Highly stable and high performance iodine redox flow batteries using host–guest interaction of (2-hydroxypropyl)-β-cyclodextrin additive

Abstract

Solutions to enhance the performance of aqueous redox flow batteries (ARFBs) using iodine as the active material for the catholyte are explored. When iodine is used, the most serious problem that occurs is undesirable I₂ film as a byproduct of redox reactions of the iodine electrolyte. To suppress the formation of I₂ film, host–guest interaction of (2-hydroxypropyl)-β-cyclodextrin (HP-β-CD) additive is suggested. With the unique structure of HP-β-CD, the aqueous solubility of I₂ increases, and the desired reactions of iodine to form the I₃⁻ ion from the complex consisting of I₂ and HP-β-CD works well, promoting stable operation of ARFBs using iodine with HP-β-CD included in the electrolyte over a long time, even with high iodine concentrations. Thus, these ARFBs address the undesirable I₂ film issue and enhance the aqueous solubility and reactivity for the desirable reaction of iodine, increasing the potential for commercialization of ARFBs.