Aryl Viologens: Unprecedented Stability of Viologen-Derivatives as Anolytes for Alkaline Redox Flow Batteries

Viologen derivatives are widely used in the anolytes of aqueous organic redox flow batteries (AORFBs). However, their applications have been restricted to neutral pH systems due to their fast degradation in basic media via a dealkylation process driven by a nucleophilic attack of hydroxide. In this study, a family of viologen-based anolytes suitable for alkaline systems is introduced, demonstrating that properly designed viologens can also be used in alkaline conditions. A variety of N-aryl viologens are prepared and characterized, showing that the dealkylation process is prevented by bonding an aryl group directly to the N-atom of the bipyridine core. Pairing B-2,5-DHPV for the anolyte and K₄Fe-(CN)₆ for the catholyte, a full alkaline AORFB having a nominal cell voltage at 0.98 V maintains stable capacity over 1400 continuous cycles with nearly 0.03%·h^-1 capacity decay, which is a very acceptable value for a viologen in an alkaline medium. Our results enable the broadening of the range of viable organic anolytes for alkaline AORFBs.