Temperature Dependency of Net Water Drag Coefficient in Polymer Electrolyte Fuel Cells

Because polymer electrolyte fuel cell (PEFC) systems for vehicles are stand-alone systems, the net water drag coefficient ( alpha NWD ) is an essential index in water management and must be negative even during high temperature operation above 100 degrees C. In this study, to verify the water balance in the cell during high temperature operation, the effect of operating temperature on the current density ( i ) - alpha NWD characteristics under gas supply conditions similar to those of a real system was examined experimentally, where the water amount supplied to and discharged from the cell was measured at a cell temperature (T cell) of 80, 100, and 120 degrees C. The experimental results obtained here indicates that the gas supply conditions to keep alpha NWD negative becomes more limited as T cell is increased. This indicates that the relative humidity of supplied hydrogen should be kept low and that the stoichiometric ratio of hydrogen should be kept high. Furthermore, the effect of membrane thickness ( tmem ) on alpha NWD became more pronounced when T cell exceeded 100 degrees C, indicating that operation at 120 degrees C is not realistic for PEFCs that use a 30-mu m-thick proton exchange membrane.