Electrochemistry of Iodide, Iodine, and Iodine Monochloride in Chloride Containing Nonhaloaluminate Ionic Liquids

The electrochemical behavior of iodine remains a contemporary research interest due to the integral role of the I^–/I₃^– couple in dye-sensitized solar cell technology. The neutral (I₂) and positive (I⁺) oxidation states of iodine are known to be strongly electrophilic, and thus the I^–/I₂/I⁺ redox processes are sensitive to the presence of nucleophilic chloride or bromide, which are both commonly present as impurities in nonhaloaluminate room temperature ionic liquids (ILs). In this study, the electrochemistry of I^–, I₂, and ICl has been investigated by cyclic voltammetry at a platinum macrodisk electrode in a binary IL mixture composed of 1-butyl-3-methylimidazolium chloride ([C₄mim]­Cl) and 1-ethyl-3-methylimidazolium bis­(trifluoromethanesulfonyl)­imide ([C₂mim]­[NTf₂]). In the absence of chloride (e.g., in neat [C₂mim]­[NTf₂]), I^– is oxidized in an overall one electron per iodide ion process to I₂ via an I₃^– intermediate, giving rise to two resolved I^–/I₃^– and I₃^–/I₂ processes, as per previous reports. In the presence of low concentrations of chloride ([Cl^–] and [I^–] are both <30 mM), an additional oxidation process appears at potentials less positive than the I₃^–/I₂ process, which corresponds to the oxidation of I₃^– to the interhalide complex anion [ICl₂]⁻, in an overall two electron per iodide ion process. In the presence of a large excess of Cl^– ([I^–] ≈ 10 mM and [Cl^–] ≈ 3.7 M), I^– is oxidized in an overall two electron per iodide ion process to [ICl₂]⁻ via an [I₂Cl]⁻ intermediate (confirmed by investigating the voltammetric response of ICl and I₂ under these conditions). In summary, the I^–/I₂/I⁺ processes in nonhaloaluminate ILs involve a complicated interplay between multiple electron transfer pathways and homogeneous chemical reactions which may not be at equilibrium on the voltammetric time scale.