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Electrochemical characterization of the interface between Bi(111) surface and ionic liquid mixtures of 1-ethyl-3-methylimidazolium tetrafluoroborate with 1-ethyl-3-methylimidazolium bromide (EMImBF 4 + x% EMImBr) has been evaluated by using the electrochemical impedance spectroscopy, cyclic voltammetry methods and density functional theory calculations. Dependence of the experimental data on the bromide concentration and on the electrode potential has been analyzed. Comparison of adsorption data with Bi(111) | EMImBF 4 + x% EMImI interface shows that adsorption activity of halide ions from EMImBF 4 follows a similar trend as from the aqueous and organic electrolyte solutions. The adsorption activity of anions increases in order Cl − < Br − < I − , despite the fact that the solvation properties of relevant media are significantly different. We discuss this trend in the light of a possible application of the RTIL mixtures as an electrolyte for electrochemical energy storage devices like supercapacitors. The ionic liquids as electrolytes have been studied since the early 20 th century, but there is still much to be understood regarding some fundamental aspects. [1][2][3][4][5][6] Theoretical models that explain the dependence of the differential capacitance values on the electrode potentials have been worked out for the aqueous and non-aqueous electrolytes and molten salts. 7,8 However, in the case of the specific adsorption, there are no detailed models for the electrical double layer capacitance, that analyze the influence of the surface-active anion concentration and electrode potential on the specific interaction energy (including charge transfer) and on the distance of the closest approach of anions onto the metal electrode surface.5,9,10 Therefore, the detailed analysis and characterization of these complex electrode | ionic liquid interfaces (with the addition of ions that might have strong chemical interactions and high interaction energies with electrode surface layer atoms, demonstrating very negative Gibbs adsorption energy values, so-called strong specific adsorption properties) is considered our high priority. Bismuth, as an electrode material, has been studied widely and has shown good electrochemical stability and high reproducibility of the data. The properties are inevitable for detailed investigation of interfacial adsorption and faradaic charge transfer processes. The amount of previously measured adsorption data from different electrolytes (including electrochemical impedance spectroscopy and in situ STM methods) enables to develop more realistic, i.e. complex models for the electrical double layer capacitance, that analyze the influence of the surface-active anion addition on the inner layer structure, dielectric permittivity, effective dipoles moment for dipole created at metal|RTIL interface, etc. 7,[11][12][13][14] To continue with the systematic research of halide ions, 1-ethyl-3-methylimidazolium bromide (EMImBr) was selected for the source of bromide ions. There are some theoretical and e...

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