## 3 Results and Discussion
### Physicochemical Characterization of the RC/IL Films
Figure 6: Light reflection as a function of wavelength for the RC/BMIMPF\({}_{6}\) film (red solid line), the RC/AliquatCl film (blue solid line) and the RC-support (black dashed line). Visible region: between the two vertical dashed black lines. Impedance spectroscopy (IS) is another non-destructive technique commonly used for the electrical characterization of materials. IS gives quantitative and qualitative information on systems associated with both bulk and surface (more exactly, the electrode/sample interface) by means of the impedance plots and using equivalent circuits as models [57; 58]. Electrical parameters (resistance, R, and capacitance, C) can be determined by analyzing the Nyquist plot (\(Z_{\text{real}}\) versus \(-\)\(Z_{\text{img}}\)), with \(Z_{\text{real}}\) and \(Z_{\text{img}}\) being related to the electrical resistance and capacitance by the following expressions [45]:  
\[Z_{\text{real}}=(\text{R}/[1+(\omega\text{RC})^{2}]) \tag{1}\]  
\[Z_{\text{img}}=-(\omega\text{R}^{2}\text{C}/[1+(\omega\text{RC})^{2}]) \tag{2}\]  
where \(\omega\) represents the angular frequency (\(\omega\) = 2\(\pi\)f). For homogeneous systems, the Nyquist plot consists of a semi-circle (a unique relaxation time), which corresponds to a parallel association of resistance and a capacitance (RC circuit) with intercepts on the \(Z_{\text{real}}\) axis at \(\text{R}_{\infty}\) (\(\omega\)\(\rightarrow\)\(\infty\)) and \(\text{R}_{0}\) (\(\omega\)\(\rightarrow\)\(0\)), with \(\text{R}\) = 0.5(\(\text{R}_{0}\) - \(\text{R}_{\infty}\)) being the electrical resistance of the system, while the maximum of the semi-circle occurs at a frequency such that \(\omega\)RC = 1 [45; 46]. However, for non-homogeneous systems (two or more relaxation times [40]), depressed semi-circles such as those shown in Figure 7a are commonly obtained, and the equivalent circuit consists of a parallel association of resistance and a non-ideal capacitor (or a constant phase element (CPE)), with the impedance of this latter being expressed by [45]: \(\text{Q}(\omega)\) = \(\text{Y}_{\text{o}}(\text{j}\omega)^{-n}\), where \(\text{Y}_{\text{o}}\) represents the admittance (inverse of impedance) while n is an experimental parameter (\(0\leq\) n \(\leq\)1), and the equivalent capacitance (\(\text{C}^{\text{eq}}\)) is determined by: \(\text{C}^{\text{eq}}\) = ([R.Y\({}_{\text{o}}\)]\({}^{1/n}\))/R [45]. Figure 7: Impedance diagrams for the RC/BMIMPF\({}_{6}\) film (\(\bullet\)), the RC/AliquatCl film (\(\blacktriangledown\)) and the RC support (\(\square\)). (**a**) Nyquist plot (\(-\)\(Z_{\text{img}}\) vs. \(Z_{\text{real}}\)); (**b**) and (**c**) Bode plots (\(Z_{\text{real}}\) vs. frequency) and (\(-\)\(Z_{\text{img}}\) vs. frequency), respectively. Insert in (**b**) and (**c**): Bode plots for the AlqCl IL (\(\bigtriangledown\)). As can be observed in Figure 7a, the Nyquist plots obtained for the RC/IL films showed slight differences depending on the IL, but they differed significantly from the plot corresponding to the RC support also presented in Figure 7a for comparison reasons. On the other hand, the \(Z_{\text{real}}\) vs. frequency plot (Bode plot) drawn in Figure 7b shows clear differences among the samples in the interfacial region (electrode/film contribution, f \(<\) 100 Hz), which is an indication of film surface modification as it was already reported for nanoporous alumina modified structures [58]; moreover, a slight shift to lower frequency values for the maximum of the \(-Z_{\text{img}}\) vs. frequency curves shown in Figure 7c for both RC/IL films with respect to that obtained for the RC-support can also be observed, which used to be attributed to a more compact film structure [46]. In this context, the Bode plot obtained for the IL AliquatCl was also included as an insert in Figure 7c, with the frequency value for the maximum of the IL curve of 500,000 Hz, much higher than that determined for the RC/AliquatCl film (\(\sim\)2800 Hz).