Turning Adversity\ninto Advantage: Investigating the\nCapacity Decay Mode of Carboxylate Functionalized-Anthraquinone in\nOrganic Redox Flow Batteries

Organic redox-molecule-based flow batteries (ORFB) are\nconsidered\na potential alternative to the inorganic counterparts in flow battery\nsystems as, technically speaking, organic materials are ubiquitous\nand can be synthesized anywhere. Nevertheless, they were also known\nto degrade in a multitude of ways in flow battery ambience. In this\nstudy, 4,4′-((9,10-dioxo-9,10-dihydroanthracene-1,5-diyl)bis(oxy))dibutyric\nacid (1,5-DCAQ) is used as an anolyte in alkaline media. As carboxylate\ninteracts intramolecularly with the carbonyl group of 1,5-DCAQ, the\naromatic portion of the molecule exhibits an association with the\nNafion membrane separator through lipophilic interaction, causing\ncapacity decay of the cell. We have modified this curse into a boon\nby functionalizing the thermally activated graphite felt anode with\nthe Nafion ionomer, thereby retaining the redox molecule around the\nelectrode instead of the membrane, achieving 99.9% of theoretical\ncapacity and 95% Coulombic efficiency at 15 mA cm<sup>–2</sup> current density. As this molecule exhibits a solubility of 0.5 M,\nthe system with suitable optimization is expected to deliver a solubility\nof up to 26.8 Ah L<sup>–1</sup>.