Calibration-free coulometric sensors for operando electrolytes imbalance monitoring of vanadium redox flow battery

It is necessary for vanadium redox flow battery (VRFB) to become more cost-effective due to long-term stable operation with minimal life-cycle maintenance for its further development. Despite the absence of self-discharge in idle mode, the available capacity of VRFB gradually decreases during the battery cycling. The capacity decrease is attributed to side reactions appearance, which increase the average oxidation state (AOS) of vanadium electrolyte. This negative effect can be minimized by periodic electrolyte capacity recovery. However, it is crucial to have reliable information on the electrolytes composition to conduct rebalancing efficiently and timely. Various methods for vanadium electrolyte assessment have been previously proposed, however, none of them can be used coupled with industrial VRFBs. In this work, we present an operando coulometric analysis of an electrolyte sample in a VRFB cell and its subsequent return to the bulk solution. The proposed approach outperforms other methods due to sensitivity of the signal to the overall vanadium content and AOS and stable performance under imbalance conditions. The analysis of AOS was verified by three methods – coulomb-counting, ex-situ spectrophotometry, and coulometry. Therefore, this method can be successfully used not only in lab-scale VRFB studies, but can also be coupled with industrial VRFB stacks. •The coulometric analysis outperform known SoC determination methods.•Electrolyte coulometric analysis was performed operando in flow cell.•The method is calibration- and sampling-free, and supporting electrolyte-independent.•The analysis remains relevant under VRFB imbalance.•The proposed sensors can be integrated in VRFB BMS.