An Electrolyte Rebalancing Procedure for Vanadium Redox Flow Batteries

A new method is proposed that restores the battery energy and capacity of a Vanadium Redox Flow Battery, by counteracting the charge imbalance caused by air-oxidation and hydrogen evolution in the negative electrolyte. The process makes use of a regeneration system specifically designed and successfully tested. This system is built around an electrolysis cell, that is called "regeneration cell", where an electro-reduction reaction of V(V) is coupled with an oxygen evolution reaction catalyzed by a Ti-IrO2 electrode. The rebalancing of three different unbalanced electrolytes was tested: V3.55+, V3.60+ and V3.65+, namely: 45% V(III)/55% V(IV), 40% V(III)/60% V(IV) and 35% V(III)/65% V(IV) respectively. The behavior of the VRFB cell during the rebalancing process was investigated both numerically and experimentally. An end-point strategy for the rebalancing process without additional sensors has been developed by using of a specific mathematical model capable of simulating the current response and concentrations profile during the rebalancing process. This strategy allowed to improve the performance of the rebalancing process. After rebalancing, the resulting electrolyte oxidation states were respectively: V3.483+, V3.482+, V3.484+. Since a perfect balanced electrolyte is V3.50+ (50% V(III), 50 % (VIV) such results demonstrated that the electrolyte imbalance was drastically reduced and the rebalancing process was successful.