Stable redox flow batteries through suppression of side reactions with lawsone anolyte and iodine catholyte

Redox flow batteries (RFBs) with high cycle stability are important to establish a stable large-scale energy storage system. In this study, electrochemical and chemical stability of lawsone that is the active material in anolyte is enhanced by optimizing electrolyte pH, while electrochemical and spectroscopic evaluations indicate that weakly alkaline condition is preferred to induce its stable redox properties. As a counterpart, iodide (I−) that has high solubility and reasonable redox potential as the active material in catholyte is suggested. Its reactions produce undesirable solid iodine (I2) on carbon felt (CF) during charging of RFBs, and the I2 promotes an uneven current density distribution and suppresses a facile flow of electrolyte, leading to unstable performance of RFBs. To overcome the issue of I2, manipulating the amount of I− that participates in electrochemical and chemical reactions independently is suggested, while validity of such determined the amount of I− is proved by observation of I2 gas and spectroscopic inspection. These strategies enable a stable cycling of RFBs over 100 cycles with excellent decay rate and discharge capacity (0.085% cycle−1 and 4.83 Ah L−1 at 40 mA cm−2), while initial discharge capacity of 20.03 Ah L−1 is achieved at 100 mA cm−2 with 0.088% cycle−1 even in high concentrations. Based on that, this study establishes asymmetric RFBs using lawsone and I− suppressing formation of solid I2.