A Secondary-Doped Polybenzimidazole Membrane with High Proton Conductivity and Ion Selectivity for Vanadium Redox Flow Batteries

Polybenzimidazole (PBI)-based membranes are one of the most promising proton exchange membranes for vanadium redox flow batteries (VRFBs) due to their excellent ion selectivity. However, the relatively lower proton conductivity limits their application. Herein, a PBI membrane with both high proton conductivity and ion selectivity is prepared through a secondary phosphoric acid-doping method. The secondary-doped PBI membrane has a lower doping level in the surface layer while a higher doping level at the inner layer, forming a significant gradient-doped structure. In this structure, the former ensures an excellent ion selectivity while the latter enables a preferable proton conductivity. As a result, the VRFB with the secondary-doped PBI membrane exhibits an ultrahigh coulombic efficiency (CE) of 99.2% at the operating current density of 200 mA cm −2 , which is significantly higher than that of the Nafion 212 membrane (97.7%), signifying an excellent ion selectivity. Meanwhile, the corresponding voltage efficiency (VE) is high up to 87.1%, which is also better than that of the Nafion 212 membrane (84.8%), indicating a high proton conductivity. Therefore, the secondary-doped PBI membrane might be a promising candidate for the highly efficient membrane for VRFB, and the secondary-doping method is simple and facile to realize engineering applications.