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Abstract A highly stable phosphonate‐functionalized viologen is introduced as the redox‐active material in a negative potential electrolyte for aqueous redox flow batteries (ARFBs) operating at nearly neutral pH. The solubility is 1.23 m and the reduction potential is the lowest of any substituted viologen utilized in a flow battery, reaching −0.462 V versus SHE at pH = 9. The negative charges in both the oxidized and the reduced states of 1,1′‐bis(3‐phosphonopropyl)‐4,4′‐bipyridine‐1,1′‐diium dibromide ( BPP−Vi ) effect low permeability in cation exchange membranes and suppress a bimolecular mechanism of viologen decomposition. A flow battery pairing BPP−Vi with a ferrocyanide‐based positive potential electrolyte across an inexpensive, non‐fluorinated cation exchange membrane at pH = 9 exhibits an open‐circuit voltage of 0.9 V and a capacity fade rate of 0.016% per day or 0.00069% per cycle. Overcharging leads to viologen decomposition, causing irreversible capacity fade. This work introduces extremely stable, extremely low‐permeating and low reduction potential redox active materials into near neutral ARFBs.
Jin et al. (Mon,) studied this question.
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