Electrochemical Characterization of 1 Wt% Mixture of 1-Ethyl-3-Methylimidazolium Chloride and 1-Ethyl-3-Methylimidazolium Tetrafluoroborate|Bi(111) Interface
The measurements were carried out in a three electrode system using electrochemically polished Bi(111) as a working electrode, Ag wire coated with AgCl as a reference electrode and Pt net as a counter electrode. The mixture was prepared by dissolving 1 wt% EMImCl (Merck KGaA, ≥ 98.0 %) in EMImBF4 (Fluka Analytical, ≥ 99.0 %) and then heating it up to 80 . The Bi(111)|EMImBF4 and EMImCl interface was studied using electrochemical impedance spectroscopy and cyclic voltammetry.
Current density (j) vs. electrode potential (E) dependencies obtained showed that the region of ideal polarization for EMImBF4 containing 1 wt% of EMImCl was from -1.0 V to -0.3 V, whereas for the mixture of EMImBF4 and EMImI (the same concentration) it was from -1.0 V to -0.2 V (vs. Ag|AgCl)4.
The experimental differential capacitance, potential (C vs. E) curves measured at constant ac frequency (210 Hz) were acquired using electrochemical impedance spectroscopy. The reversible nature of the system was determined by measuring first towards more positive potential directions and then towards more negative potential values. Comparison of data for EMImBF4 + EMImCl and EMImBF4 + EMImI4 mixtures (1 wt%) showed that the capacitance values of the former were noticeably lower than that of I- containing RTIL mixture.
Clear dependance of adsorption characteristics on frequency and potential applied can be seen on Nyquist and phase angle (δ) vs. frequency (f) plots. The nearly ideal capacitive behavior of the system appeared in a narrow region of frequencies in the part of intermediate f values (2-100 Hz) and showed only light diminishing at lower f values (0.1-2 Hz).
The series differential capacitance (Cs) values were calculated from Nyquist plots (, where ) at different fixed frequencies. Obtained Cs vs. E plots indicated that at more positive potentials, where the specific adsorption of Cl- ions takes place, the Cs values increase with the decrease of frequency applied.
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This study was partially funded by the Estonian Energy Technology Program project SLOKT10209T, IUT20-13 and Estonian Centers of Excellence in Science project: High-technology Materials for Sustainable Development TK117 and PUT55.