Substrate

1. A membrane electrode assembly comprising

a proton-conducting membrane, two electrocatalyst layers and two gas diffusion substrates such that the membrane is interposed between the electrocatalyst layers and one gas diffusion substrate is adjacent to each electrocatalyst layer;

wherein each gas diffusion substrate comprises a porous electrically conductive sheet material and a gasket member, the sheet material having a central portion and a peripheral portion, the peripheral portion having first and second planar faces;

wherein each gasket member is located on the first and second planar faces of the peripheral portion and extends beyond the edge of the peripheral portion in a direction away from the central portion, such that the edge of the peripheral portion is enveloped by the gasket member;

wherein each electrocatalyst layer does not extend beyond the central portion of the porous electrically conductive sheet material;

and wherein the membrane extends beyond the electrocatalyst layers, such that the membrane contacts the gasket members.

2. A membrane electrode assembly according to claim 1 , wherein the membrane does not extend beyond the edge of the gasket members.

3. A membrane electrode assembly according to claim 2 , wherein the gasket member impregnates the peripheral portion, in addition to being present as a coating on the first and second planar faces of the porous electrically conductive sheet material.

4. A membrane electrode assembly according to claim 2 , wherein the gasket member does not impregnate the peripheral portion, but is only present as a coating on the first and second planar faces of the peripheral portion of the porous electrically conductive sheet material.

5. A membrane electrode assembly according to claim 2 , wherein the peripheral portion of the porous electrically conductive sheet material comprises a plurality of holes through the thickness of the sheet material, wherein said gasket member extends through said holes in addition to being present on the first and second planar faces of said peripheral portion.

6. A membrane electrode assembly according to claim 2 wherein the gasket member is formed from a polymeric material.

7. A membrane electrode assembly according to claim 1 , wherein the gasket member impregnates the peripheral portion, in addition to being present as a coating on the first and second planar faces of the porous electrically conductive sheet material.

8. A membrane electrode assembly according to claim 1 , wherein the gasket member does not impregnate the peripheral portion, but is only present as a coating on the first and second planar faces of the peripheral portion of the porous electrically conductive sheet material.

9. A membrane electrode assembly according to claim 1 , wherein the peripheral portion of the porous electrically conductive sheet material comprises a plurality of holes through the thickness of the sheet material, wherein said gasket member extends through said holes in addition to being present on the first and second planar faces of said peripheral portion.

10. A membrane electrode assembly according to claim 1 wherein the gasket member is formed from a polymeric material.

11. An integrated cell assembly comprising a membrane electrode assembly according to claim 1 , wherein the membrane electrode assembly is joined to a field flow plate.

12. A fuel cell stack comprising membrane electrode assemblies according to claim 1 .

13. A fuel cell stack comprising integrated cell assemblies according to claim 11 .

14. An integrated cell assembly comprising a membrane electrode assembly according to claim 2 , wherein the membrane electrode assembly is joined to a field flow plate.

15. A fuel cell stack comprising membrane electrode assemblies according to claim 2 .

16. A fuel cell stack comprising integrated cell assemblies according to claim 14 .

17. A process for producing a membrane electrode assembly comprising the steps of:

a) preparing two gas diffusion substrates, in each case by forming a gasket member onto an electrically conductive sheet material, wherein the sheet material has a central portion and a peripheral portion, the peripheral portion having first and second planar faces, such that the gasket member is formed on the first and second planar faces of the peripheral portion and envelops the edge of the peripheral portion;

b) preparing a catalysed membrane by applying electrocatalyst layers to upper and lower surfaces of the membrane in regions that correspond to the central portion of the electrically conductive sheet material; and

c) sandwiching the catalysed membrane between the two gas diffusion substrates and forming a gas and fluid impermeable bond between the membrane and the gasket members.

18. A process for producing a membrane electrode assembly comprising the steps of:

a) preparing two gas diffusion electrodes, in each case by forming a gasket member onto an electrically conductive sheet material, wherein the sheet material has a central portion and a peripheral portion, the peripheral portion having first and second planar faces, such that the gasket member is formed on the first and second planar faces of the peripheral portion and envelops the edge of the peripheral portion;

b) applying electrocatalyst layers to the central portions of the electrically conductive sheet material, either before or after the formation of the gasket member; and

c) sandwiching a membrane between the two gas diffusion electrodes and forming a gas and fluid impermeable bond between the membrane and the gasket members.

19. A process for producing a membrane electrode assembly comprising the steps of:

a) preparing one gas diffusion substrate and one gas diffusion electrode, in each case by forming a gasket member onto an electrically conductive sheet material, wherein the sheet material has a central portion and a peripheral portion, the peripheral portion having first and second planar faces, such that the gasket member is formed on the first and second planar faces of the peripheral portion and envelops the edge of the peripheral portion;

b) preparing a membrane with one catalysed face by applying an electrocatalyst layer to a surface of the membrane that corresponds to the central portion of the electrically conductive sheet material of the gas diffusion substrate;

c) applying an electrocatalyst layer to a central portion of an electrically conductive sheet material of the gas diffusion electrode, either before or after the formation of the gasket member; and

d) sandwiching the membrane with one catalysed face between the gas diffusion substrate and the gas diffusion electrode, and forming a gas and fluid impermeable bond between the membrane and the gasket members.