High-performance anthraquinone with potentially low cost for aqueous redox flow batteries

Electrolyte cost and long-term durability are the two most challenging obstacles to the practical utilization of redox-active organics in aqueous redox flow batteries. Starting from potentially inexpensive 1,8-dihydroxyanthraquinone (1,8-DHAQ), we developed a one-pot, green, and scalable approach to synthesize a highly water-soluble and potentially low-cost anthraquinone 1,8-dihydroxy-2,7-dicarboxymethyl-9,10-anthraquinone (DCDHAQ). The demonstrated volumetric capacity of DCDHAQ in 1 M KOH is 40.2 A h L⁻¹, which is around 70 times higher than that of its precursor 1,8-DHAQ (0.567 A h L⁻¹) at pH 14. The introduction of –CH₂CO₂⁻ as solubilizing groups suppressed the disproportionation reaction of reduced anthraquinone both thermodynamically and kinetically. Consequently, the cycling stability of anthraquinone was improved significantly compared to that of the precursor. Pairing a negolyte comprising 0.75 M DCDHAQ with a posolyte comprising 0.3 M ferrocyanide at pH 14, we demonstrated a cell with an open-circuit voltage of 1.1 V and a low capacity fade rate of 0.03% per day. The synthetic method of attaching –CH₂CO₂⁻ as solubilizing groups is likely applicable to other anthraquinone derivatives and other aromatic organics as an inexpensive approach to performance enhancement in energy applications.