Detection of capacity imbalance in vanadium electrolyte and its electrochemical regeneration for all-vanadium redox-flow batteries

A vanadium electrolyte for redox-flow batteries (VRFB) with different VIII and VIV mole fractions has been studied by UV-vis spectroscopy. Spectrophotometric detection enables a rough estimate of the VIV and VIII content, which can be used to detect an electrolyte capacity imbalance, i.e. a deviation in the mole fraction of VIV or VIII away from 50%. The isosbestic point at 600 nm can be used as a reference point in the analysis of common VRFB electrolyte batches. The VRFB electrolyte is observed to have an imbalance after prolonged storage (a couple of years) in a tank under ambient conditions. A regeneration procedure, which involves pre-charging the unbalanced electrolyte and mixing part of it with a portion of initial unbalanced electrolyte, has been tested. The resulting rebalanced electrolyte has been compared with a common electrolyte in a charge-discharge cell test and is shown to be suitable for cell operation.