Fuel cell operational methods for oxygen depletion at shutdown

Abstract

A method for creating an oxygen depleted gas in a fuel cell system, including operating a fuel cell stack at a desired cathode stoichiometry at fuel cell system shutdown to displace a cathode exhaust gas with an oxygen depleted gas. The method further includes closing a cathode flow valve and turning off a compressor to stop the flow of cathode air.

1 . A method for creating an oxygen depleted gas in a fuel cell system, said method comprising:
 determining that the fuel cell system has been shutdown;   setting a cathode air flow;   applying a load to consume power generated by a fuel cell stack;   providing a desired cathode stoichiometry by adjusting stack current or the load to use stack voltage as an indication of the cathode stoichiometry so as to cause oxygen to be depleted from the cathode air in the fuel cell stack;   operating the fuel cell system at the desired voltage and the desired cathode stoichiometry so as to create a volume of oxygen depleted gas in at least the cathode side of the fuel cell stack and in the cathode exhaust; and   closing a cathode exhaust valve and shutting off the compressor.   
     
     
         2 . The method according to  claim 1  wherein operating the fuel cell system at the desired cathode stoichiometry includes operating the fuel cell stack down to a cathode stoichiometry of approximately 1 so as to create a volume of oxygen depleted gas in the fuel cell stack and the cathode exhaust. 
     
     
         3 . The method according to  claim 1  further comprising measuring the cathode flow in the stack to calculate the volume of oxygen depleted gas created while operating the fuel cell system at the desired cathode stoichiometry. 
     
     
         4 . The method according to  claim 1  wherein achieving the desired cathode stoichiometry includes determining the concentration of oxygen in the cathode exhaust. 
     
     
         5 . The method according to  claim 1  further comprising adding hydrogen to a flow of cathode air so as to control the cathode stoichiometry. 
     
     
         6 . The method according to  claim 1  wherein creating a volume of oxygen depleted gas in the cathode side of the stack and in the cathode exhaust further includes feeding the oxygen depleted gas to the anode side of the stack. 
     
     
         7 . The method according to  claim 1  further comprising providing a higher resolution secondary controller to fix the stack load. 
     
     
         8 . A method for creating an oxygen depleted gas in a fuel cell system, said method comprising:
 determining that the fuel cell system can be shutdown;   cooling a fuel cell stack;   adjusting a cathode exhaust valve and a compressor to increase the pressure on a cathode side of the fuel cell stack and applying a shutdown load to achieve a desired voltage and a desired cathode stoichiometry;   operating the fuel cell system at the desired voltage and the desired cathode stoichiometry so as to create a volume of oxygen depleted gas in the cathode side of the fuel cell system, a water vapor transfer unit and a water vapor transfer unit by-pass line;   closing the cathode exhaust valve and water vapor transfer unit by-pass valves and shutting off the compressor; and   closing a cathode inlet valve when the pressure of the oxygen depleted gas on the cathode side drops to approximately ambient pressure.   
     
     
         9 . The method according to  claim 8  further comprising feeding the oxygen depleted gas created in the cathode side to the anode side of the fuel cell stack. 
     
     
         10 . The method according to  claim 8  wherein the desired cathode stoichiometry is approximately 1. 
     
     
         11 . The method according to  claim 8  further comprising calculating the volume of oxygen depleted gas created while operating the fuel cell system at the desired voltage. 
     
     
         12 . The method according to  claim 8  further comprising adding hydrogen after the stack is cooled to maximize the hydrogen available in the fuel cell stack. 
     
     
         13 . A method for creating an oxygen depleted gas in a fuel cell system, said method comprising:
 operating a fuel cell stack at a cathode stoichiometry of approximately 1 at fuel cell system shutdown to displace a cathode exhaust gas with an oxygen depleted gas; and   closing a cathode flow valve and turning a compressor off to stop the flow of cathode air.   
     
     
         14 . The method according to  claim 13  wherein the cathode stoichiometry of approximately 1 is achieved by adjusting a load and maintaining cathode air flow at a fixed flow. 
     
     
         15 . The method according to  claim 14  wherein the initial load is higher to consume adsorbed oxygen. 
     
     
         16 . The method according to  claim 13  wherein the cathode stoichiometry of approximately 1 is achieved by adjusting cathode air flow and maintaining a fixed load. 
     
     
         17 . The method according to  claim 16  wherein the cathode air flow is adjusted by adjusting a cathode backpressure valve. 
     
     
         18 . The method according to  claim 16  wherein the cathode air flow on a net oxygen basis is adjusted by adjusting the amount of hydrogen flowing into a cathode input line. 
     
     
         19 . The method according to  claim 16  wherein the cathode air flow is adjusted by adjusting the speed of the compressor. 
     
     
         20 . The method according to  claim 13  wherein fuel cell stack voltage, the voltage of one or more fuel cells, or a stack current sensor is used as a feedback to control cathode stoichiometry. 
     
     
         21 . The method according to  claim 13  wherein cathode gas concentration or a cathode flow meter is used as a feedback to control cathode stoichiometry. 
     
     
         22 . The method according to  claim 13  further comprising displacing the oxygen in the cathode exhaust gas at elevated pressure and closing a backpressure valve upon completion of the displacement of the cathode exhaust gas with oxygen depleted gas. 
     
     
         23 . The method according to  claim 13  further comprising injecting hydrogen into the cathode exhaust after the displacement of the oxygen in the cathode exhaust gas so as to cause oxygen depleted gas to back-flow into the fuel cell stack and upstream volumes. 
     
     
         24 . The method according to  claim 13  further comprising closing a cathode inlet valve after the cathode exhaust is displaced with oxygen depleted gas and the compressor is turned off. 
     
     
         25 . The method according to  claim 13  further comprising introducing hydrogen to a cathode side of the fuel cell stack while displacing the cathode exhaust gas with oxygen depleted gas. 
     
     
         26 . The method according to  claim 13  further comprising injecting hydrogen into an anode or a cathode side of the fuel cell stack after displacing the cathode exhaust gas with oxygen depleted gas to impede oxygen from entering the stack. 
     
     
         27 . The method according to  claim 13  further comprising closing cathode inlet and cathode outlet valves after displacing the cathode exhaust gas with oxygen depleted gas to impede oxygen from entering the stack. 
     
     
         28 . The method according to  claim 13  further comprising applying a brief load after closing the cathode flow valve and turning the compressor off so as to consume any oxygen remaining in the fuel cell stack. 
     
     
         29 . The method according to  claim 13  further comprising flushing an anode side of the fuel cell stack with oxygen depleted gas. 
     
     
         30 . The method according to  claim 13  further comprising cooling the fuel cell stack prior to shutting off the compressor to limit gas contraction and water vapor condensation.