WO2025114700A1

WO2025114700A1 - Process for manufacturing supported iridium oxygen evolution reaction catalyst, a product thereof and its use

Info

Publication number: WO2025114700A1
Application number: PCT/GB2024/052977
Authority: WO
Inventors: Luke Alan LUISMAN; James George STEVENS
Original assignee: Johnson Matthey Hydrogen Technologies Ltd
Current assignee: Johnson Matthey Hydrogen Technologies Ltd
Priority date: 2023-11-30
Filing date: 2024-11-27
Publication date: 2025-06-05
Anticipated expiration: 2026-05-30
Also published as: GB202318307D0

Abstract

A process for preparing an oxygen evolution reaction (OER) catalyst comprises an oxygenated iridium component supported on a particulate solid support, which process comprising the steps of: (i) forming an aqueous mixture comprising a particulate solid support and a solution of a halide-free metal iridate; (ii) reducing the pH of the aqueous mixture to ≤ 5.0 to precipitate an oxygenated iridium component onto the particulate solid support; and (iii) isolating the product of step (ii).

Description

PROCESS FOR MANUFACTURING SUPPORTED IRIDIUM OXYGEN EVOLUTION REACTION CATALYST, A PRODUCT THEREOF AND ITS USE

FIELD OF THE INVENTION

The present invention relates to a process for preparing a supported iridium oxygen evolution reaction (OER) catalyst, a supported iridium oxygen evolution reaction catalyst which may be obtained or obtainable by the process, a catalyst coated membrane (CCM) comprising the supported iridium oxygen evolution reaction catalyst and a fuel cell or a water electrolyser comprising the CCM.

BACKGROUND TO THE INVENTION

Oxygen evolution reaction (OER) catalysts are an important component of fuel cells and water electrolysers. The OER reaction in acidic conditions is approximated by the following equation:

2H₂O O₂ + 4H⁺ + 4e-

Iridium-containing catalysts for promoting this reaction and methods of making them are known and include a wet chemical precipitation of an amorphous iridium-containing precursor compound from an alkaline aqueous medium. Drying and calcining the precipitated product is said to yield more electrically conductive crystalline I rO₂ (see e.g. the Comparative Example in US patent publication no. 2007/292744 A1).

However, supported iridium catalyst species are generally more useful in the preparation of catalyst-coated membranes (CCMs), e.g. when formulated with suitable polymers to make printable inks, because they can result in more efficient OER catalysts, e.g. by promoting higher activity at lower iridium loadings while maintaining catalyst layer thickness.

In this regard, EP2608297A1 discloses iridium oxide-based catalysts for use as anode catalysts in proton exchange membrane (PEM) water electrolysis and other applications. Composite catalyst materials disclosed comprise iridium oxide (I rO₂) and optionally ruthenium oxide (RUO₂) in combination with an inorganic oxide (for example TiO₂, AI₂Os, ZrO₂ and mixtures thereof). The inorganic oxide has a BET surface area in the range of 30 to 200 m²/g and is present in a quantity of 25 to 70 wt.-% based on the total weight of the catalyst. The catalyst materials are said to be characterised by an electrical conductivity > 0.01 S/cm and high current density.

US patent publication no. 2022/0259750 A1 discloses a method for preparing a catalyst composition, wherein in an aqueous medium containing an iridium compound, at pH 9, an iridium-containing solid is deposited on a support material, and the support material loaded with the iridium-containing solid is separated from the aqueous medium and dried, wherein, in the method, the support material loaded with the iridium-containing solid is not subjected to a thermal treatment at a temperature of more than 250°C for a period of time of longer than 1 hour.

Prior art processes for making supported iridium catalyst species typically utilise chlorides of iridium as precursors, such as I rC , I rCL and H2I rCle. For example, the Example of US patent publication 2022/0259750 A1 uses iridium(IV) chloride as a precursor. In a comparative Example in the present application, Applicant’s inventors repeated this prior art Example and an assay found considerable quantities of chloride therein, despite copious washing.

It is known for halides, such as chloride, to negatively impact proton exchange membrane (PEM) fuel cell performance and durability caused by enhanced dissolution of Pt from Pt-C catalysts (see H. Li et al, Journal of Power Sources 196 (2011) 6249-6255). The literature also includes reports of membrane system durability and integrity being impacted by contact with chloride. For example, chloride can accelerate corrosion by forming a ligand with platinum group metals, stabilising them in solution. Also, chloride can evolve to hydrochloric acid which can both dilute the purity of reactant gas and corrode internal components of an electrolyser. Additionally, chloride is a catalyst site blocker in a cathode catalyst. Specific examples can be found in the paper “Impact of impurities on water electrolysis: a review”, H. Becker et al, Sustainable Energy & Fuels, 2023, 7, 1565-1603. It is also known that halides, such as chloride, can cause corrosion of metal components, such as those present in electrolyser stacks.

There is therefore a desire in the art to reduce or prevent contacting membrane supported catalysts with halides, including chlorides. Applicant has now developed a process for preparing OER-active supported iridium catalyst components where the problems associated with the prior art are reduced or avoided.

SUMMARY OF THE INVENTION

According to a first aspect, the invention provides a process for preparing an oxygen evolution reaction (OER) catalyst comprising an oxygenated iridium component supported on a particulate solid support, which process comprising the steps of: (i) forming an aqueous mixture comprising a particulate solid support and a solution of a halide-free metal iridate; (ii) reducing the pH of the aqueous mixture to < 5.0 to precipitate an oxygenated iridium component onto the particulate solid support; and (iii) isolating the product of step (ii). In initial results, Applicant has found that OER catalysts prepared according to the process of the first aspect of the invention contain substantially less chloride than equivalent catalysts prepared using IrCh (as IrCk.HzO) precursors, even after copious washing, e.g. to < 50 pS/cm conductivity in the post-wash water. Applicant believes that chloride remains in the prior art supported iridium OER catalysts prepared using chloride iridate precursors and this chloride remains leachable when in use. Elevated levels of chloride in supported iridium OER catalysts are expected to provide similar issues in terms of catalyst and COM stability when in use as to those seen for chloride impurities in Pt/C catalysts in PEM fuel cell applications.

Additionally, the initial results of analytical analyses suggest that there are beneficial physical differences between the catalyst products of the first aspect of the invention and catalysts prepared using IrCh precursors (see Example 6 hereinbelow), with a reduced chloride content and a lower iridium dissolution during initial electrochemical testing. According to a second aspect, there is provided an oxygen evolution reaction (OER) catalyst comprising an oxygenated iridium component supported on a particulate solid support, wherein the OER catalyst as a whole has a halide content of less than 500 ppm as determined by ICP-OES. Preferably, the halide content is less than 400 ppm, less than 300 ppm, or less than 250 ppm.

Preferably, in cases in which the particulate solid support is a transition metal (TM) oxide, the ratio of atomic % iridium-to-atomic % transition metal (lr:TM) at a surface of the catalyst is at least 1.0 as determined by XPS analysis, such as in the range of and including 1 .0 to 3.0. An atomic ratio greater than 1 .0 indicates a catalyst in which the oxygenated iridium component is well distributed on the surface of the particulate solid support.

Such materials may be obtained or are obtainable by the process of the first aspect. According to a third aspect, Applicant provides an oxygen evolution reaction catalyst obtained or obtainable by the process according to the first aspect of the invention.

The OER catalyst products of the first aspect of the invention, including the product of the second and third aspects of the invention, generally include no iridium metal (lr(0)) to the limits of detection using X-Ray Diffraction (XRD).

In a fourth aspect, the invention provides a catalyst coated membrane comprising an oxygen evolution reaction catalyst according to the second or the third aspect of the invention.

According to a fifth aspect, the invention provides a fuel cell or a water electrolyser comprising a catalyst coated membrane according to the fourth invention aspect. There is also provided, in a sixth aspect, the use of an oxygen evolution reaction catalyst according to the second aspect of the invention for catalysing the reaction 2H2O — > O2 + 4H⁺ + 4e-.

DETAILED DESCRIPTION OF THE INVENTION

According to a preferred embodiment of the first aspect of the invention, the halide- free metal iridate used in step (i) is obtained by the steps of: (a) combining halide-free iridium powder and a halide-free peroxide salt to produce a powder mixture; and (b) carrying out thermal treatment on the powder mixture. A benefit of this method is that iridium powder is used as a raw material rather than iridium salts which are used in many previously described syntheses of OER catalysts. Iridium powder is generally less expensive than iridium salts and can enable a product to be formed with very low chloride content.

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 molar ratio of Ir-to-peroxide salt is preferably 1 :3 or more. Applicant found that molar ratios of 1 :less than 3, conversion was lower than desired, but is not essential. While there is no particular upper limit on the equivalents of peroxide salt, too large an excess of peroxide salt is preferably avoided on because of process safety, cost, and to avoid contamination of the product with metal ions from the peroxide salt. It is preferred that the molar ratio of Ir-to-peroxide salt is 1 :4 to 1 :10, preferably of 1 :6 to 1 :9.

The thermal reaction of iridium powder and sodium peroxide itself is known and described in the article “Chemical Dissolution of Iridium Powder Using Alkali Fusion Followed by High-Temperature Leaching” Materials Transactions \/o\. 52, No. 11 (2011) pp. 2067-2070. In this reference, iridium powder and sodium peroxide are combined at a molar ratio of lr:Na2O2 ranging from 1 :0.8 to 1 :2.0 using a planetary ball mill containing zirconia balls. The milled material was then transferred to a nickel crucible and heated in air in an electric furnace at a temperature of either 500 or 600 °C for either 4 or 24 h. Although this reference describes the fusion of iridium powder and sodium peroxide, and leaching of iridium from the resulting fused product, it does not describe the isolation of an OER catalyst.

A variety of different heating techniques may be used for the thermal treatment step. In one method, the thermal treatment step is carried out in a static oven or static furnace. In this technique the powder mixture is held within a container (e.g. a tray) within the oven or furnace. In an alternative method, the thermal treatment step is carried out using a belt furnace. In this technique the powder mixture is held within a container (e.g. a tray) and is passed through the furnace by means of a belt. The furnace may be designed for single or multi-zone operation. Belt furnaces are available commercially.

In an alternative method, the thermal treatment step is carried out using a rotary calciner. A rotary calciner typically comprises a rotating drum which is externally heated. The use of rotary calcination is preferred over the use of a static oven, static furnace or belt furnace because rotary calcination mixes the powder which helps to ensure a uniform distribution of heat, which is thought to be beneficial for achieving high conversion of the iridium. If the process is operated continuously, then the drum may be inclined to control the residence time of the powder within the drum. Rotary calciners are available commercially.

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.

According to the preferred embodiment, step (i) comprises the sub-steps of: (i)(a) dissolving the halide-free metal iridate in water to produce a solution; (i)(b) adding the particulate solid support to the solution.

The isolated product of step (iii) of the process of the first aspect of the invention can be washed to remove reagent liquor.

In one embodiment, the product of step (iii) can be combined directly as a “wet cake” with a suitable liquid medium to make a printable catalyst-containing ink. Suitable liquid media that act as ink-base for supported catalysts and can be used for the production of catalyst- coated electrodes in an electrolysis cell (e.g., a PEM electrolysis cell for water electrolysis) or a PEM fuel cell are known to the person skilled in the art. For example, the ink-base medium can contain an ionomer (e.g., a polymer that contains monomers containing sulfonic acid groups) and one or more short-chain alcohols (e.g., methanol, ethanol or n-propanol or a mixture of at least two of these alcohols).

Alternatively, the process of the first aspect of the invention can include a step (iv) of drying the product of step (ii) isolated in step (iii) to remove water therefrom, e.g. to create a dry powder. The resulting dried product can be combined with a suitable liquid medium to make a printable catalyst-containing ink. A drying step can be conducted at 50 to 120°C, for example. Alternatively, the process according to the first aspect of the invention preferably comprises a step of heat-treating the product of step (iii) or step (iv). Where heat treatment is performed on the product of step (iii), e.g. the “wet-cake”, the heat treatment step performs the dual function of both removing excess water from the product and heat treating the product.

Applicant has found that the heat treatment step can improve OER catalyst durability at a moderate reduction of electrochemical activity. In this regard, a preferred temperature range of thermal treatment is 150 to 400°C. In Applicant tests conducted using a particulate TiC>2 support (no dopant), Applicant found that when the temperature of the thermal treatment is increased to above 400°C, e.g. 500°C, OER powder conductivity and corresponding electrocatalytic catalyst activity is reduced. This observation appears to be linked to a significant decrease in the atomic % ratio of surface I r:Ti as determined by X-ray photoelectron spectroscopy (XPS), increased crystallisation of iridium oxide species as determined by XRD and increased iridium agglomeration observed by Transmission Electron Microscope (TEM)- Energy Dispersive X-Ray (EDX) spectroscopy.

Without wishing to be bound by theory, Applicant’s initial analytical testing suggests that a supported amorphous oxygenated iridium species is more electrocatalytically active than a corresponding crystalline material. However, crystalline material is more durable. Therefore, depending on the end-application of the catalyst, a customer may wish to specify a higher level of catalyst durability at the expense of a moderately lower “fresh", i.e. as manufactured, electrocatalytic activity performance and vice versa.

It follows that, if higher durability - and therefore a relatively higher level of crystallinity - is specified, for the same level of electrocatalytic activity performance, a higher initial loading of ionic oxygenated iridium species is required to compensate for the moderately lower “fresh” electrocatalytic activity performance of the partly crystalline material vs. more amorphous catalyst.

In this regard, to achieve selected levels of electrocatalytic activity in the end product, the total iridium content in the aqueous mixture formed in step (ii) is 1 to 22 gi_r I L, preferably 1 to 20 gir / L or most preferably 1 to 10 gi_r / L. The more dilute systems are preferred because the expensive iridium component is being used more sustainably and efficiently for similar or better activity.

The iridium loading in the end product of the process of the first aspect of the invention can be defined by a weight ratio of the particulate solid support to the oxygenated iridium species. In this connection, a weight percentage of iridium can be 5 to 70 wt%, preferably 8 to 50 wt%, of the total weight of the OER catalyst as a whole. One variable for appropriate selection of total iridium content in the final product may be the specific surface area (SSA) of the particulate solid support prior to deposition of the iridium species thereon, as determined by nitrogen BET (Brunauer, Emmett and Teller) methodology. In this regard, Applicant’s initial development review indicates that relatively lower specific surface area support materials are preferred as this helps to promote contact with and conductivity between dispersed iridium species at a lower total iridium loadings, thereby promoting electrocatalytic activity.

Accordingly, N2 BET specific surface areas for the particulate solid support with application in the present invention are preferably < 10 m²/g.

Depending upon application of the OER catalyst end product, the particulate solid support can be a transition metal oxide; or carbon, optionally of the appropriate N2 BET specific surface area mentioned hereinabove. Where the particulate solid support is a transition metal oxide, the selected from optionally doped TiC>2, optionally doped ZrC>2, CeO2/ZrO2 mixed oxide, Ta2<D5, Nb2C>5, AI2O3, SnC>2, SnC>2 optionally doped with antimony or fluorine and mixtures of any two or more thereof. The application environment of a OER catalyst is typically highly acidic, e.g. pH 1.0. Therefore, more acid-stable supports are preferred. In this regard, AI2O3, even when doped with, e.g. silica, is less preferred because it lacks the longer-term durability in the highly acidic aqueous OER environment. More preferred support materials are optionally doped TiO2, Nb2Os, optionally doped ZrO2, and SnO2, even more preferred support materials are optionally doped TiO2, Nb2Os, and optionally doped ZrO2, most preferably optionally doped TiO2. Optional dopants for TiO2 include silica and/or tungsten.

It will be appreciated that methods of halide determination include Inductively Coupled Plasma (ICP)-Optical Emission Spectroscopy (OES) and ICP-Mass Spectrometry (MS), which reports the halide, e.g. chloride content of both the supported iridium species and the support material. Although the reagents selected for steps (i) and (ii) can be halide-free, so that the precipitated oxygenated iridium species are also halide-free, the particulate acid-free support may use a halide-containing compound in its manufacture. For example, the preferred titania support can be manufactured via the chloride process, in which titanium compounds in titanium-containing ores are converted to titanium tetrachloride (TiCk), which is readily purified and converted to the dioxide. Although any residual chloride in the support is understood to be bound and is not believed to interact with the supported oxygenated iridium species, preferred supports have a lower chloride content so that the upper limit of chloride content detected via ICP-OES for products as such of the invention, whether obtained via the process of the first and second aspects of the invention is a maximum of 0.05 weight (500 ppm). In this connection, to avoid the introduction of halide into the supported oxygenated iridium species, the pH in step (ii) is suitably reduced using a halide-free inorganic acid, preferably nitric acid or sulfuric acid. It may be further preferred that the pH in step (ii) is reduced using nitric acid.

The process according to the first aspect of the invention can comprise a subsequent step of incorporating the catalyst into catalyst layer or catalyst-coated membrane (CCM), such as a catalyst-coated proton-exchange membrane (PEM) or a catalyst-coated anion-exchange membrane (AEM), for example in the form of a printable ink as described hereinabove. Where end-application is as a CCM, the process can comprise a subsequent step of incorporating the CCM into a fuel cell or water electrolyser.

According to application, a catalyst coated membrane (CCM) can comprise an OER catalyst according to the second or the third aspect of the invention. In a preferred use, a fuel cell or a water electrolyser comprises the CCM.

Applicant believes that the physical differences between OER catalysts comprising products of the first aspect of the invention, and catalysts of the second aspect, and the prior art prepared using chloride precursors under basic pH may reside in one or more of the following definitions:

(i) in cases in which the particulate solid support is a transition metal (TM) oxide (such as TiO2), the ratio of atomic % iridium-to- atomic % transition metal (lr:TM), such as the ratio of atomic % iridium-to-atomic % titanium, 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 (inter alia as defined in Example 5 hereinbelow). A ratio of atomic % greater than 1 .0 indicates that the process has provided a catalyst in which the oxygenated iridium component is well distributed on the surface of the particulate solid support;

(ii) the OER catalyst as a whole has a halide content of less than 500 ppm as determined by ICP-OES (see Example 5 hereinbelow), preferably, the halide content is less than 400 ppm, less than 300 ppm, or less than 250 ppm;

(iii) the mass electrocatalytic activity (jmass) at 1.47 Volts in a button cell test (inter alia as defined in Example 5 hereinbelow) is > 80 Amperes per gram I rO_x; and

(iv) the powder conductivity of the catalyst (inter alia as defined in Example 5 hereinbelow) is > 0.1 S/cm at 0.3 MPa and optionally also >10.0 S/cm at 44.5 Mpa.

In particular, the results presented in Example 6 hereinbelow suggest that the ratio of atomic % iridium-to-atomic % transition metal at a surface of the catalyst at (i) and also the halide content at (ii) - appear to be important defining characteristics of the product as such. This indicates that the oxygenated iridium component is well distributed, and the chloride content is low offering stability benefits.

DEFINITIONS

By “halide-free” herein, we mean without intentionally added halide, i.e. a chemical compound which does not include a halide in its chemical formula, and we intend - in particular - to exclude chlorides. The definition halide-free does not exclude compounds which may contain trace halide impurities.

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. Such materials are suitably provided by carrying out a thermal treatment of a mixture of a halide-free iridium powder and a halide-free peroxide salt (such as a Group 1 peroxide salt, preferably sodium peroxide).

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 term “oxygenated iridium species” herein is intended to embrace a range of oxidic compositions, both amorphous and crystalline, including without limitation iridium (IV) oxide (lrC>2), iridium (III) oxide (I^Os) and iridium oxyhydroxides. Iridium oxyhydroxides are iridium compounds having both oxo (lr=O) and hydroxo (Ir-OH) functionalities which may have a composition which can be represented, for example, by the following formula: lrOx(OH)y wherein 1 < x < 2 and 0 < y < 2, and 3 < 2x+y < 4. The catalyst produced by the process of the invention has an oxygen content that is higher than is expected for pure I rC>2. There are some indications that the material includes at least some lrO(OH)2, possibly in addition to I rC>2 and/or I^Os. It will be appreciated, however, that because of its preferred amorphous state, it is not possible for Applicant to define the iridium species present with any more specificity without unfairly restricting Applicant’s claim scope.

EXAMPLES

In order that the invention may be more fully understood, the following Examples are provided by way of illustration only.

Example 1 (According to the invention)

Acoustic Mixer at 60 Hz for 30 seconds. The resulting mixed powder was transferred to an alumina crucible and calcined at 500°C for 1 hour with 5°C/min ramp rate. The fusion product was dissolved by stirring in 1 L of de-ionised (DI) water to form an iridate solution. Separately, 10 g of Aeroxide® P25 TiC>2 (Evonik) was suspended in 0.5 L of DI H2O using a Silverson mixer. The TiC>2 slurry was stirred into the iridate solution. The combined mixture was stirred for a further for 30 minutes.

Whilst stirring, concentrated nitric acid (HNO3) was added drop-wise and a target pH of < 4.0 was maintained for 1 hour. The resulting solid was then collected by filtration and washed with DI H2O to a conductivity in the washing water of below 50 pS/cm. The isolated solid was dried in air at 150°C.

Example 2 (According to the invention)

10.42 g of solid particulate Aeroxide® P25 TiC>2 (Evonik) was dispersed in 250 ml of DI water using a Silverson mixer fitted with 3/8 inch mixing head and at 10000 revolutions per minute (RPM). 183.3 ml of DI water was used to wash the head of the Silverson mixer and beaker and to transfer the mixture and washings to a reaction vessel.

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 sodium peroxide and the mixture roughly ground until well mixed. The mixture was transferred to a nickel crucible and heated by means of a natural gas flame (temperature approximately 500 °C) with mixing using a nickel rod to avoid clumping. Once the mixture showed signs of visible red heat heating was stopped and the mixture was allowed to cool for 60-90 minutes.

40 g of the Na2O2 iridium fusion product was added to the suspension of TiO2 with stirring (500 RPM) over the course of 10 minutes. The mixture was stirred for 30 minutes to ensure complete mixing. Concentrated nitric acid (HNO3) was added drop-wise to stirred mixture. Once the mixture reached a pH of 3.25, the solution was maintained at pH 3.25 for 1 hour with additional addition of nitric acid as necessary. The mass of concentrated HNO3 added was 61 .76 g. After one hour, 1.5 litres of DI water were added to the reaction mixture with stirring at 850 RPM for 5 minutes, following which a precipitate was allowed to settle. A blue, solid product was collected by filtration using a three-piece funnel set up (100mm) and 542 Whatman filter paper. The collected product was washed with DI water until the filtrate conductivity measured below 50 pS/cm. The precipitate was dried in air in an oven at 120 °C for 16 hours. Example 3 (Comparative - corresponding to the method of Example 1 in US patent publication no. US 2022/0259750 A1)

10g of IrCU.tW (calculated as 56.5%wt Ir metal, i.e. lr(0)) was dissolved in 221 ml of DI water at room temperature with stirring at 500 RPM. 4.83 g of solid particulate Aeroxide® P25 TiC>2 (Evonik) was dispersed in 50 ml DI water with a Silverson mixer fitted with a 3/8 inch mixing head and at 10000 RPM for 15 minutes. The TiC>2 suspension was added to the IrCk solution in the Silverson mixer, following which the contents of the Silverson mixer were transferred to a reaction vessel. The Silverson head and beaker were washed with 50 ml of DI water and the washings were also added to the reaction vessel. The transferred mixture was stirred for 30 minutes in the reaction vessel, following which the pH of the mixture was adjusted with dropwise addition of 1M NaOH solution to 9.7 and this pH was maintained for 30 minutes with stirring. The aqueous medium was then heated to 70°C and the pH was further adjusted to 11 with dropwise addition of 1M NaOH solution. Stirring of the mixture at 70°C was maintained overnight and was then allowed to cool. A solid product was collected by filtration using a three-piece funnel set up (100mm) and 542 Whatman filter paper. The collected product was washed with DI water until the filtrate conductivity measured below 50 pS/cm. The precipitate was dried in air in an oven at 120 °C for 16 hours.

Example 4 (Comparative)

An unsupported, i.e. no TiC>2, electrochemical grade iridium oxide purchased from J&J Materials Inc. was used as a comparison catalyst.

Example 5 - Details of product characterisation

The products of Examples 2, 3 and 4 were characterised by various methods, as follows:

Iridium and Titanium assay

Sample elemental iridium and titanium weight % content was assayed by the known technique of inductively coupled plasma optical emission spectroscopy (ICP-OES), (see e.g. https://en.wikipedia.org/wiki/lnductively coupled plasma atomic emission spectroscopy; and https://www.thermofisher.com/uk/en/home/industrial/spectroscopy-elemental-isotope- analysis/spectroscopy-elemental-isotope-analysis-learning-center/trace-elemental-analysis- tea-information/icp-oes-information.html). The reported iridium wt% value is assumed to be all “IrOx” and the IrOx component is assumed to be 80%wt iridium metal (I r(0)). Chloride assay

Sample elemental chlorine content (ppm) was assayed by the known technique of liquid ion chromatography (see e.g. https://en.wikipedia.org/wiki/lon chromatography). The ion chromatography is coupled to a “digestion system”, i.e. an “Automated Quick Furnace” (AQF) to isolate the halogens and the isolated halogens are assayed using the ion chromatography technique.

XPS

X-Ray Photoelectron Spectrometry (XPS) analysis was conducted using a Thermo Fischer 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 was calculated from atomic % iridium and the atomic % titanium. A lower 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 Ti2p3/2 signals at 459 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.

The surface ratio of atomic % iridium: atomic % zirconium or niobium may be calculated from atomic % iridium and the atomic % zirconium or niobium. The atomic % values were determined by XPS analysis of the zirconium 3ps/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.

Powder conductivity

The conductivity of powdered samples was measured using an NH Instruments PD 600 powder measuring system at ambient temperature. For the measurement, the powdery sample is pressed into the cylindrical chamber of the measuring head under a pre-defined pressure. Conductivity is then measured as increasing pressure is applied, the conductivity increasing until an essentially constant value is obtained. For each of the samples tested the maximum pressure applied was 45 Mpa.

Button Cell Testing

An ink was prepared by combining 0.1 g of a sample of Example 2, Comparative Example 3 or Comparative Example 4 with aqueous Nation solution (11.92 wt % solids, 1 mL) to produce an ink containing 110 wt % Nation with respect to the sample. The ink was then shear-mixed in a planetary mixer using 5 mm diameter yttrium stabilized zirconia beads for 5 mins at 3000 rpm. The ink was manually stirred to break apart any sediment at which point it was mixed for a further 5 mins in the planetary mixer and these steps were repeated to a total milling time of 15 minutes.

The prepared ink was spray-coated onto a Toray paper (hydrophobic gas diffusion layer 60) at 0.2 mg cm^-2 loading and verified using X-ray fluorescence (XRF) measurements. The buttons were then soaked overnight in an equivalent solution as the test solution (1 M H2SO4) under a vacuum to allow ingress of solution into the gas diffusion layer, ensuring all of the catalyst is in contact with electrolyte.

A button was then placed in an electrochemical cell that contained 1 M H2SO4 while being purged with nitrogen (liquid nitrogen off gas) and held at 60 °C. A reversible hydrogen electrode (RHE) (hydrogen bubbled over Pt/C catalyst) and Pt wire was used as a reference and counter electrode, respectively. First, the cell was cycled between 0 and 1.35 V vs RHE at different scan rates (5-300 mV s^-1), and then, an activity sweep was performed between 1 and 1.55 V vs RHE at 1 mV s’¹.

The activity data were /R corrected by taking the high frequency intercept of an impedance scan measured at 1.45 V vs RHE; typical values were between 0.2 and 0.35 Q . Activity data were normalized to a mass activity by measuring the loading of the ruthenium by XRF and then assuming the ratio of the EDXA results. Degradation was monitored by taking a 1 mL sample of the electrolyte solution at the start of testing and then after the beginning- of-life (BOL) activity test (referred to in Table 1 below as the “1^st activity test”) for inductively coupled plasma mass spectrometry (ICP-MS) analysis. The 1 mL sample was diluted with water to 1 v.% H2SO4, and then, 1 v.% HCI was added. These samples were injected directly into an ICP-MS to obtain the concentration of metal leached into solution.

The mass electrocatalytic activity (jmass) results are reported at 1.47 Volts (Amperes per gram lrO_x).

Example 6 - Test Results

The test results for the characterisation methods of Example 5 are set out in the following Table 1 Table 1

The chloride content of the solid particulate Aeroxide® P25 TiC>2 (Evonik) raw material was assayed as containing 1100 ppm and 58.5 wt% elemental titanium. The chloride results from the method of manufacture of the TiC>2 (chloride process).

* A theoretical wt% loading based on the amount of iridium added. The calculation assumes that all Ir is present as IrOx and the IrOx component is assumed to be 80% wt Ir. t The chloride content of the sample is a dilution effect brought about by the presence of IrOx relative to TiO2 alone. The chloride content of the product of Example 2 is significantly lower than the product of Example 3. It will be recognised that the chloride present in the Example 3 sample, despite copious washing, i.e. to a conductivity of < 50 pS/cm in the post wash water, likely results from a combination of the IrCk.fW and TiO2 used, whereas the chloride present in the Example 2 sample is contributed entirely by the TiC>2.

It can be deduced from the results in T able 1 that the product of the process according to the invention have a lower chloride content and a lower iridium dissolution in comparison with the product from the process of the prior art.

It can be seen that the product of Example 2 (according to the invention) is more electrochemically active than the unsupported iridium oxide of Example 4 (Comparative), although less active than the supported product made with IrCk.fW precursor (Example 3 (Comparative)). While methods of improving electrochemical activity of the supported product of the invention continue, it is noted that the Example 3 (Comparative) product contains significantly more chloride than the product of Example 2 (according to the invention). The dissolution data shows that the product of Example 2 is more stable. XRD of materials produced by the process and dried at 150 °C (such as Example 2) indicates the formation of amorphous iridium oxide (considered to include some hydroxyl and oxide functionality). XRD of materials produced by the process and heat treated at 400 °C indicates the formation of iridium oxide (I rC>2).

For the avoidance of doubt, the entire contents of all documents identified in the description are incorporated herein by reference.

Claims

CLAIMS:

1. A process for preparing an oxygen evolution reaction (OER) catalyst comprising an oxygenated iridium component supported on a particulate solid support, which process comprising the steps of:

(i) forming an aqueous mixture comprising a particulate solid support and a solution of a halide-free metal iridate;

(ii) reducing the pH of the aqueous mixture to < 5.0 to precipitate an oxygenated iridium component onto the particulate solid support; and

(iii) isolating the product of step (ii).

2. A process according to claim 1 , wherein the halide-free metal iridate used in step (i) is obtained by the steps of: (a) combining halide-free iridium powder and a halide-free peroxide salt to produce a powder mixture; and (b) carrying out thermal treatment on the powder mixture.

3. A process according to claim 1 or claim 2, wherein step (i) comprises the sub-steps of:

(i) (a) dissolving a solid halide-free metal iridate in water to produce a solution; and

(i) (b) adding the particulate solid support to the solution.

4. A process according to claim 1, 2 or 3, comprising a step (iv) of drying the product of step (ii) isolated in step (iii) to remove water therefrom.

5. A process according to any one of the preceding claims, comprising a step of heat- treating the product of step (iii) or step (iv).

6. A process according to claim 5, wherein the heat treatment step is done at a temperature in the range of, and including, 150 to 400°C.

7. A process according to claim 2 and any one of claims 3 to 6 when dependent on claim 2, wherein the halide-free peroxide salt is sodium peroxide.

8. A process according to claim 2 or any one of claims 3 to 7 when dependent on claim 2, wherein the halide-free iridium powder and halide-free peroxide salt are combined at a molar ratio of from 1 :4 to 1 : 10.

9. A process according to any one of the preceding claims, wherein a total iridium content in the aqueous mixture formed in step (i) is 1-22 gi_r / L.

10. A process according to any one of the preceding claims, wherein a weight percentage of iridium is 5 to 70 wt%, preferably 8 to 50 wt%, of the total weight of the OER catalyst as a whole.

11. A process according to any one of the preceding claims, wherein in step (ii) the pH is reduced using nitric acid or sulfuric acid.

12. A process according to any one of the preceding claims, wherein in step (ii) the pH is reduced to less than 4.

13. A process according to any one of the preceding claims, wherein the particulate solid support is a transition metal oxide.

14. A process according to any one of the preceding claims, wherein the particulate solid support is selected from optionally doped TiC>2, optionally doped ZrC>2, CeO2/ZrO2 mixed oxide, Ta2<D5, Nb2C>5, AI2O3, SnC>2, SnC>2 optionally doped with antimony or fluorine, and mixtures of any two or more thereof, preferably TiC>2.

15. A process according to any one of claims 1 to 12, wherein the particulate support is carbon.

16. A process according to claim 13, 14 or 15, wherein the N2 BET specific surface areas for the particulate solid support added at step (i) is < 10 m²/g.

17. A process according to any one of the preceding claims, comprising a subsequent step of incorporating the catalyst into a catalyst layer.

18. A process according to any one of claims 1 to 16, comprising a subsequent step of incorporating the catalyst into a catalyst-coated membrane (CCM).

19. A process according to claim 18, comprising a subsequent step of incorporating the catalyst-coated membrane into a fuel cell or water electrolyser.

20. An oxygen evolution reaction (OER) catalyst comprising an oxygenated iridium component supported on a particulate solid support, wherein:

(i) the OER catalyst as a whole has a halide content of less than 500 ppm as determined by ICP-OES;

(ii) the particulate solid support is a transition metal (TM) oxide, and the ratio of atomic % iridium-to-atomic % transition metal (lr:TM) at a surface of the catalyst is at least 1.0 as determined by XPS analysis, such as in the range of and including 1 .0 to 3.0.

21. An oxygen evolution reaction (OER) catalyst obtained or obtainable by a process according to any one of the preceding claims.

22. An oxygen evolution reaction (OER) catalyst according to claim 21 , wherein the OER catalyst as a whole has a halide content of less than 500 ppm as determined by ICP-OES, preferably less than 400 ppm, less than 300 ppm, or less than 250 ppm.

23. An oxygen evolution reaction (OER) catalyst according to claim 21 or claim 22, wherein the particulate solid support is a transition metal (TM) oxide, and the ratio of atomic % iridium- to-atomic % transition metal (lr:TM) at a surface of the catalyst is at least 1.0 as determined by XPS analysis, such as in the range of and including 1.0 to 3.0.

24. A catalyst coated membrane comprising an oxygen evolution reaction catalyst according to any one of claims 20 to 23.

25. A fuel cell or a water electrolyser comprising a catalyst coated membrane according to claim 24.