Liquid water storage, distribution, and removal from diffusion media in PEFCS

Liquid water storage in the diffusion media (DM) of polymer electrolyte fuel cells (PEFCs) is a function of design geometry, surface geometry, and operating conditions, and the DM, water storage, and can affect transient response, degradation via ionic contaminants, pressure loss, and freeze-thaw behavior. Neutron imaging was used to quantify the liquid water distribution in a PEFC under a variety of flow rates, humidities, and currents with paper or cloth DM. For a wide range of conditions, the paper DM held roughly 60% of the total water stored under the landings and the remaining 40% in, or under, the channels. The cloth DM had a nearly 50:50 channel to land liquid water distribution. From 0.2 to 1.0 A/cm(2) current conditions, the paper DM held 174% more water per volume of DM under the landings than cloth, resulting in a very high liquid saturation and eventual flooding. Increasing flow rate decreased the total liquid water content, mostly from removal of droplets. The residual liquid water under the lands was removed with increased flow rate more readily using the cloth DM, thus it was a more effective material for low power purge. Transient testing showed the time scale of significant liquid water accumulation is on the order of minutes. (c) 2006 The Electrochemical Society.