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

Abstract

Summary 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.

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

“…For instance, when ARFB single cell using 0.8 M cerium salts and 0.4 M AQDS dissolved in 5 M MSA was operated, AQDS molecules were precipitated (Figure S5). This precipitation was linked to viscosity of AQDS molecules because their viscosity was very sensitive to the concentration of MSA showing rheopectic behaviour over time 58,59 . Namely, if the concentration of AQDS increases in highly acidic condition, its viscosity rapidly increases, and this may induce the precipitation of AQDS molecules.…”

“…This precipitation was linked to viscosity of AQDS molecules because their viscosity was very sensitive to the concentration of MSA showing rheopectic behaviour over time. 58,59 Namely, if the concentration of AQDS increases in highly acidic condition, its viscosity rapidly increases, and this may induce the precipitation of AQDS molecules. To alleviate this problem, the condition of MSA supporting electrolyte should be modified.…”

Cost, Performance, and Sustainability of Redox Flow Batteries Using 4,5-Dihydroxy-1,3-benzenedisulfonic Acid and Vanadium Enhanced by Cross Compensation of the Activation Process

“…Redox flow batteries (RFBs) are promising large-scale ESSs because the power and energy sectors can be easily controlled by only managing the size of tanks and stacks that are main parts of RFBs. − In addition, when an aqueous electrolyte is applied, RFBs can be more acceptable for large-scale ESSs because the easy flammability issue of organic electrolytes is considerably relieved. − Furthermore, a high current density can be utilized due to high ionic conductivity of water, and thus, the size of the RFB stack is reduced. Moreover, RFBs have a long lifetime of 20 years and a discharging duration time of up to 12 h, while they are cost-effective than lithium-ion batteries or fuel cells …”