Rapid preparation of desirable vanadium electrolyte using ascorbic acid as a reducing agent in vanadium redox flow batteries

In this study, preparing process of desirable vanadium electrolyte (catholyte including V⁴⁺) needed for operation of vanadium redox flow batteries (VRFBs) is simplified. To produce the catholyte, reduction of V⁵⁺ into V⁴⁺ should be performed with reducing agent. Although oxalic acid is conventionally used for the purpose, its amount required for reduction of V⁵⁺ is large and additional processes are needed. To alleviate the drawbacks, ascorbic acid is newly explored. To prove its advantages, reaction stoichiometric and optical characterizations are done. Initially, when the same concentration of V⁵⁺ is contained in electrolyte, the required concentration of ascorbic acid (19.6–1) is ten times lower than that of oxalic acid (2–1), verifying ascorbic acid is more effective than oxalic acid. Furthermore, when 125 % of initial concentration of ascorbic acid is included, conversion rate of V⁵⁺ reach ∼99.6 %. When VRFB single cells using three electrolytes (pristine V⁴⁺ electrolyte, V⁴⁺ electrolytes treated with oxalic acid, and V⁴⁺ electrolyte treated with ascorbic acid) are run, they show similar efficiencies. Even in stability tests operated for 300 cycles at 200 mA cm⁻², those including ascorbic acid indicate stable efficiencies. These results suggest that electrolyte treated with ascorbic acid is very effective for VRFBs regarding cost, performance, and stability.