Screening Viologen Derivatives for Neutral Aqueous Organic Redox Flow Batteries

Viologen derivatives have been developed as negative electrolyte for neutral aqueous organic redox flow batteries (AOFBs), but the structure-performance relationship remains unclear. Here, it was investigated how the structure of viologens impacts their electrochemical behavior and thereby the battery performance, by taking hydroxylated viologens as examples. Calculations of frontier molecular orbital energy and molecular configuration promise to be an effective tool in predicting potential, kinetics, and stability, and may be broadly applicable. Specifically, a modified viologen derivative, BHOP-Vi, was proved to be the most favorable structure, enabling a concentrated 2 m battery to exhibit a power density of 110.87 mW cm^-2 and an excellent capacity retention rate of 99.953 % h^-1 .