Thermally crosslinked sulfonated polybenzimidazole membranes and their performance in high temperature polymer electrolyte fuel cells

Abstract

The degradation pathway of phosphoric acid doped polybenzimidazole membranes in high temperature polymer electrolyte membrane fuel cells depends on the acid contents. If it is high, creep is discussed as the main reason. If it is low (membranes prepared by solvent evaporation and post-doping), the main cause may be loss of acid due to evaporation. The net transport of acid to the anode side at high current densities should also lead to local softening of the membrane, which could be mitigated by crosslinking the membrane. Here we show that sulfonated para-polybenzimidazole membranes can be stabilized by curing at 350 degrees C. In contrast to meta-polybenzimidazole and sulfonated para-polybenzimidazole, crosslinked sulfonated para-polybenzimidazole is insoluble in dimethylacetamide at room temperature and phosphoric acid at 160 degrees C. At 160 degrees C and 5% relative humidity the conductivity of crosslinked sulfonated para-polybenzimidazole and meta-polybenzimidazole is 214 mS cm(-1) and 147 mS cm(-1), respectively. At 600 mA cm(-2), the voltage decay rate is 16 mu V h(-1), much lower than published for commercial meta-polybenzimidazole (308 mu V h(-1)). Furthermore, the average voltage at 600 mA cm(-2) is 523 mV, while a previously published cured meta-polybenzimidazole membrane only reaches 475 mV.