Measurement of water transport properties through membrane electrode assemblies - Part II. Cathode diffusion media

This paper presents an in situ method to characterize the effective water diffusivity (D-e) of cathode diffusion media in an operating fuel cell. The D-e values of self-made gas diffusion layers (GDLs) and a commercial GDL were experimentally determined to be about one order of magnitude lower than the theoretical value. The effects of several factors such as the inert gas component and cathode pressure as well as the presence of microporous layer (MPL) on D-e were studied, and the mechanism of water transport through cathode diffusion media was explored. It was found that the water transport is largely controlled by molecular and Knudsen diffusion through the catalyst layer. The MPL does not exhibit a dominant resistance to water transport. Interrelations of D-e with the net water transport coefficient through the membrane and membrane resistance were also examined, suggesting that good balance between suitable membrane hydration and cathode flooding avoidance can be achieved by optimizing cathode diffusion media. The capability to measure D-e reported in this work provides a diagnostic tool for screening cathode diffusion materials. (C) 2007 The Electrochemical Society.