Redox flow batteries (RFBs) offer several advantages over enclosed batteries in grid-scale applications such as decoupled power rating (reactor size) and energy capacity (tank size), high active-to-inactive materials ratio (especially at long durations), and improved safety characteristics.1 Recent literature has focused on emerging aqueous organic RFBs (AORFB) which employ abundant redox active organic species dissolved in an inexpensive water-based electrolyte. While many new organic redox couples have been proposed, to date, quinones have achieved the greatest success with 9,10-Anthraquinone-2,7-disulfonic acid (AQDS) being the most promising and widely reported derivative. AQDS is considered a benchmark compound for AORFBs2–5 due to its good charge retention (days), low crossover rates (through Nafion), high solubility (>1 M), and low redox potential (~0.2 V vs RHE). Some studies report idealized electrochemical behavior of AQDS at low concentrations (1 mM),6 however, at higher concentrations AQDS deviates from ideal behavior and undergoes dimerization, contributing to changes in energy density, solubility, and redox potential.
In this work, we examine the nuances of AQDS in aqueous solution utilizing cyclic voltammetry (CV), bulk electrolysis (BE), chemical titration, and nuclear magnetic resonance (NMR) spectroscopy. Depending on the AQDS concentration, electrolyte choice, and time scale (electrolytic vs. voltammetric), the electrochemical behavior can vary and follow different mechanistic pathways. To elucidate the reaction mechanism, we synthesize various derivatives of AQDS and examine their electrochemistry through CV and BE, as well as their chemical structures by NMR. Further, we correlate the dimer structure of AQDS in solution with the observed electrochemistry to explain the variations in accessible capacity and to provide guidelines for maximizing storage performance. The complex, solution-phase interactions of AQDS suggest that other organic compounds may display similar (or more convoluted) phenomena. The suite of chemical and electrochemical studies developed in this study are general and can be applied to other organic candidates for RFBs to understand known issues with energy density, solubility, and stability.
We thank the Joint Center for Energy Storage Research for financial support. This research was conducted with Government support under and awarded by DoD, Air Force Office of Scientific Research, National Defense Science and Engineering Graduate (NDSEG) Fellowship, 32 CFR 168a.
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