Revisiting the attenuation mechanism of alkaline all-iron ion redox flow batteries

Alkaline all-iron ion redox flow batteries (RFBs) based on iron (III/II) complexes as redox pairs are considered promising devices for low-cost and large-scale energy storage. However, present alkaline all-iron ion RFBs suffer from the issue of capacity decay, and the deeper mechanisms are elusive. Here, the attenuation mechanism of alkaline all-iron ion flow batteries is investigated by the capacity-unbalance cells combining iron (III/II)-cyanide complexes (Fe(CN)6) in positive electrolyte and iron (III/II)-sulfonated triethanolamine complexes (Fe(DIPSO)) in negative electrolyte. It is found that the Fe(CN)63− shows strong oxidizing power in the alkaline medium and can oxidize the free ligands migrated from the negative electrolyte. Further studies reveal that the chemical reaction between Fe(CN)63− and free ligands is intermediated by the hydroxyl radical (•OH). This side reaction leads to the capacity imbalance, which is the root cause of capacity decay. Thanks to without irreversible decomposition and deposition of redox species, the capacity decay can be completely restored by rebalancing capacity through an oxygen exposure process, indicating a great potential for long-duration energy storage. This work clarifies the attenuation mechanism and offers insights for the subsequent optimization of alkaline all-iron ion RFBs.