Catalyst

1. An electrocatalyst material comprising:

(i) a support material comprising a plurality of individual support particles or aggregates;

(ii) first particles comprising a first metal and an alloying metal; and

(iii) second particles consisting of a second metal or a second metal oxide, wherein the second metal is platinum or iridium;

wherein each individual support particle or aggregate has dispersed thereon first particles and second particles,

wherein the mean average particle size of the second particles is smaller than the mean average particle size of the first particles.

2. The electrocatalyst material according to claim 1 , wherein the first metal is a platinum group metal.

3. The electrocatalyst material according to claim 2 , wherein the first metal is platinum.

4. The electrocatalyst material according to claim 1 , wherein the alloying metal is one or more selected from the group consisting of nickel, cobalt, chromium, copper, aluminium, yttrium, scandium, gadolinium, lanthanum, iron, zinc, titanium, niobium or tantalum.

5. The electrocatalyst according to claim 1 , wherein the ratio of first metal to alloying metal is from 3:1 to 1:3.

6. The electrocatalyst material according to claim 1 , wherein the first particles have a mean average particle size in the range of from 2 to 14 nm.

7. The electrocatalyst material according to claim 1 , wherein the electrocatalyst material comprises second particles consisting of platinum.

8. The electrocatalyst material according to claim 1 , wherein the electrocatalyst material comprises second particles consisting of iridium or iridium oxide.

9. The electrocatalyst material according to claim 1 , wherein the electrocatalyst material comprises second particles consisting of platinum and second particles consisting of iridium or iridium oxide.

10. The electrocatalyst material according to claim 1 , wherein the second particles have a mean average particle size in the range of from 0.5 to 10 nm.

11. The electrocatalyst material according to claim 1 , wherein the total loading of platinum group metal in the electrocatalyst material is from 20 to 70 wt % based on the total weight of the electrocatalyst material.

12. The electrocatalyst material according to claim 1 , wherein 20 to 80% of the platinum group metal in the electrocatalyst material is contained in the first particles.

13. A process for the preparation of an electrocatalyst according to claim 1 , said process comprising the steps of:

(i) depositing a first metal onto the support material to form a first precursor;

(ii) depositing an alloying metal onto the first precursor to form a second precursor;

(iii) annealing the second precursor to alloy the first metal and the alloying metal to form a third precursor comprising the support material and the first particles;

(iv) depositing a second metal or a second metal oxide onto the third precursor to form the electrocatalyst material.

14. The process according to claim 13 , wherein step (iv) comprises depositing the second metal using pre-formed metal or metal oxide nanoparticles suspended in solution.

15. The process according to claim 13 , comprising an additional step after step (iii) and before step (iv), in which the third precursor is subjected to an acid wash.

16. The process according to claim 13 , comprising an additional step (v) comprising depositing a second metal or second metal oxide different to that deposited in step (iv) onto the third precursor.

17. A catalyst layer comprising an electrocatalyst material according to claim 1 .

18. The catalyst layer according to claim 17 , wherein the catalyst layer is a cathode catalyst layer.

19. A gas diffusion electrode comprising a gas diffusion layer and a catalyst layer according to claim 17 .

20. A catalyst coated ion-conducting membrane comprising an ion-conducting membrane and a catalyst layer according to claim 17 .