Influence of Molecular Organization of Ionic Liquids on Electrochemical Properties

Abstract

Ionic liquids (ILs) are pure salts with melting points typically less than 100°C. ILs exhibit several advantages over conventional molecular liquids in disparate applications because of their remarkable physical properties, which include wide electrochemical stability windows, high ionic conductivity and negligible vapor pressure. IL applications, either already realized or currently under development, encompass many diverse areas such as analytics, catalysis, chemical synthesis, separation technologies, electrochemistry, capacitors, batteries, fuel cells, solar cells, and tribology. Many of these applications involve reactions at the IL/solid interface. Hence, a detailed understanding of the structure of this interface is important and cannot be overstated. ILs exhibit behavior that is very different from common molecular liquids. As ILs are composed entirely of charged species, they usually exhibit a more pronounced structure in the bulk and at surfaces than molecular liquids.1 ILs are subject to a range of cohesive interactions (Coulombic, van der Waals, hydrogen bonding and solvophobic forces), resulting in a well-defined nanostructure both in the bulk and at interfaces.2 The nanostructure of ILs evolves as a consequence of electrostatic interactions between charged groups that produce polar domains. Cation alkyl chains are solvophobically repelled3 from these charged domains and cluster together to form apolar regions, that in turn produce a sponge-like phase-separated nanostructure.4 This spongelike structure present in the bulk changes immediately adjacent to a smooth solid surface. Atomic force microscopy (AFM) force curves5,6 and reflectivity experiments7 both reveal the formation of discrete ion or ion pair layers immediately adjacent to the solid surface. This layered surface structure decays to the bulk sponge morphology over a length-scale of a few nanometers.8 The IL/solid interface has been the subject of extensive experimental and theoretical studies. Various spectroscopic and scattering methods have been applied to examine this interface.7,9-21 Electrochemical impedance spectroscopy (EIS) measurements have been used to study the structure and dynamics of ILs.22-24 Theoretical descriptions of the IL/electrode interface using molecular dynamics and Monte Carlo simulations,25-35 and mean field theory,36,37 have predicted “bell-” and “camel-” shaped capacitance curves and oscillating ion density profiles at the electrode surface, consistent with experimental results. However, a proper theoretical model of the electrified IL/solid interface does not yet exist and further experimental studies are required for better understanding the IL/electrode interfacial structure. During the last decade, in situ atomic force microscopy (AFM) and scanning tunneling microscopy (STM) have been extensively used to probe the IL structure at the IL/solid interface.1,5,8,38-50 The mechanism of operation of the AFM experiment is presented schematically in Fig. 1. The solid electrode substrate and the AFM tip and cantilever are completely immersed within the IL (Fig. 1a). The layers close to the surface are shown schematically as single (blue) layers. As the AFM tip moves towards the surface, it is deflected away due to the forces imparted by the interfacial IL