Tailoring the vanadium/proton ratio of electrolytes to boost efficiency and stability of vanadium flow batteries over a wide temperature range

Fast capacity decay and narrow temperature window still hinds the practical application of vanadium flow batteries (VFB). Optimization the electrolyte composition is an effective strategy to realize the stable operation of VFB. Herein, we introduce the concept of V/H ratio (vanadium ion to proton concentration ratio) in electrolyte to explore the vanadium concentration impact on overall performance of VFB. Theoretical analysis and experimental results indicate that reducing the V/H ratio is beneficial to obtain higher efficiency and better cycle stability over wide temperature (-15 °C ~ 55 °C). Meanwhile, during long-term operation, a lower V/H ratio electrolyte can achieve higher electrolyte utilization rate and deliver higher capacity despite the lower theoretical capacity density. Unlike previous studies that focused on increasing the vanadium concentration to obtain a higher theoretical capacity density, our techno-economic assessment clearly demonstrates that a relatively low vanadium concentration (i.e. 1.0–1.2 M) is more suitable for VFB, because safer, cheaper and more stable operation is particularly important for large-scale energy storage.