Concentration-Dependent Dimerization of Anthraquinone Disulfonic Acid and Its Impact on Charge Storage

9,10-Anthraquinone-2,7-disulfonic acid (AQDS) is considered a benchmark active material for aqueous organic redox flow batteries. At low concentration, AQDS demonstrates two-electron transfer at near ideal electrochemical reversibility; however, at higher concentration, AQDS displays more complex behavior presumably due to the emergence of intermolecular reactions. Here, we systematically examine the electrochemical and physical properties of AQDS solutions using a suite of electrochemical, analytical, and spectroscopic techniques. Depending on the AQDS pretreatment, concentration, solution pH, and electrolyte composition, coupled chemical and electrochemical reactions lead to different charge storage capabilities. To elucidate the underlying cause of these differences, we performed various pretreatments of AQDS, examined chemical speciation by NMR, and investigated the corresponding electrochemical properties through cyclic voltammetry and bulk electrolysis. In all cases, reversible intermolecular dimerization was detected at solution concentrations greater than 10 mM. Moreover, we found that the charge state of the formed dimers was dependent on the AQDS pretreatment and the solution pH. Under acidic conditions, 1.5 electrons per molecule of AQDS were reversibly accessible, whereas under buffered mild-alkaline conditions, only one electron per molecule of AQDS was accessible. Because of insufficient proton concentration, AQDS did not cycle reversibly in unbuffered neutral electrolyte. Even when employing chemical oxidants during a chemical titration, charge storage of two electrons per molecule could not be realized. We hypothesize that adduct formation between AQDS and CO₂, along with solution pH, play important roles in the charge accessibility.