Electrooxidation of the Iodides [C4mim]I, LiI, NaI, KI, RbI, and CsI in the Room Temperature Ionic Liquid [C4mim][NTf2]

Abstract

The electrochemical oxidation of 1-butyl-3-methylimidazolium iodide, [C4mim]I, has been investigated by cyclic voltammetry at a platinum microelectrode at varying concentrations in the RTIL 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [C4mim][NTf2]. Two oxidation peaks were observed. The first peak is assigned to the oxidation of iodide to triiodide, in an overall two-electron process: 3I- − 2e- → I3-. At higher potentials, the electrogenerated triiodide oxidizes to iodine, in an overall one-electron process: I3- − e- → 3/2I2. An average diffusion coefficient, D, for I- of 1.55 × 10-11 m2 s-1 was obtained. A digital simulation program was used to simulate the voltammetric response, and kinetic parameters were successfully extracted. The parameters deduced from the simulation include D for I-, I3-, and I2 and Keq,2, the equilibrium constant for the reaction of iodide and iodine to form triiodide. Values for these parameters are of the same order as those previously published for the oxidation of Br- in the same RTIL [Allen et al. J. Electroanal. Chem. 2005, 575, 311]. Next, the cyclic voltammetry of five different inorganic iodide salts was studied by dissolving small amounts of the solid in [C4mim][NTf2]. Similar oxidation peaks were observed, revealing diffusion coefficients of ca. 0.55, 1.14, 1.23, 1.44, and 1.33 × 10-11 m2 s-1 and solubilities of 714, 246, 54, 83, and 36 mM for LiI, NaI, KI, RbI, and CsI, respectively. The slightly smaller diffusion coefficients for the XI salts (compared to [C4mim]I) may indicate that I- is ion-paired with Li+ , Na+ , K+ , Rb+, and Cs+ in the RTIL medium.