Electrochemical Characterization of the Interface Between Pyrolytic Graphite Electrode and 1-Butyl-3-Methylimidazolium Iodide

Ionic liquids as electrolytes for modern electrochemical energy conversion/storage devices (electrochemical double layer capacitors (EDLC), hybrid capacitors, batteries, etc.) have been intensively investigated over the past few years. However, still the kinetics of formation/rearrangement of the electrical double layer (edl) between electrode and ionic liquid (IL) needs an extended examination. Our previous works were focused on developing of better electrolyte for EDLCs by mixing chemically different ionic liquids EMImBF₄ and EMImI¹. It has already been successfully tested in a system using microporous-mesoporous carbon electrodes².

The main aim of this work was to establish the electrochemical characteristics of adsorption of iodide ions from ionic liquid media without added electrolytes.

In this work electrochemical impedance spectroscopy and cyclic voltammetry (CV) methods were applied to study the electrochemical characteristics of the interface between pyrolytic graphite (PG) and 1-butyl-3-methylimidazolium iodide (BMImI) at 22±1 ^oC. All measurements were carried out inside a glove box 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 show that there is clear dependence of capacitance on the composition of IL if we compare 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF₄) and PG electrode interface to the BMImI containing system. There is an influence of the alkyl chain length and in the case of BMImI, the I^- anion demonstrates specific adsorption behaviour. The electrochemical characteristics of BMImI|Bi(111) interface show similarities to EMImBF₄ + EMImI interface. The PG|BMImI interface characteristics were measured within the potential region from ─1.0 V to +1.5 V (vs. Ag | AgCl).

The shape of the complex impedance plane, i.e. Nyquist plots, total impedance and phase angle vs. frequency plots shape depend strongly on the electrode potential and on the ac frequency (f) applied. The shape of the phase angle vs. ac f dependencies show that at low frequency range (f < 1 Hz) and potential region from ─0.8 V to +0.4 V the system has nearly ideally capacitive behaviour. At more positive potentials the mixed kinetics behaviour can be seen.

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 = √−1 and Z’’ is the imaginary part of impedance). C_s depends strongly on ac frequency and on electrode potential applied, increasing at lower ac frequencies and at anodic range of potentials.

References:

1.         L. Siinor, J. Poom, C. Siimenson, K. Lust, E. Lust, J. Electroanal. Chem., 719, 133 (2014).

2.         T. Tooming, T. Thomberg, L. Siinor, K. Tõnurist, A. Jänes, E. Lust, J. Electrochem. Soc. 161, A222 (2014).

Acknowledgments: This study was partially funded by the Estonian Energy Technology Program project SLOKT10209T; Project IUT20-13; Estonian Centers of Excellence in Science project: High-technology Materials for Sustainable Development TK117 and by the Estonian Research Council grant PUT55.