An Asymmetric Viologen-Based Negolyte With A Low Redox Potential For Neutral Aqueous Redox Flow Batteries

Aqueous redox flow batteries (ARFBs) with neutral electrolytes show promise for large-scale energy storage owing to their relatively low cost and inherent safety. However, the performance of these ARFBs has been limited by the low solubility of electroactive species and/or low cell voltages. In this study, an asymmetric viologen-based compound called MMV (1-Methyl-4,4 '-bipyridylium iodide) was assessed as the redox active compound in the negolyte of ARFBs. Inexpensive starting materials and a simple synthesis route yielded MMV at a low cost. MMV exhibited a solubility of similar to 3 M in water, and electrochemical measurements confirmed that the reaction of MMV involves the transfer of a single electron with relatively fast kinetics. Importantly, MMV with an asymmetric structure demonstrated a redox potential of -1.05 V vs SCE, one of the most negative potentials reported for an electroactive organic compound in neutral electrolytes. MMV, however, showed poor cycling performance at high concentrations. Signs of dimerization and precipitation were observed, resulting in measured capacities significantly lower than theoretical values and a rapid fade rate. It should be possible to avoid these problems by synthesizing asymmetric MMV derivatives with a higher charge to preserve high solubility and low redox potential.