Crosslinked membranes of sulfonated polyimides for polymer electrolyte fuel cell applications

Polymer electrolyte fuel cell performance of crosslinked membranes of sulfonated polyimides based on 2,2′-bis(3-sulfophenoxy) benzidine was investigated at higher temperatures of 90-110 °C, relative humidities of 82-27% RH and pressures of 0.3-0.1 MPa. The operation conditions of temperature, pressure and relative humidity largely affected the PEFC performance in connection with each other. At 90 °C and relatively high humidification of 82/68% RH for hydrogen/air, the cell performance was hardly affected by the pressure and was kept in a high level even at 0.1 MPa. At low humidification of 48% and 27% RH, the cell performance was still kept in a reasonably high level at high pressures above 0.2 MPa, but it largely decreased at low pressures below 0.15 MPa. A slight decrease in ion exchange capacity (IEC) of membrane caused a relatively large decrease in the cell performance at a low relative humidity. With an increase in temperature from 90 to 110 °C, the cell performance largely decreased. For example, the membrane (MX2) with a relatively high IEC of 1.9 mequiv g^-1 showed a low maximum output (W_max) of 0.30 W cm^-2 at 110 °C, 0.2 MPa and 49% RH, which was less than a half of the corresponding value (0.71 W cm^-2) at 90 °C. These results indicate that the back-diffusion of water formed at cathode into membrane became less effective at the lower pressure and the higher temperature and for membrane with the lower IEC, resulting in the larger decrease in through-plane proton conductivity under PEFC operation. At 110 °C, 0.3 MPa and 49% RH, MX2 showed the fairly high PEFC performance and durability.