Diffusion media with microporous layer
Specially prepared gas diffusion media improve the performance of PEM fuel cells. The media are made by first dipping an electrically conductive porous material such as carbon fiber paper into a suspension of hydrophobic polymer and drying the paper to create a desired deposition pattern of hydrophobic polymer on the substrate. Then a paste containing a fluorocarbon polymer and carbon particles is applied to a desired side of the substrate, and thereafter the paste and hydrophobic polymer are sintered together at high temperature on the paper. In particular, nonionic surfactants remain on the carbon fiber paper after the initial hydrophobic polymer is applied to the electrically conductive porous material. When the paste is coated on the dried paper, the paste is in contact with a hydrophilic surface.
1. A method for preparing a diffusion medium for use in a PEM fuel cell, comprising:
applying a polymer composition comprising a first fluorocarbon polymer, a surfactant, and a solvent to at least one surface of an electrically conductive porous substrate to form a coated substrate in the form of a coated sheet having two sides;
removing the solvent from the coated substrate without removing the surfactant;
thereafter applying a microporous layer comprising carbon particles and a second fluorocarbon polymer to the coated substrate; and
thereafter heating the substrate at a temperature sufficient to remove the surfactant and sinter the first and second fluorocarbon polymers,
wherein no sintering is carried out between removing the solvent and applying the microporous layer.
2. A method according to claim 1 , wherein the substrate comprises a carbon fiber based material.
3. A method according to claim 1 , wherein removing the solvent comprises heating above room temperature.
4. A method according to claim 1 , wherein removing the solvent comprises exposing the substrate to infrared radiation.
5. A method according to claim 3 , wherein one side of the substrate is blocked during the heating.
6. A method according to claim 3 , wherein both sides of the substrate are open during the heating.
7. A method according to claim 4 , wherein one side of the substrate is blocked during exposure to the infrared radiation.
8. A method according to claim 4 , wherein both sides of the substrate are open during exposure to the infrared radiation.
9. A method according to claim 4 , comprising a continuous process.
10. A method according to claim 1 , wherein the first fluorocarbon polymer comprises polytetrafluoroethylene.
11. A method according to claim 1 , wherein the second fluorocarbon polymer comprises polytetrafluoroethylene.
12. A method according to claim 1 , wherein the microporous layer comprises acetylene black and polytetrafluoroethylene.
13. A method according to claim 1 , comprising heating the substrate above 290° C. to remove the surfactant and sinter the fluorocarbon polymers.
14. A method according to claim 1 , comprising a continuous process.
15. A method according to claim 1 , wherein removing the solvent comprises evaporating the solvent while the coated substrate is held in contact with a pattern member comprising openings, and wherein the first fluorocarbon polymer is deposited at the openings upon evaporation of the solvent.
16. A method according to claim 1 , comprising applying the first fluorocarbon polymer in a pattern on surface of the sheet, and applying the microporous layer on the other surface.
17. A fuel cell comprising at least one diffusion medium made by a process according to claim 1 .
18. A fuel cell comprising at least one diffusion medium made by a process according to claim 4 .
19. A method for making a diffusion medium for use in a PEM fuel cell, comprising
immersing an electrically conductive porous substrate in a polymer composition comprising a first fluorocarbon polymer, solvent, and a surfactant, wherein the substrate is in the form of a sheet having two surfaces;
removing the solvent from the substrate by exposing at least one surface of the substrate to infrared radiation;
applying a coating composition comprising carbon particles and particles of a second fluorocarbon polymer onto a surface of the substrate to form a coated substrate; and
heating the coated substrate above 290° C. to remove the surfactant and sinter the fluorocarbon polymers.
20. A method according to claim 19 , wherein one surface of the substrate is exposed to infrared radiation and the other surface of the substrate is blocked during exposure to infrared radiation.
21. A method according to claim 19 , wherein one surface of the substrate is exposed to infrared radiation and the other surface of the substrate is open during exposure to infrared radiation.
22. A method according to claim 19 , comprising exposing only one surface of the substrate to infrared radiation and applying the coating composition to the surface of the substrate not exposed to infrared radiation.
23. A method according to claim 19 , wherein the first fluorocarbon polymer comprises polytetrafluoroethylene.
24. A method according to claim 19 , wherein the second fluorocarbon polymer comprises polytetrafluoroethylene.
25. A method according to claim 19 , wherein the coating composition comprises acetylene black and polytetrafluoroethylene.
26. A fuel cell comprising at least one diffusion medium made by a process according to claim 19 .
27. A fuel cell stack, comprising a plurality of fuel cells according to claim 26 .