A Highly Stable and Soluble Asymmetric Anthraquinone Derivative for Aqueous Redox Flow Batteries

A highly stable and highly water-soluble anthraquinone derivative having one hydroxy and one 3-carboxypropyloxy group (abbreviated MHMBEAQ) was designed and synthesized for the anolyte of an aqueous redox flow battery. The potassium salt of the dideprotonated species of MHMBEAQ showed an extremely high solubility of ∼1 M in 1 M potassium hydroxide aqueous solution. The electrochemical measurement in alkaline aqueous solution showed a redox potential intermediate between the related symmetrically substituted anthraquinone derivatives (2,6-dihydroxy and 2,6-di(3-carboxypropyloxy) ones). Battery tests using a cell paired with a ferro/ferricyanide catholyte showed an open-circuit voltage of 1.1 V, which is also intermediate between symmetric anthraquinone derivatives. MHMBEAQ showed high stability, where the charge/discharge capacity retained ∼70% capacity over 70 days and 3800 charge/discharge cycles. MHMBEAQ also sustained over 10,000 charge/discharge cycles under a capacity limitation of 47%.