Two-phase transport and the role of micro-porous layer in polymer electrolyte fuel cells

Two-phase transport of reactants and products constitutes an important limit in performance of polymer electrolyte fuel cells (PEFC). Particularly, at high current densities and/or low gas flow rates, product water condenses in open pores of the cathode gas diffusion layer (GDL) and limits the effective oxygen transport to the active catalyst sites. Furthermore, liquid water covers some of the active catalytic surface, rendering them inactive for electrochemical reaction. Traditionally, these two-phase transport processes in the GDL are modeled using so-called unsaturated flow theory (UFT), in which a uniform gas-phase pressure is assumed across the entire porous layer, thereby ignoring the gas-phase flow counter to capillarity-induced liquid motion. In this work, using multi-phase mixture (M²) formalism, the constant gas pressure assumption is relaxed and the effects of counter gas-flow are studied and found to be a new oxygen transport mechanism. Further, we analyze the multi-layer diffusion media, composed of two or more layers of porous materials having different pore sizes and/or wetting characteristics. Particularly, the effects of porosity, thickness and wettability of a micro-porous layer (MPL) on the two-phase transport in PEFC are elucidated.