Probing Nanoscale Ion Dynamics in Ultrathin Films of Polymerized Ionic Liquids by Broadband Dielectric Spectroscopy

Abstract

Continuous progress in energy storage and conversion technologies necessitates novel experimental approaches that can provide fundamental insights regarding the impact of reduced dimensions on the functional properties of materials. Here, we demonstrate a nondestructive experimental approach to probe nanoscale ion dynamics in ultrathin films of polymerized 1-vinyl-3-ethylimidazolium bis­(trifluoromethylsulfonyl)­imide over a broad frequency range spanning over 6 orders of magnitude by broadband dielectric spectroscopy. The approach involves using an electrode configuration with lithographically patterned silica nanostructures, which allow for an air gap between the confined ion conductor and one of the electrodes. We observe that the characteristic rate of ion dynamics significantly slows down with decreasing film thicknesses above the calorimetric glass transition of the bulk polymer. However, the mean rates remain bulk-like at lower temperatures. These results highlight the increasing influence of the polymer/substrate interactions with decreasing film thickness on ion dynamics.

Experimental evidence for bipolaron condensation as a mechanism for the metal-insulator transition in rare-earth nickelates

“…Our BDS experiments are performed on pressed powders and in an air gap geometry (Methods). This setup can produce a value of the conductivity that is reduced by as much as eight orders of magnitude compared with traditional four-probe conductivity measurements 29 . This decrease occurs because no electrical conduction can proceed through the air gap and instead only electrical displacements induced by the alternating electric field are measured.…”

Ion Dynamics of Monomeric Ionic Liquids Polymerized In Situ within Silica Nanopores

“…The native OH bonds of SiO 2 are known to have a slightly negative charge and, therefore, preferentially interact with the cation. 51 The schematic first displays an adsorbed layer of 1E3VIm cations near the pore surface (in green) closely coordinated with a second layer of anions having the C1 conformer (in orange) that correspond to a larger dipole moment of the NTf 2 anion. This picture is reinforced by results that show that imidazolium cations orient planar to silica surfaces, and the SO 2 oxygens of the NTf 2 anion in the cis conformation are closely coordinated to the imidazolium ring.…”

“…With the ionic radii calculated for NTf 2 (∼1.85 Å) and 1-ethyl-3-methyl­imidazolium (∼1.5 Å) by Nordness et al, we have sketched a 7.5 nm diameter pore filled with 1E3VIm NTf 2 monomer, which is displayed roughly to scale as a schematic inset of Figure with ∼11 ion pairs spanning the diameter of the pore. The native OH bonds of SiO 2 are known to have a slightly negative charge and, therefore, preferentially interact with the cation . The schematic first displays an adsorbed layer of 1E3VIm cations near the pore surface (in green) closely coordinated with a second layer of anions having the C1 conformer (in orange) that correspond to a larger dipole moment of the NTf 2 anion.…”

“…However, very few studies have examined the effects of surface interactions and confinement on the properties of poly-ILs. 48,49,51 To unravel the interplay between confinement on the molecular conformations and ionic dynamics of nanoconfined polyILs, the well-studied monomeric vinyl IL, 1-ethyl-3vinylimidazolium [1E3VIm] with the bis(trifluoromethanesulfonyl)imide [NTf 2 ] counteranion, was filled into unidirectional nanoporous silica membranes with mean pore diameter of 7.5 nm and subsequently polymerized in situ by free radical polymerization. Broadband dielectric spectroscopy (BDS) and Raman spectroscopy were employed to study the changes in the dynamics and ion conformations and coordination that result from both the nanoconfinement and polymerization.…”

“…For noncharged polymers under nanoconfinement, several physical characteristics have also been observed to change compared to bulk including the dynamic glass transition temperature ( T g ), self-diffusivity, dielectric relaxation strength, conformational entropy, and molecular packing density. ,− Similarly, these structural and dynamical effects that are observed under nanoconfinement have also been observed in pressure-dependent studies of noncharged polymer systems. ,− Additionally, in situ free radical polymerization within anodized aluminum oxide (AAO) nanopores have resulted in an increase in reaction kinetics with the resulting polymers exhibiting a narrower molecular weight distribution compared to those prepared under similar reaction conditions in bulk. , For bulk polyILs, experimental and computational studies have shown that when the ion conduction is dominated by hopping of ions along the polymer backbone corresponding to faster dynamics compared to hopping from chain to chain in an entangled system. ,, Thus, if nanoscale confinement can lead to favorable alignment of chains as seen in noncharged polymers, then higher ionic conductivity may result in polyILs. However, very few studies have examined the effects of surface interactions and confinement on the properties of polyILs. ,, …”