Stabilization of the Ionic Liquid – Electrode Interface from Large Nonpolar Groups in Ionic Liquids and Ionic Liquid Mixtures

Electrochemical capacitors, also called electrical double layer capacitors (EDLCs), can store energy in the interface between an electrode and an electrolyte. The interface where charge is stored influences more than simply EDLCs due to its presence at any charged surface. Conventional electrolytes, organic and aqueous based, form a well described electrode – electrolyte interface. However, conventional electrolytes are often limited in their application due to restrictions in thermal and electrochemical stability. Ionic liquids (ILs) are an emerging class of electrolytes with improved thermal and electrochemical stability compared to conventional electrolytes, however, the interface is poorly understood. Current understanding of the conventional systems cannot be completely transferred to ILs because they are not sufficiently described as point charges, and because ILs contain no solvent. Completing our understanding of the complex IL-electrode interface will greatly influence the efforts at designing task specific ILs for electrochemical devices. Here it is demonstrated that quaternary ammonium cations with large nonpolar substituent groups directly impact the anion density near the electrode at positive potentials. This work also addresses whether or not the unique IL-electrode interface is formed from a process which expels void space from the local environment, or by replacing “non-participating” neutral ion pairs with “participating” free ions. Potential driven structuring of four neat ILs and two IL mixtures has been examined on a glassy carbon electrode. Potential dependent capacitance (differential capacitance), measured by electrochemical impedance spectroscopy, was used to examine the formed IL - glassy carbon interface. Differential capacitance curves were paired with physical measurements of viscosity, density, and conductivity as represented in a Walden plot. The Walden plot was used to estimate the extent to which the ions of the IL were dissociated. There appeared to be no trend between the extent of dissociation and the differential capacitance behavior. The absence of a trend between dissociation and differential capacitance suggests that the interface is not described by a simple model depending solely on the number of free ions in the interface. Instead, the interface depends more heavily on the intermolecular interactions between the constituent ions such as ion – ion repulsions and steric repulsions. It is shown in the differential capacitance curves that the ILs containing large non polar substituent groups have a more stable interface associated with the accumulation of anions. Stabilization of the interface holds even for mixtures of ILs where the minor component contains the large nonpolar group. Results from this work offer ways to selectively tune the ion density in the IL-electrode interface at specific electrode potentials.