| This research explored the design of composite proton exchange membranes as alternatives to single component membranes such as Nafion. The driving hypothesis was that adopting a two component approach simplifies membrane design by decoupling the problems of optimizing the physical and conductive properties of proton exchange membranes. Other advantages of a composite approach include simplification of membrane synthesis, rapid prototyping, and reduced cost. The conductivity of some membranes were comparable to Nafion, and had better high temperature performance.; The composite membranes studied used inorganic particles chemically functionalized to have acid groups on their surface. While this work focused on silica particles, any inert particle, in principal, could be compatible with the two-phase composite membrane approach. Various sized particles were examined, ranging from macroscopic silica to nanoparticles. The higher surface to volume ratio of nanoparticles allowed higher loadings of acid groups in composite membranes. Another benefit of using nanoparticles was improved water retention at high temperatures, presumably due to more effective capillarity in aggregates of nanoparticles.; The proton conductivity depends on the number of charge carriers (protons) and their mobility (related to electrolyte structure). We found that immobilizing a single layer of acid groups on the surface of A200 fumed silica provided too few carriers to support conductivity. We solved this problem in two ways. First, we increased the concentration of acid groups on the surface by growing polystyrene from initiators anchored to the surface, and then sulfonated the polymers. Tethering acids to the polymers dramatically increases the number of available acid groups (2∼3 mmol/g) and membranes prepared by embedding these particles in PVDF had conductivities of ∼0.08 S/cm. A second approach was the sol-gel synthesis of nanoparticles containing alkyl thiols as a component of their structure. Oxidizing the thiols provided sulfonic acids tethered to the surface. Composites prepared from these particles also exhibited high conductivities.; The proton mobility in a composite membrane depends on the existence of conductive paths through the membrane. We observed percolation thresholds when measuring the proton conductivity of membranes, with the conductivity increasing several orders of magnitude when the particle content approached 30 wt%. Examination of membranes using TEM and confocal microscopy showed that particles in poorly conductive membranes aggregated in non-continuous domains, but in membranes with high conductivity, the domains were connected. (Abstract shortened by UMI.)... |
