The anion conductivity of acid-doped polybenzimidazole membrane and utilization in mitigating the capacity decay of vanadium redox flow battery stacks

•The SO42- conduction of acid-doped PBI membrane is first reported.•The capacity decay features of VRFBs with Nafion 212 and SP-PBI are systematically compared.•The accumulated discharge capacity in 1000 cycles is increased by 119.77%. Polybenzimidazole (PBI) membrane is one of the most promising proton exchange membranes for vanadium redox flow batteries (VRFBs) due to its excellent ion selectivity and stability. However, in this work, we first found the acid-doped PBI membrane conducts both H+ and SO42- in VRFBs. Besides, the capacity decay features of the VRFBs with acid-doped PBI membrane also present the same trend as that of anion exchange membrane, exhibiting an opposite direction of net electrolyte flux after long-term cycling than that of Nafion 212 (N212). Inspired by this finding, we build a VRFB two-cell stack consisting of an acid-doped PBI membrane and an N212 membrane to mitigate capacity decay. This method breaks the trade-off between voltage efficiency and capacity retention rate and significantly increases the accumulated discharge capacity by 119.77% compared to that with 2 pieces of N212 in 1000 cycles. This work deepens the understanding of the capacity decay mechanism of VRFB with different types of membranes and provides a simple yet highly effective strategy for mitigating the capacity decay of VRFB stacks.