Highly Stable Asymmetric Viologen as an Anolyte for Aqueous Organic and Halide-Based Redox Flow Batteries

In redox flow batteries (RFBs), energy density mainly depends on the volume and concentration of active species in electrolyte. However, the poor solubility of organic molecules limits the further development of aqueous organic redox flow batteries (AORFBs). Herein, the viologen core structure is modified with asymmetric functional groups such as propyl and triethyl ammonium propyl for enhancing the solubility up to 1.82 m in aqueous medium. Further, the modified viologen structure lowers the redox potential (-0.58 V vs Ag/AgCl) and significantly reduces radical dimerization. Interestingly, synthesized [1-propyl-1'-(3-triethylammonio)propyl-4,4'-bipyridinium tribromide] (PV3+) coupled with bromide/bromine (2Br(-)/Br-2) redox species shows cell voltage (1.66 V). The tested PV3+/PV2+center dot||2Br(-)/Br-2 RFB having Nafion-117 membrane exhibits a round-trip efficiency of 99% over 100 cycles. The self-discharge behavior of PV3+/PV2+center dot||2Br(-)/Br-2 RFB system retains 85% of the cell voltage over 20 h, indicating the lowest permeability of bromine in RFB. Thus, the present approach of viologen modification with asymmetric functionalities drastically improves the RFB performance in terms of energy density and cycle life of the system.