Novel N-alkylation synthetic strategy of imidazolium cations grafted polybenzimidazole for high temperature proton exchange membrane fuel cells

Development of simple synthetic strategy for polybenzimidazole-based membranes with high uptake and retention ability of phosphoric acid is essential for long-term practical applications of high temperature proton exchange membrane fuel cells. In this work, N, N-dimethylacetamide is used as both deacid reagent and solvent for N-alkylation reaction to synthesize imidazolium cations-grafted polybenzimidazole. The accordingly formed membranes exhibit the improved uptake and retention ability of phosphoric acid compared to pristine polybenzimidazole membrane. The optimized membrane with grafting degree of 60% has an acid uptake of 361.2 wt% with an anhydrous proton conductivity of 75.2 mS cm−1 at 160 °C, about 4.2 times higher than that of pristine polybenzimidazole membrane. After the acid retention test at 160 °C for 240 h, proton conductivity of the optimized membrane decreases by 7.5%, lower than that of pristine polybenzimidazole membrane under the same conditions (11.9% loss). The thus-assembled single cell delivers the power density of 0.429, 0.596, and 0.708 W cm−2 under 0, 100 and 200 kPa back pressures whereas the values are 0.319, 0.427 and 0.510 W cm−2 for the cell assembled from phosphoric acid-doped pristine polybenzimidazole membrane.