## 3 Results and Discussion
### Electrochemical Impedance Spectroscopy (EIS)  
EIS technique is a novel approach to examine the electrical properties of bulk and interfacial regions of various materials [27; 28; 29]. Figure 8 shows the EIS spectra of metallic copper in choline chloride-based IL at various direct current (DC) potentials. At \(-0.2\) V, there was a single semi-circle for an electron transfer process which got smaller at \(0\) V indicating an increase in electron transfer rate constant. At \(+0.2\) V, a second semi-circle was enlarged at \(+0.4\) V, and above that voltage, dominated, suggesting that an insulating layer formed on the electrode's surface. At \(+0.2\) V, there were two semi-circle responses which could represent the first and second oxidations of copper. The data acquired for impedance at different polarisation potentials in Figure 8b were fitted to an electrical equivalent circuit (EEC) involving a two Randle's circuits in series with Warburg impedance. EEC is a straight method to represent the behaviour of the medium with circuit elements, thus understanding the electrical properties of the system under study [30; 31]. From the data analysis, when applying a potential of \(+0.8\) V, the film capacitance was found to be \(4.9\times 10^{-6}\) F\(\cdot\)cm\({}^{-2}\), assuming a dielectric constant of \(8.0\); the thickness of the film was estimated to be \(1.4\)\(\upmu\)m [32]. This is about an order of magnitude more than expected from the films formed in aqueous media. The film thickness was, however, remarkably thicker than that found for stainless steel electropolishing in the same liquid under the comparable conditions, and less thick compared to cobalt in the same electrolyte. As a consequence, the cobalt surface underwent a well-mirror-like electropolishing. The thickness was estimated to be only \(16\) nm for that layer [32]. It should, however, be noted that speciation is different where for stainless steel, the iron complex formed is glycolate. Figure 7: Anodic LSVs of Cu disc in ethaline versus Ag/AgCl, at a sweep rate of \(5\) mV\(\cdot\)s\({}^{-1}\) and at various rotation speeds. The experiment was repeated in choline chloride-based IL at 70 \({}^{\circ}\)C (not shown) and the same mechanism of dissolution was gained. A comparison with copper dissolution at 20 \({}^{\circ}\)C showed a capacitance of 2.17 \(\times 10^{-5}\) F\(\cdot\)cm\({}^{-2}\) at 70 \({}^{\circ}\)C and + 0.8 V which corresponds to a layer of about 0.3 \(\upmu\)m thick [32]. It would seem logical to say that the diffusion layer in the duplex salt film model should be thinner at a higher temperature as the salt would be more soluble. The structure of the double layer at the electrode's surface in molecular solvent systems is completely different from that in ILs, in such a way that in the former, the electrode charge is compensated by both adsorbed counter ions and the diffuse layer, while in the latter, the structure may involve a monolayer of counter ions as compensation, followed by a multilayer involving cations and anions adjacent to each other [33; 34]. In Figure 8c the impedance of a copper electrode in imidazolium-based IL was measured at 70 \({}^{\circ}\)C as a function of DC potential. A single semicircle was observed at \(-\)0.2 V corresponding to the process of electron transfer that was likely the oxidation of copper. In this experiment, the polarisation potential was altered over the potential window from negative to positive. As the DC potential was shifted from negative to 0.0 V, the semicircle lessened its width as a result of a faster electron transfer process that was predicted given the increase in overall potential. From +0.4 to +1.2 V, a vertical straight line was observed, signifying the characteristic of a series RC circuit which was caused by an insulating layer on the electrode's surface. The response of the series RC circuit did not alter with potential, indicating that once the film forms, it is not permeable and insulates the electrode's surface.