Quaternary ammonium groups grafted polybenzimidazole membranes for vanadium redox flow battery applications

The sulfuric acid (SA) doped dense polybenzimidazole (PBI) membrane exhibits low vanadium crossover, and is of great interest as the separator in vanadium redox flow battery (VRFB) applications. However, further reducing the area resistance is needed to increase the voltage efficiency. Towards this target, a series of quaternary ammonium groups grafted PBI membranes are designed and synthesized by using 2,3-epoxypropyltrimethylammonium chloride (EPTMA-Cl) as the grafting agent. The membranes were prepared through a gentle and facile heating curing method without the usage of catalyst, and physicochemical properties and single cell performance of SA doped PBI-EPTMA membranes are systematically investigated. The results indicate that grafted EPTMA groups increase the hydrophilicity and free volume of membranes, resulting in enhanced SA doping content and decreased area resistance. Ultimately, VRFB assembled with the PBI-21%EPTMA membrane displays higher energy efficiencies (87.6%–75.3%) at current densities of 50–150 mA cm−2, comparing with Nafion115 (81.7–61.2%). The stable performance over 450 charge-discharge cycles of VRFB demonstrates the good stability of the PBI-EPTMA membrane.