Date of Award

2021

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Materials Science & Engineering

First Advisor

Karen I. Winey

Abstract

As society transitions to carbon neutral technologies there is a continual need for the development of energy storage technologies such as batteries and fuel cells. Single-ion conducting solid polymer electrolytes present an opportunity to improve the safety and performance of batteries, and new membrane chemistries may reduce the cost of proton exchange membranes for hydrogen fuel cells. This thesis focuses on correlating nanoscale morphology with ion and proton transport, which is critical for developing design rules for highly conductive solid polymer electrolytes. Modifying the polarity, along with the size and flexibility of the functional groups in polystyrene trisaminocyclopropenium-based polymerized ionic liquids significantly changes the glass transition temperature and ion transport properties. The geometry of the functional groups tethered to the backbone has a much larger effect on conductivity than the size of the group itself. Single-cation conducting polymers with a polyethylene backbone and a phenylsulfonic acid group precisely spaced every 5th carbon are synthesized by a ring-opening metathesis polymerization and self-assemble into percolated ionic aggregates. These aggregates are continuous pathways for the transport of metal cations Li+, Na+, and Cs+. Ion transport is decoupled from the polymer backbone, which is glassy even at high temperatures. Sulfonated monodisperse telechelic polyethylene ionomers, with a backbone length of 48 carbon atoms, also contain pathways that facilitate the decoupled transport of metal cations, though through layered ionic aggregates. When the polyethylene backbone is hexagonally packed, there is a low activation energy for cation transport along the aggregate, demonstrating the promising ability of ionic layers to facilitate ion transport in crystalline polymers. Telechelic polyethylenes end-functionalized with phosphonic acid also self-assemble into a layered aggregate morphology to transport protons in anhydrous conditions, with potential applications as an electrolyte for a fuel cell. The percolated aggregates in the sulfophenylated polyethylenes with a functional group spaced precisely every 5th carbon also nanophase separate when the mobile counterion is a proton. Percolated hydrophilic channels swell with water and facilitate the rapid transport of protons, demonstrating the potential of these precise hydrocarbon-based polymers as processible and effective proton exchange membranes.

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