In recent times, ionic liquids (IL) have been studied as alternatives to water and organic solvents because of their unique features, in particular, tunability and designability. The electrochemical window, viscosity, conductivity, and hydrophobicity of ILs can be tuned by altering their molecular structures. One of the applications of ILs is as an electrolyte solution for electrochemical devices.

In the last decade, many researchers have investigated the electrochemical characteristics of ILs and the behavior of IL/electrode interfaces. On the other hand, the effect of impurity contained in ILs on electrode surface structure has not been well uncovered. Typical impurities are halide ions, alkaline metal ions, and water. It has been well known that the halide ions are adsorbed on metal electrode surface and affect electrochemical processes. Especially, chemically adsorbed halide ions are called “specific adsorption anions”. To understand the effect and the behavior of halide ions exist at electrode/ILs interfaces is important from both scientific and industrial point of views. In this study, the behavior of halide ions on the Au(111) electrode surface in two ionic liquids (IL), 1-buthyl-1-methylpyrrolidiniun bis(trifluoromethylsulfonyl)amide ([BMP]TFSA) and 1-buthyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([BMIM]TFSA), was investigated by monitoring the electrode surface structure. Potential dependences of the X-ray diffraction intensity, which originate from the Au(111)-(1 × 1) structure and the surface normal structure, were simultaneously measured with cyclic voltammograms. We considered the dependence of ion concentration and ion species.

The electrolytes were prepared using [BMP]Cl, [BMP]Br, [BMP]I, [BMIM]Br, [BMP]TFSA, and [BMIM]TFSA. The electrolytes were dehydrated at 80 °C under vacuum for 24 h before use. A “drop cell” was used as the electrochemical cell. A working electrode was a Au(111) single crystal disk (Φ ≈ 5 mm). Surface X-ray scattering (SXS) experiments were conducted at BL14B1, SPring-8 using a κ-type diffractometer. A 30 keV X-ray was used to obtain a sufficient attenuation length in the drop cell arrangement.

                In [BMP]TFSA, cyclic voltammograms (CV) showed both halide ion species and their concentration dependence. In the ion species dependence, current values increased in the order of Cl^-, Br^-, and I^-. The cathodic current peak position also shifted to negative potential in the same order. Further, the kinetics of surface reconstruction became faster as the concentration increased. On the other hand, the potential dependence of the surface (1 × 1) structure showed no common behavior. In [BMIM]TFSA, small dependence was observed in both CVs and surface structure.

                The results revealed that halide ions are co-adsorbed with ionic liquid molecules on electrode surfaces and they increase the mobility of surface atoms. In contrast, the Au(111) surface does not reconstruct to the (p × √3) structure even in the IL containing 200 mM Br⁻, suggesting that the interaction between halide ions and surface Au atoms is weaker than that between IL molecules and surface Au atoms, that is, the surface property is mainly governed by adsorbed IL molecules. Furthermore, compared with results measured in the two ILs, it was revealed that the effect of halide ions on the Au(111) electrode surface structure depends on the strength of the interaction between IL molecules and surface Au atoms.