Particulate composite iridium oxide materials and preparation method and application thereof

Abstract

The present application discloses a method for producing an iridium oxide film on a particulate substrate having an electrical conductivity of >0.01 S / cm (as detected by powder measurements) by chemical vapour deposition, comprising: (i) providing the particulate substrate in a reaction chamber; (ii) contacting the particulate substrate with a vaporized iridium oxide precursor gas, thereby forming a homogenously distributed film of iridium precursor material on the substrate; (iii) contacting the material comprising the homogenously distributed layer with a vaporized oxidant to allow the oxidant to react with the homogenously distributed iridium precursor film on the substrate; and (iv) optionally, repeating until steps (ii) to (v); to obtain a particulate material comprising a surface film of a desired thickness comprising homogeneously distributed discrete iridium oxide clusters with an average diameter in the range of from 0.5 to 5 nm.

Description

[0001] Particulate Composite Iridium Oxide Materials and Preparation Method and Application Thereof

[0002] Field of the invention

[0003] The present invention relates to iridium oxide film particulate catalysts, a method for the preparation of particulate composite materials comprising a homogenously distributed iridium metal or iridium oxide layer, and the use of the particulate composite materials in a catalyst composition which can be used in water electrolysis.

[0004] Background of the invention

[0005] Various useful particulate catalysts comprising noble metals, in particular from the platinum group, for electrolysis have been described previously. These typically comprise an inorganic carrier, onto which catalytic metals or metal oxides are deposited by precipitation and / or crystallisation from solution.

[0006] EP4019666A1 for instance discloses a particulate catalyst wherein a coating of an average layer thickness in the range of 1.5 to 5.0 nm containing iridium oxide, iridium hydroxide, or iridium hydroxide oxide, at an iridium content of at most 50% by weight, is precipitated onto a support material having a BET surface area from 2 to 50 m2 / g and composed of an oxide of a transition metal, an oxide of a main group metal, and / or SiCh.

[0007] Similarly, EP2608297 discloses a particulate catalyst comprising a silica or titania particles with high surface areas, onto which iridium oxide has been deposited by precipitation from a precursor solution.

[0008] US20140322631A1 discloses a catalyst comprising a iridium oxide coating, and an inorganic oxide carrier at 25 to 70 wt.% based on the total catalyst weight, and a BET surface area of 30 to 200 m2 / g. This catalyst is described as having an electrical conductivity, by powder measurement, of above 0.01 S / cm.

[0009] US7976989B2 discloses a composite catalyst comprising iridium oxide particles deposited on, or dispersed around inorganic oxide particles in less than 20 wt. % having a BET surface area of 50 to 400 m2 / g, less than 20 wt. % based on the total weight of the composite catalyst. The catalyst further comprises ruthenium oxide to an Ir / Ru-atomic ratio of 4 / 1 to 1 / 4.

[0010] A disadvantage of such and similar catalysts is that during the precipitation phase in liquid solution, the deposition of the catalytic metal oxides is typically not homogenous. Also, the reaction kinetics and the mass transfer change over time as the pH and other factors of the solution change. Furthermore, nucleation and crystal growth typically affect the size of the conglomerates formed, which may render a certain portion of the catalytic metal inaccessible for catalysis. A different approach has been disclosed in US9450251B2, describing a method of manufacturing an anode material for a fuel cell, including synthesizing an electrolysis catalyst from a porous titania substrate by depositing a solid solution of iridium oxide and a valve metal oxide thereon by Atomic Layer Deposition (ALD). Similarly, US20190109330A1 discloses a catalyst material prepared by ALD comprising a support comprising a material selected from metal carbides, nitrides, oxides and combination thereof; and a thin film covalently bonded to the support at a thickness of 0.5 to 5.0 nm, the thin film comprising a catalytic metal selected from the group consisting of platinum-group metals, platinum-group metal oxides, transition metals, transition metal oxides, and combinations thereof. In both cases, ALD processes were carried out, applying vacuum purge steps.

[0011] However, applicants found that the disclosed catalysts based on metal oxide substrates may exhibit a limited electrical conductivity, which renders their use as electrocatalyst difficult, since high voltages would be required that may lead to undesired side reactions. Furthermore, it is known from fuel cell membrane research that that certain metal oxide carriers, in particular titania, may react with hydrogen peroxide leading to hydroxyl radical generation which may negatively affect the stability of hydrogen-permeable ionomer layers in proton exchange membrane electrolysers or fuel cells.

[0012] EP1701790B1 discloses precious metal based oxide catalysts for water electrolysis, comprising iridium oxide and a high surface area inorganic oxide, wherein the inorganic oxide is present in a quantity of less than 20 wt.-% based on the total weight of the catalyst. If the amount of inorganic oxide is higher than 20 wt.-%, the electrical conductivity of the catalyst and the electrode is impaired. Because of that, these catalysts contain a high amount of precious metal oxide, which renders them expensive, not permitting for a very low precious metal loading in the respective electrodes. Thus, when using the catalysts disclosed in EP 1701790B1, the precious metal consumption is still relatively high, leading to high environmental costs of the catalyst products.

[0013] W02022015161A1 discloses a process for coating iridium oxide onto a particulate titanium oxide, wherein the particulate substrate is generated in situ on a membrane or electrode, i.e. within a porous substrate, before being coated by means of atomic layer deposition techniques, resulting in core-shell nanoparticles formed within a porous substrate. The formation of a core-shell structure within a porous substrate means that the porous substrate will influence the formation of the core-shell structure. Depending on the substrate used, as well as on the reaction conditions, non-uniform shells may occur, or a coating may be applied to the porous substrate itself, instead of solely on the particulate particles. The core-shell nanoparticles may have a variety of morphologies including dump a bell -like morphology where the core is partially surrounded by the shell. Multiple cycles, the number of which is not clear, may be necessary to arrive at a sufficient core-shell nanoparticles, and the catalyst precursor is to be chosen carefully so as not to react with the carrier material. US20210252486A1 discloses the application of Fe or FeOxon various substrates. A goal of the invention of the present application is to provide materials suitable as catalysts for electrochemical reactions, such as water electrolysis, and for the use of the catalyst particles. Another goal is to provide an improved method of producing a particulate catalyst according to the invention.

[0014] Summary of the invention

[0015] In view of the above discussion, the present disclosure provides for a method for producing an iridium oxide film on a particulate substrate having an electrical conductivity of >0.01 S / cm (as detected by powder measurements) by chemical vapour deposition at a pressure in the range of from 80 to 1010 kPa, comprising:

[0016] (i) providing the particulate substrate in a reaction chamber;

[0017] (ii) contacting the particulate substrate with a vaporized iridium oxide precursor gas, thereby forming a homogenously distributed film of iridium precursor material on the substrate;

[0018] (iii) contacting the material comprising the homogenously distributed layer with a vaporized oxidant to allow the oxidant to react with the homogenously distributed iridium precursor film on the substrate; and

[0019] (iv) optionally, repeating until steps (ii) and (iii); to obtain a particulate material comprising a surface film of a desired thickness comprising homogeneously distributed discrete iridium oxide clusters with an average diameter in the range of from 0.3 to 5 nm.

[0020] It is a further object to provide a particulate material obtainable by the method according to the disclosure. It is yet a further object to provide a method for the formation of an electrolysis electrode, comprising using an electrocatalyst according to the present disclosure. In a further aspect, the subject of the invention is to provide an electrocatalyst assembly comprising catalyst particles according to the present disclosure,

[0021] It is a further object of the invention to provide a cell comprising a catalyst material according to the invention or the composition according to the invention.

[0022] It is yet a further object of the invention to provide a use of the materials according to the invention in a water electrolysis device, a fuel cell, or a bi-directional fuel cell.

[0023] Applicants have found that with the methods, materials, compositions and electrolytic cells according to the invention the goal has been achieved.

[0024] Short Description of the Figures

[0025] The present invention will now be described with reference exemplary embodiments illustrated in the attached drawings in which: Figure 1 shows the morphology of an Iridium Oxide (IrOx) catalyst according to a preferred embodiment of the present disclosure, using a transmission electron microscopic (TEM) image.

[0026] Figure 2 shows a comparison of the mass activity, electrochemical surface area (ECA) and durability of two catalyst compositions according to preferred embodiments of the present disclosure, when tested in rotating disk electrode (RDE) electrochemical tests; comparing the durability over activity with a commercial Iridium oxide catalyst, i.e. Iridium Black.

[0027] Figure 3 shows the iridium oxidation state of several catalyst compositions according to preferred embodiments of the present disclosure, ranging from amorphous Ir(lll) to more oxidized Ir(IV), using XPS (X-ray Photoelectron Spectroscopy).

[0028] Figure 4 shows the 0.5x current density of catalyst compositions with 15 to 30 times less iridium according to preferred embodiments of the present disclosure as compared to commercially available comparative catalysts, in a membrane electrode assembly (MEA) test.

[0029] Figure 5 shows the change in conductivity when the particulate starting material, as well as the calcined catalyst composition are subjected to an etching treatment.

[0030] Figure 6 shows a graph plotting the cell potential versus the current density at the beginning of life (BoL) of catalyst particles measured in a MEA test. Depicted, are catalysts with different iridium loadings and the iridium black comparison.

[0031] Figure 7 shows a graph plotting the voltage over current density at the beginning of life (BoL), at the beginning of the accelerated age test (BoT) and at the end of the accelerated age test (EoT) in the MEA test.

[0032] Detailed Description of the Invention

[0033] The invention is described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the size and relative sizes of layers and regions may be exaggerated for clarity.

[0034] It will be understood that when an element or layer is referred to as being "on", "connected to" or "coupled to" another element or layer, it can be directly on, connected or coupled to the other element or layer or intervening elements or layers may be present. In contrast, when an element is referred to as being "directly on," "directly connected to" or "directly coupled to" another element or layer, there are no intervening elements or layers present. Like numbers refer to like elements throughout. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items. It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, components, regions, layers and / or sections, these elements, components, regions, layers and / or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present invention.

[0035] Spatially relative terms, such as "beneath", "below", "lower", "above", "upper" and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as "below" or "beneath" other elements or features would then be oriented "above" the other elements or features. Thus, the exemplary term "below" can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.

[0036] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and / or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof.

[0037] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0038] Embodiments of the present invention are described herein with reference to cross-section illustrations that are schematic illustrations of idealized embodiments of the present invention. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and / or tolerances, are to be expected. Thus, embodiments of the present invention should not be construed as limited to the particular shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. For example, an etched region illustrated as a rectangle will, typically, have rounded or curved features. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the precise shape of a region of a device and are not intended to limit the scope of the present invention.

[0039] The inventors have surprisingly found that by choosing a particulate substrate having an electrical conductivity of >0.01 S / cm, as detected by powder conductivity measurements, and by chemical vapour deposition at a pressure in the range of from 80 to 1010 kPa, a particularly good electrocatalyst material can be prepared, having a beneficial ratio of mass activity over durability. Accordingly, the present invention relates generally to methods of producing thin films by chemical vapour deposition, more specifically, atomic layer deposition (ALD) processes at or near atmospheric pressure, and yet more specifically, atomic layer deposition (ALD) processes with a continuous precursor and oxidant feed addition, essentially not requiring purging steps.

[0040] Atomic layer deposition (ALD) is a particular technique generally based on chemical vapor deposition (CVD). CVD is a commonly used deposition processes, usually employed for depositing layers on a substrate from a precursor vapour material under low pressure, whereby vacuum, substrate temperature and temperature, concentration and nature of the precursor materials introduced into the reaction chamber may allow to produce a desired layer of the deposited material, with a uniform thickness and high homogeneity. ALD is a variant of CVD comprising a cyclical deposition in sequential cycles, based on atomic layer epitaxy (ALE) and chemisorption and reaction techniques. Typical ALD processes as disclosed in the literature operate in a reaction chamber under strongly reduced pressure, also referred to as vacuum chamber. Either in the chamber, or before, the substrate is heated up to suitable deposition temperature at reduced pressure. Then a reactant gas stream is introduced into the vacuum chamber and brought into contact with the substrate, leading to the deposition or adsorption of a small amount of the reactant gas on the substrate surface. This reactant gas stream may conveniently be added in the form of gaseous reactant pulses into the reaction chamber, wherein it is allowed to contact the surface of the substrate, to form a monolayer of the reactant on the substrate surface. Excess gas comprising the reactant is purged out of the reaction chamber in vapour or gas form.

[0041] Then a second gaseous reactant is added to the chamber and brought into contact with the coated substrate surface to provide a surface reaction between the second reactant and the first reactant bound to the surface where it is allowed to react. This is followed by a further purge step of removal of any excess of the second gaseous reactant and any vapour or gaseous by-products formed in the reactions with the reactant layer, and any uncoated substrate, which are purged out of the reaction chamber. These steps may be repeated until the desired thickness of the deposited reacted film is reached.

[0042] Since the gas phases are purged prior to the introduction of a following gas addition, direct reactions of reactant and oxidant in the gas phase can be minimized. The removal or purge steps may comprise subjection to further reduced pressure, and / or a purge with a gas that is inert at the reaction conditions, e.g., nitrogen, helium or argon. In such cycles, the substrate surface to be coated is exposed to a first precursor gas, then purged to remove unreacted first precursor gas, and then contacted with a second precursor, which reacts in situ with the retained or absorbed first precursor, to form a product film on the substrate surface. The cycle may be repeated to form a layer of a desired thickness.

[0043] However, processes employing high vacuum require generation and maintenance thereof, and therefore have a relatively low energy efficiency.

[0044] The present process advantageously operates at, or near atmospheric pressure, thereby reducing the need for the generation and maintenance of the usual very lower pressures employed in classical ALD processes. Furthermore, the present process does not require a purge step to remove byproducts or unreacted reactants prior to adding a following reactant pulse, effectively being a CVD process for particulate substrates. By no longer needing to remove unreacted reactants and byproducts, the process can be performed in a single reactor chamber, with a continuous feed of powder and reactant, allowing for a fully continuous operation. A particularly suitable reactor is a continuous pneumatic transport reactor, as for instance disclosed in US20210102288A1.

[0045] This process comprises at least the following steps: providing the particulate substrate in a reaction chamber; contacting the particulate substrate with a vaporized iridium oxide precursor gas, thereby forming a homogenously distributed film of iridium precursor material on the substrate; contacting the material comprising the homogenously distributed layer with a vaporized oxidant to allow the oxidant to react with the homogenously distributed iridium precursor film on the substrate; and optionally, repeating these steps sequentially; to obtain a particulate material comprising a surface film of a desired thickness comprising homogeneously distributed discrete iridium oxide clusters with an average diameter in the range of from 0.3 to 5 nm.

[0046] Accordingly, the method according to the disclosure further preferably comprises sequentially exposing the particulate substrate to a gaseous iridium precursor and to a gaseous oxidising agent, and depositing the iridium material on the substrate.

[0047] Additionally optional steps of removing excess iridium oxide precursor gas, excess reactant gas and / or reaction by-products from the reaction chamber may be applied. Preferably, however in the method according to the disclosure, excess iridium oxide precursor gas, excess reactant gas and / or reaction by-products are not removed from the reaction chamber, to enhance the production rate.

[0048] Preferably, in the method according to the disclosure, steps (ii) and (iii) are performed such that the iridium oxide precursor gas is dosed to attain a self-terminating reaction with the surface of the particulate substrate, and the oxidant is dosed to attain a self-terminating reaction with the precursor material comprising the continuous layer of iridium oxide.

[0049] Preferably, in the method according to the disclosure, steps (i) to (iv) are performed at or near ambient pressure. Particulate Substrate

[0050] Preferably, in the method according to the disclosure, an iridium oxide film is produced on a particulate substrate having an electrical conductivity of >0.01 S / cm (as detected by powder measurements) by chemical vapour deposition at a pressure in the range of from 80 to 1010 kPa.

[0051] The method according to any one of the preceding claims, wherein the substrate or iridium support material is essentially inert under oxidative and acidic conditions, preferably wherein the support material is inert during deposition of the iridium oxide layer, the calcination, and when used in a PEM electrolyser.

[0052] The substrate is a particulate material that is suitable to be coated with iridium oxide under the conditions of the process, and essentially inert to the process conditions.

[0053] Applying the present method, on the particulate substrate material having an average particle size in the range given below, the electrical conductivity of the resulting catalyst material is in an acceptable range, suitable for the production of electrodes with sufficiently high performance.

[0054] Typically, the electrical conductivity of the resulting composite precious metal oxide catalyst is >0.01 S / cm at 300 kPa pressure, preferably a conductivity of at least >0.1 S / cm, at 300 kPa (3 bar) pressure. As a result, the electrochemical mass activity (detected as current density [A / mg Ir] at 1.5 V vs. RHE) is generally in a range of 100 to 15000 mA / mg Ir.

[0055] Preferably, in the method according to the disclosure, the particulate support material comprises metal oxides, carbides, borides, nitrides and non-metallic oxides, and / or carbides.

[0056] Preferably, in an embodiment, the particulate substrate is selected from inorganic metal oxides, preferably, selected from titania (TiCh), silica (SiCh), alumina (AI2O3), zirconia (ZrCh), tin dioxide (SnCh), F- doped tin oxide (SnCh / F), ceria (CeCh), ceria doped zirconia (CeCh / ZrCh), niobium pentoxide (NbjOs), tantalum pentoxide (TajOs); and mixtures and combinations thereof; or boron carbide materials, or composites of boron carbide with other compounds, such as silicon boron carbide. Preferably, in the method according to the disclosure, the inorganic oxide or boron carbide is added in a range of 30 wt.-% to 60 wt.-%, based on the total weight of the catalyst.

[0057] Preferably, the support material particle size preferentially has a weight average particle sizer in the range of from 10 nm to 100 nm, preferably of from 40 nm to 60 nm.

[0058] In one or more embodiments, the iridium oxide-coated particulate material comprises boron at a concentration of 1 at %, 5 at %, 10 at %, 12 at %, 15 at %, 20 at %, 22 at %, or 25 at % to 28 at %, 30 at %, 35 at %, 40 at %, 45 at %, 50 at %, 60 at %, or 70 at %. For example, the iridium oxide-coated particulate boron carbide contains boron carbide at a concentration of 1 at % to 70 at %, 1 at % to 60 at %, 1 at % to 50 at %, 1 at % to 45 at %, 1 at % to 40 at %, 1 at % to 35 at %, 1 at % to 30 at %, 1 at % to 25 at %, 1 at % to 20 at %, 1 at % to 15 at %, 1 at % to 10 at %, 10 at % to 70 at %, 10 at % to 60 at %, 10 at % to 50 at %, 10 at % to 45 at %, 10 at % to 40 at %, 10 at % to 35 at %, 10 at % to 30 at %, 10 at % to 25 at %, 10 at % to 20 at %, 10 at % to 15 at %, 10 at % to 12 at %, 15 at % to 70 at %, 15 at % to 60 at %, 15 at % to 50 at %, 15 at % to 45 at %, 15 at % to 40 at %, 15 at % to 35 at %, 15 at % to 30 at %, 15 at % to 25 at %, or 15 at % to 20 at %.

[0059] The substrate should be powdery materials, should be inert and should have a very low solubility in water and in acidic environment. This is important for a long lifetime and high endurance of the PEM electrolyzer unit.

[0060] Preferably, the substrate is a material that can sustain the production conditions in a pneumatic reactor or a fluidized bed without significant particle attrition.

[0061] In some embodiments of the present invention, the iridium oxide layer is formed on a dielectric material. Examples of a dielectric material for substrate include hafnium oxide (HfCh), aluminium oxide (AI2O3), titanium oxide (TiCh), yttrium oxide (Y2O3), zirconium oxide (ZrCh), tantalum oxide (Ta2O5), niobium oxide (Nb2O5), a barium titanate (BaTiOs), a strontium titanate (SrTiOs) and / or the like. Suitable substrates in particular may comprise a high surface area inorganic oxide, such as TiCh, AI2O3, ZrCh and mixtures thereof. Particularly suitable inorganic oxide substrate materials comprise inorganic oxides are pyrogenic oxides such as TiCh, SiCh or AI2O3. However, other inorganic oxides may be used as well, such as Nb2O5, SnC>2, Fluoro-doped tin oxide (SnCh / F), zirconia (ZrCh), ceria doped zirconia (CeCh / ZrCh), and / or mixtures and combinations thereof.

[0062] A further particularly suitable class of substrates are those selected from nitride, such as titanium nitride (TiN), or carbide materials, such as titanium carbide (TiC), tantalum carbide. Preferably, such materials include silicon or boron materials, such as boron nitride or boron carbide, and / or mixtures, alloys and combinations thereof.

[0063] Boron carbide is used in a variety of industrial applications, including abrasives, refractory materials, and as a neutron absorber in nuclear power plants. Preferably, when used as an electrolysis catalyst support, a boron carbide substrate may be employed since it shows a good ratio between durability and inherent electrical conductivity.

[0064] Once formulated into a PEM stack or CCM, catalyst materials based on Boron carbide permitted to attained a degradation level approximating 1 microvolt / hour, correlating approximately with a degradation of 0.1% in lOOOh of catalyst particles.

[0065] Suitable boron carbide materials include essentially pure boron carbide materials, or composites of boron carbide with other compounds, such as silicon carbide. The chemical formula of boron carbide is typically B12C3, although for simplicity it is often referred to as the smallest unit, B4C. These boron carbide materials may also include oxygen, which may form during the CVD process or by storing in ambient conditions in air. Also, it was found that the catalyst particles according to the present invention, when prepared from the above substrates were typically exhibiting a conductivity of at least >0.1 S / cm at 300 kPa (3 bar) pressure.

[0066] Such materials are generally considered covalent ceramic materials, which are generally among the hardest materials known to mankind. Hence, these materials are particularly suitable for use in a continuous loop reactor or a pneumatic transport reactor due to the low loss of catalyst materials due to attrition.

[0067] Iridium Oxide Precursor

[0068] Advantageously, the iridium oxide precursor comprises an organometallic complex of iridium. Preferably, the iridium oxide precursor comprises an Iridium Ci to C4- alkylcyclo-pentadienyl) complex, preferably a methylcyclopentadienyl or ethylcyclopentadienyl complex. Preferably, in the method according to the disclosure, the iridium oxide precursor is a heteroleptic metallocene complex.

[0069] Preferably, in the method according to the disclosure, the (Cito C4- alkylcyclopenta- dienyl) complex further comprises a diene ligand selected from linear or cyclic alkylenes, preferably selected from 1,4-butadiene, 1,4- pentadiene, 1,5- pentadiene, 1,4-hexadiene, 1,5- hexadiene; cyclopentadiene, 1,3-cyclohexadiene, 1,4-cyclohexadiene, and oxocyclohexadiene, each diene ligand optionally being substituted with one or more substituents R1, wherein each R1is independently selected from H, O, Ci to C linear or branched alkyl, Ci to C linear or branched alkylamide, Ci to C linear or branched alkoxide, Ci to C linear or branched alkylsilyl, Ci to C linear or branched alkylsilylamide, and Ci to C linear or branched fluoroalkyl.

[0070] Particularly preferred iridium oxide precursors are selected from (methylcyclopentadienyl) (1,3- cyclohexadiene) iridium; (ethylcyclopentadienyl)(l,3- cyclohexadiene) iridium; (ethylcyclopentadienyl)(l,5-cyclooctadiene) iridium, (l,5-cyclooctadiene)(2,2,6,6-tetramethyl-3,5- heptanedionate)iridium; (ethylcyclopentadienyl)(2,3-Me-2-l,3-butadiene) iridium; (methylcyclopentadienyl) (1,5-hexadiene) iridium; (ethylcyclopentadienyl)(l,5-hexadiene)iridium, (methylcyclopentadienyl)(l,4-butadiene) iridium, (methylcyclopentadienyl) (1,4- pentadiene)iridium, (methylcyclopentadienyl)(l,5- pentadiene)iridium, (methylcyclopentadienyl)(l,4-hexadiene)iridium, (methylcyclopentadienyl)(l,5- hexadiene)iridium; (ethylcyclopentadienyl)l,4-butadiene iridium, (ethylcyclopentadienyl)(l,4- pentadiene)iridium, (ethylcyclopentadienyl)(l,5- pentadiene)iridium, (ethylcyclopentadienyl)(l,4-hexadiene)iridium, and / or (ethylcyclopentadienyl)(l,5- hexadiene) iridium, or mixtures thereof.

[0071] Preferably, the iridium oxide precursor has a melting point below 25°C, preferably below 0°C, and more preferably is a liquid at a temperature in the range of from 25°C to 45°C and ambient pressure. Preferably, in the method according to the disclosure, the iridium oxide precursor is a liquid at room temperature. Preferably, in the method according to the disclosure, wherein the iridium oxide precursor exhibits a vapour pressure at room temperature to 200° C in the range of from 1 - 100 torr.

[0072] Oxidant

[0073] Processing an iridium precursor often involves using oxygen or an oxidizing co-reagent. Use of oxygen and oxidizing co-reagents can be incompatible with the underlying surface. Therefore, there is a need in the art for iridium precursors and co-reagents that react to form iridium metal and iridium based thin films without an oxidizing co-reagent or oxidant.

[0074] Preferably, in the method according to the disclosure, the oxidant material comprises a source of molecular oxygen provided by the decomposition of the oxidant under the conditions in step (iii).

[0075] Preferably, the oxidant comprises a peroxide of hydrogen or of an organic compound. Yet more preferably, the oxidant comprises hydrogen peroxide.

[0076] Reaction Conditions

[0077] Preferably, in the method according to the disclosure, the iridium oxide precursor and / or the co-reactants are heated to increase the vapour pressure prior providing the vapour to the reaction chamber(ii).

[0078] The iridium precursor vapour or gas, and / or the oxidant vapour or gas, are preferably introduced into the process chamber to contact the substrate at a flow rate of at least 10 seem, preferably at least 50 seem, at least 100 seem, at least 200 seem, or in the range of from 300 seem to 400 seem, at least 500 seem, at least 700 seem, at least 1000 seem, at least 1200 seem, at least 1500 seem, at least 1800 seem, or at least 2000 seem.

[0079] For example, the iridium precursor or oxidant may be introduced into the process chamber and / or exposed to the substrate at a flow rate of in the range of from 10 seem to 2,000 seem, preferably, 10 seem to 1500 seem, more prefearbly 10 seem to 1000 seem, again prefearbly 10 seem to 800 seem, yet more preferably of from 10 seem to 500 seem, again more preferably of from 10 seem to 300 seem, 10 seem to 100 seem, 100 seem to 2000 seem, 100 seem to 1500 seem, 100 seem to 1000 seem, 100 seem to 800 seem, 100 seem to 500 seem, or 100 seem to 300 seem.

[0080] Carrier Gas

[0081] The iridium precursor or oxidant may be introduced into the process chamber and / or exposed to the substrate as a precursor mixture comprising the iridium precursor entrained in one or more inert carrier gases, or as oxidant mixture comprising the oxidant entrained in one or more inert carrier gases.

[0082] The carrier gas may comprise one or more gases, such as nitrogen, argon, helium, or any combination thereof. Accordingly, preferably, in the method according to the disclosure, the gaseous precursor compounds are carried into the reaction chamber using an inert carrier gas.

[0083] Catalyst Particles Preferably, in the method according to the disclosure, the continuous film of iridium oxide has an average layer thickness of below 5 nm, as determined by Transmission Electron Microscopy (TEM).

[0084] The iridium oxide-coated particulate material obtainable by the process comprises a continuous homogeneously distributed film of iridium oxide particles having an average layer thickness of below 5 nm, as determined by Transmission Electron Microscopy (TEM).

[0085] Preferably, in the method according to the disclosure, the iridium oxide comprises iridium(IV)- oxide, iridium( I II )-oxide and / or mixtures thereof.

[0086] The iridium oxide-coated particulate material preferably contains iridium at a concentration of 30 at %, 40 at %, 50 at %, 60 at %, or 70 at % to 75 at %, 80 at %, 85 at %, 87 at %, 90 at %, 95 at %, 96 at %, 97 at %, 98 at %, or 99 at %. For example, the iridium oxide-coated particulate material contains iridium at a concentration of 30 at % to 99 at %, 40 at % to 99 at %, 50 at % to 99 at %, 60 at % to 99 at %, 70 at % to 99 at %, 80 at % to 99 at %, 90 at % to 99 at %, 30 at % to 90 at %, 40 at % to 90 at %, 50 at % to 90 at %, 60 at % to 90 at %, 70 at % to 90 at %, 80 at % to 90 at %, 30 at % to 80 at %, 40 at % to 80 at %, 50 at % to 80 at %, 60 at % to 80 at %, 70 at % to 80 at %, or 75 at % to 80 at %.

[0087] The iridium material (e.g., metallic iridium or iridium oxide) is deposited or otherwise formed to a thickness of 2 A, 5 A, 8 A, 10 A, 15 A, 20 A, 25 A, 30 A, 40 A, or 50 A to 60 A, 70 A, 80 A, 90 A, 100 A, 120 A, 150 A, 200 A, 250 A, 300 A, 400 A, 500 A, or thicker. For example, the iridium material is deposited or otherwise formed to a thickness of 2 A to 500 A, 5 A to 400 A, 5 A to 300 A, 5 A to 200 A, 5 A to 150 A, 5 A to 100 A, 5 A to 80 A, 5 A to 50 A, 5 A to 25 A, 10 A to 400 A, 10 A to 300 A, 10 A to 200 A, lO A to 150 A, lO A to 100 A, lO A to 80 A, lO A to 50 A, lO A to 25 A, 20 A to 400 A, 20 A to 300 A, 20 A to 200 A, 20 A to 150 A, 20 A to 100 A, 20 A to 80 A, 20 A to 50 A, or 20 A to 25 A.

[0088] Preferably, in the method according to the disclosure, steps (ii) and (iv) are performed at a temperature in the range of from 125°C to 375°C, preferably at a temperature in the range of from 150°C to 300°C.

[0089] The process chamber and / or the substrate is preferably heated to and / or maintained at a temperature of 0°C, 5°C, 10°C, 15°C, 20°C, 25°C, 30°C, 35°C, 40°C, 45°C, or 50°C to 60°C, 70°C, 80°C, 100°C, 120°C, 150°C, 180°C, 200°C, 250°C, 300°C, 350°C, 400°C, or 500°C during the ALD process. In one or more embodiments, the process chamber and / or the substrate is preferably heated to and / or maintained at a temperature of 0°C to 500°C, 0°C to 400°C, 0°C to 350°C, 0°C to 300°C, 0°C to 250°C, 0°C to 200°C, 0°C to 150°C, 0°C to 100°C, 0°C to 80°C, 0°C to 70°C, or 0°C to 50°C during the ALD process. In some embodiments, the process chamber and / or the substrate are preferably heated to and / or maintained at a temperature at a range of from 20°C to 500°C, 20°C to 400°C, 20°C to 350°C, 20°C to 300°C, 20°C to 250°C, 20°C to 200°C, 20°C to 150°C, 20°C to 100°C, 20°C to 80°C, 20°C to 70°C, or 20°C to 50°C during the deposition process. In other embodiments, the process chamber and / or the substrate are is preferably heated to and / or maintained at a temperature in the range of from 40°C to 400°C, 40°C to 375°C, 40°C to 350°C, 40°C to 300°C, 40°C to 250°C, 40°C to 200°C, 40°C to 150°C, 40°C to 100°C, 40°C to 80°C, 40°C to 70°C, or 40°C to 50°C during the deposition process.

[0090] The present invention also relates to an iridium oxide-coated particulate material comprising a continuous iridium oxide film, wherein the film is a conformal film comprising carbon in a range of from 3 to 5 wt.% on the iridium oxide film, as determined by ASTM E1019.

[0091] The present invention also relates to the iridium oxide-coated non-metallic particulate material, wherein the iridium is present in a range of from 5 to 50 wt%, based on total catalyst weight, as determined by ICP-OES.

[0092] The iridium oxide of the present invention comprises predominantly of iridium-(IV)-oxide (IrCh), however, various amounts of iridium-(l ll)-oxide (IrjOa) may be present.

[0093] The term particulate catalyst herein means that the catalyst contains the iridium oxide particles finely deposited on or dispersed around the particulate substrate material.

[0094] Reactors

[0095] Other suitable reactors that may be used for the deposition of thin films according to the processes of the present invention include commercially available ALD reactors, and many other kinds of reactors capable of ALD growth of thin films, including CVD reactors equipped with appropriate equipment and means for pulsing the precursors, can be employed.

[0096] Preferably, the method according to the disclosure may be performed in a batch-type fluidized bed reactor, a rotary drum reactor, or in a continuous-type reactor.

[0097] Preferably, the reactor line-up is designed such that reactants are kept separate until reaching the reaction chamber, such that shared lines for the precursors are minimized. However, other arrangements are possible.

[0098] Etching

[0099] The method according to the disclosure further advantageously comprises the step of contacting the particulate material prior to metal deposition with water at an elevated temperature, to remove residual compounds and to form an etched particulate material. The method according to the disclosure further also relates to an etched activated iridium oxide-coated particulate catalyst material, wherein the continuous iridium oxide film is a conformal film comprising carbon in a range of from 0 to less than 3 wt.% on the iridium oxide film, as determined by ASTM E1019.

[0100] Calcination

[0101] The method according to the disclosure further comprises step (vii) of subjecting the iridium oxide-coated particulate substrate to a heat treatment at a temperature in the range of from 150°C to

[0102] 550°C in the presence of oxygen, to obtain an activated iridium oxide-coated particulate catalyst material. Instead of an oxidative environment using oxygen, a reductive environment can also be used such as in the presence of hydrogen. Such a heat treatment is sometimes referred to as "calcination" or "annealing".

[0103] PEM and CCM Application

[0104] The present disclosure also advantageously relates to a process for the preparation of a proton exchange membrane (PEM) electrolyzer, comprising: blending the activated iridium oxide-coated particulate catalyst material according to the disclosure with a polymeric material, preferably, a ionomer, to form a catalyst polymer dispersion, and depositing the catalyst polymer dispersion onto a PTFE membrane, to form a catalyst layer, and transferring the catalyst layer onto a proton exchange membrane, to obtain a catalyst-coated membrane (CCM). The present invention also relates to a membrane electrode assembly (MEA) suitable for use in PEM electrolyzer stacks, comprising a catalyst coated membrane (CCM) obtained as set out herein. The present invention hence also relates to the use of an activated iridium oxide-coated particulate catalyst material according to the disclosure in a proton exchange membrane electrolyzer.

[0105] For the manufacture of electrodes, catalyst-coated membranes (CCMs) and membrane- electrode-assemblies (MEAs), the iridium oxide particulate catalysts according to the subject disclosure are prefearbly processed into a polymeric matrix, such as an inks or paste by adding suitable solvents and optionally ionomer materials. The catalyst material may be deposited onto gas diffusion layers (GDLs), current collectors, ionomer membranes, blank PTFE sheets, release papers or separator plates and the like by spraying, printing, doctor-blading or other deposition processes. Usually, a drying process is subsequently applied to remove the solvents of the catalyst ink. In catalyst-coated membranes and MEAs for PEM water electrolysers, the claimed catalyst materials are applied to the anode side of the MEA. The typical loading is in the range of 0.2 to 2.5 mg of iridium metal / cm2.

[0106] Catalyst-coated membranes (CCMs) are advantageously used in electrolysis. This refers to the underlying principle not only of hydrogen production, but when inverted, also for the generation of energy by fuel cells.

[0107] In such a fuel cell, electrochemical reactions occur at the electrodes, and the chemical energy charged in a fuel and an oxidant are converted to electrical energy and heat. Electrocatalysts are used to promote the electrochemical oxidation of the fuel at the anode and the electrochemical reduction of oxygen at the cathode. In particular in regenerative fuel cells, electrodes are bi-functional and both anode and cathode must support two electrochemical reaction types at different times. When operating as a fuel cell the cathode must reduce oxygen and the anode oxidise hydrogen; when operating as an electrolyte, the cathode must evolve hydrogen and the anode evolve oxygen. It may therefore be beneficial to incorporate both a traditional hydrogen oxidation reaction catalyst and an oxygen evolution reaction catalyst in the anode of such a fuel cell, because with such an arrangement, the anode can carry out both the hydrogen oxidation and oxygen evolution reactions effectively.

[0108] Various electrocatalysts for the oxygen evolution reaction are known in the art. For example, W02005 / 049199 discloses a catalyst for water electrolysis. For example, W011 / 021034 discloses catalyst layers comprising an electrocatalyst and an oxygen evolution reaction catalyst, wherein the oxygen evolution reaction catalyst comprises iridium or iridium oxide and one or more metals M or an oxide thereof, wherein M is selected from the group consisting of transition metals and Sn, with the exception of ruthenium. EP2475034 describes a catalyst layer comprising an iridium oxide component in combination with at least one other inorganic oxide component. WO2016 / 038349 describes a process for preparing a catalyst comprising iridium oxide and a metal oxide by flame spray pyrolysis. However, the provision of improved oxygen evolution reaction catalysts is desirable, and particularly the provision of catalysts which can improve the stability of membrane electrode assemblies during repeated reversal events.

[0109] Various embodiments of the present invention will now be described with reference to the figures, which show preferred exemplary embodiments of the subject invention.

[0110] Figure 1 shows the morphology of an Iridium Oxide (IrOx) catalyst according to a preferred embodiment of the present disclosure, using a transmission electron microscopic (TEM) image.

[0111] Applicants found that the initial deposition results in a homogenous distribution of iridium oxide over the surface of the substrate, which is depicted in this figure. Herein the iridium oxide forms a discontinuous, yet conformal film of iridium oxide crystals or metal clusters.

[0112] Applicants found that this morphology may change upon calcination, wherein larger iridium oxide clusters may form, which are still homogeneously distributed over the particle surface.

[0113] Figure 2 shows a comparison of the mass activity and electrochemical surface area (ECA) performance of catalyst compositions according to preferred embodiments of the present disclosure when tested in rotating disk electrode (RDE) electrochemical tests. Comparing the durability over activity with a commercial Iridium oxide catalyst, i.e. Iridium Black. From the viewpoint of improving the performance, it appears preferable to use a conductive support having no or few such pores, contrary to the catalyst shown in the prior art. Accordingly, particle substrates such as boron carbide may have shown a higher performance.

[0114] Figure 3 shows the iridium-4f section of the XPS spectrum (Al Ka radiation) of a sample of different iridium oxidation states of several catalyst compositions according to preferred embodiments of the present disclosure, ranging from amorphous Ir(lll) to more oxidized Ir(IV), using XPS (X-ray Photoelectron Spectroscopy).

[0115] Figure 4 shows the 0.5x current density of catalyst compositions according to preferred embodiments of the present disclosure as compared to commercially available comparative catalysts, in a membrane electrode assembly (MEA) test. From top to bottom, the lines depict the performance of comparative iridium black with Pt-PTL; comparative iridium black with Ti-PTL; each at 2.4 g / cm2, vis-a- vis example 1: IrOx on B4C with Pt-PTL 5% Nation; example 2: IrOx on B4C with Pt-PTL 5% Nation; and Example 3 IrOx on B4C with Pt-PTL 10% Nation; which clearly shows a similar activity, albeit at 15 to 30 times lower iridium concentrations, ie. At 0.077-0.145 g / cm2catalyst load.

[0116] Figure 5 shows a table describing the change in conductivity when applying an etching treatment to the support and to the calcined catalyst. In each case the etching treatment enhances the powder conductivity.

[0117] Figure 6 shows a graph plotting the cell potential versus the current density at the beginning of life (BoL) of catalyst particles. Depicted are catalysts with different iridium loadings and the iridium black comparison. This clearly shows that the catalyst particles according to the present disclosure have a much higher iridium utilization rate, when tested in a PEM electrolyzer.

[0118] Figure 7 shows a graph plotting the voltage over current density at the beginning of life (BoL), at the beginning of the accelerated age test (BoT) and at the end of the accelerated age test (EoT). While the catalyst particles according to the examples herein show a somewhat lower performance at the Beginning of life (BoL), with increased iridium loading these are approaching the iridium black benchmark with each CCM iteration. Example 2 has an area loading of the catalyst particles of 0.15 mg / cm2while the iridium black comparison has an area loading of 2.4 mg / cm2. Despite a 16-fold reduction in iridium of the example catalyst, it still performs close to the iridium black comparison.

[0119] Accordingly, it is clear that a catalysts according to the present disclosure, when loaded with more iridium oxide, will give equivalent or higher current density at a much lower iridium concentration than the benchmarking iridium black catalysts that represent the state of the art.

[0120] The following, non-limiting examples illustrate the process and materials according to the present disclosure. Herein, four catalyst compositions were prepared, and compared to two reference catalysts.

[0121] Electrochemical Measurement

[0122] The electrochemical properties of the catalyst materials were determined by the current density (in mA / cm2) at 1.5 V vs. RHE (in A / mg; RHE=reversible hydrogen electrode). In these tests, the catalyst samples were dispersed in ultra purified water and fixed on a gold carbon electrode. Cyclic voltammograms were taken in sulfuric acid (c=0.05 mol / l) at room temperature (20 °C). The counter electrode was Au, the reference electrode was a reversible hydrogen electrode (GasKatel); scan speed was 20 mV / s. After 50 cycles of conditioning, the third scan of voltammograms was taken to generate quasi-stationary conditions. The electrochemical mass activity was determined by the parameter of the current at a constant voltage of 1.5 V vs. RHE.

[0123] Membrane electrode assembly (MEA) tests

[0124] For MEA tests, a catalyst coated membrane (CCM) was assembled and tested in a homemade MEA setup. For the CCM fabrication, an ink was made consisting of the catalyst powder, a solvent, Nation and other additives. The catalyst ink was screen printed on a decal sheet. The catalyst layer is then transfer from the decal sheet onto a Nation 115 membrane by hot pressing to form the CCM. The CCM is then assembled in a MEA, which includes a titanium porous transport layer (PTL), which is contained in a cell housing. After the mounting of the MEA, water of 40 °C was fed into the cell housing to wet the CCM for 30 minutes. After 30 minutes, a cell voltage of 2 V was applied while the cell housing was simultaneously heated to 80 °C with a ramp of 0.5 °C / min. The cell voltage was applied for a period of 24 hours after which the next step started.

[0125] During the beginning-of-life (BoL) performance measurement, a polarization curve (IV-curve) was recorded. The polarization curve was recorded in a step-wise V-curve going from 2 V down to 1.45 V and back up to 2 V in 3 minute steps. For every point, the recorded current of the last 30 seconds was averaged and the upward going curves were used to plot the corresponding polarization curves.

[0126] After the BoL performance measurement, the cell voltage was set to 2 V for a 40 h period to study the current over time as a measure of the degradation. After the 40 hour period, the end-of-test (EoT) performance measurement was performed the same way as the BoL performance measurement.

[0127] Measurement of Electrical

[0128] The electrical conductivity of the samples was determined using an in-house constructed setup with a force sensor (Burster sensormaster 9163), displacement sensor (Aiggend, 0-12.7 mm) and resistance sensor (HP, 4338B Milliohmmeter) connected to two brass pistons that are inserted into a plastic container. After filling the powder sample in the container (sample radius 3.0 mm, sample weight between 0.05 and 0.1 g), the pressure was increased from 0 MPa up to a pressure of 7 MPa during measurement.

[0129] Atomic Layer Chemical Vapour

[0130] An iridium precursor that was a liquid at room temperature and suitable for deposition of an iridium film at a temperature in the range of from 150° C to 375° C was stored in a flask comprising a bubbler, and kept at a temperature of 60° C under nitrogen as inert gas. When the substrate was introduced into a fluidized bed, iridium precursor vapour was entrained from the flask, and delivered to a fluidized bed reactor by bubbling the inert gas through the warm precursor liquid as a dilute vapour. After the reaction was largely terminated, the oxidant co-reactant gas was introduced into the fluidized bed in the reactor. Metallic iridium depositions in ALD technique are possible when the co-reactant is molecular and atomic oxygen, as well as with other oxidants.

[0131] Examples 1 to 4 were prepared by contacting the substrate sequentially with alternating iridium precursor gas and oxidant gas pulses, whereby increasing amounts of iridium were deposited. Following the deposition, the catalyst particles were calcined under an oxygen atmosphere.

[0132] MEA tests

[0133] Table 1 shows the difference in current density at metal load for different examples according to the disclosure, and two comparative samples.

[0134] *BoL to EoT

[0135] The Iridium Black samples used as comparative examples using for comparative example 1 a standard porous transport layer (PTL) and for comparative example 2 a platinum coated PTL. Examples 1 to 3 concern an iridium Oxide film on a boron carbide substrate having an average particle size of 45-55 nm, which were applied using chemical vapour deposition from an iridium heteroleptic metallocene complex that was a liquid at room temperature. Example 1 includes a standard titanium PTL and comprises 33% per weight of IrOx; examples 2 and 3 a platinum coated PTL and comprises 33% and 26% by weight of IrOx. Example 2: Performance of catalyst in rotating disk electrode (RDE) electrochemical tests, Oxygen

[0136] Evolution Reaction (PER) Activity and Stability Benchmarks

[0137] Examples 1 and 2 show comparable durability and much higher PER activity vis-a-vis the commercial baseline catalysts of comparative 1 and 2. Again, this clearly shows that the present catalyst materials have unique properties, as reflected by the high durability and OER activities, as compared to the present industry standards.

[0138] The foregoing is illustrative of the present invention and is not to be construed as limiting thereof. Although a few exemplary embodiments of this invention have been described, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the claims.

[0139] Therefore, it is to be understood that the foregoing is illustrative of the present invention and is not to be construed as limited to the specific embodiments disclosed, and that modifications to the disclosed embodiments, as well as other embodiments, are intended to be included within the scope of the appended claims. The invention is defined by the following claims, with equivalents of the claims to be included therein.

Claims

Claims1. A method for producing an iridium oxide film on a particulate substrate having an electrical conductivity of >0.01 S / cm as detected by powder measurements by chemical vapour deposition, comprising:(i) providing the particulate substrate in a reaction chamber;(ii) contacting the particulate substrate with a vaporized iridium oxide precursor gas, thereby forming a homogenously distributed film of iridium precursor material on the substrate;(iii) contacting the material comprising the homogenously distributed layer with a vaporized oxidant to allow the oxidant to react with the homogenously distributed iridium precursor film on the substrate; and(iv) optionally, repeating until steps (ii) to (iii); to obtain a particulate material comprising a surface film of a desired thickness comprising homogeneously distributed discrete iridium oxide clusters with an average diameter in the range of from 0.3 to 5 nm, as determined by Transmission Electron Microscopy (TEM).

2. The method according to claim 1, comprising sequentially exposing the particulate substrate to a gaseous iridium precursor and to a gaseous oxidising agent, and depositing the iridium material on the substrate.

3. The method according to claim 1 or claim 2, comprising the steps of removing excess iridium oxide precursor gas, excess reactant gas and / or reaction by-products from the reaction chamber.

4. The method according to claim 1 or claim 2, wherein excess iridium oxide precursor gas, excess reactant gas and / or reaction by-products are not removed from the reaction chamber.

5. The method according to any one of the preceding claims, wherein steps (ii) and (iii) are performed such that the iridium oxide precursor gas is dosed to attain a self-terminating reaction with the surface of the particulate substrate, and the oxidant is dosed to attain a selfterminating reaction with the particulate surface comprising the continuous layer of iridium oxide precursor material.

6. The method according to any one of claims 1 to 5, wherein steps (i) to (iv) are performed at a pressure in the range of from 80 to 1010 kPa, preferably at a pressure at or near ambient pressure.

7. The method according any one of the preceding claims, wherein the iridium oxide precursor comprises an organometallic complex of iridium.

8. The method according to claim 7, wherein the iridium oxide precursor comprises an Iridium (Ci to C4- alkylcyclopentadienyl) complex, preferably a methylcyclopentadienyl or ethylcyclopentadienyl complex.

9. The method according to claim 7 or claim 8, wherein the iridium oxide precursor is a heteroleptic metallocene complex.

10. The method according to any one of claims 8 or claim 9, wherein the (Cito C4- alkylcyclopentadienyl) complex further comprises a diene ligand selected from linear or cyclic alkylenes, preferably selected from 1,4-butadiene, 1,4- pentadiene, 1,5- pentadiene, 1,4- hexadiene, 1,5- hexadiene; cyclopentadiene, 1,3-cyclohexadiene, 1,4-cyclohexadiene, and oxocyclohexadiene, each diene ligand optionally being substituted with one or more substituents R1, wherein each R1is independently selected from H, O, Ci to C linear or branched alkyl, Ci to C linear or branched alkylamide, Ci to C linear or branched alkoxide, Ci to C linear or branched alkylsilyl, Ci to C linear or branched alkylsilylamide, and Ci to C linear or branched fluoroalkyl.

11. The method according to claim 10, wherein the iridium oxide precursor is selected from (methylcyclopentadienyl) (1,3-cyclohexadiene) iridium; (ethylcyclopentadienyl)(l,3- cyclohexadiene) iridium; (ethylcyclopentadienyl)(l,5-cyclooctadiene) iridium, (1,5- cyclooctadiene)(2,2,6,6-tetramethyl-3,5-heptanedionate)iridium; (ethylcyclopentadienyl)(2,3- Me-2-l,3-butadiene) iridium; (methylcyclopentadienyl) (1,5-hexadiene) iridium; (ethylcyclopentadienyl)(l,5-hexadiene)iridium, (methylcyclopentadienyl)(l,4-butadiene) iridium, (methylcyclopentadienyl) (1,4- pentadiene)iridium, (methylcyclopentadienyl)(l,5- pentadiene)iridium, (methylcyclopentadienyl)(l,4-hexadiene)iridium, (methylcyclopentadienyl)(l,5- hexadiene)iridium; (ethylcyclopentadienyl)l,4-butadiene iridium, (ethylcyclopentadienyl)(l,4- pentadiene)iridium, (ethylcyclopentadienyl)(l,5-pentadiene)iridium, (ethylcyclopentadienyl)(l,4-hexadiene)iridium, and (ethylcyclopentadienyl)(l,5- hexadiene) iridium.

12. The method according to any one of the preceding claims, wherein the iridium oxide precursor has a melting point below 25°C, preferably below 0°C, and more preferably is a liquid at a temperature in the range of from 25°C to 45°C and ambient pressure.

13. The method according to any one of the preceding claims, wherein the oxidant material comprises a source of molecular oxygen provided by the decomposition of the oxidant under the conditions in step (iii).

14. The method according to any one of the preceding claims, wherein the oxidant comprises a peroxide of hydrogen or a peroxide of an organic compound, preferably wherein the oxidant comprises hydrogen peroxide.

15. The method according to any one of the preceding claims, wherein steps (ii) and (iii) are performed at a temperature in the range of from 125°C to 375°C, preferably at a temperature in the range of from 150°C to 300°C.

16. The method according to any one of the preceding claims, wherein the iridium oxide precursor and / or the oxidant co-reactant are heated to increase their vapour pressure prior providing the vapour to the reaction chamber(ii).

17. The method according to any one of the preceding claims, wherein the vapour or gaseous precursor compound and / or oxidant co-reactant are carried into the reaction chamber using an inert carrier gas.

18. The method according to any one of the preceding claims, performed in a batch-type fluidized bed reactor, a rotary drum reactor, or in a continuous-type reactor.

19. The method according to any one of the preceding claims, wherein the support material is essentially inert under oxidative and acidic conditions, preferably wherein the support material is inert during deposition of the iridium oxide layer, during calcination, during etching and when used in a PEM electrolyser.

20. The method according to any one of the preceding claims, wherein the particulate support material comprises metal oxides, carbides, borides, nitrides and non-metallic oxides, and / or carbides.

21. The method according to claim 20, wherein the particulate substrate is selected from inorganic metal oxides, preferably, selected from titania (TiCh), silica (SiCh), alumina (AI2O3), zirconia (ZrCh), tin dioxide (SnCh), F-doped tin oxide (SnCh / F), ceria (CeCh), ceria doped zirconia (CeCh / ZrCh), niobium pentoxide (NbjOs), tantalum pentoxide (TajOs); and mixtures and combinations thereof; or boron carbide materials, or composites of boron carbide with other compounds, such as silicon boron carbide.

22. The method according to any one of the preceding claims, wherein the support material particle size preferentially has a weight average particle sizer in the range of from 10 nm to 100 nm, preferably of from 40 nm to 60 nm.

23. The method according to any one of the preceding claims, wherein the continuous film of homogeneously distributed iridium oxide has an average layer thickness of below 5 nm, as determined by Transmission Electron Microscopy (TEM).

24. The method according to claim 22 or 23, wherein the iridium oxide comprises iridium(IV)-oxide, iridium(lll)-oxide, lridium(0)-metal and / or mixtures thereof.

25. The method according to any one of claims 22 to 24, wherein the inorganic oxide is added in a range of 30 wt.-% to 60 wt.-%, based on the total weight of the catalyst.

26. The method according to any one of the preceding claims, further comprising the step of contacting the particulate material prior to metal deposition with water at an elevated temperature, to remove residual compounds and to form an etched particulate material.

27. The method according to any one of the preceding claims, further comprising step (vii) of subjecting the iridium oxide-coated particulate substrate to a calcination treatment at a temperature in the range of from 150°C to 550°C in the presence of oxygen, to obtain an activated iridium oxide-coated particulate catalyst material.

28. The method according to claim any one of claims 1 to 27, further comprising the step of contacting the calcined activated iridium oxide-coated particulate catalyst with water at an elevated temperature, to remove residual compounds and to form an etched activated iridium oxide-coated particulate catalyst material.

29. An iridium oxide-coated particulate material obtainable by a method according to any one of claims 1 to 28, comprising a continuous film of homogeneously distributed iridium oxide having an average layer thickness of below 5 nm, as determined by Transmission Electron Microscopy (TEM).

30. The iridium oxide-coated particulate material according to claim 29, wherein the continuous iridium oxide film is a conformal film further comprising carbon containing residues in an amount in the range of from 3 to 5 wt.% on the iridium oxide film, as determined by ASTM E1019.

31. The iridium oxide-coated non-metallic particulate material according to claim 29 or claim 30, wherein the iridium is present in a range of from 5 to 50 wt%, based on total catalyst weight, as determined by ICP-OES.

32. An etched activated iridium oxide-coated particulate catalyst material obtainable according to the method according to claim 27, wherein the continuous iridium oxide film is a conformal film comprising carbon in a range of from 0 to less than 3 wt.% on the iridium oxide film, as determined by ASTM E1019.

33. A process for the preparation of a proton exchange membrane (PEM) electrolyzer, comprising: a. blending the activated iridium oxide-coated particulate catalyst material according to any one of claims 29 to 32 with a polymeric material preferably, a ionomer, to form a catalyst polymer dispersion, and b. depositing the catalyst polymer dispersion onto a PTFE membrane, to form a catalyst layer, and c. transferring the catalyst layer onto a proton exchange membrane, to obtain a catalyst-coated membrane (CCM).

34. A membrane electrode assembly (MEA) suitable for use in PEM electrolyzer stacks, comprising a catalyst coated membrane (CCM) obtained according to the process of claim 33.

35. Use of an activated iridium oxide-coated particulate catalyst material according to any one of claims 29 to 32 in a proton exchange membrane electrolyzer.

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