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

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 67–78% 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 51V 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.