OXYGEN EVOLUTION REACTION CATALYST

1 . An oxygen evolution reaction catalyst, wherein the oxygen evolution reaction catalyst is an oxide material comprising iridium, tantalum and ruthenium:

wherein the oxygen evolution catalyst comprises a crystalline oxide phase having the rutile crystal structure;

wherein the crystalline oxide phase has a lattice parameter a of greater than 4.510 Å.

2 . The oxygen evolution reaction according to claim 1 , wherein ruthenium is present in an amount in the range of and including 1 to 15 atomic % based on the total atomic percent of iridium, tantalum and ruthenium species in the oxygen evolution reaction catalyst.

3 . The oxygen evolution reaction catalyst of claim 1 , wherein the oxygen evolution reaction catalyst has a BET surface area of at least 30 m 2 /g.

4 . A method of synthesis of the oxygen evolution reaction catalyst according claim 1 , the method comprising steps of:

providing an aqueous solution of compounds of iridium, tantalum and ruthenium;

spray drying the solution to form a dry powder; and

subjecting said powder to calcination to thereby form the oxygen evolution reaction catalyst.

5 . The method of claim 4 , wherein the step of providing an aqueous solution of compounds of iridium, tantalum and ruthenium comprises sub-steps of:

providing an aqueous solution of a compound of iridium and a compound of ruthenium; and

mixing said aqueous solution with an aqueous solution of a compound of tantalum.

6 . The method of claim 4 , wherein the aqueous solution of compounds of iridium, tantalum and ruthenium has a molar ratio Ir:Ta:Ru of 5 to 7:2 to 4:0.5 to 1.5.

7 . The method according to claim 4 , wherein calcination is performed at a temperature in the range of and including 400° C. to 800° C.

8 . A catalyst layer comprising the oxygen evolution reaction catalyst of claim 1 and a second electrocatalyst material.

9 . The catalyst layer of claim 8 , wherein the cathode layer is an anode catalyst layer, optionally an anode catalyst layer for a proton exchange membrane fuel cell.

10 . The catalyst layer of claim 8 , wherein the second electrocatalyst material is selected from:

the platinum group metals (platinum, palladium, rhodium, ruthenium, iridium and osmium);

gold or silver;

a base metal; or

an alloy or mixture comprising one or more of these metals or their oxides.

11 . The catalyst layer of claim 8 , wherein the weight ratio of the oxygen evolution reaction catalyst to the second electrocatalyst material in the catalyst layer is from 10:1 to 1:10.

12 . A gas diffusion electrode comprising a gas diffusion layer and a catalyst layer as claimed in claim 8 .

13 . A catalysed membrane comprising an ion-conducting membrane and a catalyst layer as claimed in claim 8 .

14 . A membrane electrode assembly comprising a catalyst layer as claimed in claim 8 .

15 . A fuel cell comprising a catalyst layer as claimed in claim 8 .