A low-cost average valence detector for mixed electrolytes in vanadium flow batteries.

The decay in capacity has hindered the applications of vanadium redox flow batteries (VFBs), which are promising energy storage devices with many benefits. Mixing positive and negative electrolytes can recover some of the lost capacity but is ineffective towards the increasing average valence of the mixed electrolyte caused by the side reactions. In this study, a low-cost optical average valence detector has been developed to monitor the average valence of the mixed electrolyte in VFBs. We demonstrate experimentally that with the aid of the average valence detector, the capacity of VFBs can be regularly recovered via electrolyte mixing and online electrolysis. This low-cost average valence detector has great potential for recovering the decayed capacities of VFBs in large-scale energy storage stations, which consist of thousands of VFBs.