Oxygen evolution reaction catalyst and method for its preparation

Abstract

A method for preparing a ruthenium based OER catalyst. A process for preparing a ruthenium-based oxygen evolution reaction catalyst, the process comprising the steps of: (i) combining ruthenium powder

Description

Technical field The present invention relates to ruthenium-containing oxygen evolution reaction (OER) catalysts and a method for their preparation. Background Oxygen evolution reaction (OER) catalysts are an important component of fuel cells and water electrolysers. In fuel cells, the OER catalyst can perform in either the anode or the cathode to improve tolerance to cell reversal events (see, for example, WO01 / 15247), and to prevent high cathode potentials during start-up which arise due to the presence of both hydrogen and air on the anode, and air on the cathode (see, for example, (Journal of Power Sources 158 (2006) 1306-1312). The OER reaction in acidic conditions is approximated by the following equation: 2H2O O2 + 4H+ + 4e’ Iridium oxide catalysts are widely used for this reaction. Despite being widely referred to as “iridium oxide”, most “iridium oxide” catalysts are more likely a mixture of oxidic iridium species such as lrO2 and / or lrO(OH)2. For that reason “iridium oxide” is often referred to as IrOx. Additionally, ruthenium oxide catalysts are used, as well as oxide catalysts containing iridium and ruthenium. OER catalysts used in a fuel cell or water electrolyser should ideally have a good activity per gram of metal and should also have high stability under operation conditions of the fuel cell or water electrolyser. IrOx catalysts, as well as ruthenium and mixed iridium ruthenium oxide catalysts, can be prepared in various degrees of crystallinity ranging from essentially amorphous to highly crystalline. The article “Synthesis and Optimisation of IrO2 Electrocatalysts by Adams Fusion Method for Solid Polymer Electrolyte Electrolysers” Micro and Nanosystems 2012, 4, 186-191 describes the preparation of iridium dioxide using the Adams fusion method. The Adams fusion method involves the fusion of a metal chloride precursor with sodium nitrate in air at elevated temperature and has been used to prepare various noble metal oxides. In this paper H2lrCl6 and NaNOs were combined in isopropanol, the isopropanol is removed by heating in an oven and the resulting catalyst precursor / salt mixture was then reacted in a preheated furnace. The obtained metal oxide was then cooled, washed with ultrapure water to remove excess NaNOs and then dried in an oven at 100 °C. Amorphous iridium oxide was obtained in all cases. A peak assigned to the (101) reflection of lrO2 at 20 ~ 34-35° was seen in all samples. Ruthenium oxide materials may be prepared by such methods, as well as conventional precipitation methods. Iridium ruthenium oxide materials are conventionally prepared by methods such as precipitation from a solution of ruthenium and iridium chloride salts followed by drying and calcination in air at 500°C, as disclosed in WO2004 / 010521. There is a need for new OER catalysts having a good balance between catalyst activity and operational stability, which can be manufactured by a route which is scalable, and which contain low levels of impurities such as metal and halide species. Summary of the invention In a first aspect the invention provides a process for preparing a ruthenium-based oxygen evolution reaction catalyst, the process comprising the steps of: (i) combining ruthenium powder and a peroxide salt to produce a powder mixture; (ii) carrying out thermal treatment on the powder mixture; (iii) dissolving the product from (ii) in water to produce a solution; (iv) reducing the pH of the solution from (iii) to effect a precipitation and form a solid and a supernatant; (v) separating the solid from the supernatant; and (vi) drying the solid. The oxygen evolution reaction catalyst may further comprise iridium such that it is a ruthenium-and iridium-based oxygen evolution reaction catalyst. Accordingly, the oxygen evolution reaction catalyst may be a ruthenium-based or ruthenium- and iridium-based oxygen evolution catalyst. Therefore, step (i) may further comprise combining iridium powder such that step (i) requires combining ruthenium powder, iridium powder, and a peroxide salt to produce a powder mixture. The oxygen evolution reaction catalyst is an oxide material, and preferably substantially all (i.e. greater than 99 % by weight of the oxygen evolution reaction catalyst) of the metal oxide species present are ruthenium, and when applicable iridium, oxide species. The oxygen evolution reaction catalyst may contain hydroxide species. Accordingly, the term “oxide” encompasses materials containing hydroxide species, i.e. hydroxide oxides. The atomic ratio of ruthenium to iridium, when present, is typically in the range of and including 1:9 to 9:1, preferably 2:8 to 9:1. The atomic ratio of ruthenium, or when applicable ruthenium + iridium, to oxygen, may be at least 2:3, typically at least 1:2. The atomic ratio of ruthenium, or when applicable ruthenium + iridium, to oxygen is typically at most 1:4. The higher oxygen content than expected for RuO2 can be attributed to, for example, the presence of oxide hydroxide species. A benefit of the process is that metal (ruthenium, and where applicable iridium) powder is used as a raw material rather than metal salts which are used in many previously described syntheses of OER catalysts. Metal powder is generally less expensive than metal salts. The avoidance of salts, and particularly chloride salts, means that the product can be produced with very low chloride impurities. Chloride contamination in the product may affect the catalytic properties and performance due to the possibility of HCI and / or Ch production during operation which can damage the equipment. A further benefit of this process is that, by control of the fusion step (ii) and / or by carrying out a filtration between steps (iii) and (iv), it is possible to produce a product which is free from metal. Metal is believed to be less active in the OER, so material made by this process may be able to achieve comparable OER activity using lower loadings of total metal. Metal may also dissolve under typical conditions of CCM operation which is undesirable. Some sources of metal have a geometry which can cause damage to the membrane, which is a further reason why complete conversion of the metal is desirable. A further benefit of the process is that, by appropriately selecting the material used to contain the powder mixture during step (ii), the resulting product can be made with very low contaminants of transition metals. Some transition metals, particularly copper, chromium and cobalt, are known to catalyse the formation of oxidizing species which can damage the membrane of a CCM (see the article “A comparative study of several transition metals in Fenton-like reaction systems at circum-neutral pH” Acta Chim. Slov. 2003, 50, 619-632) and the avoidance of these metals is particularly advantageous. This process produces an OER catalyst which has a good balance between activity, conductivity, and operational stability. The catalyst produced following the drying step (vi) typically has a BET surface area of 5 to 80 m2 / g, typically 5 to 70 m2 / g. Ruthenium-based catalysts (not containing iridium) have a rutile crystal structure, with main peaks at 28°, 35° and 54°. Higher temperature heat treatments (the heat treatment step is discussed in more detail herein), e.g. greater than about 250°C, e.g. around 400°C result in narrower peaks in the XRD pattern indicating a more crystalline structure, with low temperature treatments, e.g. less than about 250°C, e.g. around 150°C, providing materials having poorly crystalline characteristics with broader peaks in the XRD pattern. Ruthenium- and iridium-based catalysts exhibit an XRD pattern which depends on the iridium content and heat treatment temperature. For lower temperature heat treated materials, as the material approaches a 1:1 atomic ratio of ruthenium to iridium, the XRD pattern evolves into a “pseudo-amorphous” iridium oxide pattern. A “pseudo-amorphous” iridium oxide pattern, which predominates when there is more iridium than ruthenium, exhibits a broad maxima in its XRD pattern in the region 20 = 25-38°. For example, when the atomic ratio of ruthenium to iridium is less than 1:1, the XRD pattern will just exhibit a broad maxima in its XRD pattern in the region 20 = 25-38°. However, when the atomic ratio of ruthenium to iridium is greater than 1:1, for example greater than 4:1, the XRD pattern will contain one or more of the broadened peaks at 20 = 28°, 35° and 54° which corresponds with poorly crystalline rutile pattern visible. Despite being poorly crystalline or pseudo-amorphous, the lower temperature heat treated material has excellent operational stability and conductivity and will show relatively low levels of ruthenium, and when applicable iridium, dissolution. For higher temperature heat treated materials, e.g. greater than about 250°C, e.g. around 400°C, the XRD pattern shows a rutile crystal structure, with main peaks at 20 = 28°, 35° and 54°. The more crystalline material may be even more stable, whist still surprisingly retaining a high level of activity, and high conductivity. The process may include a subsequent step of incorporating the catalyst into a catalyst-coated membrane. The process may also include a subsequent step of incorporating the catalyst-coated membrane into a fuel cell or water electrolyser. In a second aspect the invention provides a ruthenium-based oxygen evolution reaction catalyst obtained or obtainable by a process according to the first aspect of the invention. The oxygen evolution reaction catalyst may further comprise iridium such that it is a ruthenium-and iridium-based oxygen evolution reaction catalyst. Accordingly, the oxygen evolution reaction catalyst may be a ruthenium-based or ruthenium- and iridium-based oxygen evolution catalyst. The oxygen evolution reaction catalyst is an oxide material, and preferably substantially all (i.e. greater than 99 % by weight of the oxygen evolution reaction catalyst) of the metal oxide species present are ruthenium, and when applicable iridium, oxide species. The oxygen evolution reaction catalyst may contain hydroxide species. Accordingly, the term “oxide” encompasses materials containing hydroxide species, i.e. hydroxide oxides. The atomic ratio of ruthenium to iridium, when present, is typically in the range of and including 1:9 to 9:1, preferably 2:8 to 9:1. The atomic ratio of ruthenium, or when applicable ruthenium + iridium, to oxygen, may be at least 2:3, typically at least 1:2. The atomic ratio of ruthenium, or when applicable ruthenium + iridium, to oxygen is typically at most 1:4. The higher oxygen content than expected for RuO2 can be attributed to, for example, the presence of oxide hydroxide species. In a third aspect the invention provides a ruthenium-based oxygen evolution reaction catalyst, wherein: the catalyst has a BET surface area of 5 to 80 m2 / g; the content of any individual transition metal, other than ruthenium, and optionally iridium, is 0-600 ppm when measured by inductively coupled plasma mass spectrometry (ICP-MS); the catalyst exhibits one or more of the following features in its XRD pattern: a peak at 20 = 28°, a peak at 20 = 35°, a peak at 20 = 54°, and a broad maxima in its XRD pattern in the region 20 = 25-38°. In some embodiments, the catalyst will exhibit peaks in its XRD pattern at 20 = 28°, 35° and 54°. In some embodiments, the catalyst will exhibit a broad maxima in its XRD pattern in the region 20 = 25-38°. In some embodiments the catalyst will exhibit a broad maxima in its XRD pattern in the region 20 = 25-38°, along with one or more peaks at 20 = 28°, 35° and 54°. The oxygen evolution reaction catalyst may further comprise iridium such that it is a ruthenium-and iridium-based oxygen evolution reaction catalyst. Accordingly, the oxygen evolution reaction catalyst may be a ruthenium-based or ruthenium- and iridium-based oxygen evolution catalyst. The oxygen evolution reaction catalyst is an oxide material, and preferably substantially all (i.e. greater than 99 % by weight of the oxygen evolution reaction catalyst) of the metal oxide species present are ruthenium, and when applicable iridium, oxide species. The oxygen evolution reaction catalyst may contain hydroxide species. Accordingly, the term “oxide” encompasses materials containing hydroxide species, i.e. hydroxide oxides. The atomic ratio of ruthenium to iridium, when present, is typically in the range of and including 1:9 to 9:1, preferably 2:8 to 9:1. The atomic ratio of ruthenium, or when applicable ruthenium + iridium, to oxygen, may be at least 2:3, typically at least 1:2. The atomic ratio of ruthenium, or when applicable ruthenium + iridium, to oxygen is typically at most 1:4. The higher oxygen content than expected for RuO2 can be attributed to, for example, the presence of oxide hydroxide species. The OER catalyst according to the second and third aspects of the invention has particular utility in water electrolysis and fuel cell applications, for example as part of a catalyst coated membrane for a fuel cell or water electrolyser. In a fourth aspect of the invention there is provided a catalyst coated membrane comprising an OER catalyst according to the second or third aspects. In a fifth aspect of the invention there is provided a fuel cell or water electrolyser comprising a catalyst coated membrane according to the fourth aspect. Brief Description of the Figures Figure 1 is an X-ray diffraction pattern for an oxygen evolution reaction catalyst prepared according to the present process. Figure 2 is an X-ray diffraction pattern for another oxygen evolution reaction catalyst prepared according to the present process. Figure 3 is an X-ray diffraction pattern for another oxygen evolution reaction catalyst prepared according to the present process. Figure 4 is an X-ray diffraction pattern for other oxygen evolution reaction catalysts prepared according to the present process. Figure 5 provides liner sweep voltammograms for oxygen evolution reaction catalysts prepared according to the present process, and comparative catalysts. Detailed description The term “powder” used in connection with ruthenium and iridium powder is intended to encompass both spherical powders and also irregular powders such as ruthenium or iridium sponge. It is preferred that the ruthenium or iridium powder has a total impurity content of at most 1% (i.e. 99+% ruthenium or iridium), preferably a total impurity content of at most 0.5% (i.e. 99.5+% ruthenium or iridium), more preferably a total impurity content of at most 0.1% (i.e. 99.9+% ruthenium or iridium), in each case to minimise the content of impurities in the oxygen evolution reaction catalyst. For demanding applications which require particularly low impurity levels it is preferred that the ruthenium or iridium powder has a total impurity content of at most 0.05% (i.e. 99.95+% ruthenium or iridium). The role of the peroxide salt is to oxidize the ruthenium, and when applicable iridium, powder. In preferred embodiments the peroxide salt is a Group I or Group II peroxide salt, preferably a Group I peroxide salt. Preferred salts are sodium peroxide and barium peroxide as these are commercially available. Sodium peroxide is particularly preferred. The peroxide salt may be in the form of beads or powder. The molar ratio of Ru : peroxide salt, or Ir + Ru : peroxide salt, should preferably be 1 : >3. At molar ratios of 1 : <3 conversion may not be sufficient. While there is no particular upper limit on the equivalents of peroxide salt, too large an excess of peroxide salt should be avoided on the grounds 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 Ru : peroxide salt, or Ir + Ru : peroxide salt is 1 : 3 to 1 : 10, preferably 1 : 3 to 1 : 8. It is preferred that in step (i) no other components are added besides the ruthenium, and when applicable iridium, powder and the peroxide salt. The molar ratio of ruthenium to iridium powder, when applicable, is determined by the desired atomic ratio of ruthenium to iridium in the oxygen evolution reaction catalyst. In step (ii) a thermal treatment is carried out on the mixture of ruthenium, and when applicable iridium, and peroxide salt (hereafter “powder mixture”); this is referred to herein as the fusion step. The product of this step is believed to be a mixture of oxidic species, including ruthenate species, and when applicable metal iridates (IVklrOa (M+) and MlrOs (M2+)). The fusion between 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 Vol. 52, No. 11 (2011) pp. 2067-2070. In this reference iridium powder and sodium peroxide are combined at a molar ratio of Ir: Na2O2 ranging from 1 : 0.8 to 1 : 2.0 using a planetary ball mill using zirconia balls. The milled material was 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 and it does not describe the fusion between ruthenium powder and sodium peroxide. In the present invention, a variety of different heating techniques may be used for the fusion step. In one method the fusion step is carried out in a static oven or static furnace. In this technique the power mixture is held within a container (e.g. a tray) within the oven or furnace. In an alternative method the fusion step is carried out using a belt oven or belt furnace. In this technique the powder mixture is held within a container (e.g. a tray) and is passed through the oven or furnace by means of a belt. The oven or furnace may be designed for single or multizone operation. Such ovens and furnaces are available commercially. Static ovens and belt ovens typically generate heat that is transferred to the powder mixture. Static ovens and belt ovens typically include a fan to aid in even distribution of heat throughout the oven. Static furnaces and belt furnaces may achieve heating through any suitable means known to those skilled in the part. Preferred heating methods include: combustion heating, electrical heating, electric arc heating, induction heating, microwave heating or infrared heating. In an alternative method the fusion 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, belt oven 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 ruthenium, and when applicable iridium, and more consistent product quality. If the process is operated continuously then the drum may be inclined to control the residence time of the powder within the drum. The use of rotary calcination also offers a health and safety benefit because the operator is shielded from the reactant mixture more thoroughly than other techniques. Rotary calciners are available commercially. In an alternative method the fusion step is carried out in a fluid bed calciner. In a fluid bed calciner a gas is passed through the powder mixture such that the powder mixture exhibits fluid-like properties. In an alternative method the fusion step is carried out using a hot plate with the powder mixture held within a suitable container on the hot plate. In an alternative method the fusion step is carried out using a combustion flame (e.g. a Bunsen burner) with the powder mixture held within a suitable container. The applicant has found that under the oxidizing conditions of step (ii) it is possible for the metals at the surface of the equipment used to contain the powder mixture to contaminate the product. The presence of certain transition metals in the OER catalyst should ideally be avoided because they may catalyse the formation of OH and O2H radicals which can in turn cause defects in the COM and reduce the lifetime of the COM. Contamination of the product can be avoided by appropriate selection of the material used to contain the powder mixture. By “used to contain the powder mixture” we mean the surface of any equipment which comes into contact with the powder mixture during the fusion step. For instance, in the case of a static oven, static furnace or belt furnace, contamination of the product can be avoided by using a tray in which at least the surface of the tray which contacts the powder mixture is made of a suitably selected material. In the case of a rotary calciner the inner surface of the drum is made of a suitably selected material. In one preferred embodiment the material used to contain the powder mixture is a metal oxide. A preferred material is aluminium oxide. In one preferred embodiment the material used to contain the powder mixture is a stainless steel. A preferred stainless steel is a chromium-nickel-molybdenum austenitic stainless steel. A particularly preferred stainless steel contains 16.0-18.0 wt% Cr, 10.0-14.0 wt% Ni, 2.00-3.00 wt% Mo, 0.08 wt% C, 2.00 wt% Mn, 0.045 wt% P, 0.03 wt% S, 0.75 wt% Si, 0.1 wt% N balance Fe; this steel is often referred to as type 316. The powder mixture is heated at a temperature and duration suitable to achieve the desired conversion of ruthenium, and when applicable 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. The mixture from step (ii) is typically left to cool to room temperature and is then dissolved or dispersed in water, preferably deionised water (DI). It is preferred that the mixture is stirred to ensure full dissolution of soluble ruthenium, and when applicable iridium, species; stirring for 30 mins is normally sufficient. Dissolution is typically carried out at room temperature. It is preferred that the total ruthenium, or ruthenium + iridium concentration before step (iv) commences is from 10-100 g / L, preferably 10-80 g / L, for example 10-60 g / L. The concentration of ruthenium, or ruthenium + iridium, may be important in controlling the properties of the material formed in step (iv). For the avoidance of doubt, if 100 g ruthenium, or ruthenium + iridium, powder is used in step (ii) and 5L of DI is used in step (iii) then the metal concentration is 20 g / L. In an optional step carried out in between steps (iii) and (iv), the solution may be filtered to removed unreacted ruthenium, and when applicable iridium, particles. The solution from step (iii) is acidified during which time a precipitate forms. Although the chemistry is not yet fully understood, pH control during the acidification step is thought to be important in preparing a catalyst with the correct properties. It is preferred that the pH is reduced gradually to within the range of pH 3.0 to 5.0, more preferably pH 3.0 to 3.5. Precipitation can still be achieved at higher pH, and even at alkaline pH, but the resulting product is more difficult to wash and impurities may therefore remain in the final product. In order to avoid chloride contamination in the product it is preferred that pH adjustment is carried out using an acid that is not HCI. Nitric acid is a preferred acid for pH adjustment. The precipitation produces a solid and a supernatant. Once the pH has been reduced, the mixture may optionally be diluted by addition of water, preferably DI water. It is preferred that the mixture is diluted to achieve a ruthenium, and when applicable iridium, concentration which is 40-60% of the original concentration. For example, if the concentration prior to step (iv) was 20 g / L then the mixture is diluted to achieve a concentration of 8-12 g / L. Following acidification, and optional dilution, the solid and supernatant are separated, preferably by filtration. The solid is washed with water, preferably DI, to remove soluble impurities. The filtered solid is then dried. The solid should be dried at a temperature of less than 100 °C, typically less than 75 °C, e.g. approximately 55 °C if the hydrate is desired. To obtain a product with less water content a higher temperature should be used, preferably at least 130 °C. Such a temperature is preferably at most 600 °C, or at most 400 °C. The duration of the drying step will depend on the apparatus used and the scale, but the skilled person will be able to determine suitable conditions. The material formed after step (vi) may be crystalline, poorly crystalline, or amorphous depending on the temperature of heat treatment and the proportion of iridium, when applicable, with ruthenium. If a more crystalline material is desired then a higher drying temperature may be used, and / or a calcination step may be carried out on the material from step (iv). To afford such higher crystallinity, drying temperature and / or the calcination step may, for example, be carried out at a temperature of at least 250°C. The calcination temperature is preferably at most 600 °C, or at most 400 °C. In an optional step the dried material, which may optionally have first been calcined, is ground to the appropriate particle size. Any suitable grinding method may be used. The ground material is then sieved to the desired particle size fraction. The ground material is generally filtered using a series of sieves with the final sieve being US Mesh 200 (retaining sizes above 75 pm). Any oversized powder is returned to the grinding step. The process may include a subsequent step of incorporating the catalyst into a catalyst-coated membrane. A catalyst-coated membrane generally comprises a membrane having an anode catalyst layer on a first face thereof and a cathode catalyst layer on a second face thereof. The process may also include a subsequent step of incorporating the catalyst-coated membrane into a fuel cell or water electrolyser. The catalyst has a Sbet surface area of 5 to 80 m2 / g when measured according to the method described in the experimental section. It is preferred has the catalyst has a Sbet surface area of 5 to 70 m2 / g. In some embodiments in which the catalyst is ruthenium- and iridium- based, iridium metal may also be present, which can be identified by sharp peaks at 20 = 41°, 47° and 83° in the XRD pattern. Alternatively, the catalyst may be free from iridium metal, which can be identified by the absence of sharp peaks at 20 = 41°, 47° and 83°. When the catalyst is ruthenium-or ruthenium- and iridium-based, ruthenium metal may also be present, which can be identified by sharp peaks at 20 = 38°, 42°, 44° and 58° . Alternatively, the catalyst may be free from ruthenium metal, which can be identified by the absence of sharp peaks at 20 = 38°, 42°, 44° and 58°. As noted above, the presence of metal can be avoided by filtering the solution between steps (iii) and (iv) in order to remove unreacted iridium particles, and / or by controlling the oxidation conditions in step (ii) so as to oxidize all of the iridium powder. The catalyst typically contains at least 60 wt% ruthenium, or ruthenium + iridium, when measured by inductively coupled plasma mass spectrometry (ICP-MS), typically at most 85 wt%. If a ruthenium, and when applicable iridium, powder having a suitably low impurity content is used in step (i) and appropriate materials are used to contain the mixture used in step (ii) then it is possible to produce an OER catalyst with an especially low content of transition metal impurities. The catalyst according to the third aspect of the invention has a content of any individual transition metal, other than ruthenium, and when applicable iridium, which is 0-600 ppm when measured by inductively coupled plasma mass spectrometry (ICP-MS) analysis, preferably 0-500 ppm, such as 0-400 ppm, 0-300 ppm or 0-200 ppm. For demanding applications and where a particularly long life is required, it is preferred that the content of any individual transition metal other than ruthenium, and when applicable iridium, is 0-100 ppm. In preferred embodiments the catalyst has a content of any individual transition metal other than ruthenium, and when applicable iridium, which is 0-500 ppm, preferably 0-250 ppm. For the avoidance of doubt, the combined amount of transition metals, other than ruthenium, and when applicable iridium, may be above 500 ppm. For demanding applications and where a particularly long life is required, it is preferred that the catalyst has a content of any individual transition metal other than ruthenium, and when applicable iridium, which is 0-100 ppm. It is preferred that the catalyst has a largest particle size of <75 pm. This corresponds to catalyst which passes through a US Mesh 200. This particle size is typically suitable for incorporation in a layer of a fuel cell or water electrolyser. Examples BET surface area The sample was placed in a 3 / 8” diameter small bulb tube. The amount of sample was chosen so as to fill at least 1 / 3rd of the bulb, and degassed at room temperature for 10 minutes followed by treatment at 300 °C for 1 h under N2 flow, then allowed to cool down gradually under N2 flow to room temperature. N2 BET surface area was measured using a Micromeritics™ TriStar II Plus instrument. The values of surface area calculated from this technique are m2 per gram of catalyst. XRD XRD analysis was carried out using a Bruker D8 Advance Davinci diffractometer with a Lynxeye-XE PSD detector with 0.0125° Ni Filter. The analysis was carried out an ambient temperature using Cu Ka (A = 1.5406 + 1.54439 A) radiation. Phase identification was carried out using Bruker AXS Diffrac EVA V5 (2010-2018) software and the PDF-4+ database (2020 release). Electrochemical data - linear sweep voltammocirams (LSVs) For liquid cell electrochemical testing, inks were spray coated onto a Toray paper (hydrophobic gas diffusion layer 60) at 0.2 mg cm-2 loading, verified using X-ray fluorescence (XRF) measurements. The buttons were then soaked overnight in an equivalent solution as the test solution (1 M H2SO4 (VWR AVS TITRINORM)) 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 1M 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 SHE at different scan rates (5-300 mV s-1), and then, an activity sweep was performed between 1 and 1.55 V vs SHE at 1 mV s’1. The activity data were iR corrected by taking the high frequency intercept of an impedance scan measured at 1.45 V vs SHE; typical values were between 0.2 and 0.35 D. Activity data were normalised using the loading measured from XRF. Powder conductivity The conductivity of OER catalyst material 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 an automated press. Conductivity is then measured as forced is 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. Example 1 - Synthesis ofRuO2 30g of sodium hydroxide (140 mesh) and 10g of ruthenium sponge powder were ground in a pestle and mortar, resulting in a grey powder, with some peroxide particles still intact. The mixture was then quickly placed in an alumina crucible with tamping, which was then heated by means of a muffle furnace (temperature approximately 500°C). Ater heating for about one hour, the mixture was allowed to cool. Once cooled, the mixture was dissolved in 500 ml of DI water. The solution was then acidified to pH ~3.3 with reagent grade nitric acid. A colour change to black was observed, and a precipitate was seen to have formed. The precipitate was left overnight to settle. After leaving overnight, the black precipitate had settled out, with a colourless supernatant liquid. About 300 ml of liquid was decanted, and the remaining dispersion filtered. The filtrate was colourless. The residue was then washed with room temperature water (1000 ml), and dried at either 400°C or 150°C. A lumpy black friable product was recovered which was ground and sieved through a series of sieves, the final sieve being US Mesh 200 (retaining sizes above 75 pm). Example 2 - Synthesis of IrCh / RuCh Amounts of Ru and Ir powder to provide the desired Ru / lr ratio in the catalyst were combined, and powdered Na2O2 was added and mixed by shaking until a uniform grey powder was present, the ratio of Ru+lr to Na2O2 being maintained around 1:3 by weight. The mixture was then placed in an alumina crucible with tamping (in the dry box). The crucible was then sealed under dry air in a closed container which was then heated by means of a muffler furnace (temperature approximately 500°C). Ater heating for about one hour, the mixture was allowed to cool. Once cooled, the mixture was dissolved in 500 ml of DI water. The solution was then acidified to pH ~3 with reagent grade nitric acid. A colour change to black was observed and a precipitate was seen to have formed. The precipitate settled overnight then the dispersion was filtered leaving a filtrate that was colourless. The residue was then washed with room temperature water (1000 ml), and dried at 400°C or 150°C. 11.8g of a lumpy black friable product were recovered which was ground and sieved through a series of sieves, the final sieve being US Mesh 200 (retaining sizes above 75 pm). Analysis The XRD pattern of a ruthenium oxide catalyst prepared by the procedure of Example 1 and heated to 150°C is shown in Fig. 1. Broad peaks around 20 = 28°, 35° and 54° indicate a poorly crystalline rutile structure. The line peaks are a reference for crystalline ruthenium oxide. The XRD pattern of a catalyst contain iridium and ruthenium prepared by the procedure of Example 2 and having a 1:1 atomic ratio of iridium to ruthenium, heated to 150°C, is shown in Fig. 2. A broad maxima in its XRD pattern in the region 20 = 25-38° indicates pseudo-amorphous iridium oxide character, with a broad shoulder peak at 28° indicating a poorly crystalline ruthenium oxide rutile character. The line peaks are references for crystalline ruthenium oxide (dark) and iridium oxide (light). The XRD pattern of a catalyst contain iridium and ruthenium prepared by the procedure of Example 2 and having a 7:3 atomic ratio of iridium to ruthenium, heated to 150°C, is shown in Fig. 3. A broad maxima in its XRD pattern in the region 20 = 25-38° indicates pseudo-amorphous iridium oxide character. Fig. 4 provides XRD patterns of catalysts prepared by the procedures of Examples 1 and 2, heated to 400°C. The catalysts are a ruthenium oxide, and oxide catalysts which contain iridium and ruthenium, with a majority ruthenium. All of the XRD patterns have sharp peaks indicating a crystalline rutile structure. The line peaks are references for crystalline ruthenium oxide. Fig. 5 provides linear sweep voltammograms (LSV) for catalysts prepared by the procedures of Examples 1 and 2, and conventional commercially available iridium oxide catalysts. This data demonstrates the higher mass activity of the materials of the present invention, as compared with conventional iridium oxide catalyst materials. Table 1 provides powder conductivity and BET surface area data for catalysts prepared according to the examples and heated to 400°C, and a comparative commercial iridium oxide material. Catalyst BET (m2 / g) Conductivity at 12.7MPa (S / cm) RuOx 33.2659 189.7 lrO.1RuO.90x 44.022 203 lrO.2RuO.80x 63.147 161.5 lrO.3RuO.70x 24.3959 110.8 lrO.4RuO.60x 36.3589 129.9 lrO.5RuO.50x 18.8711 104.5 lrO.6RuO.40x 15.8235 84.18 lrO.7RuO.30x 12.1068 67.1 lrO.8RuO.20x 12.8382 60.49 lrO.9RuO.10x 13.0792 65.11 IrOx commercial material 26 80 Table 1 It is evident that materials prepared according to the invention may have a higher powder conductivity than conventional iridium oxide materials, whist maintaining a relatively low surface area which will facilitate high operational durability. For the materials in which the conductivity is lower, it is evident that the surface area is also much lower thus providing a material which will demonstrate very high operational stability. These beneficial effects are seen in combination with the high mass activity shown in Fig. 5, thus demonstrating the balanced and favourable properties of catalysts of this invention.

Claims

1. A process for preparing a ruthenium-based oxygen evolution reaction catalyst, the process comprising the steps of:(i) combining ruthenium powder and a peroxide salt to produce a powder mixture;(ii) carrying out thermal treatment on the powder mixture;(iii) dissolving the product from (ii) in water to produce a solution;(iv) reducing the pH of the solution from (iii) to effect a precipitation and form a solid and a supernatant;(v) separating the solid from the supernatant; and(vi) drying the solid.

2. A process according to claim 1, wherein the catalyst further comprises iridium such that it is an iridium- and ruthenium-based oxygen evolution reaction catalyst, and step (i) further comprises combining iridium powder.

3. A process according to claim 1 or claim 2, wherein the catalyst is an oxide material, and substantially all of the metal oxide species present are ruthenium, and when applicable iridium, oxide species.

4. A process according to claim 3, wherein the catalyst is a ruthenium oxide.

5. A process according to any preceding claim, wherein the atomic ratio of ruthenium, orwhen applicable ruthenium + iridium, to oxygen, is at least 2:3 and at most 1:4.

6. A process according to any preceding claim, wherein the peroxide salt is sodium peroxide.

7. A process according to any preceding claim, wherein the ruthenium powder, or the total iridium powder plus ruthenium powder, are combined with the peroxide salt at a molar ratio of from 1 : 3 to 1 : 10.

8. A process according to any preceding claim, wherein the surface of any equipment which comes into contact with the powder mixture in step (ii) is a metal oxide or a stainless steel.

9. A process according to claim 8, wherein the surface of any equipment which comes into contact with the powder mixture in step (ii) is a chromium-nickel-molybdenum austenitic stainless steel.

10. A process according to any preceding claim, wherein step (ii) is carried out in a static oven, static furnace, belt furnace or rotary calciner.

11. A process according to any of claims 1 to 9, wherein step (ii) is carried out in a belt oven.

12. A process according to any of claims 1 to 9, wherein step (ii) is carried out in a fluid bed calciner.

13. A process according to any of claims 1 to 9, wherein step (ii) is carried out using a hot plate with the powder mixture held within a suitable container on the hot plate.

14. A process according to any of claims 1 to 9, wherein step (ii) is carried out using a combustion flame with the powder mixture held within a suitable container.

15. A process according to any preceding claim, wherein the total metal content in the solution formed in step (iv) is 10-100 g / L.

16. A process according to any preceding claim, wherein in step (iv) the pH is reduced using an acid that is not HCI.

17. A process according to claim 16, wherein in step (iv) the pH is reduced using nitric acid.

18. A process according to any preceding claim, comprising a further step of calcining the material from step (vi).

19. A process according to any preceding claim, comprising a subsequent step of incorporating the catalyst into a catalyst-coated membrane.

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

21. A metal-based oxygen evolution reaction catalyst obtained or obtainable by a process according to any of claims 1 to 18.

22. A ruthenium-based oxygen evolution reaction catalyst, wherein:the catalyst has a BET surface area of 5 to 80 m2 / g;the content of any individual transition metal, other than ruthenium, and optionally iridium, is 0-600 ppm when measured by inductively coupled plasma mass spectrometry (ICP-MS);the catalyst exhibits one or more of the following features in its XRD pattern: a peak at 20 = 28°, a peak at 20 = 35°, a peak at 20 = 54°, and a broad maxima in its XRD pattern in the region 20 = 25-38°.

23. A catalyst according to claim 22, wherein the catalyst further comprises iridium such that it is an iridium- and ruthenium-based oxygen evolution reaction catalyst.

24. A catalyst according to claim 22 or claim 23, wherein the catalyst is an oxide material, and substantially all of the metal oxide species present are ruthenium, and when applicable iridium, oxide species.

25. A catalyst according to claim 24, wherein the catalyst is a ruthenium oxide.

26. A catalyst according to any of claims 22 to 25, wherein the atomic ratio of ruthenium,or when applicable ruthenium + iridium, to oxygen, is at least 2:3 and at most 1:4.

27. A catalyst according to any of claims 22 to 26, wherein the catalyst has a BET surface area of 5 to 70 m2 / g.

28. A catalyst according to any of claims 22 to 27, wherein the catalyst is free from ruthenium, and optionally iridium, metal.

29. A catalyst coated membrane comprising an oxygen evolution reaction catalyst according to any of claims 22 to 28.

30. A fuel cell or a water electrolyser comprising a catalyst coated membrane according to claim 29.

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