 shows a cross-section of a coated electrode according to an embodiment of the invention.

[0039] Figure 4 shows a cross-section of a coated electrode according to another embodiment of the invention.

[0040] Figure 5 shows a perspective view of a cross-section of a coated electrode according to an embodiment of the invention.

[0041] Figure 6 is a graph showing combined data of catalyst coated substrate (CCS) electrochemical cells. All cells were running with an active area of 25 cm2with the electrolyte being IM KOH at 60 °C.

[0042] Figure 7 is a graph showing combined data Current -Voltage Characteristics data of CCS electrochemical cell using MEA_24 (only cobalt thin film), MEA_25 (Bilayer, cobalt and ruthenium thin films) and MEA_27 (only ruthenium thin film) electrodes prepared by a method according to an embodiment of the invention. All were cells running with an active area of 25 cm2with the electrolyte being 1 M KOH at 60 °C.

[0043] Figure 8 is a graph illustrating the lifetime performance of a 4000cm2active area AEM 3- cell stack when operating with 1.5M KOH as the electrolyte.

[0044] Figure 9 is a graph illustrating the VI curve after the initial startup phase. The diamond marker indicates a current of 0.5A / cm2at 1.79 V at the start of testing.

[0045] Figure 10 is a graph illustrating the performance of an AEM three-cell stack under a solar profile, using 1.5M KOH at 54°C and the proposed dip coating method for electrodes with a 4000 cm2active area.

[0046] In the drawings, like reference numerals designate similar parts.Detailed Description of the Invention

[0047] As described above, the present invention relates to a method of producing a thin film of a metal oxide catalyst on a substrate. The method comprises providing a viscous precursor formed from a source of metal cations and a chelating agent; coating one or more surfaces of the substrate with the viscous precursor, whereby a thin film of the viscous precursor is retained on the substrate; and heat-treating the coated substrate whereby the metal oxide catalyst is formed.

[0048] The present invention will be described below in the context of producing a thin film of a metal oxide catalyst on an electrode for use in an electrolyser. It will be appreciated that this description is also generally applicable to other substrates relevant to the context of the present invention, and that a person skilled in the art could readily adapt the teachings set out below for any such other substrate.

[0049] Figure 1 shows a process flow 100 for a method of producing a thin film of a metal oxide catalyst on an electrode, the method including the steps of providing a viscous precursor 10; coating one or more surfaces of an electrode with the precursor 12; and heat-treating the coated electrode 14. In use, methods according to embodiments of the invention enable direct synthesis of a thin film of a metal oxide catalyst on an electrode.

[0050] The thin layer metal oxide catalyst formed on the electrode may have a microcrystalline or nanocrystalline structure. In use, the morphology and dispersion of the metal oxide thin film catalyst on the substrate may be controlled by adjusting the amount of (optional) plasticiser. For example, an excess of ethylene glycol can lead to formation of nanostructures such as islands or clusters instead of a continuous nanocrystalline thin film, whereas addition of a stoichiometric amount of ethylene glycol can lead to the formation of a continuous nanocrystalline thin film. Addition of a plasticiser may also result in a catalyst having a platelet like structure, or contribute to the formation of a continuous thin film.

[0051] The viscous precursor is formed from a mixture of a source of metal cations and a chelating agent, where a reaction between the metal cations and the chelating agent forms a coordinative polymer that imparts a viscosity to the precursor.

[0052] The viscous precursor can be formed by dissolving the source of metal cations and chelating agent in water or, alternatively, organic solvents like ethanol and propanol. Mixtures of water and miscible organic solvents may also be used (e.g. about 10% (by volume) ethanol / methanol in water).

[0053] The precursor may include about 0.01 M to about 0.2 M metal cations, for example, about 0.05 M to about 0.15 M, or about 0.075 M to about 0.125 M metal cations. The precursor may, in some embodiments, include about 0.1 M metal cations.

[0054] The source of metal cations may be soluble metal cations such as a metal nitrate, a metal chloride, or the like.

[0055] The metal cations may be a noble metal, preferably a platinum-group metal, and / or a transition metal. The noble metal cations may be ruthenium, rhodium, palladium, osmium, iridium or platinum. The transition metal cations may be iron, molybdenum, copper, chromium, nickel, manganese, vanadium or cobalt.

[0056] The source of metal cations may include one species of metal cations. The source of metal cations may include two or more species of metal cations. In embodiments where there are two or more species of metal cations, the concentration of each species may be the same or maybe different. For example, the source of metal cations may include predominantly cobalt cations with a lower concentration of metal cations acting as a dopant.

[0057] The chelating agent may be a carboxylic acid, such as glycolic acid, lactic acid, gluconic acid, citric acid, amines such as monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), triethylamine (TeA) and tetrasodium ethylenediaminetetraacetic acid (EDTA), ketones such as (3-diketones, acetylacetone and acetone, aminopolycarboxylic acids, glycine, gelatine, L-ascorbic acid, D-sorbitol, or the like.

[0058] The viscous precursor may also include a plasticiser. The addition of the plasticiser to the precursor may form a polymer network about the coordinative polymer, immobilising the chelated metal cations. In use, addition of plasticiser may control formation of continuous thin films or islands or clusters of the catalyst.

[0059] The plasticiser may be of any suitable type sufficient to assist in film formation. The plasticiser may, for example, be glycerol, propylene glycol, polyethylene glycol, sorbitol, ethylene glycol, or urea.

[0060] The viscous precursor may be allowed to age or mature for a period of time before being used to coat the substrate. During this period of time, the polymer structure may undergo further development.

[0061] The substrate may, for example, be an electrode. The electrode may be a mesh, a porous substrate, a perforated plate, a membrane, a fibre paper (e.g. a stainless steel fibre paper), a carbon (e.g. carbon black, amorphous carbon power, carbon nanotubes, or the like).

[0062] The electrode may be fabricated from a metal. The electrode may be a metal mesh, a porous substrate, or a perforated plate. The electrode may be fabricated from titanium, nickel, stainless steel, cobalt, iron, silver, or combinations thereof. The electrode may be a composite metal and ceramic, such as metal oxide and metal, for example a cement of nickel and zirconium oxide. In some embodiments, the electrode may be a stainless steel and nickel mesh. In other embodiments, the electrode may be a nickel foam.

[0063] The substrate may also be a membrane. If so, any suitable membrane may be used. For instance, the membrane may be an ion-conducting polymer, ceramic, or a composite of an ionconducting polymer and ceramic.

[0064] When the substrate is a metal, a surface of the substrate (or the substrate in its entirety) may be treated prior to the step of coating with the viscous precursor. Advantageously, the surface treatment may improve the adhesion of the precursor to the substrate.

[0065] The surface treatment may include heating the metal substrate in an oven and then acid washing the substrate. The substrate may be placed in a vacuum oven prior to heating in an oven. The oven may be a muffle furnace. The substrate may be heated at a temperature of about 300 °C to about 400 °C. The substrate may be heated at a temperature of about 400 °C.

[0066] The acid wash may utilise a strong acid, such as sulphuric acid or nitric acid. The acid wash may comprise an acid having a molarity of about 0.5 M to about 5 M. The metal electrode may be acid washed for up to about 2 minutes.

[0067] The viscous precursor may be prepared by mixing the source of metal cations in water to obtain a solution of about 0.01 M to about 0.2 M metal cations, preferably about 0.1 M metal cations. A chelating agent is then added to the solution. A plasticiser may subsequently be added to the mixture of metal cations and chelating agent.

[0068] The viscosity of the precursor may be increased prior to coating the substrate. The precursor may be heated at a temperature of about 60 °C to about 100 °C to evaporate water. The precursor may be heated at a temperature of about 80 °C. The viscosity of the precursor may be increased to about 40 to about 200 centipoise by evaporation of the water, about 50 to about 150 centipoise, about 80 to 100 centipoise.

[0069] The viscous precursor may be heated at a temperature of about 60 °C to about 100 °C for a period of time sufficient to attain the desired viscosity. In use, a shorter polymerisation time lowers the precursor viscosity, while a longer polymerisation time leads to higher viscosity. In some embodiments, the precursor may be heated for about 24 to 72 hours.

[0070] The viscosity of the precursor may also be adjusted by adding more or less plasticiser, by adjusting the polymerisation time or temperature, or any suitable combinations thereof.

[0071] In some embodiments, increasing the amount of the optional plasticiser (e.g. ethylene glycol) to greater than the stoichiometric ratio may lead to a lower viscosity, while lowering the amount of plasticiser leads to a higher viscosity. In some embodiments, the stoichiometric ratio of the metal cations to plasticiser may be about 1 : 1 to about 1:10, for example about 1 :2 to about 1:10. In some embodiments, the stoichiometric ratio of the metal cations to plasticiser may be about 1:1.

[0072] The viscous precursor may be cooled to room temperature before coating the electrode.

[0073] The electrode may be coated with the viscous precursor by dip coating, spin coating, spray coating or by brushing the electrode with the viscous precursor.

[0074] Figure 2 illustrates a method of dip coating an electrode according to an embodiment of the present invention 200, wherein the electrode 20 and viscous precursor 22 are the electrode and viscous precursor described in reference to Figure 1.

[0075] The viscous precursor 22 may be at room temperature, or heated at a temperature of about 60 °C to about 80 °C.

[0076] In the dip coating method 200, electrode 20 is first immersed in the viscous precursor 22 one or more times. The electrode 20 may be immersed in the viscous precursor 22 twice during the dip coating process. The electrode 20 may be stirred in the viscous precursor 22 or otherwise allowed to soak in the precursor for a desired period of time. During the dipping, the electrode may be withdrawn from the viscous precursor at a speed of about 1 to about 10 mm per second. The electrode may be withdrawn from the viscous precursor at a speed of about 2 mm per second. The electrode 20 is removed from the viscous precursor 22 providing a coated electrode 24.

[0077] The dip coating method may be automatic, or semi-automatic. Advantageously, use of dip coating methods according to the present invention avoids use of expensive and hard to maintain equipment such as an ultrasonic spray coating machines with lower scale up costs.

[0078] The coated electrode 24 may be subjected to a drying process to reduce the solvent content in the thin film (not shown in Figure 2). The coated electrode 24 may be dried in a vacuum oven, freeze drier, supercritical fluid drier, air drier, or the like. Drying the coated electrode is understood by the inventors to assist in binding the precursor to the substrate by forming a solid polymeric resin.

[0079] The coated electrode may be dried in air at a temperature of about 60 °C to about 100 °C for about five to 20 minutes. The coated electrode may be dried at a temperature of about 100 °C for up to about 10 minutes.

[0080] The dried coated electrode 24 may then be heat treated in either an inert atmosphere, or in air. The coated electrode may be heat-treated in an oven, such as a muffle furnace. The coated electrode may be heated at a temperature of about 300 °C to about 400 °C. The coated electrode may be heated at a temperature of about 400 °C.

[0081] Heating the coated electrode 24 is understood to thermally decompose the organic polymeric network and cause the metal species to oxidise to their corresponding metal oxides in the form of a thin-film layer of the metal oxide catalyst on the electrode. The morphology of the metal oxide catalyst is produced because of the organic polymeric network having decomposed and leaving interstitial spaces. The strained form of the metal oxide catalyst is believed by theinventors to contribute to the enhanced catalytic activity they have observed (some of which is described below).

[0082] The metal oxide catalyst formed may, for example, be cobalt oxide, iron oxide, platinum, iridium black, ruthenium oxide, and other transition metals or their oxides and combinations therein.

[0083] The steps of the method may be conducted two or more times to provide an electrode having two or more thin-film layers of metal oxide catalysts.

[0084] The method may further include the step of applying an ionomer to the coated substrate, wherein the ionomer layer forms a coating on the substrate. The ionomer coating may provide erosion resistance enhancing the practical applicability of thin films in electrochemical cells. The ionomer coating may also enhance triple phase boundaries (active reaction zone) between the electrode and an interface with the electrolyser membrane.

[0085] The ionomer may be any suitable material. The ionomer may be a polymeric material capable of forming an ion-conducting layer. The ionomer may be the same material as the electrolyser membrane.

[0086] The substrate may have one or more layers of a thin-film or layer of a metal oxide catalyst. In embodiments where the electrode has two or more layers, the layers may comprise different types of catalysts, and it is envisaged that the layer coated directly on the electrode may assist in anchoring or adhering the second layer. Advantageously, an electrode having a bilayer thin film structure may have substantially greater catalytic activity over an electrode having a single layer thin film structure, such as increased current density and increased hydrogen production per unit area (i.e. in the case of its use in an electrolyser). Without wishing to be bound by theory, the inventors believe that the structure of the metal oxide thin film layers and the atomic level interaction between metal cations in the adjacent layers may enhance catalytic activity.

[0087] In Figure 3 there is illustrated a cross-section of an electrode 300 for use in an electrolyser or other electrochemical device, wherein the electrode has thin-film layer 32 of a ruthenium oxide catalyst on the electrode substrate 30. The electrode substrate 30 may be a metal mesh such as a stainless steel or a nickel mesh. The metal oxide thin film catalyst 32 may have a microcrystalline or nanocrystalline structure. The metal oxide thin film catalyst 32 may be a metal oxide other than ruthenium oxide.

[0088] A cross-section of a bilayer electrode 400 having two layers of metal oxide thin film catalysts is illustrated in Figure 4. The electrode is a stainless steel and nickel mesh substrate130, and has a first layer of a cobalt oxide thin film 134 formed directly on the electrode and a second layer of a ruthenium oxide thin film 132 coated on the first layer. The metal oxide thin film catalysts 132 and 134 may have a microcrystalline or nanocrystalline structure.

[0089] In use, the morphology and dispersion of the second layer of a bilayer catalyst electrode can be controlled by adjusting the amount of plasticiser, in embodiments where such is present in the viscous precursor. In some embodiments, the bilayer catalyst may include a first layer of a continuous thin film of a nanocrystalline cobalt oxide catalyst and a second layer of ruthenium oxide islands supported on the cobalt oxide catalyst thin film layer. In other embodiments, the bilayer catalyst may include a first layer of a continuous thin film of a nanocrystalline cobalt oxide catalyst and a second layer of ruthenium oxide continuous thin film supported on the cobalt oxide catalyst thin film layer.

[0090] The catalyst loading on the electrode may be about 0.01 mg / cm2to about 5 mg / cm2, about 0.05 mg / cm2to about 2.5 mg / cm2. In some embodiments, the catalyst loading may be about 0.07 mg / cm2.

[0091] The first and the second layer of a bilayer catalyst may have the same catalyst loading or may have different catalyst loadings. In use, a lower catalyst loading on the second layer may enable formation of island nanostructures.

[0092] In Figure 5, a perspective view of a cross-section of a bilayer electrode 500 is illustrated. The electrode 500 has a stainless steel and nickel mesh substrate 230, a first layer of a cobalt oxide thin film catalyst 234 formed directly on the electrode and a second layer of a ruthenium oxide thin film catalyst 232 coated on the first layer.

[0093] The thin film layers (or each layer, in a bilayer structure) may be of any suitable thickness, such as from about 10 nm to about 10 pm, about 50 nm to about 5 pm, about 100 nm to about 1 pm. in some embodiments the thin films may have a thickness of about 5-8 pm.

[0094] The electrode may further include an ionomer layer (not shown) applied to the one or more nanocrystalline thin-film layers. In embodiments, the ionomer layer may be applied to the outermost metal oxide thin film layer only such that it forms a coating on the electrode. In use the ionomer coating may provide a protective layer and enhances ion conduction.

[0095] The ionomer layer may be applied to the electrode by any suitable method, such as dip coating, brushing, or spray coating.ExamplesExample 1Step 1: Preparation of Porous Metal Substrate or Mesh (optional)

[0096] Surface preparation of a metal electrode (e.g., nickel foam from Sumitomo Electric Asia Pacific Pte Ltd, or nickel mesh from Italfim): This step may provide a surface structure which improves adhesion of films to the substrate. The substrate is then placed in a vacuum oven and in a muffle furnace in air and heated to temperature in range of 300 - 400 °C (400 °C preferred) to form a thin oxide layer on the surface. The substrate in then treated with sulphuric or nitic acid (0.5 M to 5 M) for up to 2 minutes. The surface of the metal electrode is rougher after treatment.Step 2: Dip coating solution synthesis

[0097] Preparation of viscous precursor: The polymeric precursor was prepared by mixing appropriate amounts of metal cation source compounds (e.g. nitrates or chlorides) in deionised water to obtain solutions having about 0.1 M concentration, and adding ethylene glycol as plasticiser to assist film formation and citric acid as a chelating agent to form a stable metal- citrate complex that have a three dimensional network structure.

[0098] The precursor was allowed to incubate with mixing to allow chelation for a period of about 24 to 60 hrs until the water has evaporated and the viscous precursor obtained.Step 3: Dip coating

[0099] Figure 2 illustrates a dip coating process. Dip coating involves dipping an electrode (such as the treated mesh from Step 1) into the viscous precursor from Step 2) using an automatic or semiautomatic dip coater. After dip coating, the catalyst is bound to the electrode using a vacuum oven and placed in a muffle furnace at high temperature. The muffle furnace is programmed to ramp at 1 °C per minute, up to 300 °C. The temperature then stays at 300 °C for 60 minutes, before cooling back down to room temperature. As citric acid is combustible and ignites at temperatures above 300 °C it acts as a fuel during heating of the coated electrode.

[0100] This entire process of coating the electrode may be conducted two times (each coat increases the catalyst loading) to ensure the catalyst doesn’t wash off when used in the electrolyser cell, as well as to get an ideal loading of the catalyst.Step 4: Ionomer coating (optional)

[0101] Step 4 is optional but may enhance the erosion resistance and / or reduce delamination of the coated thin-layer catalyst from the electrode.

[0102] This step involves coating the electrode coated with thin film catalyst from Step 3 with a polymeric protective layer made from ion-conducting ionomer (e.g. commercially available FAA3-50, Fumion, Sustainion® XA-9 or lonomr AP2-HNN8-00) and drying the ionomer coated electrode. An ionomer is a type of polymer that contains both covalent bonds (like traditional polymers) and ionic bonds. Typically, an ionomer can be of same material as a membrane material, where such is used. Coating the coated electrode with an ionomer layer can protect the catalyst film from erosion and also enhance triple phase boundaries (active reaction zone) between the electrode and membrane interface.

[0103] The ionomer layer was dried in forced air convection oven at a temperature of about 70 °C. Depending upon ionomer, however, the temperature may be from about 50 °C to about 150 °C. Ideally, the drying temperature should be less than the glass transition temperature of the ionomer.Example 2

[0104] Electrodes and electrochemical cells were prepared according to the below method and used to evaluate the current density and hydrogen generation potential of the cells. The effect of electrode (anode or cathode) substrate, catalyst type, and catalyst loading was evaluated. The effect of monolayer and biolayer catalysts was also evaluated.Electrode Fabrication

[0105] Catalyst-coated substrates were prepared by dip coating nickel foam electrodes into viscous polymeric precursors.

[0106] For the preparation of the polymeric precursors, the reagent grade metal nitrates (e.g cobalt nitrate hexahydrate, Sigma Aldrich) were dissolved in 20 mL of DI water to give a 0.1 M solution. Citric acid (molar ratio of metal nitrate to citric acid is 0.1) and ethylene glycol (ratio of the metal cations to glycol is 1:5) were added to DI water. After thorough mixing, the conical flask containing the solution was placed in a laboratory oven at 80 °C for 24 hrs. The solution was then taken out of the oven, allowed to cool down to room temperature, and used for dip coating.

[0107] For electrode fabrication, two nickel foams from Sumitomo Electric Corporation were cut in square shapes (25cm2) and were acid cleaned followed by rising with DI water and drying in oven. For dip coating, a homemade fixture was used with Ni foam tied to nickel wire. A coating speed of about 2mm per second was used. After dip coating, Ni foams were dried at 80°C for 10 minutes using a forced convection oven. After drying, the coated foams were furtherheat treated at 300 °C in a muffle furnace for 1 hr and used as anode and cathode for electrolysis tests in a single cell configuration.Cell assembly and testing

[0108] For cell assembly, a nickel plated test fixture commercially purchased from Dioxide materials (FL, USA) was used. Anion Exchange Membranes purchased from lonomr Innovations or Asahi SELEMION membrane were sandwiched between the two Nickel foams and EPDM rubber was used as gaskets. A pair of peristaltic pumps circulated the electrolyte solution on the anode and cathode sides. Small SS containers with demystifier mesh were used as gas water separators. 1.5M KOH was used as an electrolyte solution, and a line heater was used to heat the KOH solution to 80 °C. The Bio-Logic instrument Model SP 150e-3540 potentiated was used for the measurement of the Current -Voltage curve (Scan rate of 5 mV / s) and chronoamperometry. The test was controlled with EC-Lab® v 10.36 software. Polarization curves were fitted and analysed with EC-Lab® v 11.50 software. The results of these experiments are summarised below in Table 1.Table I. Electrode structure and performance data

[0109] A comparison of MEA_24 (cobalt oxide thin film only), MEA_25 (bilayer structure, see Example 4) and MEA_27 (ruthenium only) shows that current using the bilayer structure is over 90% higher than ruthenium alone. This means amount of hydrogen produced using MEA_25 (bilayer structure) at particular energy input is 100% more than the amount of hydrogen produced using MEA_27 (ruthenium alone) or more than 300% higher than MEA_24 (cobalt only even with much higher catalyst loading for cobalt).

[0110] Figure 6 and Table 1 shows that the average current densities of most cells were upwards of 500 mg / cm2. Most cells were run with the lonomr Innovations AF3 membrane, with the only exception being MEA_21 which was run with Asahi SELEMION™ AMVN membrane.

[0111] As seen from the Figure 6, the major differences in current density were in direct relation to the type of catalyst used and the amount of catalyst used.

[0112] In Figure 7 and Table 1, an enhancement effect of the bilayer structure (using ruthenium thin film on top of cobalt thin film, MEA_25) is clearly evident where it gives current density (proportional to rate of hydrogen production per unit area of the cell) of about 0.66 A.cm2at a voltage of 1.9 V (proportional energy input in kWh) compared to ruthenium alone (MEA_27) which has a current density of about 0.33 A.cm2. This proves that the method according to embodiments of the present invention is viable and produces results on par with other coating methods at lower catalyst loadings as well as lower coating costs.Example 3 - Comparative testing

[0113] The performance of an electrode fabricated using the method according to an embodiment of the invention (test electrode) was compared to a standard electrode fabricated using a traditional method of coating catalyst particles on a substrate (standard electrode). The experiment compared electrode performance in (a) Anion Exchange Membrane (AEM) electrolysis (Table 2), where the test anode was fabricated using the method according to an embodiment of the invention, and (b) Proton Exchange Membrane (PEM) electrolysis (Table 3),where the test cathode was fabricated using the method according to an embodiment of the invention. All other variables were held constant for a direct comparison of the electrode performance.Table 2. AEM ElectrolysisTable 3. PEM Electrolysis

[0114] Table 2 illustrates that a substantially lower catalyst loading was required on the test anode (0.05 mg / cm2platinum) to provide similar average current density to the standard anode (0.4 mg / cm2platinum).

[0115] Similarly, Table 3 illustrates that a substantially lower catalyst loading was required on the test cathode (0.08 mg / cm2iridium) to provide similar average current density to the standard cathode (0.5 mg / cm2iridium).Example 4 - Preparation of a bilayer catalyst-coated electrode

[0116] In one example of a bilayer catalyst coated electrode, an underlying layer formed of a cobalt oxide thin film and a top layer formed of a ruthenium oxide thin film as shown in Figures 4 and 5 may be prepared according to an embodiment of the invention.

[0117] A cobalt-based precursor is formed by adding a water solution of citric acid and ethylene glycol to a cobalt nitrate hexahydrate solution and applying heat to increase the viscosity of the precursor. The viscous precursor is then directly coated onto an electrode having a stainless steel and nickel substrate using a dip coating method, paint brush or spray coating. The coated electrode is then heat treated in either an inert atmosphere or in air at a temperature range of between about 300 to 500 °C (preferred 400 °C) to decompose the cobalt species to cobalt oxide, providing an electrode having a first layer of a nanocrystalline cobalt oxide catalyst.

[0118] A ruthenium-based precursor is formed by adding a water solution of citric acid and ethylene glycol to a ruthenium nitrosyl nitrate solution and applying heat. The viscous precursor is then directly coated onto the substrate on top of the cobalt oxide thin-film layer using dip coating, brushing or spray coating. The ruthenium oxide thin film layer can be either coated as continuous thin film or in the form of dispersed nanoparticles on the cobalt oxide thin film. The coated electrode is then heat treated in either an inert atmosphere or in air at a temperature range of between about 300 to 500 °C (preferred 400 °C) to decompose the ruthenium nitrosyl nitrate to nanocrystalline ruthenium oxide, providing a bilayer catalyst-coated electrode.

[0119] Prior art methods comprise depositing the catalyst layer using a slurry or a paint prepared by dispersing catalyst particle in liquid and then coating on the surface of the support (termed as wash coat) or deposited in particulate form using metal nitrates and heat treatment (called as incipient-wetness impregnation-pyrolysis method). Advantageously, the method according to embodiments of the present invention can form a bilayer catalyst in the form of two thin film catalyst layers. In use, it is envisaged that the cobalt thin-film catalyst layer may provide better adhesion for the ruthenium oxide thin-film catalyst layer and also can improve reaction kinetics.Example 5 - Large-scale Anion Exchange Membrane (AEM) electrolyser stack

[0120] The inventors have produced a large-scale working prototype in the form of a large-scale Anion Exchange Membrane (AEM) electrolyser stack with an active area of 4000 cm2, the electrodes of which were coated using the methods described herein. The stack has been rigorously tested with a Balance of Plant (BoP) system specifically designed to optimise its performance. The following description provides detailed insights into the design, testing, and performance characteristics of this advanced electrolyser stack.

[0121] As can be seen in Figure 8, the lifetime performance of the AEM 3-cell stack when operating with 1.5M KOH as the electrolyte is very encouraging. The data spans 120 hours, showcasing both current (in Amperes) and voltage (in Volts) over time. The performance of the stack remains relatively stable at 1.8 V and 2500 A, indicating consistent performance throughout the testing period.

[0122] The VI curves were manually recorded and documented in an Excel spreadsheet. Figure 9 illustrates the VI curve after the initial startup phase. The diamond marker indicates a current of 0.5A / cm2 at 1.79 V at the start of testing.

[0123] Figure 10 illustrates the performance of the AEM three-cell stack under a solar profile, using 1.5M KOH at 54°C. The current (A), fluctuates between 0 and 2500 A over the 80-minute experiment. Meanwhile, the voltage (V) ranges from 0 to 5 V.

[0124] These data provide insights into the efficiency and stability of the AEM stack under the specified conditions, highlighting the dynamic behaviour of current in response to the solar profile while maintaining a stable voltage. It also demonstrates that the dip coating method is robust enough to handle the real-world conditions imposed by a solar profile.

[0125] The membrane or electrode according to embodiments of the invention may be used in an electrochemical cell, such as those found in electrolysers, fuel cells, redox flow batteries.

[0126] The membrane or electrode according to embodiments of the invention may be used in a catalytic process, such as ammonia synthesis or dissociation or hydrocarbon synthesis and dissociation.

[0127] It will be understood to persons skilled in the art of the invention that many modifications may be made without departing from the spirit and scope of the invention. All such modifications are intended to fall within the scope of the following claims.

[0128] It is to be understood that any prior art publication referred to herein does not constitute an admission that the publication forms part of the common general knowledge in the art.

[0129] In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

Claims

CLAIMS:

1. A method of producing a thin film of a metal oxide catalyst on a substrate, the method comprising: providing a viscous precursor formed from a source of metal cations and a chelating agent; coating one or more surfaces of the substrate with the viscous precursor, whereby a thin film of the viscous precursor is retained on the substrate; and heat-treating the coated substrate, whereby the metal oxide catalyst is formed.

2. The method according to claim 1, wherein the viscous precursor is formed by dissolving the source of metal cations in water, adding the chelating agent and heating the resultant mixture.

3. The method according to claim 2, wherein a plasticizer is added to the water before heating.

4. The method according to claim 3 wherein the plasticizer is selected from one or more of the following: ethylene glycol and urea.

5. The method according to any one of the preceding claims, wherein the chelating agent is selected from one or more of the following: citric acid, glycolic acid, lactic acid, gluconic acid, aminopolycarboxylic acids, glycine, gelatin, P-diketones, acetylacetone and acetone, monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), triethylamine (TeA), ethylenediaminetetraacetic acid (EDTA), L-ascorbic acid and D-sorbitol.

6. The method according to any one of the preceding claims, wherein a viscosity of the viscous precursor is adjustable in order to produce a thin film having a predetermined thickness.

7. The method according to any one of the preceding claims, wherein the viscous precursor has a viscosity of between about 40 and about 200 centipoise.

8. The method according to any one of the preceding claims wherein the metal cation is selected from one or more of the following: cobalt, iron, silver, molybdenum, copper, chromium, nickel, manganese, vanadium, platinum, iridium, ruthenium, rhodium palladium and osmium.

9. The method according to any one of the preceding claims, wherein the substrate is coated with the viscous precursor by dip coating, spray coating or painting.

10. The method according to any one of the preceding claims, wherein the heat treatment comprises heating the coated substrate to a temperature of between about 300°C and about 400°C.

11. The method according to any one of the preceding claims, wherein the substrate is an electrode or a membrane for use in an electrochemical cell, or a substrate for use in a catalytic process.

12. The method according to claim 11, wherein the electrode is a metal electrode selected from a metal plate, a metal mesh or a perforated plate.

13. The method according to claim 12, wherein the electrode is formed of nickel, cobalt, iron, silver, titanium, stainless steel or alloys of any of these metals.

14. The method according to claim 11, wherein the electrode is a ceramic electrode or a composite metal and ceramic electrode.

15. The method according to claim 11, wherein the membrane is an ion conducting polymer membrane, a ceramic membrane or a composite of polymer and ceramic membrane.

16. The method according to any one of the preceding claims, wherein the metal oxide catalyst is a nanocrystalline or microcrystalline metal oxide catalyst.

17. The method according to any one of the preceding claims, further comprising a predrying step of the coated substrate, before the heat-treatment.

18. The method according to claim 17, wherein the pre-drying step comprises heating the coated substrate to a temperature of about 100°C for 5 to 20 min.

19. The method according to claim 17 or claim 18, wherein the pre-drying step comprises heating the coated substrate in a vacuum oven, freeze drier, supercritical fluid drier or air drier.

20. The method according to any one of the preceding claims, further comprising coating one or more of the surfaces of the coated substrate with a second viscous precursor formed from a source of second metal cations, and a chelating agent, wherein the second metal is different to the first metal, whereby a substrate having a plurality of thin films of metal oxide catalysts is produced.

21. The method according to any one of the preceding claims, further comprising applying an ionomer to the thin film of metal oxide catalyst formed on the coated substrate and curing the ionomer to form a coating.

22. A substrate in the form of an electrode, a membrane or a substrate for use in a catalytic process produced according to the method of any one of claims 1 to 21.

23. A substrate in the form of an electrode, a membrane or a substrate for use in a catalytic process comprising a plurality of thin films of layered metal oxide catalysts on the substrate.

24. The substrate according to claim 23, wherein a first thin-film comprises a continuous layer and a second thin-film comprises island nanostructures.

25. The substrate according to claim 23 or claim 24, comprising two thin-film layers in the form of a cobalt oxide thin-film layer and a ruthenium oxide thin-film layer.

26. The substrate according to any one of claims 23 to 25, further comprising an ionomer layer coating the plurality of thin-film layers.

27. Use of the substrate according to any one of claims 22 to 26 in an electrochemical cell or in a catalytic process.

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