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Design of Nanomaterial Catalysts for Energy and Environmental Applications

By Abdelhamid El-Sawy

Abstract

Part 1: Controlling active sites of metal-free catalysts is an important strategy to enhance activity of the oxygen evolution reaction (OER). Many attempts have been made to develop metal-free catalysts, but the lack of understanding of active-sites at the atomic-level has slowed the design of highly active and stable metal-free catalysts. We have developed a sequential two-step strategy to dope sulfur into carbon nanotube-graphene nanolobes. This bi-doping strategy introduced stable sulfur-carbon active-sites. Fluorescence emission of the sulfur K-edge by X-ray absorption near edge spectroscopy (XANES) and scanning transmission electron microscopy electron energy loss spectroscopy (STEM-EELS) mapping and spectra confirm that increasing the incorporation of heterocyclic sulfur into the carbon ring of CNTs not only enhanced OER activity with an overpotential of 350 mV at a current density of 10 mA cm-2, but also retained 100% of stability after 75 h. The bi-doped sulfur carbon nanotube-graphene nanolobes behave like the state-of-the-art catalysts for OER but outperform those systems in terms of turnover frequency (TOF) which is two orders of magnitude greater than (20% Ir/C) at 400 mV overpotential with very high mass activity 1000 mA cm-2at 570 mV. Moreover, the sulfur bi-doping strategy showed high catalytic activity for the oxygen reduction reaction (ORR). Stable bifunctional (ORR and OER) catalysts are low cost, and light-weight bi-doped sulfur carbon nanotubes are potential candidates for next-generation metal-free regenerative fuel cells. Part 2: Controlling the size, shape, morphology, and crystallinity of 1D-metal oxide nanostructures (MON) is the goal for bottom up synthesis methods. We successfully fabricated nanowires OMS-2 using UV. K2SO4 has been found to play an important role in transformation of γ-MnO2 to OMS-2 phase. The longitudinal and lateral directions of OMS-2 can be tuned by changing the amount of K2S2O8. High resolution TEM of OMS-2 showed that nanowires have a lot of defects and also lattice fringes dislocations that can enhance the catalytic activity of OMS-2. UV-assisted methods for synthesis of OMS-2 showed very high surface area 149 m2/g compared to the other counterpart methods. OMS-2 showed very high catalytic activity for oxygen evolution reaction (OER) and activity follows this order OMS-2-4h \u3e OMS-2-6h \u3e OMS-2-8h \u3e OMS-2-12h. OMS-2-4h showed high catalytic activity for pollutant removal. UV-OMS-2-4h can fully degrade methyl orange as model compound to its mineral compounds (CO2, H2O, etc.) within 5 min under visible light irradiation

Topics: Sulfur doped Carbon Nanotube, Graphene Nanolobes, Materials Characterization, Energy, Catalysis, Environmental, Manganese Oxide, Pollutant removal, Fuel cells, Electrochemistry
Publisher: OpenCommons@UConn
Year: 2016
OAI identifier: oai:opencommons.uconn.edu:dissertations-7466
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