EP4698696A1

EP4698696A1 - Catalyst materials

Info

Publication number: EP4698696A1
Application number: EP24722070.0A
Authority: EP
Inventors: Shahul Hameed ABDULRAHMAN; David Thompsett; Jasmine CLAYTON; Richard Walton
Original assignee: Johnson Matthey Hydrogen Technologies Ltd
Current assignee: Johnson Matthey Hydrogen Technologies Ltd
Priority date: 2023-04-17
Filing date: 2024-04-17
Publication date: 2026-02-25
Also published as: GB202305611D0; WO2024218486A1

Abstract

Oxygen evolution catalyst materials are provided with a pyrochlore-type structure and with (i) calcium and / or sodium as A-site elements of the pyrochlore-type structure; (ii) iridium and / or ruthenium as first B-site elements of the pyrochlore-type structure; (iii) niobium and / or tantalum as second B-site elements of the pyrochlore-type structure; and (iv) a molar ratio of A-site elements: first and second B-site elements is in the range of and including 0.8: 1 to 1:1.

Description

CATALYST MATERIALS

Field of the Invention

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

Background of the Invention

The electrolysis of water to produce high purity hydrogen and oxygen can be carried out in both alkaline and acidic systems. Those electrolysers that employ a solid proton-conducting polymer electrolyte membrane, or proton exchange membrane (PEM), are known as proton exchange membrane water electrolysers (PEMWEs). Those electrolysers that utilise a solid anion-conducting polymer electrolyte membrane, or anion exchange membrane (AEM), are known as anion exchange membrane water electrolysers (AEMWEs).

Ion-conducting membranes, such as PEMs and AEMs, are also used in fuel cells. In a proton exchange membrane fuel cell (PEM FC) the membrane is proton conducting, and protons, produced at the anode, are transported across the membrane to the cathode, where they combine with oxygen to form water.

Catalyst coated membranes (CCMs) may be employed within electrochemical devices, such as electrolysers and fuel cells. Such CCMs comprise an ion-conducting membrane, such as a PEM or AEM, with an anode catalyst layer and / or a cathode catalyst layer applied to a face of the membrane, the anode catalyst layer and cathode catalyst layer being applied to opposite faces.

For water electrolyser applications, hydrogen evolution reaction (HER) catalysts are used in such cathode catalyst layers, for example HER catalysts comprising platinum, such as platinum on a carbon support. Oxygen evolution reaction (OER) catalysts are utilised in electrolyser anode catalyst layers. Catalysts containing iridium and I or ruthenium are well known for their properties as excellent OER catalysts and are preferred materials for the oxygen evolution reaction on the anode side of a water electrolyser.

For fuel cell applications, oxygen reduction reaction (ORR) catalysts are used in cathode catalyst layers and hydrogen oxidation reaction (HOR) catalysts are utilised in anode catalyst layers. For PEM FC applications, suitable cathode and anode catalyst materials comprise a platinum group metal (PGM) or an alloy of a PGM with one or more other metals, for example platinum or an alloy of platinum with one or more other metals. Iridium- and I or ruthenium-containing OER catalysts may also be incorporated into fuel cell anodes and I or cathodes to improve cell reversal tolerance and stability during start-up I shut-down cycling.

Materials with a pyrochlore-type structure are isostructural to the mineral pyrochlore and typically have a cubic structured phase and space group Fd-3m. Such materials have A- site cations, B-site cations and oxide anions. In some cases, hydroxyl anions and I or water molecules are known to occupy sites in the pyrochlore-type structure. It may also be the case that some of the A-sites are vacant.

Iridium and I or ruthenium-containing materials with a pyrochlore-type structure are promising candidates as OER catalysts for use in electrolyser and fuel cell applications. For example, it is described in Walton R.l. et al Structural variety in iridate oxides and hydroxides from hydrothermal synthesis, Chem. Sci., 2011 , 2, 1573 that iridates of formula (Na,Ca)2lr2Oe xH2O with a pyrochlore-type structure may be formed via a hydrothermal process.

Such pyrochlore materials offer high OER catalytic activity but can suffer from loss of iridium or ruthenium from the structure, for example through dissolution during electrochemical cycling.

The demand for hydrogen-based solutions is expected to increase dramatically in response to net zero policies. This provides a challenge for technologies that exploit iridium catalysts due to the relative scarcity of this element. More effective catalysts are therefore also required which provide more activity per unit mass of iridium.

There is a need to develop iridium and ruthenium-based OER catalysts with improved performance, such as with greater stability and / or an increased catalytic activity with lower loadings of iridium metal.

Summary of the invention

The present inventors have surprisingly found that the incorporation of selected elements into iridium and / or ruthenium pyrochlore materials provides oxygen evolution catalysts which exhibit higher activity than a conventional iridium oxide electrocatalysts, and which offer improved stability.

Therefore, in a first aspect of the invention there is provided an oxygen evolution catalyst material with a pyrochlore-type structure and with:

(i) calcium and / or sodium as A-site elements of the pyrochlore-type structure; (ii) iridium and I or ruthenium as first B-site elements of the pyrochlore-type structure;

(iii) niobium and I or tantalum as second B-site elements of the pyrochlore-type structure; and

(iv) a molar ratio of A-site elements: first and second B-site elements in the range of and including 0.8: 1 to 1 : 1.

Suitably, such materials have a composition represented by formula: (AA’)_a(BB’)2ObX_c in which A is Ca, A’ is Na, B is Ir and I or Ru, B’ is Nb and I or Ta, X is O, OH, H2O or combinations thereof, 1 .6 < a < 2.0, 5 < b < 7, and 0 < c < 2.

In a second aspect of the invention there is provided an oxygen evolution catalyst material having a composition according to Formula (1):

(Cai.xNa_x)2(lri.yB’y)2O7-z.(H2O)n

Formula (1) wherein 0 < x < 0.5, 0 < y < 0.6, 0 < z < 1 , 0 < n < 2, and M is Nb, Ta, or combinations thereof.

The oxygen evolution catalyst materials may be advantageously formulated with an ionconducting polymer to form an ink. Therefore, in a third aspect of the invention there is provided an ink comprising an oxygen evolution catalyst material according to the first or the second aspects, and an ion-conducting polymer.

Such inks may be suitably applied to a substrate to produce a catalyst layer. Therefore, in a fourth aspect of the invention there is provided a catalyst layer comprising an oxygen evolution catalyst material according to the first or the second aspects and an ionconducting polymer. Suitably, the catalyst layer is an anode catalyst layer for a water electrolyser. The oxygen evolution catalyst material may also be incorporated into a catalyst layer for a fuel cell, particularly in a fuel cell anode catalyst layer in combination with a hydrogen oxidation reaction (HOR) catalyst, such as a platinum catalyst, to improve cell reversal tolerance.

Such catalyst materials and catalyst layers may be used in the formation of catalyst-coated membranes. Therefore, in a fifth aspect of the invention there is provided a catalyst-coated membrane comprising an oxygen evolution catalyst material according to the first or the second aspects, or a catalyst layer according to the fourth aspect. Suitably, the catalyst- coated membrane comprises a proton exchange membrane (PEM) or an anion exchange membrane (AEM). The oxygen evolution catalyst material is preferably provided in an anode catalyst layer applied to a face of the membrane. Such CCMs are suitable for use in a water electrolyser or a fuel cell.

The oxygen evolution catalyst materials of the first or the second aspects may be advantageously produced via a hydrothermal process. Therefore, in a sixth aspect of the invention there is provided a method of manufacturing an oxygen evolution catalyst material according to the first aspect or the second aspect, the method comprising the steps of:

(i) providing an aqueous mixture comprising at least one source of iridium and I or iridium, at least one source of calcium and I or sodium, at least one source of niobium and I or tantalum, and a base;

(ii) treating the aqueous mixture under hydrothermal conditions;

(iii) isolating the oxygen evolution catalyst material.

Description of the Figures

Figure 1 shows the results of electrochemical testing of the catalyst material formed in Example 1 and comparative materials.

Figure 2 shows the results of testing for iridium dissolution during electrochemical testing.

Detailed Description

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

The present invention provides oxygen evolution reaction (OER) catalyst materials with a pyrochlore-type structure and with (i) calcium and I or sodium as A-site elements of the pyrochlore-type structure; (ii) iridium and I or ruthenium as first B-site elements of the pyrochlore-type structure; (iii) niobium and I or tantalum as second B-site elements of the pyrochlore-type structure; and a molar ratio of A-site elements: first and second B-site elements in the range of and including 0.8: 1 to 1 : 1. Typically, the catalyst materials have a crystalline structure with space group Fd-3m.

Suitably, such materials may be represented by general formula (AA’)_a(BB’)2ObX_c in which A is Ca, A’ is Na, B is Ir and I or Ru, B’ is Nb and / or Ta, X is O, OH, H2O or combination thereof, 1 .6 < a < 2.0, 5 < b < 7, and 0 < c < 2. In such materials, one or more A-sites may be left vacant, reducing the stoichiometry of the A-site elements in the crystal structure. Furthermore, in some instances, water molecules may occupy some vacant sites to provide a hydrated or partially-hydrated metal oxide material.

It may be preferred that the molar ratio of A: A’ is in the range of and including 2: 0 to 1 : 1 . It may be preferred that the A-site cation is Ca. It may be preferred that 1.7 < a < 2.0, 1 .8 < a < 2.0, 1.9 < a < 2.0, or that a is 2.

It may be preferred that the molar ratio of B: B’ is in the range of and including 0.95: 0.05 to 0.40: 0.60, such as 0.9: 0.1 to 0.5: 0.5. It may be preferred that B is Ir, or a mixture of Ir and Ru. It may be preferred that B’ is Nb or a mixture of Nb and Ta.

It may be preferred that 6 < b < 7. It may be preferred that 0 < c < 1.8, 0 < c < 1.6, 0 < c < 1 .4, 0 < c < 1 .2, or 0 < c < 1.0. It may be preferred that X is H2O.

It will be understood by the skilled person that where the calculation of molar ratio involves more than one element, then then ratio is between the sum of the elements involved. For example, it will be understood by the skilled person that the molar ratio of A site elements: first and second B site elements is the ratio of (number of moles of Ca + number of moles of Na): (number of moles of Ir + number of moles of Ru + number of moles of Nb + number of moles of Ta).

It may be preferred that the catalyst material has a composition according to Formula 1 .

In Formula 1 , 0 < x < 0.5. It may be preferred that 0 < x < 0.45, 0 < x < 0.40, 0 < x < 0.35, 0

< x < 0.30, 0 < x < 0.25, 0 < x < 0.20, 0 < x < 0.15, 0 < x < 0.10, 0 < x < 0.05, or that x = 0.

In Formula 1 , 0 < y < 0.6. It may be preferred that 0 < y < 0.5, 0 < y < 0.4, 0 < y < 0.3, 0 < y < 0.2, 0 < y < 0.1 , 0 < y < 0.05. It may be preferred that 0.1 < y < 0.6, 0.1 < y < 0.5, 0.1

< y < 0.4, or that 0.1 < y < 0.3.

In Formula 1 , 0 < n < 2. It may be preferred that 0 < n < 1 .5, 0 < n < 1 .0, 0 < n < 0.5 or that n = 0.

In Formula 1 , B is Ir, Ru, or combinations thereof. It may be preferred that B is Ir, or is Ru. It may be preferred that B is a mixture of Ir and Ru. It may be preferred that B is Ir, or a mixture of Ir and Ru.

In Formula 1 , B’ is Nb, Ta or combinations thereof. It may be preferred that B’ is Nb or is Ta. It may be preferred that the oxygen evolution catalyst material has a general formula (Cai.xNa_x)2(Bi.yi.y2NbyiTay2)2O7-z.(H2O)z in which 0 < y1+y2 < 0.6.

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

It has been found by the inventors that the catalyst materials may be prepared using a hydrothermal method. The method comprises the step of providing an aqueous mixture comprising at least one source of iridium and I or ruthenium, at least one source of calcium and I or sodium, at least one source of niobium and I or tantalum, and a base.

Typically, the at least one source of iridium and I or ruthenium is a salt, such as an inorganic salt. Typically, the at least one source of iridium and I or ruthenium has iridium or ruthenium in a +3 oxidation state, such as an iridium (III) salt and / or a ruthenium (III) salt. The at least one source of iridium and I or ruthenium may be, for example IrC , RuCh, ^IrCh, Ru(NO)(NOs)s, or combinations thereof.

Typically, the at least one source of calcium is a salt, such as an inorganic calcium salt, or an oxide I peroxide, for example CaC>2, Ca(NOs)2, or Ca(OH)2. Typically, the optional at least one source of sodium is a salt, such as an inorganic salt. The at least one source of sodium may be, for example, NaBrCh, NaOH, or Na2<D2.

Typically, the at least one source of niobium and I or tantalum is a salt, such as an inorganic salt, or an oxide I peroxide. Typically, the at least one source of niobium and I or tantalum has the metal in a +5 oxidation state. The at least one source of M may be, for example, Nb2C>5, Ta2<D5, NbCk and I or TaCk.

It will be understood by the skilled person that the sources of elements described herein may be in the form of a hydrate.

The aqueous mixture comprises a base, such as a metal hydroxide, for example sodium hydroxide or potassium hydroxide. Potassium hydroxide may be particularly preferred. It may be preferred that the aqueous mixture has a pH of greater than or equal to 12, greater than or equal to 13, or greater than or equal to 14.

The aqueous mixture may comprise an oxidising agent. The oxidising agent may be the source of one of the elements required in the desired composition, such as the source of iridium, ruthenium, calcium, sodium, niobium or tantalum. For example, the oxidising agent may be calcium peroxide, sodium peroxide, sodium bromate, or combinations thereof. Alternatively, or in addition, the oxidising agent may be a separate reactant, for example hydrogen peroxide.

The aqueous mixture is typically formed in a vessel suitable for hydrothermal treatment, such as an autoclave or other pressure vessel.

The aqueous mixture is then treated under hydrothermal conditions. In the context of the present invention, treating under hydrothermal conditions is understood as treatment of the aqueous mixture at an elevated temperature and a steam pressure of above 1 bar. It may be preferred that aqueous mixture is treated under hydrothermal conditions at a temperature greater than 200 °C, such as in the range of and including 200 to 300 °C, such as 225 to 275 °C. It may be preferred that the aqueous mixture is treated under hydrothermal conditions for a period of at least 6 hours, or at least 12 hours, such as in the range of and including 12 to 48 hours.

It may also be preferred that the hydrothermal treatment is carried out at a steam pressure of from 1 bar to 40 bar, in particular at a steam pressure from 1 bar to 10 bar. The aqueous mixture is typically reacted in a tightly closed or pressure-resistant vessel. The reaction preferably takes place in an inert or protective gas atmosphere. Examples of suitable inert gases include nitrogen, argon, or mixtures thereof.

Following hydrothermal treatment the oxygen evolution catalyst material is isolated. Typically, the reaction mixture following the hydrothermal treatment is filtered. Optionally, the formed catalyst material is washed. Suitably, the formed catalyst material is washed with water (such as de-ionised water). The catalyst material is then typically dried, for example by heating to a temperature in the range of and including 60 to 100 °C. The product may be ground to remove large agglomerates. For example, using a pestle and mortar or by milling.

The catalyst material may be used as an oxygen evolution reaction catalyst in a water electrolyser, especially in the anode of a water electrolyser, such as a PEMWE.

The catalyst material may also be used in a fuel cell, such as a PEMFC, especially in a fuel cell anode for the purposes of cell reversal tolerance.

The oxygen evolution catalyst material may be formulated as an ink, typically by dissolving or dispersing the catalyst material in a mixture of an ion-conducting polymer and water, or a mixture of an ion-conducting polymer, water and an organic solvent, such as ethanol or propan-1 -ol. Suitable ion-conducting polymers are known to those skilled in the art and include fluorinated ion-conducting polymers, such as perfluorinated sulfonic-acid (PFSA) ionomers, partially-fluorinated sulfonated or phosphonated polymers, non-fluorinated hydrocarbon sulfonated or phosphonated polymers, or mixtures thereof.

The inks may be used to form a catalyst layer. Such layers suitably comprise the catalyst material and an ion-conducting polymer (suitable ion-conducting polymers including those set out hereinbefore in relation to inks). The catalyst layers may also comprise additional components, such as additional catalysts, radical scavengers, etc., as will be known to those skilled in the art.

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

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

The CCM may also be suitable for use in a fuel cell, such as a PEM fuel cell. In such cases the CCM typically comprises: (i) an ion-conducting membrane with a first face and a second face; (ii) an anode layer on the first face of the membrane the anode layer comprising the oxygen evolution catalyst material as described herein, a hydrogen oxidation reaction (HOR) catalyst, such as a hydrogen oxidation reaction (HOR) catalyst comprising platinum (for example platinum on carbon), and an ion-conducting polymer; and (iii) a cathode catalyst layer on the second face of the membrane, the cathode catalyst layer comprising an oxygen reduction reaction (ORR) catalyst, such as an ORR catalyst comprising platinum (for example platinum on carbon).

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

Example 1 : Synthesis of (Ca Iro.sNboAOe ^O

Iridium chloride (IrC ^FW) (0.19 g), CaC>2 and Nl^Os W were added in a 0.8 (Ir) : 0.2(Nb) : 1 : 4 molar ratio respectively to 10 M KOH (10 ml). The mixture was stirred for 1 hour. The autoclave was then placed in a fan-assisted oven at 240 °C for 4 days. The product was washed with deionised water and 3 M nitric acid and dried overnight at 70 °C in air.

XRD analysis of the formed material showed a pyrochlore-type structure.

Preparation of a button electrode incorporating the material of Example 1

Catalyst (0.1 g), Nation™ ionomer (0.1 g) and 5 yttrium stabilized zirconia balls (5 mm diameter) were added to a 10 ml mixing pot and a FlackTek SpeedMixer (DAC 330-100 SE) was used to mix the contents 3 times (3 minutes at 3000 rpm). Subsequently 2 ml of deionised water followed by 1.5 ml of propan-1-ol was added to the pot which was then mixed once (3 minutes at 3000 rpm) using the SpeedMixer. The ink was added to the Spray Gun (Bahco 1/4in Air Inlet (BSP), with 1 to 2 mm tip) and used to coat squares (9 x 9 cm) of Toray carbon paper (SUB0001 Batch SHF-2941 C) with catalyst. The carbon paper was placed on a hotplate (I KA C-MAG HS7) at 140 °C during the spray coating to allow evaporation. Circles (2 cm diameter) were cut from the carbon paper and a Fischerscope XDV XRF instrument was used to scan each button 29 times (30 seconds per scan) to determine the Ir and Nb loading.

Electrochemical testing

Wet cell testing was carried out in 0.1 M H2SO4 using a jacketed cell and a water bath to heat the water at 60 °C. The experiment was carried out using an Ag/AgCI (non-leak, ~3.5 M KCI, WPI) reference electrode. The button electrode was submerged between PTFE mesh in 35 ml 0.1 M H2SO4 under vacuum for 1 hour to allow the solution to pass through the Toray paper and into the pore structure. Five aliquots of approximately 2 ml were taken throughout the testing and each analysed by ICP-MS. The first aliquot (Assay 1) was taken from the solution used to soak the button electrode prior to the gold paperclip being attached to the button electrode to form the working electrode. The working electrode was then inserted into the cell with a Pt counter electrode and 95 ml of 0.1 M H2SO4. The cell was degassed with nitrogen prior to starting electrochemical measurements. Initial redox behaviour was characterised using cyclic voltammetry (CV). The cell was cycled between 0.0-1.35 V vs RHE at different scan rates (100, 50, 10 and 5 mV s-1) and a second aliquot (Assay 2) was taken. An activity sweep was performed between 1.0-1 .55 V vs RHE at 1 mV s-1 at the beginning of life (BOL) and a third aliquot (Assay 3) was taken. Degradation was performed with CV between 0.60-1.35 V vs RHE at 100 mV s-1 for 1000 cycles and a fourth aliquot was taken (Assay 4). The cell was cycled again between the same potentials and at each of the scan rates used for the initial redox behaviour measurements. An end of life (EOL) activity sweep was performed between 1.0-1.55 V vs RHE at 1 mV s-1 and the fifth aliquot was taken (Assay 5).

Figure 1 shows the mass activity vs applied potential of Nb-containing pyrochlore formed in Example 1 compared to a prior art iridate pyrochlore (Ca)2-xlr2Oe.xH2O) and reference I rC>2. This data shows that a high OER catalyst activity is maintained despite the substitution of iridium for niobium.

Figure 2 shows the results of ICP-MS analysis of the aliquots taken during the electrochemical testing. This indicates significantly less leaching of iridium from the material of Example 1 in comparison with prior art pyrochlore materials indicating higher catalyst stability in acidic conditions.

Example 2: Synthesis of (Cai-xNaxMNbo.sIro shOs.n^O

The material was prepared by hydrothermal synthesis at 240 °C. IrC (1.5 mmol), NbCU (1.5 mmol) and Ca(NOs)2 (3 mmol) were stirred together in 12.5 mL de-ionised (DI) H2O for 1 hour. KOH needed for a 2M final solution was dissolved in 10 mL DI H2O and added slowly to the metal solution and stirred together followed by the addition of 6 mmol of NaBrOs. The mixture was stirred for 1h and then transferred to a Teflon-lined autoclave and heated at 240 °C for 24h and cooled to room temperature. The black precipitates were recovered by centrifugation and washed with DI H2O and dried at 100 °C overnight.

XRD analysis of the formed material showed a pyrochlore-type structure.

Example 3: Synthesis of (Cai-xNaxMTao.sIro shOs.n^O

This example was prepared by a method according to Example 2 but with the use of TaCU (1 .5 mmol) instead of NbCU.

XRD analysis of the formed material showed a pyrochlore-type structure.

Example 4: Synthesis of (Ca.Nahdri- H2O pyrochlores (y=0, 0.1 , 0.2, 0.5, 0.8; B’ =

Nb, Ta) A series of pyrochlore materials, (Ca.Na^lri.yB’y^Oe W (y=0, 0.1 , 0.2, 0.5, 0.8; B’ = Nb, Ta) were prepared hydrothermally at 240 °C. In a typical synthesis, a mixture of I rC and NbCk/ TaCk (total Ir and Nb/Ta content of 3mmol) was made in 10 mL of deionised water and added to KOH solution. Then 3 mmol of Ca(NOs)2 and 6 mmol of NaBrOs were added and kept stirring for 1 h. The volume of the mixture was adjusted to 22.5 mL to ensure 2M KOH concentration in the final solution. The mixtures were heated at 240 °C for 24h in a teflon-lined 45mL autoclave and cooled naturally to room temperature. The precipitates were recovered by centrifugation and washed with deionised H2O and dried overnight at 100 °C.

Example 5: Synthesis of (Ca.NahfRuosNbos Os and (Ca.NahfRuo.sTaos Os

Mixed Nb/Ta ruthenates were prepared by hydrothermal synthesis at 240 °C. A mixture of RuCk and NbCk/ TaCk (total Ru and Nb/Ta content of 3mmol) was made in 10 mL of deionised water and added to KOH solution. Then 3 mmol of Ca(NOs)2 and 6 mmol of NaBrOs were added and kept stirring for 1 h. The volume of the mixture was adjusted to 22.5 mL to ensure 2M KOH concentration in the final solution. The mixtures were heated at 240 °C for 24h in a teflon-lined 45mL autoclave and cooled naturally to room temperature. The precipitates were recovered by centrifugation and washed with DI H2O and dried overnight at 100 °C.

Electrode preparation

Catalyst (100 mg) and deionised water (72 mg) were mixed before addition of a Nation ionomer dissolved in 1 -propanol and water. The catalyst: Nation ratio was 1 :10 in the final electrode. The contents were mixed homogeneously in a 10 mL mixing pot with 5 yttrium stabilized zirconia balls (5 mm diameter) using a FlackTek SpeedMixer (DAC 330-100 SE). The inks were made by mixing the contents at 3000 rpm for 15 minutes (5 min x 3).

To coat the electrodes, the ink was added to a Spray Gun (Bahco 1/4in Air Inlet (BSP), with 1 to 2 mm tip) and sprayed on to a 7 x 7 cm² square of Toray carbon paper (SUB0001 Batch SHF-2941 C). The carbon paper was placed on a hotplate (IKA C-MAG HS7) at 140 °C during the spray coating to allow evaporation of the solvents. After drying the electrodes, circles (2 cm diameter) were cut from the carbon paper and a Fischerscope XDV XRF instrument was used to scan each button 29 times (30 seconds per scan) to determine the Ir and Nb/Ta loading. The results of OER catalyst activity testing are shown in Table 1. This table shows that the materials have promising catalytic activity despite the substitution of iridium for niobium or tantalum. Samples at lro.2 show a lower activity than the other samples tested.

Table 1 - Results of OER catalyst activity testing

Claims

1 . An oxygen evolution catalyst material with a pyrochlore-type structure and with:

(i) calcium and I or sodium as A-site elements of the pyrochlore-type structure;

(ii) iridium and I or ruthenium as first B-site elements of the pyrochlore-type structure;

(iv) a molar ratio of A-site elements: first and second B-site elements is in the range of and including 0.8: 1 to 1 : 1.

2. An oxygen evolution catalyst material according to claim 1 having a composition (AA’)_a(BB’)2ObX_c in which A is Ca, A’ is Na, B is Ir and I or Ru, B’ is Nb and / or Ta, X is O, OH, H2O or combinations thereof, 1 .6 < a < 2.0, 5 < b < 7, and 0 < c < 2.

3. An oxygen evolution catalyst material according to claim 1 or claim 2 having a composition according to Formula (1):

(Cai.xNa_x)2(Bi.yB’y)2O7-z.(H2O)n

Formula (1) wherein:

0 < x < 0.5;

0 < y < 0.6;

0 < z < 1 ;

0 < n < 2;

B is Ir, Ru or combinations thereof; and

B’ is Nb, Ta, or combinations thereof.

4. An oxygen evolution catalyst material according to claim 2, wherein B’ is Nb.

5. An oxygen evolution catalyst material according to any one of the preceding claims wherein 0 < y < 0.3.

6. An oxygen evolution catalyst according to any one of the preceding claims wherein B is Ir.

7. An oxygen evolution catalyst according to any one of the preceding claims with a crystalline structure with space group Fd-3m.

8. A catalyst ink comprising an oxygen evolution catalyst material according to any one of claims 1 to 7 and an ion-conducting polymer.

9. A catalyst layer comprising an oxygen evolution catalyst material according to any one of claims 1 to 7 and an ion-conducting polymer.

10. A catalyst-coated membrane comprising an oxygen evolution catalyst material according to any one of claims 1 to 7, or a catalyst layer according to claim 9.

11. A water electrolyser or a fuel cell comprising a catalyst-coated membrane according to claim 10.

12. A method of manufacturing an oxygen evolution catalyst material according to any one of claims 1 to 7, the method comprising the steps of:

(i) providing an aqueous mixture comprising at least one source of iridium and I or ruthenium, at least one source of calcium and I or sodium, at least one source of niobium and / or tantalum, and a base;

(ii) treating the aqueous mixture under hydrothermal conditions;

(iii) isolating the oxygen evolution catalyst material.