Doctoral Dissertations

Date of Award

12-2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemical Engineering

Major Professor

Joshua R. Sangoro

Committee Members

Roberto S. Benson, S. Michael Kilbey, Alexei P. Sokolov

Abstract

Polymerized ionic liquids, polyILs, are a novel type of solid polymer electrolyte with possible applications in energy conversion or storage devices. The key to unlocking the true potential ionic conductivity and mechanical strength of polyILs lies in the strategic design of the chemical structure which, facilitates fast ion transport in a thermally stable material. To shed light onto the structure-property relationship in polymerized ionic liquids, this dissertation presents experimental studies on the impact of molecular structure and spatial confinement on ion dynamics in ammonium- and imidazolium-based polymerized ionic liquids with various chemical structures.Broadband dielectric spectroscopy is used alongside X-ray scattering and differential scanning calorimetry to investigate the impact of alkyl pendant group length and poly-cation chemical structure on counter-ion mobility. It is found that the ion mobility in ammonium-based polymerized ionic liquids is more sensitive to variation of the molecular surrounding of the poly-cation, compared to their imidazolium-based counterparts. Furthermore, it is shown that the cation chemistry plays a more significant role than the cation location relative to the backbone, which is an important design handle when selecting the molecular chemistry of the material for a specific application.Ultra thin polymer films have a large ratio of interface to bulk material, which due to electrostatic interactions and confinement effects, can drastically alter dielectric properties. Broadband dielectric spectroscopy in combination with a nano-structured electrode configuration is used to investigate ion dynamics in ultra-thin films of polymerized ionic liquids as thin as 7.5 nm. Ion dynamics remain unaltered at low temperatures, while a decrease in the characteristic ion hopping rate is observed above the Tg of the bulk polyIL. With this experimental approach the structural relaxation of an ammonium based polymerized ionic liquid film of 15nm thickness is measured. The ionic conductivity and structural relaxation data presented in this work provides valuable strategic information for designing electrode materials compatible with solid polymer electrolyte thin films, and smart choice of chemical structures to avoid parasitic losses due to electrode polarization in thin film geometries.

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