Ion transport in nanochannels probed by In Situ Nanodielectric Spectroscopy
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Abstract
Understanding the way that ions penetrate in narrow pores is essential for the development of systems where precise control of transport properties is required (e.g. in high-performance materials used in energy storage, nanofluidics, and sensing technologies). This Thesis explores
the ion transport behavior of ionic liquids (ILs), polymerized ionic liquids (PILs), and their mixtures (ILs/PILs) in nanopores, using in situ nanodielectric spectroscopy. The research addresses how the geometry of confinement, the molecular structure/design, and the interfacial interactions collectively govern ion transport, phase behavior, and polymer dynamics.
Confined ILs display different physical properties when compared to their bulk state. They include suppressed crystallization, shifted glass temperature, and reduced ionic conductivity. These changes can be largely attributed to interfacial interactions between the ions and the pore walls. Ion adsorption at the surface leads to a reduction in the number of mobile charge carriers, directly impacting ionic conductivity. Furthermore, spatial confinement affects nucleation, in some crystallizable ILs resulting in the suppression of crystallization in smaller pores. Notably, the geometry of confinement also plays a key role in determining ion dynamics. ILs within conically shaped nanopores, for example, exhibit higher ionic conductivity than within cylindrical ones. This enhancement is likely due to the uneven surface charge distribution in conically-shaped pores, which reduces the strength of Coulomb interactions in the vicinity of the narrower openings.
In PILs, the size and structure of the tethered cation significantly influence both imbibition dynamics and ionic conductivity. Cations whose charged imidazolium rings are located close to the polymer backbone restrict chain mobility and lead to a higher viscosity. In contrast, cations having additional phenyl spacers between the backbone and the imidazolium ring promote greater chain flexibility and improved ionic mobility. A strong decoupling between segmental dynamics and ion transport is observed below the glass temperature under strong confinement. This phenomenon is consistent with the “shoving model”, according to which ion movement in confined environment is facilitated by the ability to deform the local matrix. IL/PIL mixtures are dynamically heterogeneous in the bulk. Their infiltration into nanopores leads to enhanced local segregation and eventually to compositional demixing. During imbibition, ILs are preferentially drawn into the pores by capillary forces and dominate early transport, while a minority of PILs infiltrate more slowly and tend to adsorb onto the pore walls, leading to reduction on ionic conductivity. The results suggest the possibility to separateII a mixture of ionic compounds (IL and PIL in this case) by the difference in the imbibition kinetics of its constituent components in nanopores and in the absence of solvent. These insights deepen our understanding of ion transport under confinement. In the long term they could offer some guiding principles for the rational design of next-generation electrolytes and nanostructured materials for energy storage.