The Activity of Halide Ions and the Effect on the Electrochemical Stability of Bi(111) Electrode in a Mixture of Ionic Liquids

Abstract

Ionic liquids are versatile solvents and electrolytes for many electrochemical applications such as secondary batteries, capacitors, solar cells and electrochemical deposition of metal coatings[1–3]. In order to rationally design and further develop such systems, however, a detailed understanding of the processes that govern the interfacial processes and stability of ionic liquid interfaces is required. Although a large number of experimental and theoretical studies of ionic liquid interfaces exist, the research regarding the electrochemistry of ionic liquid mixtures has been limited. It is thus the focus of the presentation to provide information about the concentration dependent activity of halide ions in a mixture of ionic liquids and to explore the effect of ion activity on the nanoscale stability of the Bi(111) electrode surface. Potentiometry, cyclic voltammetry, electrochemical impedance spectroscopy and in situ scanning tunneling microscopy (STM) have 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 affects the electrochemical stability and 2D layer formation at a Bi(111) electrode surface in a mixture of ionic liquids. It is observed that, different from aqueous and organic electrolyte solutions, the ionic liquid media promote ion aggregate formation and complex cations exist in many different valence states, dependent upon the concentration of complex forming ions. The effects of the ion activity are evident from in situ STM measurements, as the surface of a Bi(111) electrode is stable in both ionic liquids that contain no halide additions as well as those with a saturated amount of halide ionic liquids, but is unstable in mixtures containing a small or medium amount of halide ionic liquid additions. Electrochemical measurements also suggest that different processes and mechanisms are required in order to describe the concentration dependence of the specific adsorption of halide ions at a Bi(111) electrode[4]. 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, which results in the dissolution and re-deposition of bismuth ions to preferentially occur are terrace edge sites, thus maintaining the monocrystalline structure of the Bi(111) electrode surface. Acknowledgements This work was supported by the Estonian Ministry of Education and Research (projects no. IUT 20-13, PUT55 and PUT1107), European Development Fund (project 3.2.1101.12-0019), and Estonian Centres of Excellence (projects no. TK117T and TK141).