Excellent ion selectivity of Nafion membrane modified by PBI via acid-base pair effect for vanadium flow battery

• Nafion/PBI composite membrane is prepared via a facile solution casting method. • Ion selectivity and conductivity of Nafion membrane is improved by PBI. • PBI content is optimized and 1 wt% PBI shows the best properties and stability. • The EE of VFB with optimized membrane achieves 81.31% at 200 mA cm −2 . As the key material of Vanadium flow battery (VFB), the properties of ion exchange membrane limit the battery performance and cost of VFB to some extent. Nafion membrane of Dupont, one of the most employed membranes in VFB, is also exposed to some issues such as high cost and low ion selectivity. To improve ion selectivity of Nafion membrane, polybenzimidazole (PBI), functional groups of which can be ionized to a positive charge and prevent vanadium ions from crossing the membrane, is introduced to prepare reinforced Nafion membrane (Nafion/PBI membrane). By this way, excellent conductivity and vanadium ion selectivity of Nafion membrane are optimized via the “acid-base pair” effect. Furthermore, a series of PBI contents have been optimized and the battery with Nafion/PBI membrane of 1 wt% PBI exhibits excellent performance (coulombic efficiency: 97.72%, energy efficiency: 81.31% at 200 mA cm −2 ). In the long-term cycles, higher capacity retention and superior stability of the battery with Nafion/PBI membrane (1 wt% PBI) are delivered than Nafion 212 membrane. Thus, Nafion/PBI membrane is indeed a promising candidate to promote the whole properties of VFB by surmounting the trade-off between proton conductivity and vanadium ion permeability.