(178k) Molecular Modeling of the Ionic Liquid [EMIM+][TFMSI-] Inside a Slit Nanoporous Electrode: Effect of Charge Density, Pore Size and Pore Loading On Properties of Confined ILs

Authors: 
Monk, J. D., Louisiana State University
Singh, R., Louisiana State University
Hung, F. R., Louisiana State University


Molecular dynamics (MD) simulations were performed to investigate the structural and dynamical properties of ILs confined inside charged graphitic surface. A fundamental understanding of properties of ILs inside nanopores is relevant for applications of ILs as electrolytes for energy storage in EDLC and DSSCs. In order to improve the performance of these devices it is essential to study the effect of different variables such as pore size and pore loading of IL. 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.   

Model electrodes represented as slit-like graphitic nanopores and the IL 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [EMIM+][TFMSI-] are studied using molecular dynamics (MD) simulations. Our goal is to understand the effect of pore size, pore loading and surface charge density on the structural, dynamical and electrostatic properties of ILs confined inside nanoporous materials.  Pore size and pore loadings induce slight changes in the local structure of confined, but have a deeper influence on the dynamics of IL. Change in charge density show drastic effect on structural, dynamical and electrical properties of ILs.

Our results indicate that High-density region of cations and anions are formed at the negative and positive surface respectively creating a polarized region, which leads to formation of electrochemical double layer. 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. Profound effect in dynamics of ions is observed with change in charge density. We observed that the dynamics of cations is slowest in the direction perpendicular to the surface in the layer close to the negative surface and that of anions close to the positive surface