Overlimiting current by iodide electrode oxidation in aqueous media: an electrogenerated iodine interphase with positively charged channels stimulating in situ electrokinetic iodide transport

Abstract

Herein, we demonstrate an electrogenerated porous iodine interphase with positively charged (e.g., protonated in an aqueous acidic medium) electrolyte channels on a Pt ultramicroelectrode (UME) that stimulates in situ electrokinetic transport of I⁻, resulting in a significant enhancement of anodic current associated with electro-oxidation of I⁻ in an aqueous medium. Our scientific findings would be critical for developing various ‘fast charging’ I⁻-based aqueous rechargeable batteries. The cyclic voltammograms (CVs) obtained using a 10 mM I⁻ + 1 M HClO₄ solution represent the electro-oxidation of I⁻ to I₂via I₃⁻ and precipitation of solid I₂ on the electrode due to its limited solubility under the conditions in which the mass transport of I⁻ was mainly governed by diffusion. However, as the I⁻ concentration increased to 1 M, the voltammetric behavior for oxidation of I⁻ deviated from the previously reported electrode reaction model. The abnormal voltammogram became explicit as the concentration of HClO₄ increased to 4 M or higher, showing a linear increase in the overlimiting anodic current as the electrode potential was positively biased. In addition, the formation of protonated iodine, I₂(H⁺)_n, was estimated from the onset of a potential shift in the negative direction with increasing bulk H⁺ concentration in the solution. Molecular dynamics (MD) simulations demonstrated feasible porous iodine structures with electrolyte channels in an aqueous solution containing both I₂ and HClO₄. Voltammetric and MD simulation analyses suggested the electrogeneration of a porous I₂(H⁺)_n interphase with positively charged electrolyte channels on the Pt UME, which stimulates in situ electrokinetic transport of I⁻ through the channels. This was supported by finite element analyses of the transport of I⁻ through a simplified model channel with a positively charged surface, which demonstrated a linearly increased I⁻ flux as a function of a positively biased electrical potential difference between the electrode and the entrance of the channel. In addition, we observed that the electrokinetic phenomena occurring during the electro-oxidation of I⁻ also occurred in aqueous media containing other electrolytes (e.g., NaClO₄). The slope of the voltammetric curve and the maximum value of the observed electrokinetic current due to electro-oxidation of I⁻ in HClO₄ were higher than those in NaClO₄ solution because of higher positive surface charge density of the porous iodine interphase in the aqueous medium containing HClO₄ than that containing NaClO₄. Chronoamperometric analysis revealed the transient phase transition region where an effective porous I₂(H⁺)_n interphase was formed from its disconnected clusters that contributed to the electrokinetic I⁻ current. In addition, it showed that the structure of the I₂(H⁺)_n interphase would be stable for electrokinetic transport of I⁻. Based on our analyses, we provide a more detailed mechanistic picture of the electro-oxidation of I⁻ to I₃⁻via an I₂ interphase in an aqueous acidic medium with the following phase transition of I₂: I₂(H⁺)_n cluster → porous I₂(H⁺)_n stimulating electrokinetic transport of I⁻ → dense I₂ with an I⁻-insulating nature.