A bipolar nitronyl nitroxide small molecule for an all-organic symmetric redox-flow battery

An all-organic symmetric redox-flow battery (RFB) that employs nitronyl nitroxide (NN) units as a bipolar redox-active charge-storage material was designed and investigated. An organic molecule possessing two bipolar redox-active NN units connected via a tetraethylene glycol chain was synthesized for this purpose. Owing to the ethylene glycol chain, this molecule demonstrates good solubility in organic solvents. The electrochemical behavior of the obtained compound was investigated via cyclic voltammetry (CV) measurements and it features quasi-reversible redox reactions of the NN+/NN redox couple at E½=0.37 V and the NN/NN− redox couple at E½=−1.25 V versus AgNO3/Ag, which led to a promising cell voltage of 1.62 V in a subsequent battery application. A static solution-based battery exhibits a stable charge/discharge performance over 75 consecutive cycles with a high energy efficiency of 82% and an overall energy density of the electrolyte system of 0.67 W h l−1. In addition, a pumped RFB test demonstrates an overall energy density of the electrolyte system of 4.1 W h l−1 and an energy efficiency of 79%.