On In–situ Redox Balancing of Vanadium Redox Flow Battery Using D‐Fructose as Negative Electrolyte Additive

Abstract

AbstractVanadium redox flow battery (VRFB) is an energy storage system, wherein V2+/V3+ and VO2+/VO2+ are used as negative and positive electrolyte respectively. It is well known that, V2+/V3+ redox reaction is sluggish in comparison to that of VO2+/VO2+ reaction. As the redox potential of V2+/V3+ redox‐couple is more negative to that of H+/H2 redox‐couple, during the V2+ formation hydrogen evolution occurs concomitantly, which affects the capacity retention of VRFB inducing redox couples concentration imbalance between the positive and negative electrolytes. In this study, we have explored the beneficial effect of D‐fructose as an additive to the negative electrolyte. D‐fructose (i) enhances the interfacial area of the graphite felt negative electrode‐electrolyte interface by wetting, thereby the current due to V2+/V3+ redox reaction at a given overpotential, (ii) suppresses the H2 evolution at negative electrode during charging of VRFB (iii) controls the VO2+ accumulation at the positive electrolyte and (iv) chemically reduces the VO2+ arriving at negative electrode side through crossover, thereby avoiding the direct reaction between V2+ and VO2+. A capacity retention of 86 % and 25 % is achieved at the end of the 25th cycle in the presence and absence of D‐fructose in the negative electrolyte, respectively. This way of in‐situ redox balancing alleviates the requirement for external redox balancing of the electrolyte, and help VRFB to deliver constant capacity with cycles.