Detailed understanding of interfacial processes between electrodes and ionic liquids is required for the design of better batteries, capacitors, solar cells and deposited metal coatings. Specific adsorption of iodide ions has been investigated in our research-group for a long time in different aqueous and organic solvent solutions [1]. Recently we have showed how iodide ions behave in different ionic liquid (IL) mixtures and demonstrated the influence of specifically adsorbed anions on the electrochemical behaviour of Bi|RTIL and PG|RTIL interfaces [2,3]. There are some reports for platinum and glassy-carbon [4] electrodes, but for a more fundamental understanding, it is better to use monocrystalline electrodes. There is still a lot to look into for a better understanding of how the concentration dependent activity of halide ions in a mixture of ionic liquids influences the systems under study.

For the electrochemical characterization cyclic voltammetry and impedance spectroscopy methods were used. In situ scanning tunnelling microscopy (STM) has been applied in the present study in order to explore the concentration dependent activity of halide ions (iodide, bromide, chloride) and to demonstrate how the activity of halide ions affect the electrochemical stability and 2D layer formation at a Bi(111) electrode surface in a mixture of ionic liquids. Analysis of experimental and calculated data show that the electrical double layer structure at electrode|electrolyte interface depends noticeably on the chemical nature of the electrode, electrode potential applied as well as on the chemical nature of ionic liquid used. Furthermore, even without solvation sheaths, the anion adsorption plays a key role in determining the values of phase angle and capacitance. The phase angle values −85 degrees have been measured being characteristic for the physical adsorption step rate limited mechanism. In situ STM measurements demonstrate that the formation of a tightly packed 2D layer of specifically adsorbed ions is only formed in concentrated mixtures of halide ionic liquids.

Acknowledgements

This study was partially funded by the Estonian Energy Technology Program project SLOKT10209T, Projects IUT20-13, PUT55, PUT1033, PUT1107, and Estonian Centres of Excellence in Science project TK141 “Advanced materials and high-technology devices for energy recuperation systems”

References:

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[2] C. Siimenson, L. Siinor, K. Lust, E. Lust, The electrochemical characteristics of the mixture of 1-ethyl-3- methylimidazolium tetrafluoroborate and 1-ethyl-3-methylimidazolium iodide, J. Electroanal. Chem. 730 (2014) 59–64. doi:10.1016/j.jelechem.2014.07.035.

[3] L. Siinor, J. Poom, C. Siimenson, K. Lust, E. Lust, Electrochemical Characteristics Pyrolytic Graphite | Mixture of 1-Ethyl-3-methylimidazolium tetrafluoroborate and 1-Ethyl-3-methylimidazolium iodide Interface, J. Electroanal. Chem. 719 (2014) 133–137. doi:10.1016/j.jelechem.2014.01.035.

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