Rebalancing/Regeneration of Vanadium Flow Batteries

Rebalancing and regeneration are essential to counteract the evolution of electrolyte imbalance in flow batteries (FBs). These effects have different physical and chemical causes and produce a progressive decay of capacity. In this chapter, the different causes of the electrolyte imbalance are discussed, which lead to concentration imbalance, oxidative imbalance, and reductive imbalance. The effect of crossover, namely the transfer of vanadium and solvent ions across the separator and the side reactions that can take place inside the cells during charge–discharge cycles are presented. These different events call for different recovery processes: physical treatments for concentration imbalance and chemical/electrochemical treatments for the oxidative and reductive imbalance can be used. In this regard, several methods have been developed and published, which follow either direction. Some methods are focused on physical treatments such as the remixing of the positive electrolyte with the negative one, or the interdiffusion of the surplus vanadium ions through the membrane. Other methods are focused on chemical and electrochemical regeneration procedures which make use of chemical reducing agents, catalysts, or electrochemical processes. The latter require the use of an electrolysis system to restore vanadium oxidation state to the correct ratio in the positive and negative electrolyte.