Improved All-Vanadium Redox Flow Batteries using Catholyte Additive and a Cross-linked Methylated Polybenzimidazole Membrane

Abstract

Through controlling the cross-linking and thickness of low-cost methylated polybenzimidazole anion exchange membranes and by using imidazolium chloride as a catholyte additive, we improved the energy efficiency at a current density of 100 mA cm(-2) for all-vanadium redox flow batteries (RFBs) to 82%, compared to 76% with a Nafion 212 membrane and 6778% for previously reported polybenzimidazole membranes with standard electrolyte. Moreover, thermal stability analysis of the charged catholytes, by direct observations of vanadium precipitation and variable temperature V-51 nuclear magnetic resonance, confirmed an optimized content of 0.16 M of the electrochemically inert additive in 1.6 M vanadium solution, consistent with the electrochemical data. The imidazolium chloride additive with such a low concentration is sufficient to induce interaction with vanadium species in catholyte. This explains the improved electrolyte stability and enhanced cycling efficiency.