A comparative study of the cathodic behaviour of EuCl3 in two imidazolium chloride ionic liquids, the 1-butyl-3-methylimidazolium (C4mimCl) and the 1-ethyl-3methylimidazolium (C2mimCl), on a glassy carbon electrode.

Abstract

This work presents a comparative study of the electrochemical behaviour, on a glassy carbon electrode, of dissolved EuCl3 in two chlorobasic ionic liquids, the 1-ethyl-3-methylimidazolium chloride, C2mimCl, and the 1-buthyl-3-methylimidazolium chloride, C4mimCl, over the temperature ranges 363–398 K and 343–363 K respectively. In both media, the electro-reduction of EuCl63 − takes place via only one electrochemical step EuCl63 −/EuCl64 −. The electrochemical system EuCl64 −/Eu(0) has not been observed within the electrochemical window of both ILs, due to the prior reduction of the respective imidazolium cation from the solvents, which inhibits the electro-extraction of Eu(0) from the media on the GC electrode. The paper describes how several electrochemical techniques (e.g. cyclic voltammetry (CV), convolutive potential sweep voltammetry (CPSV), steady state voltammetry (SSV), chronopotentiometry (CP) and chronoamperometry (CA)) have been used experimentally: i) verifying assumptions used in their theoretical analysis, and ii) ensuring reproducible conditions at the electrode/electrolyte interface, with the aim to obtain high-precision measurements of the diffusion coefficient of EuCl63 −. In order to know if there are discrepancies between the obtained data, a two-way analysis of variance, ANOVA, has been carried out. The analysis has shown that with a 95% confidence level there are no significant differences between the diffusion coefficients obtained by the different techniques. On the other hand, the diffusion coefficient of EuCl63 − increases with the temperature following the Arrhenius law, being the activation energy for diffusion 40.4 ± 2.6 and 60.2 ± 1.8 kJ mol− 1 in C2mimCl and C4mimCl respectively. The dimensionless Schmidt numbers, defined as the ratio between solvent viscosity and solute diffusivity (Sc = ν/D), have also been calculated to characterise the solute global mass transport through its environment. On the GC electrode, the electro-reduction of EuCl63 − to EuCl64 − is a quasi-reversible process. Accurate values of the kinetic parameters (i.e. the intrinsic rate constant of charge transfer, k0, and the charge transfer coefficient, α), as well as the reversible half wave potential, Er1/2, have been obtained for the first time in the mentioned ionic liquids, by simulation of the cyclic voltammograms and logarithmic analysis of the voltammograms and convoluted curves