Revealing Electrochemically Generated Local High Order Polyiodides (I-2n+1(-), n=1-3) on Pt Ultramicroelectrode

Abstract

Zn-polyiodide redox flow batteries (RFB) with a highly concentrated ZnI2 electrolyte have been considered as promising grid scale energy storage devices. Despite a continuous energy density enhancement in Zn-polyiodide RFBs after their first introduction as a prototype by Li et al. in 2015, a fundamental understanding of electrochemical/chemical reactions occurring in Zn-polyiodide RFBs still has not been well known. One of the first subjects to understand with regard to the battery system would be chemical speciation analyses, which would dynamically change depending on the concentration of Zn2+ and I- in an electrolyte solution during charging/discharging. In this article, we present the speciation of electrochemically generated polyiodides (I-2n+1(-)) locally existing on a Pt ultramicroelectrode (UME) during electro-oxidation of I- in an acidic aqueous solution with highly concentrated ZnI2 via voltammetric titration. When I- is electrochemically oxidized, an iodine film immediately forms on a Pt UME, which is designated as I-2-F. After the formation of I-2-F on a Pt UME, I- would transfer from the aqueous to the I-2-F phase and electrooxidized to form I-2n+1(-) at equilibrium based on the model suggested by Gileadi et al. From the mechanistic model, we could estimate the concentration of I- in I-2-F, C1-(I2-F) as a function of the bulk concentration of free I- in ZnI2 solutions, C-1-(aq),C-free by a measurement of the steady state current, i(ss )associated with the electro-oxidation of I-. Then, -log C1-(I2-F) can be plotted as a function of C-1-(aq),C-free. The graph shows several plateaus, which are indicative of I--buffer regions resulting from I-2n+1(-)-dissociation reactions to I- and nI(2) with corresponding equilibrium constants, K-eq,K-n in I-2-F. Eventually, the fractional distribution of I-2n+1((I2-F))- as a function of CI2(I2-F) can be obtained. We performed the presented electrochemical analysis within an acidic ZnI2 solution with C-1-(aq),C-free < 4 M and found that I-3(-), I-5(-), and I-7(-) would form in I-2 -F, among which I-7(-) would be the main I-2n+1(-). Nonetheless, we found I-3(-) to be the most stable polyiodide species in an aqueous solution from the computational calculations, indicating that both I-5(-) and I-7(-) only exist locally in I-2-F during the electro-oxidation of I- on a Pt UME. Lastly, we estimated the fraction of I-2n+1(-) in I-2-F as a function of the overall rate constant of I-3(-)(aq)-formation in various concentrations of free I- in aqueous ZnI2 solutions.