Oxygen evolution reaction catalyst and method for its preparation

Abstract

The present invention provides a process for preparing a catalyst, as well as a catalyst, the catalyst comprising an oxygen evolution reaction electrocatalyst, a hydrogen oxidation reaction electrocatalyst, and a particulate solid support, wherein the oxygen evolution reaction electrocatalyst and the hydrogen oxidation reaction catalyst are both supported on the particulate solid support. In the process, the oxygen evolution reaction electrocatalyst is deposited before the hydrogen oxidation reaction electrocatalyst by using a halide-free metallate as precursor.

Description

[0001] 1 P102300

[0002] Oxygen evolution reaction catalyst and method for its preparation

[0003] Technical field

[0004] The present invention relates to iridium-containing oxygen evolution reaction (OER) catalysts and a method fortheir preparation, in particular so-called “hybrid” catalysts which contain both an OER electrocatalyst and a separate platinum group metal electrocatalyst on a support material.

[0005] Background of the Invention

[0006] 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:

[0007] 2H2O O2+ 4H++ 4e-

[0008] 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 lrO2and / 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 and oxide catalysts containing iridium with other metals, such as tantalum.

[0009] 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, and mixed iridium and, for example, tantalum oxides, can be prepared in various degrees of crystallinity ranging from essentially amorphous to highly crystalline.

[0010] The article “Synthesis and Optimisation of I rO2Electrocatalysts 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 H2lrCle 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 I rO2at 20 ~ 34-35° was seen in all samples. 2 P102300

[0011] 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. Iridium tantalum oxide materials may be prepared by a process of spray drying a solution of metal chloride precursors, for example as disclosed in WO2011 / 021034.

[0012] It is known that there are a number of situations where incorporating an OER catalyst into a fuel cell at either the anode or the cathode can prove beneficial. For example, WO01 / 15247 describes how incorporating an additional or second catalyst composition at the anode for purposes of water electrolysis / oxygen evolution can improve tolerance of a fuel cell to cell voltage reversal. Cell voltage reversal can occur if a cell receives an inadequate supply of fuel (for example, as a result of blockage of fuel access to a part of that cell). If this occurs, reactions other than fuel oxidation may take place at the fuel cell anode, because the potential on the anode is raised to high values, including water electrolysis and oxidation of anode structural components. Oxidation of anode structural components is undesirable as this can result in significant degradation of the anode. By incorporating a catalyst composition at the anode which promotes the oxygen evolution reaction, degradation of the anode can be reduced or avoided, by promotion of water electrolysis over anode structural component oxidation.

[0013] Another example of a situation in which promotion of water electrolysis may be beneficial is for fuel cells where it is not practical or economic to provide purging of hydrogen from the anode gas space with an inert gas such as nitrogen during shut down, or when a cell is restarted after being idle for some time. Both of these situations can result in a mixed composition of hydrogen and air on the anode whilst air is present on the cathode. Under these circumstances an internal cell can exist, as described by Tang et al (Journal of Power Sources 158 (2006) 1306-1312), which leads to high potentials on the cathode. The high potentials can cause carbon to oxidise which is highly damaging to the structure of the catalyst layer where the catalyst layer contains carbon. If the cathode layer is able to support oxygen evolution however, the high potentials can be used to drive water electrolysis rather than carbon corrosion.

[0014] Finally, in regenerative fuel cells, the electrodes are bi-functional and both anode and cathode must support two electrochemical reaction types at different times. When operating as a fuel cell the cathode must reduce oxygen and the anode oxidise hydrogen; when operating as an 3 P102300

[0015] electrolyser the cathode must evolve hydrogen and the anode evolve oxygen. It may therefore be beneficial to incorporate both a traditional hydrogen oxidation reaction (HOR) electrocatalyst and an OER electrocatalyst in the anode of such a fuel cell, because with such an arrangement, the anode can carry out both the HOR and OER effectively.

[0016] 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 electrocatalysts, e.g. by promoting higher activity at lower iridium loadings while maintaining catalyst layer thickness. However, prior art processes for making supported iridium catalyst species typically utilise chlorides of iridium as precursors, such as I rCL, I rCU and H2I rCh. For example, the Example of US patent publication 2022 / 0259750 A1 uses iridium(IV) chloride as a precursor. 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).

[0017] There is a need in the art for improved OER electrocatalysts which are suitable for use in a fuel cell anode layer. Such OER electrocatalysts desirably maintain high activity but allow for low loading of the OER electrocatalysts in the catalyst layer, and can be made using cost effective processes which do not affect the activity of the HOR electrocatalyst also present in the catalyst layer. Reduced halide content of the OER electrocatalyst is also desirable.

[0018] Summary of the Invention

[0019] According to a first aspect, the invention provides a process for preparing a catalyst, the catalyst comprising an oxygen evolution reaction electrocatalyst, a hydrogen oxidation reaction electrocatalyst, and a particulate solid support, wherein the oxygen evolution reaction electrocatalyst and the hydrogen oxidation reaction electrocatalyst are both supported on the particulate solid support, and wherein;

[0020] the oxygen evolution reaction electrocatalyst is an oxygenated material which comprises iridium and / or ruthenium;

[0021] wherein the process comprises a step A of:

[0022] A(i) forming an aqueous mixture comprising a particulate solid support and a solution of halide-free metallate(s) of the requisite metal(s) for the oxygen evolution reaction electrocatalyst;

[0023] A(ii) reducing the pH of the aqueous mixture to < 7.0 to precipitate an oxygenated component comprising the requisite metal(s) onto the particulate solid support, to provide a supported oxygen evolution reaction electrocatalyst product; and 4 P102300

[0024] A(iii) optionally isolating the product of step A(ii);

[0025] wherein step A is followed by a step B of:

[0026] B(i) depositing a hydrogen oxidation reaction electrocatalyst on the solid support of the product prepared in step A to provide the catalyst;

[0027] Such so called “hybrid” supported catalysts also exist which contain two different metal and / or metal alloy electrocatalysts with different modes of action on the same support material. Examples are disclosed in W02019 / 102197 and WO2017 / 203257. However, in the present invention the oxygen evolution reaction (OER) electrocatalyst is an oxygenated material and there is a specific advantage that the OER electrocatalyst can be deposited on the support before the hydrogen oxidation reaction (HOR) electrocatalyst in a manner which allows tailoring of the properties of the OER electrocatalyst. For example by tuning the crystallinity through heating, without negatively affecting the support material and / or the HOR electrocatalyst. Moreover, the obtained catalyst may permit thrifting of platinum group metal in a fuel cell anode catalyst layer. For example, the OER electrocatalyst will block degradation sites on support materials such as carbon, and so less HOR electrocatalyst can be used because no extra HOR electrocatalyst is required to block said sites. Also, the obtained catalyst and may facilitate more effective preparation of such fuel cell anode catalyst layers due to not having to formulate an ink containing a separate iridium and / or ruthenium containing OER electrocatalyst in addition to a supported HOR electrocatalyst.

[0028] Moreover, OER electrocatalysts prepared according to the process of the first aspect of the invention contain substantially less chloride than equivalent electrocatalysts prepared using metal chlorides such as IrC (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 OER electrocatalysts prepared using metal chloride, for example, chloride iridate precursors and this chloride remains leachable when in use. Elevated levels of chloride in supported OER electrocatalysts 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.

[0029] According to a second aspect, there is provided a catalyst, the catalyst comprising an oxygen evolution reaction electrocatalyst, a hydrogen oxidation reaction electrocatalyst, and a particulate solid support, wherein the oxygen evolution reaction electrocatalyst and the hydrogen oxidation reaction electrocatalyst are both supported on the particulate solid support, wherein the catalyst as a whole has a halide content of less than 500 ppm as determined by ICP-OES, wherein the oxygen evolution reaction electrocatalyst is an 5 P102300

[0030] oxygenated material which comprises iridium and / or ruthenium. Preferably, the halide content is less than 400 ppm, less than 300 ppm, or less than 250 ppm.

[0031] According to a third aspect, applicant provides a catalyst obtained or obtainable by the process according to the first aspect of the invention.

[0032] The 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)) and / or ruthenium metal (Ru(0)) to the limits of detection using X-Ray Diffraction (XRD).

[0033] In a fourth aspect, the invention provides a catalyst layer comprising a catalyst according to the second or the third aspect of the invention.

[0034] In a fifth aspect, the invention provides a catalyst coated membrane comprising the catalyst layer according to the fourth aspect.

[0035] According to a sixth aspect, the invention provides a fuel cell or a water electrolyser comprising a catalyst coated membrane according to the fifth invention aspect.

[0036] Brief Description of the Figures

[0037] Figure 1A is a schematic diagram of a catalyst material of the invention.

[0038] Figure 1B is a schematic of a different catalyst material with a contrasting arrangement of electrocatalyst and support particles.

[0039] Figure 2 is an x-ray diffraction pattern for a catalyst of the invention, overlaid with known characteristic peaks of platinum, iridium oxide and graphite.

[0040] Detailed Description of the Invention

[0041] The OER electrocatalyst may comprise an additional metal M. Metal M may be selected from the transition metals and tin. Metal M is preferably selected from the group of group IVB, VB and VI B metals, Mn, Ni and Sn, more preferably selected from the group of Ti, Zr, Hf, Mn, Ni, Nb, Ta and Sn, more preferably selected from the group of Ti, Ta and Sn, even more preferably metal M is Ta. When M is present, the OER electrocatalyst is an oxygenated iridium and metal M material, an oxygenated ruthenium and metal M material, or an oxygenated iridium, ruthenium and metal M material, preferably an oxygenated iridium and metal M material. Preferably, the OER electrocatalyst may only comprise an additional metal M when it comprises iridium and not ruthenium.

[0042] The term “requisite metal(s)” means the metals required to make the OER electrocatalyst. For example, if the OER electrocatalyst is an oxygenated material comprising iridium, the requisite metal is iridium and if the OER electrocatalyst is an oxygenated material 6 P102300

[0043] comprising ruthenium, the requisite metal is ruthenium. If the OER electrocatalyst contains iridium and ruthenium then the requisite metals are iridium and ruthenium and if the OER electrocatalyst comprises an additional metal M, then the requisite metals include the metal M.

[0044] Typically, the OER electrocatalyst comprises iridium and / or ruthenium and does not comprise an additional metal M. Accordingly, the requisite metal(s) are typically iridium and / or ruthenium and the OER electrocatalyst is an oxygenated iridium material, an oxygenated ruthenium material, or an oxygenated iridium and ruthenium material, preferably an oxygenated iridium material or an oxygenated ruthenium material. For avoidance of doubt, by oxygenated iridium material, oxygenated ruthenium material and oxygenated iridium and ruthenium material, we mean that substantially all of the metal in the oxygenated material is iridium, ruthenium, or iridium and ruthenium respectively. That is to say, greater than 95 wt% of the metal, preferably greater than 99wt% of the metal is iridium, ruthenium, or iridium and ruthenium respectively.

[0045] According to a preferred embodiment of the first aspect of the invention, the halide-free metallate(s) used in step A(i) is / are obtained by the steps of: (a) combining powder(s) of the requisite metals(s) and a peroxide salt to produce a powder mixture; and (b) carrying out thermal treatment on the powder mixture. Such metal powders and peroxide salts do not typically contain halide and so may be referred to as halide-free. A benefit of this method is that metal powder is used as a raw material rather than metal salts which are used in many previously described syntheses of OER electrocatalysts. Metal powder, in particular in the case of iridium and ruthenium, is generally less expensive than metal salts and can enable a product to be formed with very low chloride content.

[0046] The role of the peroxide salt is to oxidize the metal 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.

[0047] The molar ratio of total metal(s)-to-peroxide salt is preferably 1:3 or more. Applicant found that molar ratios of 1:less than 3, conversion is 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 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 total metal(s)-to-peroxide salt is 1:3 to 1:10, preferably of 1:3 to 1:8.

[0048] 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. 7 P102300

[0049] In this reference, iridium powder and sodium peroxide are combined at a molar ratio of lr:Na2C>2 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 electrocatalyst.

[0050] 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.

[0051] 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.

[0052] 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 metal. 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.

[0053] The powder mixture is heated at a temperature and duration suitable to achieve the desired conversion of 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.

[0054] According to the preferred embodiment, step A(i) comprises the sub-steps of: A(i)(a) dissolving the powder(s) of the requisite metal(s) in water to produce a solution; A(i)(b) adding the particulate solid support to the solution.

[0055] In step A(ii), the pH may preferably be reduced to < 5, for example < 4. The product of step A(ii) of the first aspect of the invention is preferably isolated in a step A(iii), e.g. the dispersion is filtered and a “wet-cake” is produced. The isolated product of step A(iii) can be washed to remove reagent liquor.

[0056] The process of the first aspect of the invention can include a step A(iv) of drying the product of step A(ii) isolated in step A(iii) to remove water therefrom, e.g. to create a dry 8 P102300

[0057] powder. The resulting dried product is then used in step B(i) to deposit the HOR electrocatalyst on the solid support. A drying step can be conducted at 50 to 120°C, for example.

[0058] The process according to the first aspect of the invention preferably comprises a step of heat-treating the product of step A(ii) or step (iii), or step A(iv). Where heat treatment is performed on the product of step A(ii) or A(iii), the heat treatment step performs the dual function of both removing excess water from the product and heat treating the product.

[0059] Without wishing to be bound by theory, Applicant’s initial analytical testing suggests that a supported amorphous oxygenated metal 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.

[0060] 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 metal species is required to compensate for the moderately lower “fresh” electrocatalytic activity performance of the partly crystalline material vs. more amorphous catalyst. In an embodiment of the present invention the temperature range of thermal treatment is greater than 300°C, suitably greater than 325°C. Typically the thermal treatment will be under 450°C, especially when a carbon support is used, because higher temperatures may degrade the support.

[0061] It is a particular advantage of the present invention that a hybrid catalyst can be prepared in which the level of crystallinity of the OER electrocatalyst is tailored by control of the heating step, without the heating step having any detrimental effects on the particulate support or the HOR electrocatalyst. Therefore, the benefits of a hybrid catalyst can be realised (e.g. thrifting of iridium in a fuel cell anode catalyst layer and facilitation of more effective preparation of catalyst layers) without trade-off for overall activity of the catalyst. US2020 / 00411881 describes platinum and an iridium oxide material on a catalyst support. The process in US2020 / 00411881 is different in that it involves a hydrolysis of iridium chloride in ethylene glycol to deposit iridium onto a pre-formed Pt / C catalyst. Chloride containing iridium precursor salts are used in the process, along with a chloride containing acid. The process is also different in US2020 / 00411881 because the material which contains both the platinum and iridium is then heated to oxidise the iridium onto an iridium oxide. Accordingly, catalysts of the present invention will not only have a lower chloride content, but the HOR electrocatalyst will have different structural properties due to not being subjected to any 9 P102300

[0062] heating steps required to form the OER electrocatalyst, for example it will retain a higher metal surface area. Also, the HOR electrocatalyst will stay where it is deposited and not migrate towards the OER electrocatalyst during the heat treatment step. Moreover, the OER electrocatalyst will have different structural properties due to it being prepared in an entirely different way. Additionally, the authors of US2020 / 00411881 demonstrate it is advantageous to anneal the material in air (increases voltage reversal time), and they achieve the best performance at 300°C, with significant loss in performance at higher temperature, which may be due to oxidation of the carbon support. In the present invention it is possible, for example, to anneal to at least 325°C (which a customer may desire in order to raise the crystallinity of the OER electrocatalyst) without oxidising the support, in part due to the initial deposition of the OER electrocatalyst with any associated heat treatment being carried out prior to deposition of the HOR electrocatalyst. Accordingly, there is no requirement to anneal the HOR electrocatalyst nanoparticles in air, which is likely to oxidise and potentially reduce metal area / activity.

[0063] In this regard, to achieve selected levels of electrocatalytic activity in the end product, the total metal content in the aqueous mixture formed in step A (ii) is 0.1 to 10 gir / L, preferably 0.1 to 5 gir / L or most preferably 0.1 to 2.5 gir / L.

[0064] The HOR electrocatalyst may be deposited on the solid support in step B(i) by methods known to those skilled in the art, for example as disclosed in WO2013 / 045894, WO2014 / 184546 and WO2014 / 122428. This includes wet-chemical routes in which a salt of the HOR electrocatalyst is dispersed with the product of step A in water followed by hydrolysis of the salt and selective reduction to deposit the HOR electrocatalyst on the support. Halide-free salts may be used in order to provide electrocatalysts with the preferred low halide content. In the present invention, the HOR electrocatalyst is deposited onto the particulate solid support, not the OER electrocatalyst. For example, greater than 95wt% of the HOR electrocatalyst, by total weight of the HOR electrocatalyst in the catalyst, is supported on the support, preferably greater than 99wt%. The product of step A may then be re-diluted as necessary in an appropriate solvent for the addition of the HOR electrocatalyst.

[0065] The HOR electrocatalyst comprises metal particles. The metal in the metal particles is typically a platinum group metal or an alloy of a platinum group metal. Preferred platinum group metals are platinum, palladium and ruthenium, which may be alloyed with other noble metals or base metals. A preferred electrocatalyst metal is platinum, which may be alloyed with other noble metals or base metals. Preferably, the HOR electrocatalyst is platinum, i.e. it is not an alloy. A base metal is tin or a transition metal which is not a noble metal. A noble metal is a platinum group metal (platinum, palladium, rhodium, ruthenium, iridium or osmium), 10 P102300

[0066] silver or gold. Suitable base metals include copper, cobalt, nickel, zinc, iron, titanium, molybdenum, vanadium, manganese, niobium, tantalum, chromium and tin. Preferred base metals are nickel, copper, cobalt, and chromium. More preferred base metals are nickel, cobalt and copper.

[0067] The total platinum group metal loading (for example total iridium in the OER electrocatalyst plus platinum in the HOR electrocatalyst) in the end product of the process of the first aspect of the invention, and in the catalyst of the second and third aspects of the invention, can be defined by a weight percentage of platinum group metal by the total weight of the catalyst including the support. The weight percent of total platinum group metal can be 10 to 70 wt%, for example 10 to 60 wt% by total weight of the catalyst. The weight ratio of platinum group metal in the OER electrocatalyst to platinum group metal in the HOR electrocatalyst (for example iridium in the OER electrocatalyst to platinum in the HOR electrocatalyst) can be, for example, in the range of and including 1:1 to 1:40, typically 1:5 to 1:40.

[0068] Depending upon application of the catalyst end product, the particulate solid support can be a transition metal or tin or aluminium oxide; or carbon. By solid, we take the conventional meaning of a particulate solid support which is that the particles are not a nanoframe or hollow nanostructure that comprises networks of pores which interconnect throughout the particle. However, there may be some porosity in the support. For example the support particles may have a solid core with a porous structure towards the surface. Where the particulate solid support is a transition metal oxide, the transition metal oxide may be selected from optionally doped TiC>2, optionally doped ZrC>2, CeO2 / ZrO2 mixed oxide, Ta2Os, Nb2C>5, AI2O3, MnC>2, SnC>2, SnC>2 optionally doped with antimony or fluorine and mixtures of any two or more thereof. The application environment of a OER electrocatalyst 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 transitions metal support materials are optionally doped TiO2, Nb2Os, optionally doped ZrO2, and SnO2, most preferably optionally doped TiO2. Optional dopants for TiO2 include silica and / or tungsten.

[0069] The particulate solid support may be a carbon support, in particular an electrically conductive carbon support. Suitably, the electrically conductive carbon support is a carbon powder which may be, for example, a carbon black or graphitised carbon black for example a commercially available carbon black (such as from Cabot Corp. (Vulcan® XC72R) or Akzo Nobel (the Ketjen® black series)). Another suitable carbon support is an acetylene black (e.g. 11 P102300

[0070] those available from Chevron Phillips (Shawinigan Black®) or Denka). The electrically conductive carbon support can be prepared by the method disclosed in WO2013 / 045894.

[0071] The OER electrocatalyst and the HOR electrocatalyst are both supported on the particulate solid support. In particular, the particulate solid support comprises a plurality of individual support particles or aggregates wherein each individual support particle or aggregate has supported thereon (i) OER electrocatalyst particles and (ii) HOR electrocatalyst particles. By the phrase ‘individual support particle or aggregate’, we mean the conventional interpretation in the art, which is the smallest single moiety which is unable to be broken down into two or more smaller moieties. The term ‘aggregate’ is used to refer to primary particles that have associated into a cluster composed of two or more primary particles, and which are permanently bound to each other; the total specific surface area of the aggregate is less than the sum of the surface areas of the primary particles before they were aggregated. Two or more individual support particles or aggregates can combine to provide an agglomerate. Agglomerates comprise loosely held individual support particles or aggregates held together by weak forces and can be readily broken down into the individual support particles or aggregates under imposition of low energy agitation. Whether or not a particular solid support forms aggregates and / or agglomerates will depend on the nature of the particular material used.

[0072] Fig. 1A is a schematic diagram showing both OER electrocatalyst and HOR electrocatalyst particles being supported on each individual support particle or aggregate of the support material. Thus, each individual support particle or aggregate of the support material has two kinds of particle dispersed thereon: OER electrocatalyst and HOR electrocatalyst particles. Fig. 1B shows a contrasting arrangement, different from the invention, in which each individual support particle or aggregate has one kind of particle dispersed thereon, either OER electrocatalyst or HOR electrocatalyst. Such a contrasting catalyst may be prepared, for example, by simply mixing two different catalyst powders.

[0073] It will be appreciated that methods of halide determination include ion chromatography. Although the reagents selected for steps A(i) and A(ii) can be halide-free, so that the precipitated oxygenated metal species are also halide-free, the particulate support may use a halide-containing compound in its manufacture. For example, a 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 metal species, preferred supports have a lower chloride content so that the upper limit of chloride content detected via ICP-OES for 12 P102300

[0074] 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).

[0075] In this connection, to avoid the introduction of halide into the supported oxygenated species, the pH in step A(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 A(ii) is reduced using nitric acid.

[0076] 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), for example in the form of a printable ink. Suitable liquid media that act as ink-base for supported catalysts and can be used for the production of a catalyst layer for a PEM fuel cell are known to the person skilled in the art. For example, the ink-base medium can contain an ion-conducting polymer, e.g., an ion-conducting polymer that contains monomers containing sulfonic acid groups, e.g. a perfluorosulphonic acid (PFSA) or a hydrocarbon ion-conducting polymer, and one or more short-chain alcohols (e.g., methanol, ethanol or n-propanol or a mixture of at least two of these alcohols).

[0077] 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. Preferably, the catalyst layer is an anode catalyst layer of a PEM fuel cell, and the application in a CCM is as the anode of a PEM fuel cell CCM.

[0078] 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 PEM fuel cell comprises the CCM.

[0079] 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.

[0080] By “halide-free metallate” herein we mean a compound comprising a metal-containing oxyanion and counter ion(s) of a different metal which is without intentionally added halide. For example, a halide-free metal iridate is a compound comprising an iridium-containing oxyanion and metal counter ion(s) which is without intentionally added halide. Preferably, the halide-free metallate is sodium metallate, an example being sodium iridate. Preferably, the halide-free metallate has a halide content of less than 100 ppm or, more preferably, less than 50 ppm or less than 10 ppm. 13 P102300

[0081] The term “powder” used in connection with metal powder is intended to encompass both spherical powders and also irregular powders such as iridium sponge.

[0082] The term “oxygenated metal species” herein is intended to embrace a range of oxidic compositions, both amorphous and crystalline, including without limitation, metal oxides and metal oxyhydroxides, i.e. metal compounds having both oxo (e.g. lr=O) and hydroxo (e.g. Ir-OH) functionalities. In the case of iridium, such species 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. In the case of iridium oxide, the OER electrocatalyst 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. 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. The same breadth of oxide species may be present, as possible, for ruthenium and the additional metal M when present, and it will be equally evident that it is not possible for Applicant to define the species in any more specificity without unfairly restricting Applicant’s claim scope.

[0083] The term “supported” herein is intended to mean that particles of the electrocatalyst are dispersed on (and / or in the pores of) a support material and bound or fixed to the support material by physical or chemical bonds. For instance, the electrocatalyst may be bound or fixed to the support material by way of ionic or covalent bonds, or non-specific interactions such as van der Waals forces.

[0084] Examples

[0085] In order that the invention may be more fully understood, the following Examples are provided by way of illustration only.

[0086] Example 1 to 4 prepare non-hybrid supported oxygenated iridium OER electrocatalysts, along with directly comparable catalysts, and are used to show the preparation of and benefit of such catalysts per se, which will carry over to their use in hybridtype catalysts of the present invention.

[0087] Example 1 - Non-hybrid oxygenated iridium OER electrocatalyst

[0088] 10g of iridium Sponge was mixed with 30g of Na2O2 beads using a Resonance 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 14 P102300

[0089] mixer. The TiC>2 slurry was stirred into the iridate solution. The combined mixture was stirred for a further for 30 minutes.

[0090] 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.

[0091] Example 2 - Non-hybrid oxygenated iridium PER electrocatalyst

[0092] 10.42 g of solid particulate Aeroxide® P25 TiO2 (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.

[0093] A Na2O2 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.

[0094] 40 g of the Na2O2 iridium fusion product prepared according to 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.

[0095] Example 3 (Corresponding to the method of Example 1 in US patent publication no. US 2022 / 0259750 A1)

[0096] 10g of lrCl4.H2O (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 IrCU 15 P102300

[0097] 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.

[0098] Example 4 (Comparative)

[0099] An unsupported, i.e. no TiC>2, electrochemical grade iridium oxide purchased from J&J Materials Inc. was used as a comparison catalyst.

[0100] Example 5 - Details of product characterisation

[0101] The products of Examples 2, 3 and 4 were characterised by various methods, as follows:

[0102] Iridium and Titanium assay

[0103] 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.

[0104]

[0105] analysis / spectroscopy-elemental-isotope-analysis-learning-center / trace-elementai-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)).

[0106] Chloride assay

[0107] Sample elemental chlorine content (ppm) was assayed by the known technique of liquid ion chromatography (see e.g. The

[0108]

[0109] 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. 16 P102300

[0110] Button Cell Testing

[0111] 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.

[0112] The prepared ink was spray-coated onto a Toray paper (hydrophobic gas diffusion layer 60) at 0.2 mg cm-2loading and verified using X-ray fluorescence (XRF) measurements. The buttons were then soaked overnight in an equivalent solution as the test solution (1M H2SO4) under a vacuum to allow ingress of solution into the gas diffusion layer, ensuring all of the catalyst is in contact with electrolyte.

[0113] 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.

[0114] 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 “1stactivity 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.

[0115] The mass electrocatalytic activity (jmass) results are reported at 1.47 Volts (Amperes per gram I rOx) .

[0116] Example 6 - Test Results

[0117] The test results for the characterisation methods of Example 5 are set out in the following Table 1. 17 P102300

[0118] Table 1

[0119] "

[0120]

[0121] 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).

[0122] * 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.tW and TiO2 used, whereas the chloride present in the Example 2 sample is contributed entirely by the TiO2.

[0123] It can be deduced from the results in Table 1 that the product of the process used in the invention have a lower chloride content and a lower iridium dissolution in comparison with the product from the process of the prior art.

[0124] 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. 18 P102300

[0125] Example 5 - Hybrid catalysts according to the invention

[0126] 5g of iridium sponge was mixed with 15g of Na2<D2 beads using a resonance 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.

[0127] 20 g of fusion product (5 g Ir) was dispersed in 250 mL H2O and stirred for 30 minutes.

[0128] 7.3 g of carbon support was dispersed in 722 mL water using a Silverson mixer. The iridium dispersion was added to the carbon dispersion over 1 hour with stirring. 1M nitric acid was used to lower the pH of the mixture over 1 hour to pH 3.25, and the product was aged for further hour following acid addition. The product was filtered and washed to achieve filtrate conductivity of 20 iS before drying in an oven at 105°C for 16 hours. The IrOx / C catalyst was then annealed in a rotary tube furnace under flow of air at 325°C for 2 hours.

[0129] Platinum was then deposited on the material by a wet chemical deposition route as disclosed in WO2014 / 122428. Pt can then be subsequently reduced under flow of 5% H2 / N2 at 50°C.

[0130] Fig. 2 shows an x-ray diffraction pattern for a catalyst material prepared in accordance with this method. The pattern is overlaid with known characteristic peaks of platinum, iridium oxide and graphite. It is evident from the pattern that discrete platinum and iridium oxide materials exist with the carbon support. There are no characteristic iridium metal reflections present, which is evidence that the present invention allows for preparation of a hybrid catalyst without undesired reduction of the formed iridium oxide during the platinum deposition step.

[0131] XRD analysis was carried out using a Bruker D8 Advance Davinci diffractometer with a Lynxeye-XE PSD detector with 0.0125 mm 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 V6 (2010-2024) software and the PDF-4+ database (2024 release). The literature reflections used for overlaying with the diffraction pattern are: Pt, PDF 00-004-0802; lrO2, PDF 00-015-0870; C, Graphite-3R, PDF 01-073-5918. The literature reflections for iridium metal which are not present in Fig. 2 are literature reference PDF 00-006-0598 and include 2040.662, 47.313, 69.142, 83.445, 88.066, 127.568 and 158.685.

Claims

19 P102300Claims:

1. A process for preparing a catalyst, the catalyst comprising an oxygen evolution reaction electrocatalyst, a hydrogen oxidation reaction electrocatalyst, and a particulate solid support, wherein the oxygen evolution reaction electrocatalyst and the hydrogen oxidation reaction electrocatalyst are both supported on the particulate solid support, wherein;the oxygen evolution reaction electrocatalyst is an oxygenated material which comprises iridium and / or ruthenium;wherein the process comprises a step A of:A(i) forming an aqueous mixture comprising a particulate solid support and a solution of halide-free metallate(s) of the requisite metal(s) for the oxygen evolution reaction catalyst;A(ii) reducing the pH of the aqueous mixture to < 7.0 to precipitate an oxygenated component comprising the requisite metal(s) onto the particulate solid support, to provide a supported oxygen evolution reaction electrocatalyst product; andA(iii) optionally isolating the product of step A(ii);wherein step A is followed by a step B of:B(i) depositing a hydrogen oxidation reaction electrocatalyst on the solid support of the product prepared in step A to provide the catalyst;2. A process according to claim 1, wherein the halide-free metallate(s) used in step A(i) is / are obtained by the steps of: (a) combining powder(s) of the requisite metal(s) and a 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 A(i) comprises the sub-steps of:(i) (a) dissolving halide-free metallate(s) of the requisite metal(s) in water to produce a solution; and(i) (b) adding the particulate solid support to the solution.

4. A process according to any preceding claim, wherein the product of step A(ii) is isolated in step A(iii).

5. A process according to claim 4, comprising a step A(iv) of drying the product of step A(ii) isolated in step A(iii) to remove water therefrom.20 P1023006. A process according to any preceding claim, comprising a step of heat-treating the product of step A(ii), A(iii) or step A(iv).

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

8. A process according to claim 2 and any of claims 3 to 7 when dependent on claim 2, wherein the peroxide salt is sodium peroxide.

9. A process according to claim 2 or any of claims 3 to 8 when dependent on claim 2, wherein the powder(s) of the requisite metal(s) and peroxide salt are combined at a molar ratio of from 1:4 to 1:10.

10. A process according to any of the preceding claims, wherein a total metal content in the aqueous mixture formed in step A(i) is 1-22 girI L.

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

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

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

14. A process according to any 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 of claims 1 to 12, wherein the particulate solid support is a carbon support.

16. A process according to any of preceding claim, comprising a subsequent step of incorporating the catalyst into a catalyst layer, preferably an anode catalyst layer.

17. A process according to claim 16, comprising a subsequent step of incorporating the catalyst layer into a catalyst-coated membrane.

18. A process according to claim 17, comprising a subsequent step of incorporating the catalyst-coated membrane into a fuel cell.

19. A catalyst obtained or obtainable by a process according to any of claims 1 to 15.21 P10230020. A catalyst according to claim 19, wherein the 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.

21. A catalyst, the catalyst comprising an oxygen evolution reaction electrocatalyst, a hydrogen oxidation reaction electrocatalyst, and a particulate solid support, wherein the oxygen evolution reaction electrocatalyst and the hydrogen oxidation reaction electrocatalyst are both supported on the particulate solid support, wherein the catalyst as a whole has a halide content of less than 500 ppm as determined by ICP-OES, wherein the oxygen evolution reaction electrocatalyst is an oxygenated material which comprises iridium and / or ruthenium.

22. A catalyst layer comprising the catalyst according to claim 19, 20 or 21, preferably an anode catalyst layer.

23. A catalyst coated membrane comprising the catalyst layer according to claim 22.

24. A fuel cell comprising a catalyst coated membrane according to claim 23.

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