The kinetics of the Cu2+/Cu+ redox couple in deep eutectic solvents

Journal article, 2011

Kinetics of electron transfer of the Cu(I)/Cu(II) redox couple at a platinum electrode has been studied with chronoamperometry, cyclic voltammetry and impedance spectroscopy in a deep eutectic solvent consisting of choline chloride and ethylene glycol. At 25 °C, the reaction was found to be quasi-reversible with a relatively high rate constant k0 of 9.5 ± 2 × 10−4 cm s−1, and a charge transfer coefficient α of 0.25 ± 0.05. Diffusion coefficients for the Cu(I) and Cu(II) complexes were determined to be 2.7 ± 0.1 × 10−7 and 1.5 ± 0.1 × 10−7 cm2 s−1, respectively. The viscosity of the electrolyte was 41 ± 3 mPa s. The temperature dependency was also investigated. The activation energy of mass transfer was found to be 27.7 ± 1 kJ mol−1 and that of electron transfer 39 ± 7 kJ mol−1. Speciation of the Cu(I) and Cu(II) complexes was determined using UV–VIS spectroscopy, and the prevailing Cu(I) complex was found to be [CuCl3]2− and that of Cu(II) [CuCl4]2−. Highlights ► Kinetics of Cu2+/Cu+ redox process in a deep eutectic solvent. ► Comparison of cyclic voltammetry and impedance spectroscopy. ► Diffusion coefficient of Cu+ twice that of Cu2+. ► Heterogeneous kinetics fast, transfer coefficient is 0.25. ► Activation energy of mass transfer identical to activation energy of viscous flow.