Molecular Dynamics Of Ionic Liquids In Nanoporous Electrodes

Abstract

Molecular dynamics simulations were performed to investigate structural, dynamical and electrical properties of ionic liquids (ILs) confined inside different nanoporous materials. These systems have potential uses in environmental and energy applications. The main goal is to understand how these IL properties are affected by a) characteristics of nanoporous materials such as chemical nature of the pore walls (e.g., carbon, titania), pore size and pore morphology, and b) characteristics of electrolyte, such as amount of IL inside the pores (pore loading) and concentration of organic solvent, in the case of mixtures of ILs with organic solvents. The results obtained in this work indicate that the properties of the nanoporous materials have a profound effect on the structure and dynamics of ILs. Formation of layers of ILs near the interface is observed in all systems studied, however density and thickness of these layers depends on variations in pore size, amount of IL inside the nanopores, density of electrical charges in the porous walls, pore morphology, the material of the pore walls, and concentration of solvent (in the case of mixtures of ILs with organic solvents). Our results indicate that variation in pore size, pore loading and pore morphology induce only slight changes in the local structure of liquid, but other variables such as variations in surface charge density and changing the material in the porous walls has a significant effect on the structure of the confined ions. In all systems the structure of ions far away from the pore walls closely resembles that of the bulk IL. We observed that the cations have faster dynamics than the anions in each system studied, except in very small slit graphitic pores. Dynamics of ions near the pore walls are slower as compared to those of the ions in the center regions of the pores. Varying pore loading of ions have pronounced effects on the dynamics of the ions in the center of the pore, with slight effect on the ions close to the pore surfaces. We observed that increases the density of electrical charges in the pore walls lead to an important reduction in the mobility of the ions, especially in the direction perpendicular to the pore walls. The slow dynamics of ILs can be enhanced by increasing pore sizes, or by adding organic solvents such as acetonitrile.