Electronic and Structural Foundations of Nitrogen-Rich Nanocarbons for Energy Related Applications.

摘要

The development of high surface area mesoporous carbons is the subject of extensive attention, in part due to their high performance in a variety of electrochemical systems. Nitrogen doping in such systems has been previously shown to impart additional electrochemical character. This work expands upon the previous development of nitrogen-rich mesoporous carbons from polyacrylonitrile-b-poly(n-butyl acrylate) precursors. Unique to this approach is the placements of the residual nitrogens along the zigzag edges of the comprising nanographenes. This dissertation was primarily focused on the understanding of a variety of structural components in this heterogeneous system, and the factors affecting them. Chapter 2 studied the electronic structure of nanographenes using Huckel theory. It was found that the electronic states located at the Fermi level are comprised of wavefunctions where the electron density is located at the zigzag edges, highlighting their importance. Altering the electronegativity of the zigzag edges, which mimics edge substitution, was found to control energy of these Fermi level electronic states. The control over the edges, in combination with control over the nanographene sizes, points to synthetic routes that can widely tune the electronic properties of nanographenes. Chapter 3 addressed the need for characterization tools that can be used to determine the sizes and distribution of the nanographenes. Laser desorption ionization time-of-flight mass spectrometry (LDI-TOF MS) was used generate mass spectra for samples of varying degrees of graphitic content with respect to the amorphous component. These spectra were compared with molecular sizes determined by X-ray diffraction. It was found that carbon films prepared from organic precursors, particularly polyacrylonitrile, generate mass spectra consistent with that from X-ray diffraction. It was generally concluded that the presence of spa hybridized amorphous carbon serves as an internal matrix, softly ionizing the graphitic species while avoiding significant fragmentation. In Chapter 4, it was shown through LDI-TOF MS that the sizes of the nanographenes can be controlled by the molecular weight of the polymer precursor as well as the pyrolysis temperature. Chapter 5 used grazing incidence X-ray scattering to study uniaxially aligned lamellar strips of nanocarbon prepared by zone casting. It was found that nanographenes formed using a block copolymer templating approach lead to nanographenes oriented such that the edges are exposed to the pore wall. This orientation is highly desirable for electrochemical and electrocatalytic devices, due to the high reactivity of the edges.;Chapter 6 began with the development of a four-channel Fourier transform computational model of grazing incidence X-ray scattering, which was used to verify the applicability of Porod's law to a grazing incidence geometry. Analysis of scattering patterns of block copolymer precursors and the carbons prepared at various pyrolysis temperatures demonstrated, through both the domain spacing and the length of inhomogeneity calculated in the context of Porod's law, that the nanoscale morphology was preserved. Chapter 7 investigated the performance of this carbon material as the anode in lithium-ion batteries. Reversible specific capacities as high as 330 mAh g-1 were observed. A high initial capacitance followed by a large irreversible capacitance, common to mesoporous carbons from organic precursors, was also observed. The rate capability determined was not found to be of improvement compared to commercially available products.