Fundamental characterization of evaporative water removal from fuel cell diffusion media

In polymer electrolyte fuel cell systems, a gas purge commonly is applied to remove excess water and achieve high performance and durability. The porous diffusion media (DM) typically store a significant fraction of the total liquid water during operation, but quantitative fundamental research examining water removal behavior from DM is not yet available in literature. The objective of this study is to investigate the fundamental behavior of evaporative water removal from fuel cell diffusion media with a special test rig developed to minimize in-plane gradients so that through-plane evaporative behavior can be analyzed. Evaporative water removal was characterized by a surface evaporation region, a constant evaporation rate region characterized by capillary flow to the evaporation front, and a falling rate region characterized by separated evaporating droplets. A semi-empirical correlation for the characteristic water removal, a generic plot of purge efficiency which describes the effectiveness of purge parameters, and a potential scheme for a more durable and less parasitic purge which describes characteristic water removal of each fuel cell component were developed. The results of this study can be used to predict and understand water removal from diffusion media and help develop a less parasitic purge protocol than presently utilized.