The creation of advanced solid polymer electrolytes is of critical importance for the development of many technologies, especially fuel cells and hydrogen electrolyzers. While hydrogen fuel cells are a top candidate to replace the internal combustion engine in many applications, they are currently too expensive for mainstream adoption due to the use of perfluorinated sulfonic acid-based (PFSA) polymer electrolytes, which are expensive, and require expensive platinum catalysts and titanium cell components. Utilizing hydrocarbon alkaline membranes can dramatically reduce costs, but such membranes that achieve chemical stability and ion conductivity comparable to PFSAs have proven elusive. It has been shown that polyatomic cations integrated into polymer backbones, when sterically protected, can provide high ion conductivity and excellent chemical stability. As these materials consist of cations directly integrated into rigid polymer backbones, the phase separation observed in high-performing polymers such as PFSA is not possible, and it is not clear how high conductivity is achieved. This thesis provides a comprehensive investigation into the nanostructure of such materials via a combination of X-ray scattering at controlled humidity and atomistic molecular dynamics simulations, which reveal a sponge-like nanostructure, near-complete percolation at low degrees of hydration, and no evidence of long-range phase separation. A preliminary analysis of the ion dynamics reveals an unexpectedly strong relationship between accessible volume and ion mobility, suggesting that ion mobility is almost completely defined by the accessible volume in these materials.
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Thesis advisor: Frisken, Barbara
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