A fuel cell unit typically comprises an anode, a cathode, and an electrolyte membrane. A reactant gas, such as oxygen gas and air, is supplied to the cathode gas flow field next to the cathode. A fuel gas such as hydrogen gas or methanol is supplied to the anode gas flow field next to the anode. A plurality of fuel cell units can be stacked together by using bipolar plates to form a fuel cell stack. The bipolar plates also include specific patterns of anode and cathode gas flow fields. The anode flow fields of individual fuel cell units in the fuel cell stack can be connected together to form anode gas flow field of the fuel cell stack. Similarly, the cathode gas flow fields of individual fuel cell units can be connected together to form the cathode gas flow field of the fuel cell stack.
When a fuel cell is shut down after normal operation, the supply of fuel gas to the anode is cut off. The anode gas flow field is sometime purged with air to remove potentially hazardous fuel gas from the fuel cell system. When a fuel cell is to be started up, flows of fuel gas to the anode gas flow field and reactant gas to the cathode gas flow field are resumed. Such startup and shutdown processes are found to cause permanent decay of fuel cell performance, particularly, in terms of significant decrease in fuel cell voltage and power output. It is believed that such decay in fuel cell performance is caused by the presence of both an air rich zone and a fuel gas rich zone in the anode gas flow field during startup and shutdown. A reverse current situation or a possible electrochemical reaction generated between the air rich zone and fuel gas rich zone is believed to result in corrosion of the metal catalyst and/or catalyst support at the anode.