Evaluation of an Aqueous Biphenol- and Anthraquinone-Based Electrolyte Redox Flow Battery

Flow cell batteries are of particular interest for applications of large-scale energy storage from renewable sources (e.g., wind, solar, etc.), as these energy sources are often intermittent or vary periodically. Recently, aqueous soluble organic redox species have been studied as alternative electrolytes to overcome the limitations of current vanadium chemistry, namely, low energy density, moderate charge/discharge efficiency, cost, and toxicity. Here, sulfonated derivatives of 4,4′-biphenol and 1,8-dihydroxy­anthraquinone were synthesized and electrochemically characterized to evaluate their potential battery performance. Long-term charge/discharge cycling measurements of the flow cell battery were obtained to determine the energy efficiency, charge capacity, and chemical stability of the electrolyte system. These electrolytes produced a standard cell potential of 0.905 V with a current density up to 0.8 A/cm2 and energy density of 45 Wh/L using 1 M electrolyte solutions, which is competitive with current vanadium-based electrolyte systems. The biphenol derivative appears to undergo a hydroxylation side reaction during charging cycles; however, once this reaction is complete Coulombic efficiency of the flow battery stabilizes at >95%.