Electrochemical Characterization of 1 Wt% Mixture of 1-Ethyl-3-Methylimidazolium Chloride and 1-Ethyl-3-Methylimidazolium Tetrafluoroborate|Bi(111) Interface

As the global energy need is in a constant growth new and more effective ways of energy production and storage have to be developed. Electrical double-layer capacitors (EDLC) are widely discussed and some even used as energy storage devices due to their high power densities and good cycling stability. However, as a drawback they often suffer from limited energy densities which for example could be improved by developing and applying new electrolytes with broader region of application potentials (ΔE) in comparison with aqueous electrolytes^1-3. Different room temperature ionic liquids (RTIL) could be theoretically used as a good alternatives possessing besides possibly higher ΔE also lower vapor pressure or higher thermal stability³. Therefore, the purpose of this work was to determine the electrochemical characteristics of the 1 wt% mixture of  1-ethyl-3-methylimidazolium tetrafluoroborate and 1-ethyl-3-methyl-imidazolium chloride as one of the newest potential electrolytes for EDLCs using Bi(111) as an electrode with well reproducible characteristics.

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 (C_s) values were calculated from Nyquist plots (, where ) at different fixed frequencies. Obtained C_s vs. E plots indicated that at more positive potentials, where the specific adsorption of Cl^- ions takes place, the C_s values increase with the decrease of frequency applied.

References

1. M. Kim, I. Oh, J. Kim, J. Mater. Chem. A 3, 3944 (2015)
2. K. Fic, G. Lota, M. Meller, E. Frackowiak, Energy Environ. Sci. 5, 5842 (2012)
3. N. D. Khupse, A. Kumar Indian J. Chem. 49A, 635 (2010)
4. C. Siimenson, L. Siinor, K. Lust, E. Lust, J. Electroanal. Chem. 730, 59 (2014)

Acknowledgments

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.