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

Abstract

Because polymer electrolyte fuel cell (PEFC) systems for vehicles are stand-alone systems, the net water drag coefficient ( α NWD ) is an essential index in water management and must be negative even during high temperature operation above 100 °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 ) - α 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 °C. The experimental results obtained here indicates that the gas supply conditions to keep α 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 ( t mem ) on α NWD became more pronounced when T cell exceeded 100 °C, indicating that operation at 120 °C is not realistic for PEFCs that use a 30-μm-thick proton exchange membrane.