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%.
Highlights
State-of-the-art redox-flow batteries (RFBs) such as the wellinvestigated all-vanadium RFB1–4 contain metal salts and corrosive acidic electrolytes
Most aromatic nitronyl nitroxide (NN) exhibit a moderate to poor solubility in nitrile- and carbonate-based solvents, which are preferred for RFB applications due to their electrochemical stability
Compound 4 was synthesized in four synthesis steps from inexpensive commercially available materials
Summary
State-of-the-art redox-flow batteries (RFBs) such as the wellinvestigated all-vanadium RFB1–4 contain metal salts and corrosive acidic electrolytes. As these metals are generally obtained as products of mining, their relative abundance in the lithosphere does not represent their actual availability and the process to achieve pure materials such as cobalt and vanadium is expensive. The demand for charge-storage materials will grow significantly and, the price of these metals will rise.[5,6] Further disadvantages of classical RFBs are the deficient civil and environmental standards associated with ore mining, the applied hazardous and highly corrosive acidic electrolytes, and the expensive membranes such as the commonly used Nafion (DuPont, Wilmington, DE, USA) cation-exchange membrane.[6,7,8,9,10]. A brief calculation for the anthraquinone disulphonate/bromine system yields a price of
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