Electrochemical Characterization of the PG | EMImBF4 + x% EMImI Interface

The need for alternative high energy and power density energy sources has led scientists to study extensively the electrochemical energy conversion/storage devices such as electrochemical double layer capacitors (EDLC), batteries, fuel cells, etc. The values of power and energy densities of EDLC depend strongly on the properties of the electrode material and electrolyte used. EDLCs based on non-aqueous electrolytes or room temperature ionic liquids have a wider region of ideal polarizability (ΔE) compared to those based on traditional aqueous electrolytes ^1-4. The wider ΔE in turn improves the energy and power density values of EDLCs. The specific adsorption of halide ions at single crystal electrodes from aqueous and non-aqueous electrolyte solutions has been demonstrated before ^5,6. However, the specific adsorption of ions is a complicated process (depending on the electrode potential, chemical composition and concentration of halides etc.) and the kinetics of formation/rearrangement of electrical double layer between electrode and ionic liquid (IL) needs an extended examination.

                The main aim of this work was to characterize the adsorption of iodide ions from the mixture of two ionic liquids based on the same electrochemically stable 1-ethyl-3-methylimidazolium cation.

                Electrochemical impedance spectroscopy and cyclic voltammetry methods have been applied to study the electrochemical characteristics of the interface between pyrolytic graphite (PG) and mixture of 1-ethyl-3-methylimidazolium tetrafluoroborate and 1-ethyl-3-methylimidazolium iodide (1-10 wt%). All measurements were carried out in a 3-electrode electrochemical cell using a PG as a working, Pt net as a counter, and Ag wire coated with AgCl as a reference electrodes.

                Current density (i) vs. electrode potential (E) dependencies (Fig. 1) show that two peaks in CV are caused by the addition of x wt% of EMImI into EMImBF4 (for PG | EMImBF₄ interface there is no peaks within the potential region from ─1.5 V to +1.5 V (vs. Ag | AgCl)). Reduction and oxidation processes are more intensive in more concentrated solutions. A strong dependence of current density on the potential scanning rate (υ) for PG | EMImBF4+EMImI interface was observed in the region of peaks. Also the capacitance values calculated from CV data (C_cv= i / υ) show the strong dependence on υindicating a very slow adsorption process occurs.

                The shape of the complex impedance plane, i.e. Nyquist plots, total impedance and phase angle vs. frequency dependences depend strongly on the electrode potential and on the concentration of the I^-ions in the solution.

                The values of series differential capacitance (C_s) were calculated from Nyquist dependencies at different fixed ac frequencies (C_s = (Z''i2πf)⁻¹; where i = √−1and Z’’ is the imaginary part of impedance). C_s depends strongly on ac frequency and electrode potential, increasing at lower ac frequencies and at anodic range of potentials. C_s values depend also on the added amount of I^-(1-10%) to the solution, increasing from 1 to 5wt% and diminish in more concentrated solutions.

References

1. L. Siinor, J. Poom, C. Siimenson, K. Lust, E. Lust, J. Electroanal. Chem., 719, 133 (2014).
2. L. Siinor, C. Siimenson, K. Lust, E. Lust, Electrochem. Commun. 35, 5 (2013).
3. R. Palm, H. Kurig, K. Tõnurist, A. Jänes, E. Lust, Electrochim. Acta, 85, 139 (2012).
4. R. Palm, H. Kurig, K. Tõnurist, A. Jänes, E. Lust, Electrochem. Commun. 22, 203 (2012).
5. L. Siinor, K. Lust, E. Lust, J. Electroanal. Chem., 601, 39 (2007).
6. M. Väärtnõu, E. Lust, J. Electroanal. Chem., 565, 211 (2004).

Acknowledgements

Estonian Energy Technology Program project SLOKT10209T, Estonian Centers of Excellence in Science project: High-technology Materials for Sustainable Development TK117, Functional Micro/Mesoporous Nanomaterials for Novel Energy Conversion and Storage Systems IUT 20-13, Functionalization of carbon nanomaterials: synthesis, characterization and application SLOKT12180T.