<p> In this dissertation density functional theory (DFT) was applied to study the electronic structure and catalytic properties of graphene containing different types of defects. These defects includes hetero-atoms such as nitrogen, sulfur doped graphene, point defects such as Stone-Wales defects, single vacancy, double vacancies and substituting pentagon ring at zigzag edge, line defects such as pentagon-heptagon carbon ring chains, pentagon-pentagon-octagon carbon ring chains locating at the middle of graphene. The mechanisms of oxygen reduction reaction (ORR) were studied on these defective graphene, and electron transfer processes were simulated. Using DFT methods, we also explored the effect of strains to ORR electronic catalytic properties on pure and nitrogen doped graphene. </p><p> Our simulaltion results show that nitrogen, sulfur doped graphene, graphene containing point defects, substituting pentagon ring at zigzag edge, graphene containing line defects, pentagon-heptagon chain or pentagon-pentagon-octagon chains which have odd number of heptagon or octagon carbon ring perform high catalytic properties for ORR. Four electron transfer reactions could occur, and there are also two electrons transfer occuring on these defective graphene. The Stone-Wales defect itself cannot generate the catalytic activity on the graphene, but can facilitate the formation of hetero atom doping on graphene, which could show high catalytic activities to ORR. The catalytic active sites on defective graphene are atoms possessing high spin or charge density, where the spin density plays more important effect on the catalytic properties. For the N-doped graphene, the identified active sites are closely related to doping cluster size and dopant-defect interactions. Generally speaking, a large doping cluster size (number of N atoms >2) reduces the number of catalytic active sites per N atom. In combination with N clustering, Stone-Wales defects can strongly promote ORR. For four-electron transfer, the effective reversible potential ranges from 1.04 to 1.15 V/SHE, depending on the defects and cluster size. The catalytic properties of graphene could be optimized by introducing small N clusters in combination with material defects. For S-doped graphene, sulfur atoms could be adsorbed on the graphene surface, substitute carbon atoms at the graphene edges in the form of sulfur/sulfur oxide, or connect two graphene sheets by forming a sulfur cluster ring. Catalytic active sites distribute at the zigzag edge or the neighboring carbon atoms of doped sulfur oxide atoms, which possess large spin or charge density. For those being the active catalytic sites, sulfur atoms with the highest charge density take two-electron transfer pathway while the carbon atoms with high spin or charge density follow four-electron transfer pathway. Stone-Wales defects not only promote the formation of sulfur-doped graphenes, but also facilitate the catalytic activity of these graphenes. The ORR catalytic capabilities of the graphene containing point or line defects denpend on whether the defects could introduce spin density into the system or not. The axial strain field applied on the graphene could change its electronic properties. Neither the compressive nor the tensile strain along the zigzag or armchair direction could facinitate the catalytic activities of perfect graphene without any defects. Tensile strain along zigzag direction could change the electronic properties of nitrogen doped graphene, which are favorable to its ORR catalytic property. </p><p> Our simulation results explored the ORR on defective graphene in essence and provide the theoretical base for searching and fabricating new high efficient catalysts using the carbon based materials for fuel cells.</p>
| Identifer | oai:union.ndltd.org:PROQUEST/oai:pqdtoai.proquest.com:3718274 |
| Date | 06 October 2015 |
| Creators | Zhang, Lipeng |
| Publisher | The University of Akron |
| Source Sets | ProQuest.com |
| Language | English |
| Detected Language | English |
| Type | thesis |
Page generated in 0.0021 seconds