Pre-removal of polybenzimidazole anion to improve flexibility of grafted quaternized side chains for high performance anion exchange membranes

Abstract Quaternized polybenzimidazole anion exchange membranes exhibit excellent stability, however, always suffer from poor hydroxide conductivity as compared with other polymer categories. In this work, it is found that polybenzimidazole anion transition state, though essential to provide grafting sites, has strong ionic interaction with the conventional quaternary ammonium cations containing grafting reagents. A novel grafting strategy is proposed to pre-remove polybenzimidazole anions by fully grafting of non-cationic ether-containing side chains, and then attaches quaternary ammonium cations to the end of pendent side chains. Further investigations with molecular dynamics simulation, TEM and SAXS indicate that by eliminating cation-anion binding, the flexible pendent quaternized cations exhibit excellent ability to aggregate into ionic clusters and transport hydroxide ions. The trimethylammonium grafting membrane shows conductivity of about 82.4 mS cm−1 at 80 °C, around 6-fold that of the conventional grafted membranes. Grafting with alkaline stable piperidinium, 93.2% conductivity retention is achieved even soaking in 2 M KOH at 60 °C for 720 h. The H2/O2 single cell shows high peak power density of about 806.1 mW cm−2 at large current density of about 2025.0 mA cm−2, which is the highest value among the polybenzimidazole membranes reported in recent years.