Shutdown methods and designs for fuel cell stacks

Abstract

Improved water distribution can be obtained within the cells of a fuel cell series stack by maintaining a suitable temperature difference between the cathode and anode sides of each cell in the stack during shutdown. This can be accomplished by thermally insulating the “hot” end and sides of the stack and by providing a thermal mass adjacent to the “hot” end.

1 - 9 . (canceled) 
     
     
         10 . A method of shutting down a fuel cell stack comprising a plurality of fuel cells, each fuel cell having a cathode side and an anode side, the method comprising:
 ceasing generation of electricity from the stack;   contacting the fuel cell at one end of the stack with a thermal mass;   allowing the stack to cool over a cool-down period; and   maintaining a temperature difference across each fuel cell such that water is transferred from the membrane electrode assemblies to one of the anode and cathode flow fields in each of the plurality of fuel cells during the cool-down period.   
     
     
         11 . The method of  claim 10  wherein the anode side is hotter than the cathode side in each fuel cell during the cool-down period. 
     
     
         12 . The method of  claim 10  wherein the stack further comprises a hot end and a cold end and wherein the hot end of the stack is thermally insulated. 
     
     
         13 . The method of  claim 10  wherein the stack further comprises an insulating layer surrounding the plurality of fuel cells. 
     
     
         14 . The method of  claim 10  wherein each fuel cell comprises cathode and anode reactant flow fields and the colder reactant flow field in each fuel cell is purged during the cool-down period. 
     
     
         15 . The method of  claim 10  wherein the thermal mass comprises a volume of coolant in thermal communication with the fuel cell at one end of the stack. 
     
     
         16 . The method of  claim 10  wherein at least about half of the water present in the membrane electrode assemblies during operation of the fuel cell stack is transferred to one of the anode and cathode flow fields during the cool-down period. 
     
     
         17 . The method of  claim 10  wherein the amount of water remaining in the membrane electrode assemblies after the cool-down period is less than or equal to about 3 mg/cm 2 . 
     
     
         18 . A method of shutting down a fuel cell stack comprising a plurality of fuel cells disposed in an enclosure, the enclosure having a gas inlet proximate to one end of the fuel cell stack and a gas outlet proximate to the other end of the fuel cell stack, each fuel cell having a membrane electrode assembly having a cathode side and an anode side interposed between anode and cathode flow fields, respectively, the method comprising:
 ceasing generation of electricity from the stack;   allowing the stack to cool over a cool-down period;   supplying a gas to the gas inlet of the enclosure at a first temperature lower than a fuel cell stack temperature and exhausting the gas from the gas outlet at a second temperature higher than the first temperature; and   maintaining a temperature difference between the cathode side and the anode side of each fuel cell during the cool-down period, wherein the direction of the temperature difference in each fuel cell is the same.