A highly proton-/vanadium-selective perfluorosulfonic acid membrane for vanadium redox flow batteries

Abstract

A highly proton-/vanadium-selective perfluorosulfonic acid (Nafion) membrane modified by phosphotungstic acid (PWA)-anchored nano Kevlar fibers (NKFs) and nano silica particles (NSP) for a vanadium redox flow battery (VRB) was fabricated through solution casting. The nanohybrid composed of NKFs, NSP and PWA exhibited excellent compatibility with the polymer matrix, and effectively suppressed the vanadium ion crossover. In addition, the anchoring of PWA was of great significance for maintaining the ion exchange capacity and constructing continuous proton transport channels. The optimal Nafion-(NKFs@NSP/PWA) membrane presented a high proton conductivity (0.079 S cm⁻¹) and a low vanadium permeability (6.27 × 10⁻⁷ cm² min⁻¹), resulting in an ion selectivity of 1.26 × 10⁵ S min cm⁻³, which is much higher than that of recast Nafion (0.34 × 10⁵ S min cm⁻³). Moreover, the VRB assembled with the modified membrane showed a lower self-discharge rate, higher battery efficiency and superior capacity retention than that with the recast Nafion over a range of current densities from 40 mA cm⁻² to 100 mA cm⁻².