POLYMER ELECTROLYTE MEMBRANES INCORPORATED WITH NANOMETER-SIZE PARTICLES OF PT AND OR METAL-OXIDES - EXPERIMENTAL-ANALYSIS OF THE SELF-HUMIDIFICATION AND SUPPRESSION OF GAS-CROSSOVER IN FUEL-CELLS/

Self-humidification behaviors of polymer electrolyte membranes (PEMs) were clarified experimentally in polymer electrolyte fuel cells (PEFCs ) operated with dry H-2 and O-2. New PEMs (Pt-PEM, TiO2-PEM, Pt-TiO2-P EM) were prepared by dispersing small amounts of Pt (1-2 nm in diamete r) and/or metal-oxides such as TiO2 (5 nm in diameter) in Nafion 112 o r a recasted Nafion film (normal-PEM, ca. 50 mu m in thickness). Distr ibution profiles of the specific resistances (rho) in thickness direct ion were measured with monitoring Pt-probes inserted into the PEMs, In normal-PEM, the rho increases with increase of current density at the anode side and vice versa at the cathode side, and this was pronounce d at high current density. However, it was found that the rho decrease d monotonically at every portion in Pt-PEM with increase of current de nsity, although the rho of anode side is larger than that of cathode s ide. Further uniform rho distribution was achieved in Pt-TiO2-PEM, i.e ., less than or equal to 20 Omega cm at a practically operational curr ent density. The mechanisms of such a distinctive self-humidification and a suppression of crossover of reactant gases in the new PEMs were clarified by monitoring consumed H-2 and O-2 and produced water in exh austing gases from PEFCs in comparison with normal-PEM. In Pt-TiO2-PEM s, crossover H-2 and O-2 were recombined on Pt particles and all of th e water generated inside the PEMs was exhausted from the anode. The Ti O2 particles enhanced the back-diffusion of water produced by faradic reaction at the cathode by the hygroscopic property, resulting in very efficient humidification of the PEM of the anode side dried by the el ectroosmotic drag, It was also found that the new PEMs improve the cat hode potential distinctively, which was ascribed to elimination of the short-circuit reaction of crossover gases in the cathode catalyst lay er, resulting in a small non-faradic consumption of H-2 and no disturb ance of reactant O-2 diffusion by the produced water vapor.