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 Ah/L, which is around 70 times higher than that of its precursor 1,8-DHAQ (0.567 Ah/L) 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%/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.