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

Abstract

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%. A compound that can replace corrosive metal salts in fuel-cell-like ‘flow’ batteries has been discovered by a team in Germany. Redox flow batteries produce electrochemical energy by moving an electrolyte between separated cathode and anode compartments. These devices, which can be recharged nearly instantly by replacing the electrolyte, are set to become more sustainable thanks to a chemical synthesis developed by Ulrich S. Schubert from Friedrich Schiller University and colleagues. The team designed an organic molecule capable of acting as both battery anode and cathode through aromatic nitronyl nitroxide groups that release or take in electrons at moderate voltages. Attaching the aromatic redox units to a glycol chain gave the compound the solubility needed to make a flow battery with better capacity and efficiency than previous all-organic attempts. Redox-flow batteries (RFBs) seem to be tailor-made for the storage of sustainably generated electrical energy and are, in particular combined with photovoltaics and wind farms, well suited for the establishment of smart grids, which are essential to render the ‘Energiewende’. Therefore, an all-organic symmetric RFB with a bipolar molecule containing two redox-active nitronyl nitroxide units, connected via a tetraethylene glycol chain, as a charge-storage material, was developed. The synthesized molecule was electrochemically characterized in detail and its potential as organic redox-active material for an application in a symmetric RFB was investigated in a static and in a pumped laboratory test cell as well.