Highly Proton-Selective meta-Polybenzimidazole-Based Membranes for Vanadium Redox Flow Batteries

Abstract

Vanadium redox flow batteries (VRFBs) have promising properties for large-scale energy storage devices such as separate rating of power and capacity, easy scale-up, high efficiency, fast response to input and output changes, and possibility to reverse electrolyte imbalances due to crossover and water transport by simply remixing catholyte and anolyte [1]. Despite of that, suitable membranes that have high oxidative stability and low crossover of redox-active species is required. Among many membrane candidates, polybenzimidazole (PBI) is expected to meet the requirements. The narrow, nano-sized morphology of PBI inhibits the diffusion of large vanadium ions and once the backbone is protonated with acids such as H2SO4, it can effectively repel the positively charged vanadium ions while excess acid inside the matrix conducts protons [2]. However, the conductivity is much lower than that of Nafion membranes and needs to be improved.

In order to enhance the properties PBI membrane, several researches have been conducted, such as pre-treating the membrane by phosphoric acid [3] and NaOH [4]. In this work, a novel process was investigated to increase the conductivity and maintain a high proton/vanadium selectivity.

The treated membrane showed 5 times higher conductivity (9.1 mS/cm) and 1.8 times higher Young’s Modulus (1.92 GPa) as compared to the membrane prepared by standard 2M acid doping [2]. In terms of proton over vanadium selectivity, an optimally treated membrane showed 4.6x106 S.min/cm3, which is 30-160 times higher than that reported previously for standard Nafion 212 in literature [5,6].