OXYGEN EVOLUTION REACTION CATALYST

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

wherein the oxygen evolution reaction 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; and

wherein the oxygen evolution reaction catalyst has a BET surface area of at least 50 m 2 /g.

2 . The oxygen evolution reaction catalyst of claim 1 , comprising:

iridium present in an amount in the range of and including 60 to 95 atomic % based on the total atomic composition of iridium and tantalum species in the oxygen evolution reaction catalyst; and

tantalum present in an amount in the range of and including 5 to 40 atomic % based on the total atomic composition of iridium and tantalum species in the oxygen evolution reaction catalyst.

3 . The oxygen evolution reaction catalyst of claim 1 , wherein the crystalline oxide phase comprises a single phase oxide structure comprising iridium and tantalum.

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

providing an aqueous solution of compounds of iridium and tantalum;

spray drying the solution to form a dry powder; and

subjecting said powder to calcination at a temperature of less than 500° C. 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 and tantalum comprises sub-steps of:

providing an aqueous solution of a compound of iridium, and mixing said aqueous solution with an aqueous solution of a compound of tantalum.

6 . The method according to claim 4 , wherein the step of subjecting said powder to calcination to thereby form the oxygen evolution reaction catalyst comprises sub-steps of: subjecting the powder to calcination at less than 500° C. for a first specified time period, and subsequently subjecting the powder to further calcination at less than 500° C. for a second specified time period.

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

8 . The catalyst layer of claim 7 , wherein the catalyst layer is an anode catalyst layer, optionally an anode catalyst layer for a proton exchange membrane fuel cell.

9 . The catalyst layer of claim 7 , 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.

10 . The catalyst layer of claim 7 , 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.

11 . A gas diffusion electrode comprising a gas diffusion layer and a catalyst layer as claimed in claim 7 .

12 . A catalysed membrane comprising an ion-conducting membrane and a catalyst layer as claimed in claim 7 .

13 . A membrane electrode assembly comprising:

a catalyst layer as claimed in claim 7 ,

a gas diffusion electrode comprising a gas diffusion layer and the catalyst layer, or

a catalysed membrane comprising an ion-conducting membrane and the catalyst layer.

14 . A fuel cell comprising:

a catalyst layer as claimed in claim 7 ,

a gas diffusion electrode comprising a gas diffusion layer and a catalyst layer,

a catalysed membrane comprising an ion-conducting membrane and a catalyst layer, or

a membrane electrode assembly comprising the catalyst layer, a gas diffusion electrode comprising a gas diffusion layer and the catalyst layer, or a catalysed membrane comprising an ion-conducting membrane and the catalyst layer.