Asymmetric Porous Polybenzimidazole Membranes with High Conductivity and Selectivity for Vanadium Redox Flow Batteries

Developing functional membrane with high conductivity and excellent stability is critical for improving the performance of vanadium redox flow batteries (VRFBs). Herein, an asymmetric porous polybenzimidazole (PBI) membrane composed of an ultra‐thin dense layer and a sponge‐like porous layer is proposed and prepared. The dense layer takes the main role of inhibiting vanadium crossover, whereas the porous layer provides not only a fast proton conduction channel but also a mechanical support to the dense layer. Hence, the asymmetric PBI membrane exhibits a high proton conductivity (36.4 mS cm −1 at room temperature) and an extremely low vanadium permeability (0.26 × 10 −7 cm 2 min −1 ). The VRFB with this membrane achieves an energy efficiency of 83.17% at 400 mA cm −2 and 78.59% at 500 mA cm −2 . Furthermore, the battery enables to be continuously cycled for 1600 times without a significant degradation at 400 mA cm −2 . All these results prove that the PBI membrane with an asymmetric porous structure is an ideal membrane for redox flow batteries.