Performance evaluation of aqueous organic redox flow battery using anthraquinone-2,7-disulfonic acid disodium salt and potassium iodide redox couple

An aqueous organic redox flow battery (AORFB) using anthraquinone-2,7-disulfonic acid disodium salt (2,7-AQDS) and potassium iodide (KI) as the negative and positive active species is suggested. The active species are dissolved into an aqueous potassium chloride (KCl) solution, while ethylene glycol (EG) and polyvinylpyrrolidone (PVP) are added to improve the solubility of 2,7-AQDS and prevent the side reactions of KI. The aqueous solubility of redox couple improved by the utilization of additive plays a role in increasing both capacity and the performance of AORFB, while as kinetic parameters to affect the capacity, electron transfer rate constant (k_s) and diffusion coefficient (D) are measured. As a result, when EG is used, the solubility of 2,7-AQDS increases from 0.3 to 0.8 M in KCl solution and PVP acts as a barrier preventing the production of iodine gas that is a product generated by the side reaction of KI. The increase in solubility by the use of EG is because it has two hydroxyl groups and they form hydrogen bonding with the oxygen of sulfonyl or carbonyl group within 2,7-AQDS, while the alkyl group of its backbone that is in non-polar nature interacts with the phenyl group within 2,7-AQDS. In addition, EG increases the redox activity of KI because this significantly increases the nucleophilic ability of the iodide anion. Regarding the effect of PVP, when PVP is added to the solution containing iodine gas, povidone-iodine complex is formed and this complex impedes the side reaction of KI and maintains iodate that is a reactant for redox reaction as it is in the system. With that, when the AORFB using the 2,7-AQDS and KI including EG and PVP is run for ten cycle, it shows excellent performances like the discharge capacity of 0.7 AhL⁻¹ and the coulombic and energy efficiencies of 98% and 82%.