Addition of weak acids in electrolytes to prevent osmosis in aqueous organic redox flow batteries

Abstract Water transfer issues related to unbalanced ionic strengths in aqueous organic redox flow batteries (AORFBs) are often overlooked. In this work, a simple strategy based on the addition of a weak acid in the negolyte was studied to prevent osmosis in AORFBs. Ferrocyanide [Fe(CN)6]4- and anthraquinone-2,7-disulfonic acid disodium salt (AQDS) were used as positive and negative active materials, respectively, and aspartic acid was chosen as a weak acid. The addition of acid aspartic as well as strong bases such as KOH, NaOH or LiOH to reach the complete deprotonation of the acid and the desired pH contributes to balancing the cations total concentration of both compartments. Battery cycling experiments were carried out with 0.2 – 0.3 M AQDS and 0.4 – 0.6 M [Fe(CN)6]4−. When no weak acid was used, osmosis quickly leading to a shortage of negative electrolyte was observed, while in presence of aspartic acid both electrolyte volumes remained unchanged during the time of experiment.