Extremely Stable Anthraquinone Negolytes Synthesized from Common Precursors

Synthetic cost and long-term stability remain two of the most challenging barriers for the utilization of redox-active organic molecules in redox flow batteries for grid-scale energy storage. Starting from potentially inexpensive 9,10-dihydroanthracene, we developed a new synthetic approach for two extremely stable anthraquinone negolytes, i.e., 3,3'-(9,10-anthraquinone-diyl)bis(3-methylbutanoic acid) (DPivOHAQ) and 4,4'-(9,10-anthraquinone-diyl)dibutanoic acid (DBAQ). Pairing with a ferrocyanide posolyte at pH 12, DPivOHAQ and DBAQ can transfer up to 1.4 and 2 M electrons with capacity fade rates of 0.014% per day and 0.0084% per day, respectively, and exhibit 1.0 V of open-circuit voltage. By adjusting the supporting electrolytes to pH 14, DPivOHAQ exhibited a record low capacity fade rate of <1% per year. We attribute the capacity loss of these flow batteries primarily to the formation of anthrone, which can be suppressed by increasing the pH of the electrolyte and reversed by exposure to air.