An Electrochemical Description of 1-Alkyl-3-Methyimidazolium Chloride Ionic Liquids

Abstract

Three common Ionic Liquids (ILs) were selected 1-Methylimidazolium chloride ([Hmim]Cl), 1-Butyl-3-methylimidazolium chloride ([Bmim]Cl) and 1-Octyl-3-methylimidazolium chloride ([Omim]Cl) for investigation to explore the effects of alkyl chain length and cation protonation on corrosion rate. The aqueous IL solutions have demonstrated a range of identifiably distinct behaviours as a function of changing solution concentration in the range of 70-99.5 mol% water. These behaviours are evidenced by large changes in conductivity, density, and viscosity; these are measurable properties indicating an evolution of hydrogen bonding networks in these systems. These bulk solution properties are critical to the interaction with a metal surface; this was explored using electrochemical testing with Cu and Zn working electrodes. Herein, the focus will be on the use of cyclic voltammetry (at 75 mol%, 85 mol%, 97 mol% and 99.5 mol% water in IL) to describe metal dissolution, deposition, and hydrogen evolution in ILs. The system of equations implemented, to interpret the electrochemical data obtained, have wide-reaching implications in understanding the fundamental behaviours causing observed corrosion behaviours; additionally, the prediction of IL solution conductivity, a vital property for applications of ILs, is of significant value.A kinetic analysis of these solution systems are benefited by the simplicity of the system with no bulk electrolyte, i.e. only the two components: water and IL. Ohmic migration in the bulk and discharge on electrode surface are the two contributions which need to be considered in the Butler-Volmer equation. (n.b. A limitation in the treatment is that thus far no account is taken for the process of solid deposition via a nucleation stage.) In this approximation the following equations can be derived. Each discharging particle must pass two consecutive processes: (1) migration in the field, with characteristic time τohm, and (2) discharge, with the time τBV. The total time will be τtot = τohm + τBV, where τohm -1 = kohm and τBV -1 = kBV. Expressions for Icathodicscan and Ianodicscan results, where φ =E-Ec , for which Ec is the potential at zero current. Two examples are included (Figure 1) to highlight the performance of the models with experimental data. The first case illustrates a particularly good agreement with a cyclic voltammogram experimentally measured for a Cu working electrode in an aqueous-IL solution, the second case is included to demonstrate that the characterstic reversal (near I=0) can be produced. Some deviation, particularly at high current, is likely due to capacitive current. The fitting also gives conductivity, which can be compared and shows good agreement with independent experimental measurements. As such, this successful prediction of the electrochemical behaviours provides an important step in the utilisation of cyclic voltammetry as a method for delivering insight into these two-component IL-containing solutions systems through fundamentally derived expressions. Figure 1