Tackling Faradaic Imbalance in Redox Flow Batteries by the Use of a Solid Reducing Agent

Redox flow batteries (RFBs) represent a promising technology for large-scale energy storage. However, they suffer from capacity fading due to various factors, including desynchronization in the state of charge of the anolyte and catholyte, often caused by irreversible electrochemical side reactions. This study proposes a novel strategy to mitigate and reverse the effects of the faradaic imbalance by, for the first time to the best of our knowledge, introducing a solid reducing agent, LiFePO₄ (LFP), in the catholyte compartment. The use of a heterogeneous reaction facilitates the removal of the reaction product, in contrast to homogeneous reducing agents. The strategy is implemented in a battery comprising K₄Fe-(CN)₆ as the catholyte and a viologen, 1,1'-bis-(3-sulfonatopropyl)-4,4'-bipyridinium (BSPV), as the anolyte in 1M KCl supporting electrolyte at neutral pH. The presence of trace oxygen in the anolyte leads to the accumulation of K₃Fe-(CN)₆ in the catholyte, resulting in a faradaic imbalanceused here as a case study. Introducing LFP pellets into the catholyte chemically reduces the accumulated K₃Fe-(CN)₆ back to K₄Fe-(CN)₆ via a spontaneous redox process, accompanied by the oxidation of LFP to FePO₄, as confirmed by XRD analysis. Implementation of this method in a flow cell with a capacity-limiting catholyte results in a significant recovery of the lost capacity, which is attributed to the reduction of accumulated K₃Fe-(CN)₆ by the LFP pellets. This study presents a promising approach to addressing the faradaic imbalance in RFBs, potentially leading to improved performance and extended operational lifetime of these systems.