Symmetric aqueous redox flow battery using hydroiodic acid and anthraquinone-2,7-disulfonic acid as redox couple

Iodide and triiodide ions (I⁻ and I₃⁻ ions) are interesting active redox materials for aqueous redox flow batteries (ARFBs) due to high solubility in various supporting electrolytes. However, under alkaline and neutral electrolytes, the redox reaction of I⁻ and I₃⁻ ions is unstable due to undesirable potassium iodate produced during the reaction, whereas the reaction is stably operated under an acidic electrolyte state. As a redox couple, anthraquinone-2,7-disulfonic acid (AQDS) is considered due to excellent stability and high solubility, and perchloric acid is determined as an electrolyte because this suppresses another undesirable water-splitting reaction. For providing abundant I⁻ and I₃⁻ ions, hydroiodic acid (HI) is used because hydrogen ions contained in HI can act as a proton donor, increasing the conductivity in the solution leading to better kinetics of the redox reaction of I⁻ and I₃⁻. When asymmetric ARFB using this redox couple is operated, gradual decay in both capacity and efficiencies is observed. This is due to the crossover of I⁻ ions to the AQDS side. To overcome the problem, symmetric ARFB using the mixture of HI and AQDS in both electrolytes is adopted, and with that, the crossover issue is solved because the ionic balance of electrolyte is well maintained by the use of symmetric electrolyte, while its energy efficiency and discharging capacity are excellent as 67.6% and 26.6 Ah L⁻¹, respectively, with capacity retention of 99.99% for 100 cycles.