Steric hindrance of viologen: Enabling aqueous flow batteries in inert environments

Aqueous organic redox flow batteries (AORFBs) is a new momentum force for renewable energy generation, utilizing reversible electrochemical reactions of organic electrolytes to store electrical energy. Viologen is a promising candidate for AORFBs. However, viologen face accelerated degradation in the unavoidable presence of oxygen. Here, a water-soluble isopropyl viologen (Ipr-Pyr) is designed to avoid the nucleophilic attack of hydroxide ions by steric hindrance effect, thereby reducing the influence of oxygen. Under nitrogen conditions, the Ipr-Pyr/K 4 Fe(CN) 6 battery operate stably for 100 cycles with an average coulombic efficiency (CE) of 90.5 %, voltage efficiency (VE) of 69.2 % and energy efficiency (EE) of 62.9 %. The decay rate of charge-discharge capacity per cycle is 0.114 %. Nevertheless, the capacity of the Et-Pyr/K 4 Fe(CN) 6 battery decline to around 0.2 mAh at the 30th cycle. The results prove that the steric hindrance is also highly effective in reducing the impact of oxygen in AORFBs.