Electrochemical Characterisation of Adsorption of Halide Ions at Bi(111) Electrode from Ionic Liquids and Their Mixtures
Authors: Siimenson Carolin , Siinor Liis , Lust Enn 
Published: February 2020

In recent years, application of ionic liquids as electrolytes in electrochemical devises has become more and more routine. Indeed, non-aqueous electrolytes can provide higher cell potential without decomposing. For electrochemical double layer capacitors (EDLCs) the widening of the cell potential increases exponentially the energy and power densities. There are several studies analyzing the strong adsorption of halide ions at Bi single crystal planes from classical electrolyte solutions demonstrating a maximum in C, E -curves in the presence of specifically adsorbed I−, Br− or Cl− anions. [1–4]
Our recent studies have shown that the addition of specifically adsorbed iodide ions strongly influences the electrochemical characteristics of RTILs systems increasing the specific capacitance and decreasing the series resistance values [1]. The main aim of this study was to investigate the adsorption of Br− ions and electrochemical double layer parameters in ionic liquid media.
The cyclic voltammetry and electrochemical impedance spectroscopy methods have been applied to investigate the electrical double layer formation and adsorption kinetic parameters of Br− anions at the electrochemically polished Bi(111) electrode surface. 1-ethyl-3-methylimidazolium bromide, 1-ethyl-3-methylimidazolium tetrafluoroborate were used to prepare the mixture of ionic liquids in order to increase the differential capacitance of the system, decrease the viscosity and lower the price of the ionic liquids mixture [5]. Furthermore, EMIm + halides have shown very good thermal stability, i.e. they are suitable electrolytes for higher temperatures as well.
The measured ideal polarizability region from – 1 to 0 V is in a good agreement with impedance data. The mixed kinetic processes take place on both systems (Bi(111)|EMImBF4 + EMImI and Bi(111)|EMImBF4 + EMImBr).
The series differential capacitance (C
s) values have been calculated from Nyquist dependencies at different fixed ac frequencies from 0.2 to 10000 Hz (C
s = (Z''i2πf)−1; where i = √−1). C
s depends on the electrode potential and ac frequency being somewhat higher for lower frequencies. The maximum capacitance values are aproximatley 15% lower than those for EMImBF4 + EMImI system.

Acknowledgments: This study was partially funded by the Estonian Energy Technology Program project SLOKT10209T, Estonian Basic Research project SF0180002s08, PUT55, IUT20-13 and Estonian Centers of Excellence in Science project: High-technology Materials for Sustainable Development TK117
References
[1] C. Siimenson, L. Siinor, K. Lust, E. Lust, J. Electroanal. Chem. 730 (2014) 59.
[2] L. Siinor, V. Ivaništšev, K. Lust, E. Lust, J. Solid State Electrochem. 14 (2010) 555.
[3] K. Lust, M. Väärtnõu, E. Lust, Electrochimica Acta 45 (2000) 3543.
[4] T. Thomberg, T. Tooming, K. Tõnurist, L. Siinor, A. Jänes, E. Lust, in:, Meet. Abstr., The Electrochemical Society, 2014, pp. 173–173.
[5] A. Efimova, L. Pfützner, P. Schmidt, Thermochim. Acta 604 (2015) 129.