Combination of Electrochemical Surface Plasmon Resonance and Molecular Dynamics Simulation to Evaluate the Quasi-Static Capacitance at the Ionic Liquid/Electrode Interface

Abstract

1. Introduction Composed of ions solely, ionic liquids (ILs) are low-melting point salts exhibiting excellent properties for electrolytes such as ultra-low volatility, non-flammability, thermal/electrochemical stability. The structure and dynamics of the electric double layer (EDL) are critical in the process of charge/mass transfer. However, different from the conventional electrolytes, ILs exhibit strong Coulomb correlation with high concentration and density, hence Gouy-Champman-Stern model is not applicable to describe the EDL of ILs. For the experimental measurement of the differential capacitance in the EDL, electrochemical impedance spectroscopy (EIS) is the most popular method. But the fast ac potential oscillation in EIS measurements can’t avoid the influence of the slow dynamics of the EDL such as ultraslow relaxation[1] and hysteresis.[2] In the present study, electrochemical surface plasmon resonance (ESPR) is proposed as an effective measurement method of static differential capacitance at the IL/electrode interface. The influences of the slow relaxation are demonstrated at different scan rates of potential. In order to achieve quantitative measurement, molecular dynamics (MD) simulations of the IL between two single-layer graphene electrodes are performed. Moreover, through the data analysis of MD, the variations of ionic layer structure and ionic orientation are revealed and described in detail. 2. Methods 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C4mim+][TFSA−]) was chosen as the IL material for research. In the ESPR experiments, a laser beam was incident through a prism and totally reflected on a 50-nm golden film deposited on a SF15 glass. The golden film was the working electrode, a Pt wire was set as the counter electrode, and an Ag/AgCl wire was inserted into the IL as the quasi-reference electrode. The experiments were performed at different scan rates to determine the influence of slow dynamics.[3] The all-atom MD simulations were performed by using DL_POLY classic. The results in all the charged conditions were simulated from the same initial configuration, which was prepared from IL bulk simulations, vacuum-liquid interface simulations, and uncharged graphene-IL interface simulations.[4] There were 720 ion pairs between two single-layer graphene electrodes within 125 Å. In each charged condition, the charge was allocated evenly to every C atom on the electrodes. The temperature was controlled (NVT ensemble) by using the Berendsen thermostat (0.5 ns as the quick equilibration process) and the Nose-Hoover thermostat (2 ns). Thus, the trajectories in the last 1 ns were chosen as the statistical data for analysis. 3. Results and Discussion By ESPR experiments, the change in SPR angle is recorded simultaneously with cyclic voltammetry, exhibiting a sigmoidal shape with the maximum slope at around −0.6 V (PZC). We find that the variation ranges by the negative scans are larger than those by the positive scans, and the variation range by positive scan increases as the scan rate decreases. These phenomena indicate that the influence of slow dynamics is insignificant for negative scans as was previously observed for a different IL.[1] To study the cause of the sigmoidal change in SPR angle, we propose a mathematical model,[3] by which a linear relationship between SPR angle and the surface charge density at the IL/electrode interface can be established. Consequently, the relative value of the quasi-static differential capacitance is shown in Fig. 1. Comparing with the previous study,[2] we determine the potential of zero charge in this system as −0.61 V, noted as the grey dashed line. This is the first time to obtain the static differential capacitance at the IL/solid electrode, in spite of the relative value. The differential capacitance exhibits a camel-shaped potential dependence, agreeing with the mean-field lattice gas model.[5] For quantitative measurement, MD simulations are performed to produce ionic distributions and surface charge density with respect to potential. By the data fitting of surface charge density and ionic concentration, we estimate the X − 1 as 3.1 mdeg∙cm2∙μC−1. Furthermore, by the observation of the ionic distribution, we confirm the butyl group of C4mim+ in the first ionic layer would provide extra room for the cations in the second layer, resulting in that the state of “ionic crowding” is hardly to occur and the interval of ionic layers varies with the potential on the negatively charged electrode.