Heat and Water Transport in Hydrophobic Diffusion Media of PEM Fuel Cells

Limiting current measurements in small-scale proton exchange membrane (PEM) fuel cells operating at high stoichiometries are used to determine oxygen transport resistance over a wide range of operating conditions, some of which cause water vapor to condense inside the cathode diffusion medium. When water does condense, the cell undergoes a rapid rise in oxygen transport resistance as the current density increases. Graphs of such behavior are useful for comparing the performance of different diffusion medium materials. A one-dimensional analysis of heat and water vapor transport produces a simple condition that determines when, depending on the ratio of thermal conductivity to effective dry diffusion coefficient, the diffusion medium should stay dry with increasing current density and when it should eventually get wet. Limiting current experiments on four different materials over a fairly broad range of temperatures and pressures correlate well with the results of this analysis. The wet/dry criterion derived in this study is also applicable to the related question of whether or not the microporous layer stays dry even when the adjoining diffusion medium is wet.