Electrochemical monitoring and mechanistic understanding of iodine film formation as a metastable intermediate during I3 −/I− redox reaction in aqueous ZnI2 media

Zn-polyiodide redox flow batteries (RFBs) are a promising grid-scale electrical energy storage (EES) option because of their significant energy density, which is due to the high solubility of ZnI₂ in water. In spite of the importance of the RFBs, a mechanistic understanding of both Zn²⁺/Zn and I₃⁻/I⁻ half redox reactions have not been fully achieved yet. The I₃⁻/I⁻ half redox reaction occurring on a cathode in this RFB is particularly complicated due to metastable iodine films, which only form on an electrode when an electrochemical potential is biased enough to drive the electro-oxidation of I⁻. Due to the thermodynamically unstable nature of iodine films in I⁻-rich aqueous solutions, the film is also difficult to characterize by ex-situ analyses, and its physicochemical properties are largely unknown. In this article, we report electrochemical in situ monitoring of the formation and dissolution of a metastable iodine film as an intermediate during I₃⁻/I⁻ redox reactions in an aqueous solution using a rotating ring disk electrode (RRDE). We suggested a reaction model for electro-oxidation of I⁻ to I₃⁻ via iodine films and its electro-reduction to I⁻ based on the mechanism proposed by Gileadi et al. Chronoamperograms (CAs) and cyclic voltammograms (CVs) associated with the I₂/I⁻ redox reaction involving only formation and dissolution of iodine films on a disk electrode were obtained using an RRDE. From the CAs and CVs, we successfully obtained trajectory information on the formation and dissolution of an iodine film during the I₃⁻/I⁻ redox reaction on a disk electrode under transient and potential-time variation conditions. In addition, we found evidence of a possible transition in the iodine film from an I⁻-conductor to an I⁻-semiconductor during formation based on the quantitatively monitored iodine formation curve as a function of time during electro-oxidation of I⁻.