Porous electrode substrate and process for production thereof, porous electrode substrate precursor sheet, membrane-electrode assembly, and polymer electrolyte fuel cell

1. A process of producing a porous electrode substrate, the process comprising:

dispersing first short carbon fibers, and producing a first precursor sheet not having a three-dimensional entangled structure of the first short carbon fibers;

treating the first precursor sheet such that the first short carbon fibers in the first precursor sheet are entangled and that a second precursor sheet having a three-dimensional entangled structure of the first short carbon fibers is obtained;

dispersing second short carbon fibers on the second precursor sheet such that a porous electrode precursor sheet including the second precursor sheet and a third precursor sheet not having a three-dimensional entangled structure of the second short carbon fibers and stacked on the second precursor sheet is obtained; and

carbonization treating the porous electrode substrate precursor sheet at a temperature of at least 1000° C. to obtain the porous electrode substrate.

2. The process according to claim 1 ,

wherein the dispersing of the second short carbon fibers on the second precursor sheet comprises feeding a slurry in which the second short carbon fibers are dispersed in a liquid medium onto the second precursor sheet.

3. The process according to claim 1 ,

wherein the dispersing of the first short carbon fibers comprises dispersing the first short carbon fibers together with short carbon fiber precursors b 1 , fibrillar carbon fiber precursors b 1 ′, or both, and

the dispersing of the second short carbon fibers comprises dispersing the second short carbon fibers together with short carbon fiber precursors b 2 , fibrillar carbon fiber precursors b 2 ′ or both.

4. The process according to claim 1 , further comprising:

hot press molding the porous electrode substrate precursor sheet at a temperature of less than 200° C., prior to the carbonization treating.

5. The process according to claim 4 , further comprising:

oxidation treating the porous electrode substrate precursor sheet at a temperature of at least 200° C. and less than 300° C., after the hot press molding and before the carbonization treating.

6. A porous electrode substrate, obtained by the process according to claim 1 .

7. The process according to claim 1 , wherein the first short carbon fibers and the second short carbon fibers independently have an average fiber length of from 2 mm to 12 mm and an average fiber diameter of from 3 μm to 9 μm.

8. The process according to claim 3 , wherein the short carbon fiber precursors b 1 , the fibrillar carbon fiber precursors b 1 ′, the short carbon fiber precursors b 2 , and the fibrillar carbon fiber precursors b 2 ′ each independently comprises at least one polymer selected from the group consisting of an acrylic polymer, a cellulose polymer, and a phenolic polymer.

9. The process according to claim 2 , wherein the liquid medium comprises water.

10. The process according to claim 1 , wherein the treating of the first precursor sheet comprises spraying a liquid to the first precursor sheet at a pressure of from 1 MPa to 7 MPa.

11. The process according to claim 1 , wherein the porous electrode precursor sheet has a thickness of from 20 μm to 400 μm.

12. The process according to claim 1 , wherein the carbonization treating is performed at a temperature of from 1000° C. to 3000° C.

13. The process according to claim 4 , wherein the hot press molding is performed at a temperature of 120° C. to 190° C.

14. The process according to claim 4 , wherein the hot press molding comprises pressing the porous electrode substrate precursor at a pressure of from 20 kPa to 10 MPa.

15. The process according to claim 5 , wherein the oxidation treating is performed at a temperature of from 240° C. to 270° C.

16. The process according to claim 2 ,

wherein the dispersing of the first short carbon fibers comprises dispersing the first short carbon fibers together with short carbon fiber precursors b 1 , fibrillar carbon fiber precursors b 1 ′, or both, and

the dispersing of the second short carbon fibers comprises dispersing the second short carbon fibers together with short carbon fiber precursors b 2 , fibrillar carbon fiber precursors b 2 ′ or both.

17. The process according to claim 2 , further comprising:

hot press molding the porous electrode substrate precursor sheet at a temperature of less than 200° C., prior to the carbonization treating.

18. The process according to claim 17 , further comprising:

oxidation treating the porous electrode substrate precursor sheet at a temperature of at least 200° C. and less than 300° C., after the hot press molding and before the carbonization treating.

19. The process according to claim 3 ,

wherein the short carbon fiber precursors b 1 and the short carbon fiber precursors b 2 each independently have a fiber length of from 2 mm to 20 mm and a diameter of no more than 5 μm, and

the fibrillar carbon fiber precursors b 1 ′ and the fibrillar carbon fiber precursors b 2 ′ each independently have a fiber length of from 1 mm to 20 mm.

20. The process according to claim 16 ,

wherein the short carbon fiber precursors b 1 and the short carbon fiber precursors b 2 each independently have a fiber length of from 2 mm to 20 mm and a diameter of no more than 5 μm, and

the fibrillar carbon fiber precursors b 1 ′ and the fibrillar carbon fiber precursors b 2 ′ each independently have a fiber length of from 1 mm to 20 mm.