Imidazolium Ionic Liquid Interfaces with Vapor and Graphite: Interfacial Tension and Capacitance from Coarse-Grained Molecular Simulations

Abstract

International audienceA recently developed coarse-grained model (J. Phys. Chem. B, 2010, 114, 12629-12631), previously validated against experimental data for a number of bulk properties, is used in molecular dynamics simulations of two different interfaces involving the ionic liquid [BMI][PF6]. First, simulations of the liquid-vapor interface demonstrate that the model is able to predict the surface tension of the fluid (for which we obtain a value of 39.4 mN*m-1 at 400 K). Second, simulations were performed at constant potential differences applied between two graphite electrodes. From simulations with different applied potentials, the differential capacitances of the positive and negative electrodes can be calculated. It appears that both capacitances (C+ = 3.9 μF*cm-2 for the positive electrode and C- = 4.8 μF*cm-2 for the negative electrode) agree very well with simulations results obtained with an all-atom model. The coarse-grained model also accurately reproduces the two-dimensional structure observed at the graphite-ionic liquid interface, namely, a defective hexagonal lattice with a lattice spacing of approximately 10 Å

Highly confined ions store charge more efficiently in supercapacitors

“…Note that for the ions, we use the revised version of the force field by Roy and Maroncelli, for which the calculated diffusion coefficients match very well the experimental ones on a wide range of temperatures. This model has also been tested in previous simulations involving interfaces with carbon electrodes both for the pure ionic liquid 34 and for solutions in ACN 33 . The results show that the structure of the liquid at the interface and the capacitive properties of the system are in very good agreement with other simulations conducted with all-atom models.…”

Understanding the different (dis)charging steps of supercapacitors: influence of potential and solvation

“…Following our previous works, we use for the ionic liquid the coarsegrained model of Roy and Maroncelli, which yields the correct structural, thermodynamic and dynamic properties of [BMI] [PF 6 ] [30,31]. Three and one sites describe the cation and the anion, respectively [18,19]. For acetonitrile, also following our previous work, the coarse-grained model is the one of Edwards et al [32].…”

“…To calculate the accurate capacitance of a nanoporous disordered electrode, it is necessary to use a planar electrode as a counter electrode with which the Poisson potential can be defined [17]. Several studies on the equilibrium arrangement of the species inside supercapacitors electrodes have already been carried out in our group [18,19]. Furthermore, the effect of solvation on static properties (structure of the liquid, global capacitances, density profiles) have been studied on planar electrodes [20].…”

Evaluation of the constant potential method in simulating electric double-layer capacitors

“…23 Several studies of EDLCs employing the CPM have been reported. Merlet et al [24][25][26][27] studied a nanoporous carbon electrode in contact with electrolyte consisting of an ionic liquid or an ionic liquid/acetonitrile mixture. Vatamanu and co-workers [28][29][30][31] investigated ionic liquid electrolytes with carbon or gold electrodes using the smooth particle mesh Ewald (SMPE) 32,33 method to simplify the calculation.…”