Quantifying effect of faradaic imbalance and crossover on capacity fade of vanadium redox flow battery

In this work, we aimed to reveal two main contributors to the capacity fade of a vanadium redox flow battery (VRFB). These contributors are the oxidative imbalance caused by the hydrogen evolution reaction (HER) competing with V3+ reduction during charging, and crossover, particularly the net transfer of vanadium ions from negolyte to posolyte. To investigate this, we performed VRFB cycling under various operation conditions with sequential monitoring of the electrolytes composition. We discovered that shortly after cycling starts, the crossover makes the negolyte capacity-limiting. This leads to high polarization of the negative electrode inducing HER and increasing average oxidation state (AOS) of the electrolyte. Our examination shows that the magnitude of the oxidative imbalance correlates with that of the crossover, both being dependent on the state-of-charge (SoC). Therefore, by changing operation conditions, we can slightly influence the crossover, while dramatically affect the oxidative imbalance. We also found that the resulting magnitude of the capacity fade is a trade-off between these two side-processes. This necessitates careful optimization of the battery and electrolyte composition, along with its cycling regime. From the data obtained, we conclude that the only proper approach to achieve lower capacity fade is by ensuring that the negolyte is capacity-limiting over long-term cycling. We hope that the data on the capacity fade mechanisms presented here will facilitate development of more stable and costeffective VRFBs.