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-/I-3(-) couple in dye-sensitized solar cell technology. The neutral (I-2) and positive (I+) oxidation states of iodine are known to be strongly electrophilic, and thus the I-/I-2/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-, I-2, 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 ([C(4)mim]Cl) and 1-ethyl-3-methylimidazolium bis-(trifluoromethanesulfonyl)imide ([C(2)mim][NTf2]). In the absence of chloride (e.g., in neat [C(2)mim][NTf2]), I- is oxidized in an overall, one electron per iodide ion process to I-2 via an I-3(-) intermediate, giving rise to two resolved I-/I-3(-) and I-3(-)/I-2 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 I-3(-)/I-2 process, which corresponds to the oxidation of I-3(-) 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-] approximate to 10 mM and [Cl-] approximate to 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 I-2 under these conditions). In summary, the I-/I-2/I+ processes in nonhaloaluminate as involve a complicated interplay between multiple electron transfer pathways and homogeneous chemical reactions which may not be at equilibrium on the voltammetric time scale.