Enhancing Solubility of Anthrarufin by Tethering Alkyl Phosphonate and Mitigating Capacity Decay with Additive in Aqueous Organic Redox Flow Batteries

Abstract

Aqueous organic redox flow battery is well-known for its high-power density, excellent charge-discharge, and long cycle life. The use of redox-active organic materials benefits from their low cost, vast abundance, variable solubility with functionalization and potential to store energy. Here, we introduced an ether-linked alkyl phosphonic acid group onto anthrarufin to form (((9,10-dioxo-9,10-dihydroanthracene-1,5-diyl)bis(oxy))bis(propane-3,1-diyl))bis(phosphonic acid) (1,5-DPAQ) to increase the solubility from 0.07 M to 0.69 M in 1 M KOH to enhance energy density. However, an oxygen evolution reaction occurring at the catholyte, oxidation of hydroquinone of 1,5-DPAQ with diffused oxygen, and charge transfer complex formation between 1,5-DPAQ and its hydroquinone form is limiting the discharge capacity. To mitigate this charge-transfer complex, an additive N,N,N’,N’-tetramethyl-1,3-propanediamine was introduced for the first time at the anolyte side, resulting in a capacity regain and increase in cell voltage with cycling due to the availability of 1,5-DPAQ during cell cycling. Keywords: Aqueous organic redox flow battery; additive; charge-transfer complex; oxygen evolution; phosphonate group; power density; solubility.