Proper tailoring of the water transport mechanism in the reactant flow channel and the porous diffusion media (DM) of a polymer electrolyte fuel cell (PEFC) is of great importance to significantly increase stack power density, durability and performance stability under normal and cold-start operations. This study is motivated by the need to establish a fundamental understanding of the water transport mechanism on and through the thin-film fuel cell DM. In the first phase of this study, the liquid water droplet behavior and instability in the reactant flow channel of a PEFC are investigated. The critical conditions leading to the droplet removal are determined, and an empirical correlation relating the surface tension and DM PTFE content is developed. The results indicate that operational conditions, droplet size, channel geometry and level of surface hydrophobicity of the DM directly affect the droplet instability.

In the second part of this study, the capillary water transport mechanism through the minute pores of the fuel cell DM is investigated. Direct measurements of capillary pressure-saturation benchmark data are generated over a wide range of conditions to develop a validated transport model for multi-phase flow in these materials. The effects of hydrophobicity, compression and operating temperature on the capillary transport characteristics of a DM are elucidated. Based on the extensive database, a modified Leverett approach applicable to fuel cell DM has been constructed. The salient feature of the new approach is that it includes the non-uniform wettability characteristics of the thin-film DM and incorporates the subsequent changes in capillary transport characteristics of the DM over a wide range of operating conditions.