277918 Structure and Dynamics of an Ionic Liquid Confined Inside Model CMK-3 and CMK5 Ordered Mesoporous Carbons
Ionic liquids (ILs) have been considered as alternative electrolytes for energy storage in electrochemical double-layer capacitors, where the electrolyte is in contact with the nanoporous electrodes. Relevant macroscopic properties of these systems (e.g., electrical capacitance and resistance) are determined by the molecular-level properties of the confined ILs. However, many aspects of how the properties of nanoconfined ILs differ from their bulk counterparts remain to be fundamentally understood.
We report classical molecular dynamics simulation results of a typical IL, [dmim+][Cl-], confined inside model CMK-3 and CMK5 ordered mesoporous carbons. These materials consist of hexagonally-arranged nanorods (CMK-3) or nanopipes (CMK-5), where the walls are made of amorphous carbon. Our results suggest that variables such as pore size and the amount of IL inside the porous materials (pore loading) affect the structural properties of the confined IL (local density, ion orientation and radial distribution functions). We also discuss how pore size and pore loading affect the dynamical properties of the confined IL (mean squared displacements, diffusivities). Our results indicate that the dynamics of the confined ions are heterogeneous and depend strongly on the distance of the ions with the pore walls. The ions in the center regions of the pore have dynamics and relaxation times that are similar to those observed in the bulk IL at the same temperature. In contrast, the dynamics slow down appreciably as the ions get closer to the pore walls. Our results also suggest that these dynamical heterogeneities are linked to variations in local structural properties. The structure of the ions in the center of the pore is similar to that observed in the bulk IL; however, significant structural differences are observed between the ions near the pore walls and those in the center of the pore.