Modified viologen as an efficient anolyte for aqueous organic redox flow batteries

In this study, we adopted an asymmetric viologen [1-propyl-1′-(3-triethylammonio) propyl-4,4′-bipyridinium tribromide, (PV3+)], wherein the presence of triethylammonium group enhances the radical solubility and effectively diminishes the radical dimerization confirmed by UV–Vis spectroscopy, and the presence of the propyl group lowers the half reduction potential. The PV3+ (0.5 mM) when coupled with 4-OH-TEMPO (0.5 mM) shows a cell voltage of 1.16 V and delivered a maximum energy efficiency of 61.25 % at 8 mA cm−2 current density even at very low concentration. The as configured flow cell offers a stable long cycling performance up to 100 cycles with 95 % coulombic efficiency.