Nanostructured π-Electron Materials for Energy Applications Derived from Macromolecular Self-Assembly
Globalization and climate change have driven the need to develop new technologies which can provide clean, plentiful energy from renewable sources. This work was focused on the application of nanostructured π-electron materials derived from carbon-based macromolecules towards capturing, converting, and storing energy. Nanostructures are beneficial in this role as they provide high interfacial area and unique electronic properties which can be harnessed to perform chemistry relevant to energy conversion and storage. This work was focused on the characterization, materials development, and device application of two nanostructured systems: (i) poly(3-hexyl)thiophene (P3HT) blended with phenyl(C-61)butyric acid methyl ester (PCBM), and (ii) copolymer-templated nitrogen-doped nanoporous carbons (CTNCs). In both systems, nanomorphology has pronounced impact on the performance of devices made from such materials. P3HT/PCBM blends find application as a photovoltaic material, where the phase-separated morphology is crucial for efficient photogenerated charge collection. Despite widespread recognition of the importance of morphology in P3HT/PCBM photovoltaics, a robust understanding of the unique packing motif of P3HT on the morphology of blended structures has yet to emerge. This thesis addresses this deficiency by developing methods which connects real-space atomic force microscopy images with inverse space x-ray scattering patterns to analyze poorly ordered two-phase systems. The application of this method allowed for quantitative measurement of the phase ratios of P3HT/PCBM nanostructured blends, utilizing the Porod length of inhomogeneity and the Bragg length associated with pseudo-fibrillar P3HT morphologies. The results showed that P3HT possesses void space originating from molecular weight dispersity inherent to polymerization, which accounts for solid-phase solubility of PCBM in P3HT. X-ray scattering and atomic force microscopy were also used in part to characterize CTNCs. Past success using CTNCs as electrocatalysts and supercapacitors motivated research towards increasing their surface area by utilizing a lower molecular weight precursor polymer. Atom-transfer radical polymerization was utilized to synthesize block copolymers consisting of polystyrene and polyacrylonitrile, but it was found their surface areas were lower than those achieved in previous work. Careful structural analysis by variable temperature x-ray scattering showed that crystallization of polyacrylonitrile drives morphological changes on heating, increasing domain spacing. Further thermal analysis showed that polystyrene interferes with the crosslinking of polyacrylonitrile, which may cause morphological collapse leading to low surface area. A feature of CTNCs noted in past and present studies is their sizeable surface area consisting of pores <1 nanometer (micropores). Differential scanning calorimetry showed that reactive chain ends left behind by polymerization might play a role in disrupting the crosslinking process, resulting in a material with sizeable microporosity, which could be used to engineer dual pore-size materials. Finally, CTNCs were utilized for heterogeneous catalytic production of hydrogen from water, with electrons provided by both light and external circuitry. Their performance was correlated with nitrogen heteroatom content, conductivity, and nanomorphology, and shown to match that of noble metals. The lessons learned about nanomorphology in P3HT/PCBM and CTNCs highlight the importance of nanomorphology in energy devices and will serve as insight for materials design in future studies.
- Doctor of Philosophy (PhD)