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

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journal contribution
posted on 2016-02-10, 18:33 authored by Cameron L. Bentley, Alan M. Bond, Anthony F. Hollenkamp, Peter J. Mahon, Jie Zhang
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.