Nonionic oligo(ethylene glycol)-substituted viologen negolytes for aqueous organic redox flow batteries

Viologen derivatives are attractive active materials for negolytes in aqueous organic redox flow batteries (AORFBs) owing to their fast kinetics and suitable reduction potentials. However, viologens with alkyl substitutions only (e.g., methyl viologen and ethyl viologen) suffer from poor stability and the insoluble nature of the second-electron product limits their energy density. A key to enabling durable and high-capacity viologen-based AORFBs lies in the molecular engineering of viologens. Here we demonstrated a series of oligo(ethylene glycol) (OEG) substituted viologens with improved cycling stability via the steric-hindrance effect and enhanced water solubility of the second-electron product. Electrochemical and symmetric cell investigations validated the high reversibility of tri(ethylene glycol) groups modified viologen (Vi-OEG3) and its improved solubility of the second-electron products in neutral aqueous solutions. When paired with a ferrocene-based posolyte, a full flow cell with Vi-OEG3 showed a much reduced capacity decay rate of 0.00253% per day or 0.000105% per cycle compared with ethyl viologen. This work expands the molecular design of viologens and shows an attractive viologen-based AORFB for low-cost and large-scale renewable energy storage systems.