Turning Adversity into Advantage: Investigating the Capacity Decay Mode of Carboxylate Functionalized-Anthraquinone in Organic Redox Flow Batteries

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