A polybenzimidazole-covalent organic framework hybrid membrane with highly efficient proton-selective transport channels for vanadium redox flow battery

Polybenzimidazole (PBI) is considered as the one of the most promising materials for proton exchange membranes (PEMs) owing to its negligible crossover and outstanding chemical stability. However, the low proton conductivity of PBI membrane restricts its commercial application in vanadium flow battery (VFB). Herein, the PBI hybrid membranes with high proton conductivity were prepared by introducing crystalline covalent organic frameworks (TpPa and TpPa-SO3H (STpPa)). Owing to the highly ordered sub-nanometer pores within TpPa and STpPa, faster proton highway was constructed in the hybrid membrane. Meanwhile, the sulfonic acid groups introduced to STpPa further provided additional proton transport sites and vanadium permeation barriers to achieve high ion selectivity. Accordingly, the cell assembled with PBI hybrid membrane with 3% STpPa achieved superior voltage efficiency (95.22%–74.84%) and energy efficiency (91.38%–74.38%) at 40–240 mA cm−2, and exhibited superb electrochemical stability during 800 cycles at 160 mA cm−2. This work offers a facile and efficient strategy to construct high-performance PBI-based PEMs by incorporating functional porous materials, which has incredible prospect for use in VFBs.