Electrochemical monitoring and mechanistic understanding of iodine film formation as a metastable intermediate during I-3(-)/I- redox reaction in aqueous ZnI2 media

Abstract

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 Zn-2(+)/Zn and I-3(-)/I- half redox reactions have not been fully achieved yet. The I-3(-)/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 I-3(-)/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 I-3(-) via iodine films and its electro-reduction to I- based on the mechanism proposed by Gileadi et al. Chronoam-perograms (CAs) and cyclic voltammograms (CVs) associated with the I-2/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 I-3(-)/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-.