Iron Complex Posolyte Species Possessing Low Capacity Fade Rate Combined with Large Positive Redox Potential for Aqueous Organic Flow Batteries

Redox flow batteries are promising technologies for energy storage and conversion applications. Among different types of flow batteries, iron-based redox-active materials can have cost and environmental advantages. We introduce a high-potential tris(2,2'-bipyridine-4,4'-diyldimethanol) iron dichloride compound as a posolyte species with a redox potential more than 0.5 V more positive than that of ferro/ferricyanide. The new species, abbreviated Fe(Bhmbp) 3 , operating at near-neutral pH, was paired with bis(3-trimethylammonio)propyl viologen tetrachloride as the negolyte species. The resulting aqueous flow battery exhibits an open-circuit voltage of 1.36 V and it shows excellent cycling stability with a capacity fade rate of 0.0007% per cycle or 0.07% per day. In comparison with previously reported organic and metalorganic compounds, Fe(Bhmbpy) 3 exhibits a superior combination of high positive redox potential and low capacity fade rate. We investigated the decomposition pathways during cell cycling to an unprecedented level of depth for an aqueous posolyte. We found that the compound degrades through self-discharge, dimerization and ligand dissociation. The proposed decomposition pathways explain the capacity fade phenomena and offer opportunities for future improvement of metalorganic posolytes with high redox potential and good cycling stability