A phosphotungstic acid coupled silica-Nafion composite membrane with significantly enhanced ion selectivity for vanadium redox flow battery

An ultra-high ion-selective Nafion composite membrane modified by phosphotungstic acid (PWA) coupled silica for vanadium redox flow battery (VRB) was constructed and prepared through solution casting . The composite membrane exhibits excellent proton conductivity and vanadium ions blocking property by incorporating the nanohybrid composed of silica and PWA into the Nafion ionomer . Simple tuning for the filling amount of the nanohybrid endows the obtained membranes preeminent vanadium barrier property including a minimum vanadium permeability of 3.13 × 10 −7  cm 2  min −1 and a maximum proton conductivity of 0.081 S cm −1 at 25 °C. These indicate an ion selectivity of 2.59 × 10 5  S min cm −3 , which is 6.8 times higher than that of recast Nafion (0.33 × 10 5  S min cm −3 ). As a result, the VRB with the composite membrane shows superior battery performance containing a lower self-discharge rate, higher capacity retention and more robust cyclic stability compared with recast Nafion over a range of current densities from 40 to 100 mA cm −2 .