Structures and preparation methods for catalyst coated membranes for fuel cells
Simplified methods for preparing a catalyst coated membrane (CCM) for solid polymer electrolyte fuel cells. The CCM has two reinforcing, expanded polymer sheets and the methods involve forming the electrolyte membrane from ionomer solution during assembly of the CCM. Thus, the conventional requirement to obtain, handle, and decal transfer solid polymer sheets in CCM preparation can be omitted. Further, CCM structures with improved mechanical strength can be prepared by orienting the expanded polymer sheets such that the stronger tensile strength direction of one is orthogonal to the other. Such improved CCM structures can be fabricated using the simplified methods.
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 expanded polymer sheets bonded to opposite sides of the solid polymer electrolyte membrane wherein each of the first and second expanded polymer sheets is a web form product having a machine direction and a transverse direction, and having anisotropic tensile strength between the machine and transverse directions, and
first and second catalyst layers comprising first and second catalysts respectively and bonded to the first and second expanded polymer sheets respectively on the sides opposite the solid polymer electrolyte membrane, wherein pores of the first expanded polymer sheet comprise membrane ionomer and first ionomer, and the pores of the second expanded polymer sheet comprise membrane ionomer and second ionomer; the method comprising:
coating a first catalyst ink slurry comprising the first catalyst and the first ionomer onto a first supporting substrate;
applying the first expanded polymer sheet to the first catalyst ink coating;
coating a membrane ionomer solution comprising the membrane ionomer onto the first expanded polymer sheet;
orienting the second expanded polymer sheet such that the stronger tensile strength direction of the second expanded polymer sheet is orthogonal to the stronger tensile strength direction of the first expanded polymer sheet;
applying the so oriented second expanded polymer sheet to the membrane ionomer coating;
annealing the membrane ionomer coating after the second expanded polymer sheet has been applied, thereby forming the solid polymer electrolyte membrane;
coating a second catalyst ink slurry comprising the second catalyst and the second ionomer onto the second expanded polymer sheet;
drying the first catalyst ink coating after applying the first expanded polymer sheet to the first catalyst ink coating, thereby forming the first catalyst layer; and
drying the second catalyst ink coating, thereby forming the second catalyst layer.
2. The method of claim 1 wherein the step of drying the first catalyst ink coating is performed before the step of coating the membrane ionomer solution.
3. The method of claim 1 wherein the step of drying the first catalyst ink coating is performed as part of the step of annealing the membrane ionomer coating.
4. The method of claim 1 wherein the step of applying the second catalyst ink slurry comprises coating the second catalyst ink slurry onto the second expanded polymer sheet after the step of annealing the membrane ionomer coating.
5. The method of claim 1 comprising:
before applying the second expanded polymer sheet to the membrane ionomer coating, the steps of:
applying the second expanded polymer sheet to a second supporting substrate;
coating the second catalyst ink slurry onto the second expanded polymer sheet applied to the second supporting substrate; and
drying the second catalyst ink coating, thereby bonding the second expanded polymer sheet to the second catalyst layer on the second supporting substrate;
removing the second expanded polymer sheet with bonded second catalyst layer from the second supporting substrate; and
applying the second expanded polymer sheet with bonded second catalyst layer to the membrane ionomer coating before annealing the membrane ionomer coating.
6. The method of claim 1 comprising, before annealing the membrane ionomer coating, the steps of:
coating the second catalyst ink slurry onto the second expanded polymer sheet;
drying the second catalyst ink coating thereby forming the second catalyst layer bonded to the second expanded polymer sheet; and
applying the second expanded polymer sheet with bonded second catalyst layer to the membrane ionomer coating.
7. The method of claim 1 wherein the first catalyst layer is a cathode layer or an anode layer.
8. The method of claim 1 wherein the membrane ionomer, the first ionomer and the second ionomer are the same ionomer.
9. The method of claim 6 wherein the ionomer is perfluorosulfonic acid ionomer or hydrocarbon ionomer.
10. A solid polymer electrolyte fuel cell comprising a catalyst coated membrane assembly made according to the method of claim 1 .
11. The method of claim 1 wherein the step of coating a first catalyst ink slurry and the step of coating a second catalyst ink slurry are performed sequentially.
12. The method of claim 1 wherein the step of coating a first catalyst ink slurry and the step of coating a second catalyst ink slurry are performed simultaneously.