Conductivity of Ionic Liquids In the Bulk and during Infiltration in Nanopores

Abstract

The conductivity of ionic liquids (ILs) in nanoporesis essentialwhen considering their application as materials for energy. However,no consensus has been reached about the influence of confinement onthe mobility of the ions. A series of ILs bearing the same cation,1-butyl-3-methylimidazolium ([BMIM](+)), and six differentanions ([Cl](-), [Br](-), [I](-), [BF4](-), [PF6](-), and [TFSI](-)) with radii from0.168 to 0.326 nm were investigated with respect to their self-assembly,the thermodynamics, and the ionic conductivity in the bulk, duringflow and under confinement in cylindrical nanopores with sizes inthe range from 400 to 25 nm. In the bulk, the [BMIM](+)[X](-) exhibits weak ordering as a result of cation-anioncorrelations (charge alteration peak), and nanophase separation ofpolar/apolar groups. Liquid-to-glass temperatures were found to differby & SIM;50 K, their viscosities by a factor of & SIM;270, andtheir conductivities by a factor of 24 (all at a temperature of 303K). Electrostatic interactions were largely responsible for variationsin the glass temperature, the viscosity, and the conductivity. ConfinedILs behave differently from the bulk. The majority of ILs in the bulkwere prone to crystallization during heating but were unable to crystallizein the smaller pores. Changes in dc-conductivity were used as markersof the phase state. This allowed the construction of the effectivephase diagrams under confinement. The ILs penetrate the pores withan effective viscosity of the order of their viscosity in their bulkstate. However, within the pores the dc-conductivity was reduced relativeto bulk, indicating the immobilization of ions at the pore walls.Hydrophobization of the pore walls by hexamethyldisilazane could partiallyrestore the conductivity. ILs are model systems where the phase stateand ion mobility can be controlled by confinement.