Structural change and proton conductivity of phosphosilicate gel–polyimide composite membrane for a fuel cell operated at 180 °C

Proton-conductive composite membranes were prepared from phosphosilicate gel powders and a commercially available polyimide precursor. The conductivity of a composite membrane containing 75 wt.% of gel powder was about 2.5 × 10−3 S cm−1 at 180 °C under more than 0.4%RH. However, under dry N2 atmosphere at 180 °C, the conductivity decreased with time because of the formation of Si5O(PO4)6 crystals. The open circuit voltage (OCV) of a fuel cell in which the phosphosilicate gel–polyimide composite sheet was used as electrolyte and Pt-loaded carbon paper sheets were used as electrodes in the membrane electrode assembly, was about 0.85 V at 80 °C with a flow of hydrogen and air, and the OCV decreased to about 0.70 V at temperatures higher than 100 °C. At 180 °C under 2%RH, the cell OCV was 0.67 V; power density of about 14 mW cm−2 was obtained with current density of 40 mA cm−2. Consequently, the fuel cell using the composite sheet as an electrolyte was verified to operate at temperatures from room temperature to 180 °C.