Surface Structure at the Ionic Liquid−Electrified Metal Interface

Abstract

Room-temperature ionic liquids are a new class of liquids with many important uses in electrical and electrochemical devices. The liquids are composed purely of ions in the liquid state with no solvent. They generally have good electrical and ionic conductivity and are electrochemically stable. Since their applications often depend critically on the interface structure of the liquid adjacent to the electrode, a molecular level description is necessary to understanding and improving their performance. There are currently no adequate models or descriptions on the organization of the ions, in these pure ionic compounds, adjacent to the electrode surface. In normal electrolytic solutions, the organization of solvent and ions is adequately described by the Gouy–Chapman−Sterns model. However, this model is based on the same concepts as those in Debye–Huckel theory, that is a dilute electrolyte, where ions are well-separated and noninteracting. This is definitely not the situation for ionic liquids. Thus our goal was to investigate the ionic liquid−metal interface using surface-specific vibrational spectroscopy sum frequency generation, SFG. This technique can probe the metal−liquid interface without interference from the bulk electrolyte. Thus the interface is probed in situ while the electrode potential is changed. To compliment the vibrational spectroscopy, electrochemical impedance spectroscopy (EIS) is used to measure the capacitance and estimate the “double layer” thickness and the potential of zero charge (PZC). In addition, the vibrational Stark shift of CO adsorbed on the Pt electrode was measured to provide an independent measure of the “double layer” thickness. All techniques were measured as a function of applied potential to provide full description of the interface for a variety of imidazolium-based (cation) ionic liquids. The vibrational Stark shift and EIS results suggest that ions organize in a Helmholtz-like layer at the interface, where the potential drop occurs over the a range of 3–5 Å from the metal surface into the liquid. Further, the SFG results imply that the “double layer” structure is potential-dependent; At potentials positive of the PZC, anions adsorbed to the surface and the imidazolium ring are repelled to orient more along the surface normal, compared with the potentials negative of the PZC, at which the cation is oriented more parallel to the surface plane and the anions are repelled from the surface. The results present a view of the ionic liquid−metal electrode interface having a very thin “double layer” structure where the ions form a single layer at the surface to screen the electrode charge. However, the results also raise many other fundamental questions as to the detailed nature of the interfacial structure and interpretations of both electrochemical and spectroscopic data.