Electrochemical Characterization of the Bi(111) | 1-Butyl-3-Methylimidazolium Iodide Interface
Authors: Siinor Liis , Läll Laura , Lust Enn 
Published: February 2020

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 ionic liquid media without added electrolytes.
Electrochemical impedance spectroscopy and cyclic voltammetry methods have been applied to study the electrochemical characteristics of the interface between electrochemically polished Bi(111) electrode and 1-butyl-3-methylimidazolium iodide (BMImI). All measurements were carried out inside a glovebox using a 3-electrode electrochemical cell: a Bi(111) 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 the system is stable within a wide range of potentials. ΔE is 1.7V i.e from ─1.4 V to +0.3 V (vs. Ag | AgCl), which is extremely high for the bismuth electrode, known as a catalytically, chemically and electrochemically inactive electrode material.
The shape of the complex impedance plane, i.e. Nyquist plots, total impedance vs. log frequency and phase angle vs. log frequency dependences depend on the electrode potential applied. The values of phase angle are high as -85o within a wide range of potentials at medium and low ac frequencies. Also the Nyquist plots show only a slight deviation from the ideal capacitive behavior of the system at f < 1 Hz within wide ΔE applied.
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). In the studied system C
s depends strongly on ac frequency and electrode potential, increasing at lower ac frequencies and at anodic range of potentials. C
s values for Bi(111) | BMImI are much higher compared to Bi(111) | 1-ethyl-3-methylimidazolium tetrafluoroborate system.

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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.