Recent efforts toward the commercialization of hydrogen fuel cells, a sustainable energy technology, have led to interest in the effects of industrial processing parameters on the morphology and properties of fuel cell ionomers. The ionomer functions as a solid electrolyte membrane on the order of microns thick and as a thin film on the order of tens of nanometers in the catalyst layer. Industrial manufacture of the membrane and catalyst layer is typically a roll-to-roll process that involves casting a colloidal dispersion of the fuel cell ionomer in predominantly mixed alcohol/water solvent systems onto a backing film or substrate, followed by evaporation of the solvent and annealing of the ionomer at elevated temperatures. The current benchmark fuel cell ionomers are a class of polymers with pendant perfluorinated side chains terminating in sulfonic acid groups, called perflurosulfonic acid ionomers (PFSAs). The purpose of this dissertation is to investigate the effects of industrial processing parameters such as dispersion solvent composition, solvent evaporation temperature, and annealing temperature on fuel cell-relevant properties of PFSA solid electrolyte membranes and model thin films. Particular focus is given to newer-generation PFSAs and the effect of their different chemical structures on the morphology and properties of dispersions, membranes, and thin films. Dipole-dipole interactions between colloidal aggregates modulated by solvent composition were found to significantly influence the viscosity of PFSA dispersions. A framework of PFSA-solvent interactions is developed to predict the onset of dipole-dipole interactions as a function of PFSA chemical structure and solvent composition. Increased steric hindrance of shorter PFSA side chain chemical structures is found to inhibit morphological development, resulting in membranes with poorer wet and dry mechanical properties. A shorter side-chain PFSA is suggested to require higher processing temperatures to achieve performance equivalent to a PFSA with slightly longer side chain. The morphology and properties of model PFSA thin films are demonstrated to decay to quasi-equilibrium values upon physical aging at both low and high relative humidity (RH). Thin film swelling curves are demonstrated to be superposable by implementing an aging time-RH shift factor, allowing for prediction of quasi-equilibration times under given fuel cell operating conditions. / Doctor of Philosophy / Interest in environmentally friendly, sustainable energy sources has led to significant industrial, academic, and governmental efforts to commercialize hydrogen fuel cells. Hydrogen gas is split into protons and electrons in the anode catalyst layer. The electrons flow through an external circuit to produce electricity, while the protons are transported from the catalyst layer through a solid electrolyte membrane to the anode to react with oxygen to form water. A key component of hydrogen fuel cells is an ion-containing polymer called an ionomer that is required for the transport of (1) protons in the solid electrolyte membrane and (2) protons and reactant gases in the catalyst layer. The solid electrolyte membrane and catalyst layer can be industrially produced by a continuous process that involves dispersing the ionomer in a mixed alcohol/water solvent and coating it onto a backing film, followed by evaporation of the solvent and annealing of the ionomer. The present work is an investigation of the effect of industrially-relevant processing parameters on the morphology and properties of a class of ionomers called perfluorosulfonic acid ionomers (PFSAs), which phase separate into hydrophilic domains that serve as transport pathways and hydrophobic domains that impart thermomechanical stability. Practical aspects of the processing and function of PFSAs, including viscosity of the PFSA dispersion, minimum processing temperature to achieve solvent stability, and physical aging of the PFSA during fuel cell operation are shown to be fundamentally related to the PFSA chemical structure and morphology.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/110879 |
| Date | 22 June 2022 |
| Creators | Novy, Melissa Hoang Lan |
| Contributors | Chemistry, Moore, Robert Bowen, Bortner, Michael J., Kusoglu, Ahmet, Ellis, Michael W., Lin, Feng |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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