A Phosphonate‐Functionalized Quinone Redox Flow Battery at Near‐Neutral pH with Record Capacity Retention Rate

TLDR

The study introduces a highly stable phosphonate‑functionalized anthraquinone as the negative electrolyte for near‑neutral aqueous redox flow batteries. The authors synthesize 2,6‑DPPEAQ, a phosphonate‑functionalized anthraquinone with high solubility at pH 9 and above, enabling its use as a negolyte. Chemical stability tests show the negolyte remains stable at pH 9 and 12, and when paired with potassium ferri/ferrocyanide it delivers a 1.0 V open‑circuit voltage and an unprecedented capacity fade rate of 0.00036 % per cycle (0.014 % per day), while its pH oscillates between 9 and 12 in the absence of atmospheric oxygen, demonstrating its suitability for large‑scale energy storage.

Abstract

Abstract A highly stable phosphonate‐functionalized anthraquinone is introduced as the redox‐active material in a negative potential electrolyte (negolyte) for aqueous redox flow batteries operating at nearly neutral pH. The design and synthesis of 2,6‐DPPEAQ, (((9,10‐dioxo‐9,10‐dihydroanthracene‐2,6‐diyl)bis(oxy))bis(propane‐3,1‐diyl))bis(phosphonic acid), which has a high solubility at pH 9 and above, is described. Chemical stability studies demonstrate high stability at both pH 9 and 12. By pairing 2,6‐DPPEAQ with a potassium ferri/ferrocyanide positive electrolyte across an inexpensive, nonfluorinated permselective polymer membrane, this near‐neutral quinone flow battery exhibits an open‐circuit voltage of 1.0 V and a capacity fade rate of 0.00036% per cycle and 0.014% per day, which is the lowest ever reported for any flow battery in the absence of rebalancing processes. It is further demonstrated that the negolyte pH drifts upward upon atmospheric oxygen penetration but, when oxygen is excluded, oscillates reversibly between 9 and 12 during cycling. These results enhance the suitability of aqueous‐soluble redox‐active organics for use in large‐scale energy storage, potentially enabling massive penetration of intermittent renewable electricity.