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CBE Dissertation Defense: Xubo Luo

Title: "Multiscale Simulations of Polymeric Electrolytes for Electrochemical Devices ."

Polymeric electrolytes are polymers containing the ionic moieties used for ion conduction in electrochemical devices. The properties of these materials are influenced by the molecular structure, and the strategic design of novel materials requires the in depth understanding of the relation between structure and properties. This dissertation reports on computational simulations for two types of materials: hydrated anion exchange membranes (AEMs) for fuel cells and the polymerized ionic liquids (PolyILs). Various chemical structures were covered for systematic investigations.

For AEMs, the ion conducting channels or domains are essential for the efficient ion transport, and thus the morphology of the hydrated membrane is worthy of study. In this work, dissipative particle dynamics simulations were carried out to investigate the meso-scale microphase separation for hydrated AEMs, which were the functionalized triblock copolymer polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene. The morphology was studied at a variety of hydration levels, and the connectivity of the water domains were analyzed. The morphology was tuned by an alkyl spacer, degree of functionalization, percentage of polystyrene, different anions, as well as the hydration level. However, a simple rule for the influence of the chemical composition was not elucidated. The size of the hydrophilic phase relative to the hydrophobic phase was speculated as an influential factor. Introducing more ionic species generally promotes water percolation. This effect was not solely determined by the water amount, but the chemistry of the AEMs was also seen to play an important role.

Classical molecular dynamics simulations were performed on a variety of different polyILs to study the mechanism of ion transport. The effects of different anions were examined in poly(1-ethyl-3-vinylimidzolium), paired with bromide, tetrafluoroborate, hexafluorophosphate and bis(trifluoromethanesulfonyl), respectivly. As an alternative polymerized cation, poly((2-alkyldimethyl-ammoniumethyl) methacrylate bis(trifluoromethylsulfonyl)imide was simulated with different tail lengths. Comparisons of ion association and hopping types in the transport of the ions were found to be inconsistent with the size of the ion. For all materials, the diffusivity was more likely to correlate to the effective hopping satisfying a criterion of distance, but not the instant hopping of ion dissociation. Generally, a large polyatomic counterion with a long linker and/or tail promoted the ion transport. These molecular structures reduced the dynamical heterogeneity and the average string length of the cooperative motion of the mobile ions.

Xubo Luo is from Tianjin, China. He earned his bachelor and master’s degrees from Tianjin University in China. He started his PhD research at the University of Tennessee in the fall semester of 2015. Shortly after that, he joined the research group of Professor Stephen J. Paddison. His research involves both meso-scale and atomic simulations of polymeric materials commonly employed as the electrolyte in electrochemical devices.

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