Redox flow batteries using cerium salts and anthraquinone-2,7-disulfonic acid as new redox couple dissolved in mixture solution of methanesulfonic and perchloric acids

Aqueous redox flow batteries (ARFBs) using acidic supporting electrolytes are one of promising future batteries because the acidic supporting electrolytes can increase the solubility of active materials and their ionic conductivity, followed by energy density and efficiencies of ARFBs using these. Nevertheless, discovering proper redox couple with a high open circuit voltage (OCV) in acidic media is still challengeable because a low OCV is a main drawback of the conventional redox couples dissolved in acidic media. To increase OCV without sacrificing the solubility of active materials, a new redox couple consisting of cerium salts and anthraquinone-2,7-disulfonic acid (AQDS) that are dispersed in methanesulfonic acid (MSA) is suggested. With cerium salts and AQDS dissolved in acidic MSA, a high OCV of 1.45 V is achieved, while MSA affects the increases in solubility and stability of cerium salts. To further increase the solubility of AQDS and facilitate its redox reactivity, the mixture of MSA and perchloric acid (HClO₄) is used as supporting electrolyte. Based on that, the solubility of AQDS is improved from 0.35 to 0.5 M, while ARFB using 1 M cerium ions and 0.5 M AQDS dissolved in the mixture solution of 4 M MSA and 1 M HClO₄, shows excellent performances such as discharge capacity of 20.3 Ah L⁻¹, energy efficiency of 86.5%, and capacity retention of ~99% over 100 cycles.