Coarse-grained simulations of an ionic liquid-based capacitor: I. Density, ion size, and valency effects

Abstract

We introduce a hierarchy of generic coarse-grained models of ionic liquids of increasing complexity. We use them in molecular dynamics simulations to study the differential capacitance of a capacitor consisting of an ionic liquid between two planar electrodes. The primary goal is to explain the complex dependence of the differential capacitance Cd on the electrode potential U in simple terms, e.g. in terms of the size and valency of the ions. For this purpose we introduce the symmetric model A, which qualitatively reproduces the Cd(U) dependence predicted by the mean-field theory but also reveals strong quantitative deviations. We further introduce size asymmetry in model A by increasing the cation size. In model B we vary the cation valency, keeping the sizes of both ions constant. We show that simultaneous increases in size and valency may compensate for each other, leading to a Cd(U) very similar to that for the symmetric case. We interpret distinct features in Cd(U) on the basis of the density profiles of the ions and charge density profiles. We focus on the first two ion layers at the electrode, and demonstrate that the polarization of the ionic liquid proceeds through replacement of one ion type by the other, in contrast to the simple increase in ion concentrations typical for dilute systems. The understanding gained for the simple models serves as a reference for interpretation of complex effects of ion size, valency and shape. This is carried through in part II (a separate article) where we show how the planar shape of ions in model C brings new features to the Cd(U) curve and also to the polarization mechanism.

The effect of finite pore length on ion structure and charging

“…Our results on the pore walls suggest that non-electrostatic interactions and image forces can have a profound impact on the in-pore ion structure. It will thus be interesting and fruitful to understand the effects due to the differences in ionic liquid models [56,57], and whether such simple and computationally inexpensive models can capture the charging behaviour correctly.…”

Fused coarse-grained model of aromatic ionic liquids and their behaviour at electrodes

“…48 Breitsprecher et al studied the ILs based capacitor via CG simulations, they also clarified that the capacitance also depends on the reference point for the potential strongly, since there is a particle-free region between the surfaces and ILs. 49 . We also studied the influence from the particle size of wall (σ w ) on the capacitance recently, which has the similar effect as the distance of the reference points.…”

Dense ionic fluids confined in planar capacitors: in- and out-of-plane structure from classical density functional theory

“…All these devices exploit the capacitive properties of the electric double layers (EDLs), which are established in the vicinity of electrode surfaces by ionic charges, drawn from the ionic electrolytes the electrodes are immersed in [11]. Accordingly, many studies have focused on the detailed description of the EDL and its properties during the last years [12][13][14][15][16][17][18][19][20][21].…”