Asymmetric porous membranes with ultra-high ion selectivity for vanadium redox flow batteries

To develop high ion-selective proton exchange membranes is of great importance for vanadium redox flow batteries. In this work, a novel asymmetric porous sulfonated poly (ether sulfone) (SPES) membrane with ultra-high ion selectivity is prepared via ionic-liquid-induced phase separation. The characteristics of the prepared porous membrane, including microstructure, proton conductivity, VO2+ permeability, mechanical strength and single cell performance, are studied. The influence of the ionic liquid on the porous structure of the membrane is illustrated. The results show that the vanadium ion permeability of the porous SPES membranes decreases by an order of magnitude as compared with Nafion212 and its proton conductivity exceeds 20 mS cm−1. With similar energy efficiency to Nafion212, the cell with the porous SPES membrane possesses higher coulombic efficiency. At 100 mA cm−2, the coulombic efficiency and energy efficiency of the single cell equipped with the porous SPES membrane are 98.8% and 84.1%, respectively and the membranes show excellent cycling stability. •A novel method for preparing microporous ion-selective membranes was proposed.•Ultra-low VO2+ permeability of 1.41 × 10−8 cm2 min−1 was achieved.•The energy efficiency of the cell reached as high as 84.13% at 100 mA cm−2.•The membrane showed excellent cycling stability in VRFBs.