Water Dynamics in a PEM Fuel Cell: Effect of Current and Humidity Transients

Abstract

The water dynamics of the membrane electrode assembly (MEA) of a 50 cm2 polymer electrolyte membrane (PEM) fuel cell was studied in response to current and humidity step transients as a function of inlet relative humidity (RH) and cell operating temperature. MEA water content change was monitored by high frequency resistance (HFR) and in situ neutron imaging. The MEA "wetting" response was significantly faster (10-30 seconds) than the reciprocal "drying" response (several minutes). The wetting response to increasing current appears faster than that to increasing gas inlet RH. Wetting and drying responses to gas inlet RH transients are greater at low current densities showing the dominating effect of current wetting over gas inlet humidity effects. Operating the fuel cell at 40oC with no inlet gas humidification, a current wetting response was observed and 34 amps of current sustained. This showed that reaction water produced at the cathode is sufficient to support high current density if the drying effect of inlet gases is low. It also suggests that back diffusion of reaction water to the MEA is sufficient to account for the fast drop in HFR. In situ neutron imaging results show that GDL wetting is also faster than drying but on a slower time scale than that for the MEA. However, MEA and GDL drying are on a similar slow time scale and are coupled because water diffusing out of the MEA must move through the GDLs.