For next generation polymer electrolyte fuel cells (PEFC), membrane-electrode assemblies (MEAs) should be operated at high temperature and low humidity conditions. At low humidity condition, the cell performance decreases drastically due to the high resistance of proton conduction. Low equivalent weight (EW), which corresponds to high ion-exchange capacity, perfluorosulfonic acid (PFSA) polymer can improve proton conductivity, especially at low humidity condition, although membrane swelling will make problems. If the membrane becomes thinner, water transport between cathode and anode can be accelerated, which prevents the membrane from drying. However, increase fuel crossover results in poor durability of MEAs. A thin membrane with reduced swelling can be fabricated using pore-filling technique, where an inert and mechanically strong porous substrate is employed to fill the polymer electrolyte inside the pores [1]. Here, we develop thin pore-filling membranes with low EW PFSA polymer and evaluate fuel cell performances using the prepared membranes (Fig.1). As the substrate, we employed porous and thin ultra-high molecular weight polyethylene films (6 μm thick and 66% porosity) which can show high mechanical strength below 100 ^oC. As the filling polymer electrolyte, low EW (EW500~900) PFSA polymers were employed.

The prepared pore-filling membrane having 7 μm thick showed very little area change ratio before and after water immersion. The prepared membrane exhibited higher proton conductivity than that of the commercial Nafion® 211 (EW1100 PFSA polymer, 25 μm thick) in wide range of temperature and relative humidities.

 MEAs with the pore-filling membranes were fabricated with catalyst layer including EW500 PFSA polymer as ionomer. The hydrogen crossover currencies with the 7 μm thick pore-filling membrane showed almost the same value with the MEA with 25 μm thick Nafion® 211. The substrate effectively suppresses the membrane swelling and reduce the crossover. The MEA with the pore-filling membrane showed high I-V performances with low humidity conditions. When H₂-O₂ fuel cell operated with 30% relative humidity at 100 ^oC, power density reached 1,000 mW/cm². These results are attributed to improved proton conductivity, water transport and low crossover with the thin pore-filling membrane using tough substrate and low EW PFSA.

 [1] T. Yamaguchi et al., Adv. Materials, 15, 1198 (2003); N. Hara, T. Yamaguchi, et al., J. Phys. Chem. B, 113, 4656−4663 (2009)