(548h) On the Influence of Pore Size, Pore Loading and Surface Charge Density On the Structure and Dynamics of the Ionic Liquid [EMIM+][TFMSI-] Inside a Slit Graphitic Nanopore

Ionic liquids (ILs) are gaining relevance as alternative electrolytes in energy-related devices such as electrochemical double-layer capacitors (EDLCs) and dye-sensitized solar cells (DSSCs). A fundamental understanding of the structure and dynamics of ILs inside nanopores is essential to advance these technologies beyond their current state of the art. Structural properties of an IL confined inside a nanoporous electrode affect macroscopic properties such as the capacitance in an EDLC; and the dynamical properties of the confined IL is one of the factors that determines the macroscopic electrical resistance in electrochemical devices.   

Molecular dynamics (MD) simulations were performed to study the IL 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide [EMIM⁺][TFMSI^-] confined inside a slit graphitic nanopore. Our goal is to understand the effect of pore size, pore loading and surface charge density on the structure and dynamics of the confined IL. As previous studies have largely concentrated on the structure and electric properties of ILs confined inside charged electrodes, we have especially focused on the dynamics of the system, which were studied in detail by measuring mean squared displacements (MSDs), van Hove self correlation functions (VHSCFs) and self-intermediate scattering function (SISFs).

Our results indicate that structural properties such as the local density profiles, the local orientation, and the radial distribution functions of the cations and anions vary strongly with pore size, pore loading and charge density in the pore walls. These variables also affect the dynamical properties of the confined IL. Our results suggest that the dynamics of the ions inside the slit pore are highly heterogeneous and depend strongly on their position with respect to the pore walls, as well as on the surface charge density of the walls. For example, our results show that the layer of cations that is closer to a negatively charged wall, as well as the layer of anions closer to a positively charged wall, exhibit extremely slow dynamics, long relaxation times and strong deviations from Gaussian dynamics. In contrast, the ions in the center of the pore have mobilities in the x-y plane that are comparable and sometimes even larger than those exhibited by the ions in a bulk IL.