Isopropyl alcohol and copper hexacyanoferrate boost performance of the iron tris‐bipyridine catholyte for near‐neutral pH aqueous redox flow batteries

Aqueous redox flow batteries (ARFBs) are poised to be a major energy storage technology when coupled with rapidly proliferating renewable wind and solar power generation. The iron (II/III) tris-bipyridine redox couple, Fe(bpy)32+/3+, possesses several highly attractive properties for its use as a catholyte in ARFBs: high redox potential, low cost, mild pH operation, and negligible irreversible chemical degradation. Nevertheless, its main drawbacks - low solubility and poor voltage efficiency - remain to be improved. In this work, we report on the success of isopropyl alcohol (IPA) and copper hexacyanoferrate (CuHCF) electrolyte additives that address these shortcomings and ultimately lead to improved ARFB performance. IPA affords a nearly 3-fold increase in solubility (energy density) and longer cycle lifetimes (lower capacity fade rate), without sacrificing voltage efficiency against the control case. CuHCF dramatically enhances Fe(bpy)32+/3+-based ARFB voltage efficiency by 15 + % through an apparent catalytic mechanism not observed before. We showcase these additives in a novel, near-neutral pH ARFB system, 2,7-AQDS-Fe(bpy)32+/3+, that affords the highest energy density and efficiency to-date for this catholyte. In addition, unlike most previous studies, multiple batteries were constructed to reproduce and confirm the results. The findings open avenues for further improvement of this promising catholyte through the tuning of additive structure, composition, and battery integration.