High Energy Density Aqueous Flow Battery Utilizing Extremely Stable, Branching-Induced High-Solubility Anthraquinone near Neutral pH
Emily F. Kerr, Zhijiang Tang, Thomas Y. George, Shijian Jin, Eric M. Fell, Kiana Amini, Yan Jing, Min Wu, Roy G. Gordon, Michael J. Aziz
Abstract
An anthraquinone featuring a chiral carboxylate-capped methyl-branched side chain with an ether linkage, 2,2′-((9,10-dioxo-9,10-dihydroanthracene-2,6-diyl)bis(oxy))dipropionic acid (2,6-D2PEAQ), was synthesized and evaluated for use in aqueous redox flow batteries. It was found to have an extraordinary solubility of 2 M (4 M electrons), corresponding to a theoretical volumetric capacity of 107.2 Ah/L for the negative electrolyte, which is 10 times that of its unbranched counterpart. The 2,6-D2PEAQ molecule demonstrated stability against thermal decomposition and was extremely stable under cell cycling conditions. A capacity fade rate of 0.02%/day over 14 days was demonstrated in a 1.1 M 2,6-D2PEAQ nearly capacity-balanced cell when paired with a ferro-/ferricyanide posolyte at pH 7. Compared to other aqueous redox-active organic molecules, its demonstrated fade rate is lower than that of any molecule with a demonstrated volumetric capacity of ≥55 Ah/L, and its volumetric capacity is greater than that of any molecule with a demonstrated fade rate of ≤0.5%/day.