Mixtures of an ionic liquid with an organic solvent are widely used as electrolytes in supercapacitors, where they are often confined in porous electrodes with pore widths only slightly larger than the sizes of bare ion or solvent molecules. The composition of the electrolyte in these pores, which may depend on pore width and choice of electrolyte, can affect supercapacitor performance but remains poorly understood. Here we perform all-atom molecular dynamics simulations of solutions of two different ionic liquids in acetonitrile under confinement between graphene sheets forming slit pores of various widths. We observe significant oscillations in the in-pore ionic liquid mole fraction with varying pore width. Ions are excluded from very narrow pores, while for pore widths close to the bare ion sizes we observe an in-pore ionic liquid mole fraction over three times greater than that in the bulk. At slightly larger pore widths, we observe for different ionic liquids either a nearly complete exclusion of ions from the pore or a slight depletion of ions, while ion population again increases as pore width further increases. We develop an analytical model that can qualitatively predict in-pore ionic liquid mole fraction based on the effective molar volumes and the molecule-pore wall interaction energies of each species. Our work suggests a new avenue for tuning nanopore ionophobicity with potentially significant implications for designing systems involving nanoconfined liquid electrolytes such as supercapacitors, where in-pore ion population is predicted to affect charging dynamics.
ACS Applied Materials and Interfaces
ionic liquids, nanoscale confinement