Simulations of Ionic Liquids Confined in Surface-Functionalized Nanoporous Carbons: Implications for Energy Storage

Porous carbons are used in a wide range of applications, including electrochemical double-layer capacitors for energy storage, in which electrolyte ion properties under confinement are crucial for the performance of the systems. While many synthesis techniques lead to the presence of surface functional groups, their effect on the adsorption and diffusion of electrolyte ions is still poorly understood. In this study, we investigate the effect of surface chemistry on the dynamical and structural properties of adsorbed ions through molecular dynamics simulations of a neat ionic liquid in contact with several zeolite-templated carbons, used here as model materials for the larger range of existing ordered microporous carbons. The steric and specific influence of functional groups is explored using three structures: a structure without any functional groups; a structure with ester, hydroxyl, anhydride acid, and carboxyl functional groups; and a structure where the oxygen and hydrogen atoms are replaced by carbon atoms. We calculate quantities of adsorbed ions and diffusion coefficients, which show variations with the addition of functional groups in agreement with experimental data for similar systems. We observe that the functionalization has a limited impact on the structure of the confined electrolyte but affects the ion mobility more markedly, and that the relative importance of the structure and chemical nature of the functional groups is not the same depending on the ion type and property considered.