## 3 Results and Discussion  
### Anodic Dissolution Mechanism  
Cyclic and Linear Sweep Voltammetry  
Figure 1a exhibits the cyclic voltammetric response a of metallic, copper disc electrode in choline chloride-based IL at 20 \({}^{\circ}\)C. Within the anodic potential range, two oxidation processes can be clearly seen. The anodic current begins to increase at \(-\)0.3 V, peaking at ca. 0 V. The current sharply drops in a manner which is a characteristic of a quasi-passivation process. This might be due primarily to presence of EG, and partly, the chloride ion. The second anodic current rises to a peak at +0.3 V and falls down to a steady state current of approximately 23 mA\(\cdot\)cm\({}^{-2}\). The second anodic peak beyond +0.25 V could be linked to extra oxidation of copper from Cu(I) to Cu(II). The other ting peaks are artefacts that might be linked to complexity of dissolution process of bulk copper metal within such a viscous liquid. On the cathodic scan, the current was approximately constant until ca. 0.2 V, when a noisy phenomenon occurred at the same potential that passivation film formation occurred: on the reversed sweep. Indeed, the process is not an artefact but is very reproducible. The main cathodic process begins at ca. 0.0 V, peaking at \(-\)0.4 V, with a shoulder at \(-\)0.2 V. The noise is likely owed to redissolution of the Cu(II) species formed on the electrode's surface. In the interpretation of the whole processes that occurs, it is helpful to compare the cyclic voltammograms of the metallic copper electrode in choline chloride-based DES with that for at Pt electrode in a solution of 0.1 M CuCl\({}_{2}\cdot\)2H\({}_{2}\)O in the electrolyte. [PERSON] et al. documented the electrodeposition of copper using CuCl\({}_{2}\cdot\)2H\({}_{2}\)O in the electrolyte, observing that two distinct processes take place, with Cu(II) undergoing a one-electron reduction to Cu(I)at +0.43 V, followed by a one-electron reduction to metallic copper at \(-\)0.45 V [24]. The significance of this finding is that both processes are reversible. It is worth-mentioning, however, that direct comparison between the redox potentials in Figure 1 and the previously reported study are not helpful due to the use of considerably different reference electrodes (a silver wire quasi-reference electrode in the latter case). Figure 1a reveals an over-laid cyclic voltammetry CV (dashed line) of 0.1 M CuCl\({}_{2}\cdot\)2H\({}_{2}\)O in choline chloride-based DES versus Ag/Ag\({}^{+}\) at 20 \({}^{\circ}\)C. It can obviously be seen that the onset potential on the anodic scan are similar to large extent, indicating that the metallic copper first dissolves as Cu(I) in the complex form. The quasi-passivation which is observed for the bulk copper electrode dissolution occurs at the same potential as the Cu(I)/(II) oxidation in solution. One can conclude that the second  
Figure 1: Cyclic voltammetric responses of a Cu disc (solid line) and 0.1 M CuCl\({}_{2}\cdot\)2H\({}_{2}\)O (dashed line) in choline chloride-based ionic liquid (IL) on a platinum electrode at 20 \({}^{\circ}\)C (**a**), and a Cu disc (solid line) and 0.2 M CuCl\({}_{2}\cdot\)2H\({}_{2}\)O in imidazolium-based IL (dashed line) at 70 \({}^{\circ}\)C (**b**), all at a sweep rate of 5 mV\(\cdot\)s\({}^{-1}\). process in metallic copper dissolution occurs owing the change in oxidation state of the metal. When Cu(I) salts solubilise in choline chloride-based DES, the speciation was found to be in the [CuCl\({}_{2}\)]\({}^{-}\) form while the Cu(II) salts tend to produce [CuCl\({}_{4}\)]\({}^{2-}\)[20,24]. Both of these complexes are known to be soluble largely and so the cause of the quasi-passivation is not immediately clear. From Figure 1a,b, one can see the differences which are caused due to a specific cation effect or because of the lower chloride concentration in choline chloride-based DES compared to imidazolium-based IL. To confirm this further, 1-butyl-3-methylimidazolium chloride (C\({}_{4}\)mimCl) was diluted by adding EG.