Long-lifetime, potentially low-cost anthraquinone flow battery chemistry developed from study of effects of water-solubilizing group and connection to core

Abstract

Water-soluble anthraquinones (AQs) hold great promise serving as redox-active species in aqueous organic redox flow batteries. Systematic investigations into how the properties of redox molecules depend on the water-solubilizing groups and the way in which they are bound to the redox core are, however, still lacking. We introduce water-solubilizing groups linked to anthraquinone by C=C bonds via Heck cross-coupling reactions and convert C=C bonds to CC bonds through hydrogenation. The anthraquinone and the ending groups are connected via branched or straight chains with either unsaturated or saturated bonds. We investigate the influence of water-solubilizing chains and ionic ending groups on redox potentials of molecules and identify three important trends. (1): The electron-withdrawing ending groups can affect redox potentials of AQs with two unsaturated hydrocarbons on the chains through π-conjugation. (2): For chains with two saturated or unsaturated straight hydrocarbons, water-solubilizing ending groups increase redox potentials of the AQs in the order of PO32 <CO2<SO3. (3): AQs with saturated and unbranched chains at high pH possess desirably low redox potentials, high solubilities, and high stability. Disproportionation leads to the formation of anthrone, which can be regenerated to anthraquinone. Tautomerization results in the saturation of alkene chains, stabilizing the structure. We utilize these observations to identify a potentially low-cost and long-lifetime negolyte that demonstrates a temporal fade rate as low as 0.0128%/day when paired with a potassium ferrocyanide posolyte.