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Large Variations in the Composition of Ionic Liquid-Solvent Mixtures in Nanoscale Confinement.

ABSTRACT: 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 ions or solvent molecules. The composition of the electrolyte inside these pores, which may depend on the 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 widths. Ions are excluded from very narrow pores, while for pore widths that tightly fit a single layer of ions, 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 the in-pore ionic liquid mole fraction based on the effective molar volumes and the pore wall interaction energies of each species. Our work suggests a new avenue for tuning the ionic liquid mole fraction in nanopores with potentially significant implications for designing systems involving nanoconfined liquid electrolytes such as supercapacitors where in-pore ion population can affect charging dynamics.

SUBMITTER: Fang A

PROVIDER: S-EPMC6761308 | biostudies-literature | 2019 Jul

REPOSITORIES: biostudies-literature

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Large Variations in the Composition of Ionic Liquid-Solvent Mixtures in Nanoscale Confinement.

Fang Alta A Smolyanitsky Alex A

ACS applied materials & interfaces 20190719 30

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 ions or solvent molecules. The composition of the electrolyte inside these pores, which may depend on the 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 ...[more]

PMID: 31287650

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Project description:The formation of ionic liquid (IL) mixtures has been proposed as an approach to rationally fine-tune the physicochemical properties of ILs for a variety of applications. However, the effects of forming such mixtures on the resultant properties of the liquids are only beginning to be understood. Towards a more complete understanding of both the thermodynamics of mixing ILs and the effect of mixing these liquids on their structures and physicochemical properties, the spatial arrangement and free volume of IL mixtures containing the common [C4C1im]+ cation and different anions have been systematically explored using small angle X-ray scattering (SAXS), positron annihilation lifetime spectroscopy (PALS) and 129Xe NMR techniques. Anion size has the greatest effect on the spatial arrangement of the ILs and their mixtures in terms of the size of the non-polar domains and inter-ion distances. It was found that differences in coulombic attraction between oppositely charged ions arising from the distribution of charge density amongst the atoms of the anion also significantly influences these inter-ion distances. PALS and 129Xe NMR results pertaining to the free volume of these mixtures were found to strongly correlate with each other despite the vastly different timescales of these techniques. Furthermore, the excess free volumes calculated from each of these measurements were in excellent agreement with the excess volumes of mixing measured for the IL mixtures investigated. The correspondence of these techniques indicates that the static and dynamic free volume of these liquid mixtures are strongly linked. Consequently, fluxional processes such as hydrogen bonding do not significantly contribute to the free volumes of these liquids compared to the spatial arrangement of ions arising from their size, shape and coulombic attraction. Given the relationship between free volume and transport properties such as viscosity and conductivity, these results provide a link between the structures of IL mixtures, the thermodynamics of mixing and their physicochemical properties.

Dataset Information

Large Variations in the Composition of Ionic Liquid-Solvent Mixtures in Nanoscale Confinement.

Publications

Large Variations in the Composition of Ionic Liquid-Solvent Mixtures in Nanoscale Confinement.

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