Fuel cell system and method for removal of impurities from fuel cell electrodes

Abstract

A fuel cell system and method of removing impurities from a catalyst are provided. The fuel cell system comprises a fuel cell stack comprising a pair of end plates and at least one unit cell. The unit cell contains a gas diffusion layer in contact with a membrane electrode assembly which is constructed of a polymer electrolyte membrane enclosed between two electrodes. The at least one unit cell is stacked between the end plates. The fuel cell system further comprises a voltage supply means and a means of impressing a cyclically varying voltage from the voltage supply means on the fuel cell stack. The cyclically varying voltage removes impurities that adhere to catalysts on the electrode surfaces in the fuel cell stack.

1 . A fuel cell system which generates electricity by supplying fuel gas and oxidant gas to a fuel cell stack comprising: 
 a fuel cell stack comprising a pair of end plates and at least one unit cell containing a gas diffusion layer in contact with a membrane electrode assembly which is constructed of a polymer electrolyte membrane enclosed between two electrodes, wherein said at least one unit cell is stacked between the end plates;    a voltage supply means; and    a means of impressing a cyclically varying voltage from the voltage supply means on said fuel cell stack.    
     
     
         2 . The fuel cell system according to  claim 1 , wherein said two electrodes comprise one each of a fuel electrode and an oxidant electrode.  
     
     
         3 . The fuel cell system according to  claim 1 , further comprising means of supplying fuel gas and oxidant gas to said fuel cell stack.  
     
     
         4 . The fuel cell system according to  claim 1 , wherein the voltage supply means is selected from the group consisting of a battery and a generator.  
     
     
         5 . The fuel cell system according to  claim 4 , wherein said battery is a secondary battery charged by said fuel cell stack or a generator.  
     
     
         6 . The fuel cell system according to  claim 1 , wherein said means of impressing a cyclically varying voltage varies the voltage per unit cell of the fuel cell stack between the range of from about −1.5 V to about 1.5 V.  
     
     
         7 . The fuel cell system according to  claim 1 , wherein said means of impressing a cyclically varying voltage varies the voltage at a rate of from about 1 mV/s to about 1000 mV/s.  
     
     
         8 . The fuel cell system according to  claim 1 , further comprising a means for measuring the current flowing in said fuel cell stack when the cyclically varying voltage is impressed on the fuel cell stack.  
     
     
         9 . The fuel cell system according to  claim 1 , further comprising a means for measuring the time for which the cyclically varying voltage is impressed on the fuel cell stack.  
     
     
         10 . The fuel cell system according to  claim 1 , further comprising a means for measuring the number of cycles for which the cyclically varying voltage is impressed on the fuel cell stack.  
     
     
         11 . A motor vehicle comprising the fuel cell system of  claim 1 .  
     
     
         12 . The motor vehicle according to  claim 11 , wherein the motor vehicle is an automobile.  
     
     
         13 . A method of impressing a cyclically varying voltage on a fuel cell stack comprising: 
 providing a fuel cell stack comprising a pair of end plates and at least one unit cell containing a gas diffusion layer in contact with a membrane electrode assembly which is constructed of a polymer electrolyte membrane enclosed between two electrodes, wherein said at least one unit cell is stacked between the end plates; and    applying a cyclically varying voltage across said fuel cell stack using voltage supplied by a voltage supply means.    
     
     
         14 . The method of impressing a cyclically varying voltage on a fuel cell stack according to  claim 13 , wherein the cyclically varying voltage is impressed on the fuel cell stack before the fuel cell stack starts to generate electricity or after the fuel cell stack stops generating electricity.  
     
     
         15 . The method of impressing a cyclically varying voltage on a fuel cell stack according to  claim 14 , wherein the voltage supply means is a battery or a generator.  
     
     
         16 . The method of impressing a cyclically varying voltage on a fuel cell stack according to  claim 15 , further comprising a step of charging the battery with electricity generated by the fuel cell stack.  
     
     
         17 . The method of impressing a cyclically varying voltage on a fuel cell stack according to  claim 13 , further comprising controlling the voltage so that the voltage per unit cell is cyclically varied between about −1.5 V and about 1.5 V.  
     
     
         18 . The method of impressing a cyclically varying voltage on a fuel cell stack according to  claim 13 , further comprising controlling the voltage so that the voltage cyclically varies at a rate of between about 1 mV/s and about 1000 mV/s.  
     
     
         19 . The method of impressing a cyclically varying voltage on a fuel cell stack according to  claim 13 , further comprising controlling the voltage so that the voltage varies linearly between the lowest and highest impressed voltages.  
     
     
         20 . The method of impressing a cyclically varying voltage on a fuel cell stack according to  claim 13 , further comprising measuring the current flowing in the fuel cell stack.  
     
     
         21 . The method of impressing a cyclically varying voltage on a fuel cell stack according to  claim 20 , further comprising stopping the impressing of a cyclically varying voltage on the fuel cell stack when the measured current falls below a predetermined amperage.  
     
     
         22 . The method of impressing a cyclically varying voltage on a fuel cell stack according to  claim 13 , further comprising measuring the time for which the cyclically varying voltage is impressed on the fuel cell stack and stopping the impressing of the cyclically varying voltage when a predetermined period of time has elapsed.  
     
     
         23 . The method of impressing a cyclically varying voltage on a fuel cell stack according to  claim 20 , further comprising measuring the number of cycles for which the cyclically varying voltage is impressed on the fuel cell stack and stopping the impressing of the cyclically varying voltage when a predetermined number of cycles has elapsed.  
     
     
         24 . A method of electrochemically removing impurities that adhere to an electrode surface in a fuel cell system, comprising: 
 providing a fuel cell stack comprising a pair of end plates and at least one unit cell containing a gas diffusion layer in contact with a membrane electrode assembly which is constructed of a polymer electrolyte membrane enclosed between two electrodes, wherein said at least one unit cell is stacked between the end plates; and    applying a cyclically varying voltage across said fuel cell stack using voltage supplied by a voltage supply means to remove impurities from the electrode surface.    
     
     
         25 . A method of electrochemically removing impurities that adhere to a catalyst, comprising: 
 providing a catalyst with a surface and impurities adhered to said surface; and    applying a cyclically varying voltage across said catalyst surface using voltage supplied by a voltage supply means to remove said impurities from said catalyst surface.