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

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 HClO 4 solution represent the electro-oxidation of I − to I 2 via I 3 − and precipitation of solid I 2 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 HClO 4 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, I 2 (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 I 2 and HClO 4 . Voltammetric and MD simulation analyses suggested the electrogeneration of a porous I 2 (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. , NaClO 4 ). The slope of the voltammetric curve and the maximum value of the observed electrokinetic current due to electro-oxidation of I − in HClO 4 were higher than those in NaClO 4 solution because of higher positive surface