Electrochimica Acta 56 (2011) 4942–4948 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta The kinetics of the Cu 2+ /Cu + redox couple in deep eutectic solvents David Lloyd a , Tuomas Vainikka a , Lasse Murtomäki a,∗ , Kyösti Kontturi a , Elisabet Ahlberg b a Aalto University, Department of Chemistry, Kemistintie 1, PO Box 16100, 00076, Aalto, Finland b University of Gothenburg, Department of Chemistry, SE-412 96 Göteborg, Sweden article info Article history: Received 31 January 2011 Received in revised form 28 March 2011 Accepted 29 March 2011 Available online 7 April 2011 Keywords: Deep eutectic solvent Ionic liquid Electrochemical kinetics Copper Cyclic voltammetry Impedance spectroscopy abstract 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 consist- ing of choline chloride and ethylene glycol. At 25 ◦ C, the reaction was found to be quasi-reversible with a relatively high rate constant k 0 of 9.5 ± 2 × 10 -4 cm s -1 , and a charge transfer coefficient ˛ 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 cm 2 s -1 , respectively. The viscosity of the electrolyte was 41 ± 3 mPa s. The temper- ature 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) com- plexes was determined using UV–VIS spectroscopy, and the prevailing Cu(I) complex was found to be [CuCl 3 ] 2- and that of Cu(II) [CuCl 4 ] 2- . © 2011 Elsevier Ltd. All rights reserved. 1. Introduction Room temperature ionic liquids (RTILs) are molten salts that are liquid at 20 ◦ C [1]. The ions are assumed to show a high degree of dissociation. Due to the wide selection of cations and anions now available it is possible to design liquids with a range of physical properties. A property of particular interest in the field of elec- trochemistry is the window of electrochemical stability. This has been shown to be exceptionally large for some ionic liquids [1] and expands the range of metals which can be electrodeposited. Electrodeposition of metals from ionic liquids became an area of periodic activity from the 1930s onwards [2]. The first major break- through was the development of aprotic haloaluminate eutectics in the 1950s by Hurley and coworkers [3]. A further improvement was the development of the aluminium chloride and 1-methyl- 3-ethylimidazolium chloride (AlCl 3 and EMIM-Cl) system in the 1980s. This mixture forms a RTIL by formation of either AlCl 4 - or Al 2 Cl 7 - complexes and has two, relatively wide low melt- ing regions. The biggest drawback to chloroaluminate systems is their reactivity with water, which drastically limits possible Abbreviations: RTIL, room temperature ionic liquid; EMIM-Cl, 1-methyl-3- ethylimidazolium chloride; Tf2N, bis(trifluoromethyl)sulfonylamide; ChCl, choline chloride; DES, deep eutectic solvent; CPE, constant phase element; CA, chronoam- perometry; IS, impedance spectroscopy; CV, cyclic voltammetry; RDE, rotating disc electrode. ∗ Corresponding author. Tel.: +358 9 470 22575; fax: +358 9 470 22580. E-mail address: lasse.murtomaki@aalto.fi (L. Murtomäki). applications. From the 1990s onwards attention has shifted to the use of other anions, for instance bis(trifluoromethyl)sulfonylamide [N(CF 3 SO 2 ) 2 - ], which result in liquids that are stable in the pres- ence of air and water. Deep eutectic solvents (DES) are a subclass of ionic liquids formed by complexation of the anion in a halide salt by means of hydrogen bonding. The prototypical salt is choline chloride (ChCl or (2-hydroxyethyl)trimethylammonium chloride) and typi- cal hydrogen bond donors are either primary amines [4], diols [5] or carboxylic acids [6]. DES based on these combinations show a sig- nificant capacity to dissolve metal chlorides, making them simple to use for studies of metal complex electrochemistry and electrode- position in particular [7]. The primary advantages of DES are the extremely low cost of the precursors and their general biodegrad- ability. DESs are inferior to other RTILs on a number of important points. Firstly, they have a much narrower window of electrochemical stability, which limits the range of metals that can be deposited. Secondly, they are hygroscopic, which does not appear to lead to electrolyte decomposition, but does complicate the reproducible performance of measurements in anything other than a carefully controlled atmosphere. Thirdly, they can exhibit significant volatil- ity if the hydrogen bond donor molecule is, for instance, ethylene glycol. One of the biggest problems in the reporting of DES properties is the wide difference between melting point and fusion tempera- ture, sometimes up to 125 ◦ C [6]. Since a DES is formed by mixing the two components at elevated temperature and then cooling, 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.03.133