Composite membrane

1. A composite membrane having a first membrane face, said composite membrane comprising at least one ion-conducting polymer and fibers, wherein the composite membrane has a first region extending from the first membrane face to a thickness of at least 10% and less than 100% of the membrane thickness and the density of fibers in the first region is lower than the density of fibers in the composite membrane as a whole, wherein the first membrane face has a surface area and the first region of the composite membrane covers 100% of the surface area of the first membrane face and wherein the first region of the composite membrane contains less than 10% of the fibers in the composite membrane as a whole.

2. A composite membrane according to claim 1 , wherein the fibers are a network of randomly orientated individual fibers.

3. A composite membrane according to claim 1 , wherein the thickness of the first region is at least 20% of the membrane thickness.

4. A composite membrane according to claim 1 , wherein the thickness of the first region is at least 30% of the membrane thickness.

5. A composite membrane according to claim 1 , wherein the first region of the composite membrane contains less than 5% of the fibers in the composite membrane as a whole.

6. A composite membrane according to claim 1 , wherein the density of the fibers in the first region is zero.

7. A composite membrane according to claim 1 , wherein the composite membrane consists of the first region extending from the first membrane face and a remaining region.

8. A composite membrane according to claim 1 , wherein the composite membrane has a second membrane face opposite the first membrane face and comprises a second region extending from the second membrane face to a thickness of at least 10% and less than 90% of the membrane thickness and the density of fibers in the second region is lower than the density of fibers in the composite membrane as a whole, wherein the second membrane face has a surface area and the second region of the composite membrane covers 100% of the surface area of the second membrane face and wherein the second region of the composite membrane contains less than 10% of the fibers in the composite membrane as a whole.

9. A composite membrane according to claim 1 , wherein the at least one ion conducting polymers is a proton conducting polymer.

10. A composite membrane according to claim 1 , wherein the total thickness of the membrane is less that 200 μm.

11. A composite membrane according to claim 1 , wherein the fibers are amorphous silica fibers.

12. A composite membrane according to claim 1 , wherein the fibers are incorporated into the composite membrane at a fiber loading of 2 to 25 wt %.

13. A catalysed membrane comprising a composite membrane according to claim 1 .

14. A membrane electrode assembly comprising a composite membrane according to claim 1 .

15. A membrane electrode assembly comprising a cathode, an anode, and a composite membrane according to claim 7 , wherein the first region is adjacent to the cathode.

16. A membrane electrode assembly comprising a cathode, an anode, and a composite membrane according to claim 7 , wherein the first region is adjacent to the anode.

17. A composite membrane according to claim 1 , wherein the at least one ion-conducting polymer is present in both the first region and in the rest of the composite membrane.

18. A composite membrane according to claim 17 , wherein the at least one ion-conducting polymer present in the first region is the same as the at least one ion-conducting polymer in the rest of the composite membrane.

19. A composite membrane according to claim 17 , wherein the at least one ion-conducting polymer present in the first region is different from the at least one ion-conducting polymer in the rest of the composite membrane.