WO2026008968A1

WO2026008968A1 - Catalyst and process

Info

Publication number: WO2026008968A1
Application number: PCT/GB2025/051433
Authority: WO
Inventors: Gary Evans; Luke Alan LUISMAN; James George STEVENS
Original assignee: Johnson Matthey Hydrogen Technologies Ltd
Current assignee: Johnson Matthey Hydrogen Technologies Ltd
Priority date: 2024-07-01
Filing date: 2025-06-30
Publication date: 2026-01-08
Anticipated expiration: 2027-01-01

Abstract

Oxygen evolution reaction (OER) catalyst materials are provided comprising an iridium- containing compound on a particulate catalyst support, the OER catalyst material having the following characteristics: (i) a BET surface area in the range of and including 5 to 20 m2/g; (ii) an iridium content in the range of and including 25 to 50 wt%; and (iii) a Tmax in the temperature-programmed reduction profile of the OER catalyst material is in the range of and including 145 to 180 °C.

Description

CATALYST AND PROCESS

Field of the Invention

The present invention relates to catalyst materials which are suitable for use as oxygen evolution reaction (OER) catalysts, for example in a water electrolyser or a fuel cell, and to improved processes for their manufacture.

Background of the Invention

During water electrolysis, the overall reaction to produce oxygen and hydrogen can be divided into two half-cell reactions: the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Oxygen evolution reaction (OER) catalysts are an integral part of the anode component of water electrolysis cells, and require high catalytic activity as the OER which occurs at the anode requires a remarkably high overpotential compared to the HER. The OER reaction in acidic conditions is approximated by the following equation:

2H₂O O₂ + 4H⁺ + 4e-

Iridium-containing compounds are well known for their advantageous properties as OER catalysts providing high OER catalytic activity and stability under electrolyser operating conditions. Due to the scarcity of iridium, and the growing demand for electrolytically produced hydrogen, there is a need to reduce the amount of iridium present in electrolyser anodes. However, reduction in iridium loading can lead to poor electrochemical performance. When the iridium loading of an electrolyser is thrifted, it has been observed that losses due to the reduction of in-plane conductivity can be more important than the inevitable kinetic losses.

One approach to the thrifting of iridium is through the use of supported catalysts. Such materials provide an iridium-containing compound on a particulate support, such as a transition metal oxide. A significant challenge associated with supported iridium- containing catalysts is to maximise conductivity, a key requirement of efficient operation of an electrolyser anode layer, which is detrimentally impacted in comparison to unsupported iridium-containing catalysts by the distribution of the iridium-containing compound over the surface of the support particle. Various approaches to the production of supported iridium-containing catalysts have been published. For example, it is described in EP1701790 (Umicore AG & CO KG) that iridium oxide-based catalysts may advantageously be produced. The catalyst materials use a high surface area inorganic oxide with a BET surface area in the range of 50 to 400 m²/g with the inorganic oxide support present in a quantity less than 20 wt%. Such materials would be expected to have high conductivity, but the high iridium content means that significant iridium-thrifting has not been achieved.

It is further described in US2024/044027A1 (HERAEUS DEUTSCHLAND GMBH & CO KG & UNIV MUENCHEN TECH) that particulate catalysts may be provided containing a support material and an iridium-containing coating. The support material has a BET surface area ranging from 2 m²/g to 50 m²/g. This publication provides a formula for iridium content related to the BET surface area of the support. Examples include the use of a TiO2 support with a BET surface area of 20 m²/g and an iridium content of 30 wt% (IE1), and a TiO2 support with a BET surface area of 5 m²/g and an iridium content of 10 wt% (IE3).

It is also described in WO2018077857A1 (BASF SE) that catalyst compositions may be formed which comprise tin oxide particles which are at least partially coated by a noble metal oxide layer. In the Examples, the tin oxide particles have iridium oxide in an outer shell and the catalysts are formed by heating at 600 °C.

It is known that heat treatment of iridium-containing compounds plays an important role in balancing OER catalyst activity and stability under electrolyser operating conditions, and depending on the application either high activity or high stability may be more important to the end user. High temperature heat treatments (such as at a temperature of at least 350 °C) lead to predominantly crystalline materials with improved stability. However, in the case of supported catalysts at lower iridium loadings (< 50 wt%), high temperature heat treatment can lead to disruption of the surface structure as the iridium containing material starts to crystallise leading to a loss of required conductivity.

There remains a need to provide improved supported iridium catalysts which help to facilitate iridium thrifting in electrolyser applications, and in particular supported iridium catalysts which provide high conductivity.

Summary of the invention The present inventors have identified that by controlling iridium loading, catalyst surface area, the nature of the iridium-containing compound and heat treatment temperature, supported catalysts which include a predominantly crystalline iridium-containing compound may be produced with particularly high conductivity at low iridium loadings (< 50 wt%).

Therefore, in a first aspect of the invention, there is provided an oxygen evolution reaction (OER) catalyst material comprising an iridium-containing compound on a particulate catalyst support, the OER catalyst material having the following characteristics:

(i) a BET surface area in the range of and including 5 to 20 m²/g;

(ii) an iridium content in the range of and including 25 to 50 wt%;

(iii) a Tmax in the temperature-programmed reduction profile of the OER catalyst material in the range of and including 145 to 180 °C.

Such materials have particular utility as a component of a catalyst coated membrane, for example for a water electrolyser or a fuel cell, and in particular for a proton exchange membrane (PEM) water electrolyser.

Therefore, in a second aspect of the invention, there is provided a catalyst-coated membrane comprising an OER catalyst material according to the first aspect.

In a third aspect of the invention, there is provided a fuel cell or a water electrolyser comprising a catalyst-coated membrane according to the second aspect.

The catalyst materials may be advantageously produced by a process involving deposition of an iridium-containing compound onto particulate support with a selected surface area range and then a controlled heat treatment. Therefore, in a fourth aspect of the invention, there is provided a process for the preparation of an OER catalyst material according to the first aspect, the process comprising the steps of: a) depositing an iridium-containing compound onto a particulate catalyst support, the particulate catalyst support having a BET surface area in the range of and including 5 to 17 m²/g; b) isolating the product of step a); c) heat-treating the product of step a) at a temperature in the range of and including

350 to 450 °C. Detailed Description

Preferred and/or optional features of the invention will now be set out. Any of the preferred and/or optional features of any aspect may be combined, either singly or in combination, with any other preferred and/or optional features of any aspect of the invention unless the context demands otherwise.

The present invention provides an oxygen evolution reaction (OER) catalyst material. OER catalysts catalyse the generation of molecular oxygen through oxidation of water and may be advantageously used as an anode component of a water electrolyser, such as a protonexchange membrane (PEM) water electrolyser (PEMWE) or an anion-exchange membrane (AEM) water electrolyser (AEMWE).

The OER catalyst material comprises an iridium-containing compound. Preferably, the iridium-containing compound is a doped or undoped iridium oxide (IrOx) or a doped or undoped iridium hydroxide oxide, or a mixture thereof. Iridium oxyhydroxides are iridium compounds having both oxo (lr=O) and hydroxo (Ir-OH) functionalities and may have a composition which can be represented, for example, by the following formula: lrO_x(OH)_y wherein 1 < x < 2 and 0 < y < 2, and 3 < 2x+y < 4. The presence of an iridium oxyhydroxide may be determined by methods known to those skilled in the art, such as infra-red spectroscopy, 10s X-ray photoelectron spectroscopy (XPS) or NMR spectroscopy following a hydrogen-deuterium exchange.

The iridium-containing compound is provided on a particulate catalyst support. The skilled person will understand that the iridium-containing compound is present on the surface of the particulate catalyst support. Suitably, the iridium-containing compound is in the form of nanoparticles, but may also be in the form of, for example, a thin layer at the surface of the catalyst support. Preferably, the iridium-containing compound is uniformly distributed on the surface of the catalyst support material. Uniform distribution may be determined using imaging techniques, such as scanning electron microscopy.

Preferably, the particulate catalyst support is a metal-containing particulate catalyst support. Preferably, the particulate catalyst support comprises, or consists of, a doped or undoped oxide, nitride or boride of a main group metal or a transition metal. More preferably, the particulate catalyst support comprises, or consists of, a doped or undoped oxide of a main group metal or a transition metal. A transition metal oxide support is particularly preferred. Preferably, the particulate catalyst support comprises, or consists of a doped or undoped oxide of titanium, zirconium, niobium, tantalum, aluminium, manganese, cerium, tungsten, silicon, tin, or molybdenum oxide. More preferably, the particulate catalyst support is doped or undoped a TiC>2, ZrC>2, Nb20s, Ta2Os, AI2O3, MnC>2, Ce2Os, CeC>2, WO3, SiC>2, SnC>2 (such as Sb-doped SnC>2) or MoOs. A TiC>2 or ZrC>2 particulate support is particularly preferred. ZrC>2 supports have been found by the present inventors to be particularly resistant to potential dependant dissolution during electrochemical cycling under electrolysis conditions.

The OER catalyst material has BET surface area in the range of and including 5 to 20 m²/g. It has been found that at an iridium content the range of 25 to 50 wt%, the conductivity is significantly reduced at catalyst material BET surface areas greater than 20 m²/g. Preferably, the BET surface area is in the range of and including 10 to 20 m²/g, or 13 to 19 m²/g. The BET surface area is determined using the BET method using N2 as the adsorption gas.

Preferably, the particulate catalyst support has a BET surface area in the range of and including of 5 to 17 m²/g, or more preferably 10 to 15 m²/g. Such support materials provide high conductivity when combined with an iridium content the range of 25 to 50 wt%.

The OER catalyst material has an iridium content in the range of and including 25 to 50 wt%. It has been found that for supported iridium catalysts that have been heat treated to increase stability, an iridium content in the range 25 to 50 wt% provides a suitable balance between iridium thrifting and OER catalyst activity. Preferably, the OER catalyst material has an iridium content in the range of and including 25 to 45 wt%, 28 to 42 wt%, or 30 to 40 wt%. It will be understood that the by the skilled person that the iridium content in wt% is the weight of iridium as a percentage of the total weight of the OER catalyst material.

The OER catalyst material has a Tmax in the temperature-programmed reduction (TPR) profile of the OER catalyst material in the range of and including 145 to 180 °C. Temperature-programme reduction is a technique which provides information on the nature of the iridium-containing compound by assessing how easily it is reduced. As shown in Table 1 , the nature of the iridium-containing compound, and therefore the T max in the TPR profile, may be varied by changing heat treatment temperature. A Tmax in the TPR profile in the range of and including 145 to 180 °C indicates that the iridium-containing compound predominantly consists of crystalline iridium oxide domains, typically with amorphous iridium oxyhydroxide as a minor component. Preferably, the Tmax is at least 150 °C, at least 155 °C, or at least 160 °C, such as in the range of and including 155 to 175 °C or in the range of and including 160 to 170 °C. The Tmax is determined by temperature programmed reduction (TPR) measurements. Suitably, the sample is heated to 300°C at a heating rate of 10°C/min under an atmosphere of 10% H2/N2.

It is particularly preferred that the OER catalyst material has a TiC>2 or ZrC>2 particulate support and has the following characteristics:

(i) a BET surface area in the range of and including 5 to 17 m²/g;

(ii) an iridium content in the range of and including 25 to 45 wt%;

(iii) a Tmax in the temperature-programmed reduction profile of the OER catalyst material in the range of and including 145 to 180 °C, such as in the range of such as in the range of and including 150 to 175 °C, or 160 to 170 °C.

Suitably, the OER catalyst material has an average particle size (Dv50) less than 10 .m, or preferably less than 5 .m, such as in the range of and including 0.5 to 10 .m, or in the range of and including 1 to 5 .m. Unless otherwise specified herein, the term Dv50 as used herein refers to the median particle diameter of the volume-weighted distribution. The Dv50 may be determined by using a laser diffraction method. For example, the Dv50 may be determined by suspending the particles in water and analysing the particle size distribution by laser diffraction, for example using a Malvern Mastersizer 3000.

Preferably, the OER catalyst material has a chloride content of less than 500 ppm. More preferably, the chloride content is less than 400 ppm, less than 300 ppm, or less than 250 ppm. The chloride content may be determined by liquid ion chromatography. A chloride content less than 500 ppm is beneficial as chloride impurities are known to be detrimental to polymer electrolyte membrane durability and catalyst layer performance. The chloride content may be measured by liquid ion chromatography.

The OER catalyst materials described herein have high electronic conductivity. Preferably, the OER catalyst material has an electronic conductivity of at least 3 S cm^-1 at 12.7 MPa and 18 °C. The upper limit for electronic conductivity of the OER catalyst materials is not particularly limited but is typically 25 S cm^-1 or less, or 20 S cm^-1 or less. The electronic conductivity is measured using a powder measuring system configured to determine specific resistance under controlled pressure (12.7 MPa) and ambient temperature (18 °C), for example using an NH Instruments PD-600. Preferably, the OER catalyst material has an atomic % of metallic iridium (oxidation state 0) as a proportion of total iridium (metallic + oxidised) is less than 2 % as determined by (X- ray photoelectron spectroscopy) XPS analysis. It may be preferred that the atomic % of metallic iridium is less than or equal to 1.8 %, 1.6 %, 1.4 %, or 1.2 %. The atomic % of metallic iridium may be suitably determined by XPS analysis of iridium 4f spectra of the composite material.

In the event that the particulate catalyst support is a transition metal oxide, it is preferred that the ratio of atomic % iridium-to- atomic % transition metal (lr:TM), such as the ratio of atomic % iridium-to-atomic % titanium (in the case of the transition metal oxide being TiC>2) or the ratio of atomic % iridium-to-atomic % zirconium (in the case of the transition metal oxide being ZrC>2), at a surface of the catalyst is at least 1.0, such as in the range of and including 1 .0 to 3.0, or 1.0 to 2.0 as determined by XPS analysis. A ratio of atomic % greater than 1.0 indicates that the iridium-containing compound is well distributed on the surface of the particulate solid support.

The OER catalyst material may be used in a water electrolyser, especially in the anode of a water electrolyser, such as a PEMWE or AEMWE.

The OER catalyst material may also be used in a fuel cell, such as a PEMFC, especially in a fuel cell anode for the purposes of cell reversal tolerance. For a discussion of the use of lrO2 materials in fuel cells for the purpose of cell reversal tolerance see WO2012/107738 (Johnson Matthey PLC). The OER catalyst material may also be used in other electrochemical applications, such as the electrochemical conversion of carbon dioxide to other compounds.

The OER catalyst material may be formulated as an ink, typically by dissolving or dispersing the OER catalyst material in a mixture of an ion-conducting polymer and water, or a mixture of an ion-conducting polymer, water and an organic solvent, such as ethanol or propan- 1- ol. Suitable ion-conducting polymers are known to those skilled in the art and include fluorinated or non-fluorinated acidic ion-conducting polymers, such as perfluorinated sulfonic-acid (PFSA) ionomers, or hydroxide-conducting polymers, such as polymers containing quaternary ammonium functional groups.

The inks may be used to form a catalyst layer. Such layers suitably comprise the OER catalyst material and an ion-conducting polymer. The catalyst layers may also comprise additional components, such as additional catalysts, radical scavengers, etc., as will be known to those skilled in the art.

The catalyst layer may form a component of a catalyst coated membrane (CCM) which comprises a polymer electrolyte membrane with the catalyst layer on a first face thereof and, optionally, a second catalyst layer on a second face thereof. Suitably, the polymer electrolyte is a proton exchange membrane (PEM) or an anion exchange membrane (AEM). The polymer electrolyte membrane may include additional components (e.g. recombination catalysts, radical scavengers, reinforcements, multiple layers) as will be known to those skilled in the art.

Typically, the catalyst layer is the anode layer of a CCM for a water electrolyser. In such cases the CCM typically comprises: (i) a polymer electrolyte membrane with a first face and a second face; (ii) an anode layer comprising the OER catalyst material and an ionconducting polymer on the first face of the membrane; and (iii) a cathode catalyst layer comprising a hydrogen evolution reaction (HER) catalyst, such as a HER catalyst comprising platinum (for example platinum on carbon), on the second face of the membrane.

The cathode catalyst layer typically comprises additional components, such as an ionconducting polymer, to improve ionic conductivity within the layer. The cathode layer may be applied to the catalyst-coated membrane using, for example, coating methods such as a slot-die (slot, extrusion) coating process, inkjet printing, gravure printing, curtain coating, or a spray coating process. The cathode catalyst layer may be applied by directly coating the membrane, or the cathode catalyst layer may be formed on a suitable backing material and then applied to the membrane using a decal process.

Separate film layers, typically formed from non-ion conducting polymers, may be positioned around the edge region of a CCM, for example on exposed surfaces of the polymer electrolyte membrane where no catalyst is present (but will also often overlap on to the edge of the catalyst layers) to provide a seal to prevent escape of reactant and product gases, to reinforce and strengthen the edge of the CCM and provide a suitable surface for supporting subsequent components. An adhesive layer may be present on one or both surfaces of the seal film layer.

In a water electrolyser, additional transport layers are positioned each side of a CCM to facilitate reagent and product transfer to and from the catalysts, and to provide electrical contact. These additional transport layers may be known as porous transport layers or gas diffusion layers. These layers may or may not be directly attached to the CCM. Other components of a water electrolyser may include bipolar plates and current collector plates. Stacks of such assemblies make up an electrolyser system including power and control systems.

Suitable transport layers at the anode side of a WE CCM are known to the skilled person and are typically formed from a metal-based porous structure. Such transport layers must be sufficiently conducting and in a form that is compatible with positioning adjacent to the CCM (without, for example, sharp edges or protrusions that would damage the membrane during use). Such metal-based porous structures may be in the form of, for example, felts or non-woven cloths, mesh, foams and sintered compacts of metal-containing particles. For PEMWE applications, suitable transport layers comprise titanium. For AEMWE applications, suitable transport layers comprise nickel or stainless steel.

Suitable transport layers at the cathode side of a WE CCM are known to the skilled person and are typically non-woven papers or webs comprising a network of carbon fibres and a thermoset resin binder (e.g. the TGP-H series of carbon fibre paper available from Toray Industries Inc., Japan or the H2315 series available from Freudenberg FCCT KG, Germany, or the Sigracet® series available from SGL Technologies GmbH, Germany or AvCarb® series, or woven carbon cloths). The carbon paper, web or cloth may be provided with a further treatment either to make it more wettable (hydrophilic) or more wet-proofed (hydrophobic). The nature of any treatments will depend on the type of electrochemical device and the operating conditions that will be used.

The OER catalyst materials as described herein may be prepared using a process comprising the step of: a) depositing an iridium-containing compound onto a particulate catalyst support, the particulate catalyst support having a BET surface area in the range of and including 5 to 17 m²/g. The use of a particulate catalyst support having a BET surface area in the range of and including 5 to 17 m²/g enables the production of materials with the desired BET surface area at the selected iridium loading.

Suitably, the deposition in step a) comprises the sub-steps of a) (i) forming an aqueous mixture comprising a particulate catalyst support, such as a particulate transition metal oxide support, and a solution of a metal iridate; a) (ii) adjusting the pH of the aqueous mixture to < 10 to precipitate the iridium-containing compound onto the particulate catalyst support, such as a particulate transition metal oxide. Preferably, the pH of the aqueous mixture is adjusted to < 7, or more preferably < 5. The adjustment in pH to a value < 5 reduces the amount of residual iridium remaining in the solution. Th pH is suitably adjusted using nitric acid or sulfuric acid.

Preferably the metal iridate is halide-free metal iridate. By “halide-free metal iridate” herein we mean a compound comprising an iridium-containing oxyanion and metal counter ion(s) which is without intentionally added halide. Preferably, the halide-free metal iridate is sodium iridate. Preferably, the halide-free metal iridate has a halide content of less than 100 ppm or, more preferably, less than 50 ppm or less than 10 ppm.

Preferably, the step a)(i) comprises the sub-steps of combining iridium powder and a peroxide salt to produce a powder mixture; and carrying out thermal treatment on the powder mixture to form a metal iridate. The use of a fusion step between a metal peroxide and iridium powder avoids the use of more expensive iridium salts and also means the preparation can be free from intentionally added chlorides. The term “powder” used in connection with iridium powder is intended to encompass both spherical powders and also irregular powders such as iridium sponge. The role of the peroxide salt is to oxidize the iridium powder. Preferably, the peroxide salt is a Group I or Group II peroxide salt, most preferably a Group I peroxide salt. A preferred peroxide salt is sodium peroxide which is commercially available. The powder mixture is heated at a temperature and duration suitable to achieve the desired conversion of iridium metal to oxidic species. It will be appreciated that the temperature and duration may differ depending on the choice of equipment used and the scale. The skilled person will be able to determine suitable conditions for a given equipment and scale. Following the thermal treatment of the powder mixture the metal iridate is typically dissolved in water to provide a solution and the solution mixed with a particulate support to form the aqueous mixture prior to the adjustment in pH.

The process further comprises the step of b) isolating the product of step a). Typically the product is isolated by filtration. The isolated solid is typically washed, for example with deionised water, and then dried (for example at a temperature between 50 and 100 °C.

The process further comprises the step c) heat-treating the product of step (b) at a temperature in the range of and including 350 to 450 °C. Such a temperature range provides for a predominantly crystalline material. A temperature greater than 450 °C, such as greater than 500 °C leads to reduction in catalytic activity. Such materials may be differentiated from the materials as described herein as they have a higher TPR Tmax as set out in Table 1. Typically, the heat treatment is carried out for a period in the range of 2 to 10 hours.

The present invention will now be described with reference to the following examples, which are provided to assist with understanding the present invention and are not intended to limit its scope.

Examples

Measurement of BET surface area

The determination of the specific surface area by the BET method is carried out by the following process: after degassing to form a clean, solid surface, a nitrogen adsorption isotherm is obtained, whereby the quantity of gas adsorbed is measured as a function of gas pressure, at a constant temperature (usually that of liquid nitrogen at its boiling point at one atmosphere pressure). A plot of l/[Va ((Po/P)-I)] vs P/Po is then constructed for P/PO values in the range 0.05 to 0.3 (or sometimes as low as 0.2), where Va is the quantity of gas adsorbed at pressure P, and Po is the saturation pressure of the gas. A straight line is fitted to the plot to yield the monolayer volume (Vm), from the intercept l/Vm C and slope (C-I)/Vm C, where C is a constant. The surface area of the sample can be determined from the monolayer volume by correcting for the area occupied by a single adsorbate molecule. More details can be found in 'Analytical Methods in Fine Particle Technology', by Paul A. Webb and Clyde Orr, Micromeritics Instruments Corporation 1997.

Electronic conductivity

The conductivity of the OER catalyst powders was measured using an NH Instruments PD 600 powder measuring system at ambient temperature (18 °C). For the measurement, the powder sample is placed into the cylindrical chamber of the cell which is placed in automated press. Conductivity is then measured as applied forced applied to cell, the conductivity increasing until an essentially constant value is obtained. For each of the samples tested the maximum pressure applied was 45 MPa. Conductivity reported at a pressure 12.7 MPa.

TPR Measurement

Temperature programmed reduction (TPR) measurements on the OER catalysts were performed by blending catalyst powder with an inert diluent at an approximate mass ratio of 9: 1 diluent to catalyst before loading into a glass tube. The tube was purged with nitrogen, then 10% H2/N2. The samples were then heated to 300°C at 10°C/min then held at 300°C for 10 minutes, before cooling. Outlet gases were passed through a CO2 and moisture trap before passing into a thermal conductivity detector (TCD). Thus, the measured signal from the detector corresponds to the consumption of hydrogen from reduction of the catalyst. The Tmax was determined from a plot of mass normalised signal (mV/g) vs sample temperature (°C). This is defined as the temperature at which the mass normalised signal reaches its maximum, corresponding to reduction of the predominant component in the catalytically active element in the catalyst.

A series of iridium oxide I oxyhydroxide materials, each prepared using the method from a Na2<D2 iridium fusion product with low pH precipitation as described herein and then heat treated, were tested using the TPR method. The results are shown in Table 1 below:

Table 1 - TPR data on a series of iridium oxide I oxyhydroxide materials heat treated at different temperatures * denotes tested sample is unsupported.

Iridium loading

Sample iridium weight % was assayed by inductively coupled plasma optical emission spectroscopy (ICP-OES) using an Agilent 5800 instrument.

Chloride assay

Sample elemental chlorine content (ppm) was assayed by liquid ion chromatography (The ion chromatography is coupled to a “digestion system”, i.e. an “Automated Quick Furnace” (AQF, Mitsubishi AQF-2100H) to isolate the halogens and the isolated halogens are assayed using the ion chromatography technique (Dionex 2100 Ion Chromatography system, Thermo). XPS

X-Ray Photoelectron Spectrometry (XPS) analysis was conducted using a Thermo Fisher Scientific NEXSA spectrometer (Al Ka X-ray Source) and the spectra analysed using Thermo Fischer Scientific’s proprietary Thermo Avantage software.

The surface ratio of atomic % iridium: atomic % titanium, zirconium or niobium were calculated from atomic % iridium and the atomic % titanium, zirconium or niobium. A higher value of this surface ratio indicates greater dispersion of iridium on the catalyst support. The atomic % values were determined by XPS analysis of the titanium 2p spectra (background range: 475 eV-450 eV, main Ti2ps/2 signals at 459 eV), zirconium 3 ₃/2 spectra (background range: 325 eV-340 eV, main Zr3ps/2 signals at 332.7 eV), or niobium 3d spectra (background range: 202 eV-214 eV, main Nb3d₅/2 signals at 207.2 eV)and the iridium 4d₃/2 spectra (background range: 305 eV-325 eV, signal at 313.7 eV) and the Schofield factors using the Thermo Avantage Software (v 5.9917 .

Examples 1 to 7

A series of supported IrOx catalyst materials was produced using the following method and with variation of the support and iridium loading as set out in Table 1

General method: ~8 g of particulate TiC>2 or ZrC>2 was dispersed in deionised (DI) water (150mL) using a high shear mixer at 10,000 rpm for 15 minutes. The mixture was transferred to a baffled process vessel and the reaction stirred at 250 rpm using an overhead stirrer.

A Na2<D2 iridium fusion product was prepared by combining iridium powder (300 g, 400 mesh corresponding to particle sizes below 23 pm) with 900 g of milled sodium peroxide and the mixed using WAB T2F Turbula mixer to get homogenous mixture. The mixture was transferred to a alumina crucible and heated using electrical muffle furnace (temperature approximately 500 °C). The mixture was heated at 500°C for 2 hours.

A portion of the Na2<D2 iridium fusion product was added to the suspension of the transition metal oxide (amount depending on the desired iridium loading). The mixture was stirred for 30 minutes to ensure complete mixing. Concentrated nitric acid (HNO3) was added dropwise to the stirred mixture. Once the mixture reached a pH of 3.25, the solution was maintained at pH 3.25 for 30 minutes with additional addition of nitric acid as necessary. The product was collected by vacuum filtration and washed with DI water until the filtrate conductivity measured below 50 pS/cm. The precipitate was dried at 60 °C under vacuum (~20 mBar) for 16 hours. The product was then heated at 400 °C for a period of 4 hours in air. Comparative Example 1 (C. Ex.1): A sample was produced using the general method as described for Examples 1 to 7 but with a higher surface area support.

Comparative Example 2 (C. Ex.2): A sample was produced using the general method as described for Examples 1 to 7 but with a low iridium loading.

Comparative Example 3 (C. Ex.3): Samples were produced using the general method as described for Examples 1 to 7 but with a heat treatment of 120 °C for a period of 4 hours instead of 400 °C. Such materials showed a Tmax from TPR analysis of < 120 °C.

Results

The results of testing of the catalyst materials are shown in Table 2. The materials produced in Examples 1 to 7 provide excellent conductivity. Moving to a higher support surface area (Comparative Example 1) or to a lower iridium loading (Comparative Example 2) leads to a significant drop off in conductivity values. A material heat treated at 120 °C (Comparative Example 3) shows a Tmax of < 120 °C.

Table 2: Results of testing of Examples 1 to 7 and Comparative Examples 1 to 3

Claims

1. An oxygen evolution reaction (OER) catalyst material comprising an iridium-containing compound on a particulate catalyst support, the OER catalyst material having the following characteristics:

(i) a BET surface area in the range of and including 5 to 20 m²/g;

(ii) an iridium content in the range of and including 25 to 50 wt%;

(iii) a Tmax in the temperature-programmed reduction profile of the OER catalyst material is in the range of and including 145 to 180 °C.

2. An OER catalyst material according to claim 1 , wherein the iridium-containing compound is a doped or undoped iridium oxide or a doped or undoped iridium hydroxide oxide, or a mixture thereof.

3. An OER catalyst material according to claim 1 or claim 2, wherein the particulate catalyst support is selected from doped or undoped TiO2, ZrO2, Nb20s, Ta2Os, AI2O3, MnO2, Ce2Os, CeO2, WO3, SiO2, SnO2 or MoOs.

4. An OER catalyst material according to any one of the preceding claims, wherein the particulate catalyst support has a BET surface area in the range of and including of 5 to 17 m²/g.

5. An OER catalyst material according to any one of the preceding claims, wherein the OER catalyst material has a BET surface area in the range of and including 10 to 20 m²/g.

6. An OER catalyst material according to any one of the preceding claims, wherein the OER catalyst material has an iridium content in the range of and including 30 to 40 wt%.

7. An OER catalyst material according to any one of the preceding claims, wherein the OER catalyst material has a Tmax in the range of and including 150 to 170 °C.

8. An OER catalyst material according to any one of the preceding claims, wherein the OER catalyst material has a chloride content of less than 500 ppm as determined by liquid ion chromatography.

9. An OER catalyst material according to any one of the preceding claims, wherein the particulate catalyst support is a transition metal oxide support, preferably ZrC>2.

10. An OER catalyst material according to claim 9, wherein the OER catalyst material has a ratio of atomic % iridium-atomic % transition metal (lr:TM) of at least 1.0 as determined by XPS analysis, such as in the range of an including 1.0 to 3.0.

11. A catalyst-coated membrane comprising an OER catalyst material according to any one of claims 1 to 10.

12. A fuel cell or a water electrolyser comprising a catalyst-coated membrane according to claim 11.

13. A process for the preparation of an OER catalyst material according to any one of claims 1 to 10, the process comprising the steps of: a) depositing an iridium-containing compound onto a particulate catalyst support, the particulate catalyst support having a BET surface area in the range of and including 5 to 17 m²/g; b) isolating the product of step a); c) heat-treating the product of step a) at a temperature in the range of and including 350 to 450 °C.

14. A process according to claim 13, wherein the deposition in step a) comprises the substeps of a) (i) forming an aqueous mixture comprising a particulate catalyst support, such as a particulate transitional metal oxide support and a solution of a metal iridate; a) (ii) adjusting the pH of the aqueous mixture to < 10 to precipitate the iridium- containing compound onto the particulate catalyst support.

15. A process according to claim 14, wherein the pH of the aqueous mixture in step a) (ii) is reduced to < 7, preferably < 5.

16. A process according to claim 14 or claim 15, wherein step a) (i) comprises the sub-steps of combining iridium powder and a peroxide salt to produce a powder mixture; and carrying out thermal treatment on the powder mixture.