Electrochemical surface plasmon resonance measurements of camel-shaped static capacitance and slow dynamics of electric double layer structure at the ionic liquid/electrode interface

Abstract

Electrochemical surface plasmon resonance (ESPR) is applied to evaluate the relative static differential capacitance at the interface between 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ionic liquid (IL) and a gold electrode, based on the relationship between the SPR angle and surface charge density on the electrode. Potential-step and potential-scan ESPR measurements are used to probe the dynamics of the electric double layer (EDL) structure that exhibit anomalously slow and asymmetrical characteristics depending on the direction of potential perturbation. EDL dynamics respond at least 30 times more slowly to changes of potential in the positive direction than in the negative direction. ESPR experiments with the positive-going potential scan are significantly affected by the slow dynamics even at a slow scan. The surface charge density that reflects the relative static capacitance is obtained from the negative-going potential scans. The evaluated quasi-static differential capacitance exhibits a camel-shaped potential dependence, thereby agreeing with the prediction of the mean-field lattice gas model of the EDL in ILs. ESPR is shown to be an effective experimental method for determining relative values of the static differential capacitance.