Computational design of C-substituted paraquat/diquat derivatives for neutral aqueous organic redox flow batteries

Abstract

The redox-active electrolyte materials are critical components in redox flow batteries (RFBs). Quaternary bipyridinium derivatives, such as viologens (4,4′-bipyridine double quaternary salts), represent an important class of organic anolyte materials that can be well-matched with cheap and non-corrosive pH-neutral aqueous electrolytes in RFB applications. Herein, to search for promising quaternary bipyridinium compounds, we investigated the redox potentials and kinetic rate constants of 228 C-substituted quaternary bipyridinium derivatives that either contain paraquat or diquat parent structures by applying density functional theory (DFT) calculations. The analysis of the results reveals the effects of the terminal functional groups, number of methylene units, substitution sites and parent structure on the electrochemical properties. On this basis, we identify several promising candidates for the anolyte materials of neutral aqueous organic RFBs and further assess their aqueous solubility and structural stability through DFT theoretical calculations.