Highly Soluble 1,4-Diaminoanthraquinone Derivative for Nonaqueous Symmetric Redox Flow Batteries

Nonaqueous redox flow batteries (RFBs) based on redox-active organic molecules are regarded as a promising technology for large-scale grid energy storage. 1,4-Diaminoanthraquinones (DAAQs) are particularly interesting as active species because they can have up to five different electrochemically accessible oxidation states, but their practical usability is limited because of their low solubility in commonly used polar organic solvents. We present a DAAQ derivative, 1,4-bis((2-(2-(2-methoxyethoxy)ethoxy)ethyl)amino)anthracene-9,10-dione (Me-TEG-DAAQ), which can be synthesized from inexpensive precursors and overcomes this disadvantage of low solubility. This derivative has a low melting point (25 °C), a concentration exceeding 2.2 mol L–¹ in the pure state, and is miscible in any ratio with polar organic solvents such as acetonitrile (MeCN) and 1,2-dimethoxyethane. Cyclic voltammetry experiments show that two anodic and two cathodic one-electron redox transitions are electrochemically accessible and highly reversible, with an interval of 1.8 V between the two inner redox couples and 2.7 V between the two outer redox couples. Proof-of-concept galvanostatic cycling experiments were conducted on dilute solutions of Me-TEG-DAAQ in a simple symmetric electrochemical cell. When only the two inner redox couples are considered, the electrochemical cell can achieve specific capacities close to the theoretical value (2.68 A h L–¹), with only limited capacity fading (i.e., <20%) over 100 cycles. When the two outer redox couples are considered as well, the cell can achieve its theoretical capacity (5.36 A h L–¹), but faster capacity fading occurs. This combination of high reversibility, high theoretical cell potential (2.7 V), high theoretical energy density (>49 W h L–¹), and limited capacity fading in an electrochemical cell based on one single active species is unprecedented in the RFB literature.