Evaluation of static differential capacitance at the [C4mim+][TFSA-]/electrode interface using molecular dynamics simulation combined with electrochemical surface plasmon resonance measurements

Molecular dynamic (MD) simulations have been performed for 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C₄mim⁺][TFSA^-]), an ionic liquid (IL), on a charged graphene electrode to achieve the quantitative analysis of the static differential capacitance using the electrochemical surface plasmon resonance (ESPR). The MD simulations have provided the surface charge density on the electrode and ionic distributions in the electric double layer, both of which are indispensable for the evaluation of static differential capacitance using ESPR but are difficult to be measured by experimental techniques. This approach has allowed the quantitative analysis and explanation of the SPR angle shift in ESPR. The major contribution to the SPR angle shift is found to be the change in ionic concentrations of the first ionic layer on the electrode, owing to higher polarizabilities of ions in the first ionic layer than those in the overlayers. Moreover, the ionic orientation on the electrode and ionic multilayer structure have also been investigated in detail. The butyl group of C₄mim⁺ in the first ionic layer is found to provide extra room for C₄mim⁺ in the second ionic layer but exclude TFSA^-, which affects the interval and regularity of ionic multilayers.

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

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.

An electric double layer structure and differential capacitance at the electrode interface of tributylmethylammonium bis(trifluoromethanesulfonyl)amide studied using a molecular dynamics simulation

A molecular dynamics simulation at the electrode interface of a quaternary ammonium ionic liquid, tributylmethylammonium bis(trifluoromethanesulfonyl)amide ([N₁₄₄₄⁺][TFSA^-]), has been performed. Unlike the commonly used cations, such as 1-alkyl-3-methylimidazolium and 1,1-alkylmethylpyrrolidinium cations, N₁₄₄₄⁺ has multiple long-alkyl groups (three butyl groups). The behavior of ions at the electrode interface, especially these butyl groups, has been investigated. N₁₄₄₄⁺ at the first layer mainly has two types of orientations, lying and standing. The lying orientation is dominant at moderately negative potentials. However, the standing one becomes dominant at the more negative potentials. Due to this orientational change, the number of N₁₄₄₄⁺ increases at the first layer as the potential becomes negative even at the potentials where the anions are completely depleted there. The change in orientation results in the upward deviation of the differential capacitance from the theoretical prediction at the negative potentials. The results suggest that the orientational preference caused by the steric constraint between alkyl groups plays an important role in the behavior of the electric double layer of the ionic liquids.