Catalyst coated membranes and preparation methods for fuel cells
Simplified methods are disclosed for preparing a catalyst coated membrane that is reinforced with a porous polymer sheet (e.g. an expanded polymer sheet) for use in solid polymer electrolyte fuel cells. The methods involve forming a solid polymer electrolyte membrane by coating membrane ionomer solution onto a first catalyst layer and then applying the porous polymer sheet to the membrane ionomer solution coating, while it is still wet, such that the membrane ionomer solution only partially fills the pores of the porous polymer sheet. A second catalyst ink is then applied which fills the remaining pores of the porous polymer sheet. Not only are such methods simpler than many conventional methods, but surprisingly this can result in a marked improvement in fuel cell performance characteristics.
1. A method of preparing a catalyst coated membrane assembly for a solid polymer electrolyte fuel cell, the catalyst coated membrane assembly comprising a solid polymer electrolyte membrane comprising a proton-conducting membrane ionomer, first and second catalyst layers comprising first and second catalysts respectively and first and second ionomers respectively and bonded to opposite sides of the solid polymer electrolyte membrane, and a porous polymer sheet at the interface between the second catalyst layer and the solid polymer electrolyte membrane and bonded to both, and wherein the pores of the porous polymer sheet comprise the solid polymer membrane electrolyte and the second catalyst layer; the method comprising:
coating a first catalyst ink comprising the first catalyst and the first ionomer onto a supporting substrate;
drying the first catalyst ink coating, thereby forming the first catalyst layer;
coating a membrane ionomer solution comprising the membrane ionomer onto the first catalyst layer;
applying the porous polymer sheet to the wet membrane ionomer solution coating wherein the membrane ionomer solution partially fills the pores of the porous polymer sheet;
drying and annealing the membrane ionomer coating, thereby forming the solid polymer electrolyte membrane;
applying a second catalyst ink comprising the second catalyst and the second ionomer to the porous polymer sheet wherein the second catalyst ink fills the remaining pores of the porous polymer sheet; and
drying the second catalyst ink coating, thereby forming the second catalyst layer.
2. The method of claim 1 wherein the first catalyst layer is a cathode layer and the second catalyst layer is an anode layer.
3. The method of claim 1 wherein the porous polymer sheet is an expanded polymer sheet.
4. The method of claim 1 wherein the membrane ionomer comprises perfluorosulfonic acid ionomer or hydrocarbon ionomer.
5. The method of claim 1 wherein the membrane ionomer solution comprises greater than about 16% by weight of membrane ionomer solids.
6. The method of claim 5 wherein the membrane ionomer solution comprises about 24% by weight of membrane ionomer solids.
7. The method of claim 1 wherein the membrane ionomer solution has a viscosity greater than about 90 centipoise.
8. The method of claim 1 wherein the membrane ionomer solution comprises a solvent mixture of an alcohol and water in a ratio greater than about 60:40 by weight of alcohol to water.
9. The method of claim 8 wherein the alcohol is n-propanol.
10. A catalyst coated membrane assembly made according to the method of claim 1 .
11. The catalyst coated membrane assembly of claim 10 wherein the first catalyst layer is a cathode layer and the second catalyst layer is an anode layer.
12. The catalyst coated membrane assembly of claim 11 wherein the membrane ionomer comprises a perfluorosulfonic acid ionomer.
13. The catalyst coated membrane assembly of claim 12 wherein the first and second ionomers comprise a perfluorosulfonic acid ionomer with a different equivalent weight than that of the membrane ionomer.
14. The catalyst coated membrane assembly of claim 11 wherein the porous polymer sheet is an expanded polymer sheet.
15. The catalyst coated membrane assembly of claim 14 wherein the expanded polymer sheet is an expanded polytetrafluoroethylene sheet.
16. The catalyst coated membrane assembly of claim 14 wherein the porosity of the expanded polymer sheet is between 60 and 90%.
17. The catalyst coated membrane assembly of claim 16 wherein the porosity of the expanded polymer sheet is about 70%.
18. The catalyst coated membrane assembly of claim 14 wherein the expanded polymer sheet is from about 4 to 8 micrometers thick.
19. The catalyst coated membrane assembly of claim 18 wherein the expanded polymer sheet is about 4 micrometers thick.
20. The catalyst coated membrane assembly of claim 14 wherein the pores in about 0.2 micrometers of the thickness of the expanded polymer sheet are filled with the anode catalyst layer.
21. The catalyst coated membrane assembly of claim 14 wherein the pores in greater than about 1% of the thickness of the expanded polymer sheet are filled with the anode catalyst layer.
22. The catalyst coated membrane assembly of claim 21 wherein the pores in about 5% of the thickness of the expanded polymer sheet are filled with the anode catalyst layer.
23. A solid polymer electrolyte fuel cell comprising a catalyst coated membrane assembly made according to the method of claim 1 .
24. A method of improving voltage reversal tolerance in a solid polymer electrolyte fuel cell comprising a catalyst coated membrane assembly, the method comprising making the catalyst coated membrane assembly according to the method of claim 2 .