## 3 Results and Discussion
### Electrochemical Stability
#### 3.1.3 Effect of Contaminants on the Anodic Stability  
The effect of the moisture content on the anodic stability of the IL electrolytes was investigated by means of CV measurements (Figure 3) performed on a selected sample (i.e., 0.1 NaClQ\({}_{4}\)-0.9 EMITFSI), which had been previously exposed to the external atmosphere. The adsorbed moisture content was estimated around 200-300 ppm by Karl-Fisher coulomb-metric titrations. Comparing these results with those obtained on the analogous anhydrous (<2 ppm moisture) electrolyte (Figure 2), a current increase of about one order of magnitude is observed up to the fourth CV cycle with a decrease in the subsequent ones (Figure 3A). A similar behaviour is observed even if the voltammetric measurements are run up to lower cut-off voltages (i.e., 4.2 (Figure 3B) and 4.0 V (Figure 3C)), despite the current value throughout the cell being significantly reduced. Therefore, the increase in current recorded in the initial cycles is attributable to the presence of humidity and/or molecular oxygen, which are both able to catalyse the degradation processes (oxidation) of the electrolyte and result in a decrease in electrochemical stability. Conversely, the decrease in the current value, recorded after a few voltametric cycles, is probably ascribable to the formation of an electronically insulating passive layer on the working electrode. These experimental findings, also reported in the literature for other ionic liquids [26; 27], clearly  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Electrolyte Sample** & **EA\({}_{1}\)** & **EA\({}_{2}\)** & **EA\({}_{1}\)** & **EA\({}_{2}\)** \\ \hline
0.9 EMITSI:0.1 NaClO\({}_{4}\) & 3.984 & 4.142 & 4.311 & 4.589 \\
0.9 EMITSI:0.1 NaClO\({}_{4}\) & 3.453 & 3.789 & 3.166 & 4.093 \\
0.9N\({}_{1114}\)TSI:0.1 NaClO\({}_{4}\) & 4.092 & 4.372 & 4.514 & \textgreater{}4.6 \\
0.9N\({}_{1114}\)TSI:0.1 NaClO\({}_{4}\) & 4.464 & \textgreater{}4.6 & \textgreater{}4.8 & \textgreater{}4.6 \\
0.9N\({}_{1114}\)TSI:0.1 NaClO\({}_{4}\) & 4.381 & \textgreater{}4.6 & 4.598 & \textgreater{}4.6 \\
0.9 PIP\({}_{13}\)TSI:0.1 NaClO\({}_{4}\) & 3.947 & \textgreater{}4.6 & 4.479 & \textgreater{}4.6 \\
0.9 PIP\({}_{14}\)TSI:0.1 NaClO\({}_{4}\) & 3.248 & 3.489 & 4.212 & \textgreater{}4.6 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Anodic voltage values (EA) determined by the CV measurements (reported in Figure 2) run on 0.1 NaClO\({}_{4}\)-0.9 IL electrolytes at 20 \({}^{\circ}\)C. The voltage, referred to the Na\({}^{+}\)/Na\({}^{\circ}\) redox couple, were taken when the current density through the cell reached 0.01 (EA\({}_{1}\)) and 0.02 (EA\({}_{2}\)) mA cm\({}^{-2}\) during the 1 st and 2 nd anodic scan, respectively. The error bar is equal to 0.001 V.  
Figure 2: Current-voltage profiles obtained by anodic CV measurements run on Na/C cells containing different 0.1 NaClO\({}_{4}\)-0.9 IL electrolytes (panels from **(A–G)**). Scan rate: 1 mV s\({}^{-1}\). Temperature: 20 \({}^{\circ}\)C. The voltage values are referred to the Na\({}^{+}\)/Na\({}^{\circ}\) redox couple.  
show how the presence of impurities (such as humidity and/or oxygen) can negatively affect the properties of electrolytes, particularly with regard to electrochemical stability. Further investigation is in progress in our laboratories to promote better understanding of this issue.