Atomic Force Microscopy Study of Ionic Liquid Structure at Carbon Interface

Ionic liquids are attractive electrolytes for electrochemical capacitors because of the large operational voltage windows leading to an increased energy density in these systems. The charge storage mechanism and structure of ionic liquids at the interface is still poorly understood. Atomic force microscopy (AFM) force spectroscopy is capable of probing the layered structure of ionic liquids which forms at the electrode interface. Figure 1 shows the ion density profiles from molecular dynamics for the cation (a) and anion(b) of the ionic liquid 1-ethyl-3methyl-imidazolium bis(trifluoromethanesulfonyl)imide (Emim⁺Tf2N^-) as a function of potential and distance from a carbon surface.  Molecular dynamics predicts an alternating structure of anions and cations extending several nm from the surface.  A histogram of 50 force distance curves on a highly oriented pyrolytic graphite (HOPG) for the same ionic liquid is shown in Figure 1c.  Distinct steps are observed during the approach of the AFM tip due to interaction with the ion layers present at the interface. By combining this technique with insights from molecular dynamics simulations the effect of applied potential on the structure of the ionic liquid within the electrical double layer is investigated and we are able to observe reconstruction of ions in response to an applied bias. This technique is also used to map the structure of the ionic liquid over the carbon surface to investigate the effect of defects such as step edges on the structure of the ionic liquid.

Figure 1: Molecular dynamics ion density profiles as a function of potential and distance from a carbon surface for the Emim⁺ cation (a) and Tf2N^- anion (b).  Histogram of 50 consecutively measured force distance curves at a single location on an HOPG surface in Emim⁺Tf2N^- ionic liquid (c).