Investigation of ICRFB hybrid rebalancing systems

Abstract

Due to their intermittent nature, the increased use of renewable energy sources requires further development in energy storage systems. Redox flow batteries (RFBs) have emerged as a promising solution, where the iron-chromium RFB (ICRFB), due to its lower electrolyte costs, offers a cheaper alternative to the established all-vanadium RFB (VRFB). However, the ICRFB system requires a rebalance system to mitigate its capacity decay. This study aimed to assess the operational suitability of rebalancing with either iron or vanadium as proposed in a patent by Wei and Li. The iron-based rebalancing successfully restored lost capacity of an imbalanced electrolyte but faced challenges in recovering the spent electrolyte due to the required addition of iron powder, which led to an increased iron concentration requiring correction with chromium chloride in HCl, effectively making new electrolyte. While the vanadium rebalancing method was suitable, it resulted in vanadium crossover into the RFB when using cation exchange membranes (Nafion 212, 115 or 117) in the rebalance cell. Long-term operation (200 cycles), using an anion exchange membrane (FAP450), showed reduced crossover but presented challenges such as active species loss, HCl loss, and membrane fouling.