Date of Completion

8-22-2016

Embargo Period

8-22-2019

Keywords

Sulfur doped Carbon Nanotube, Graphene Nanolobes, Materials Characterization, Energy, Catalysis, Environmental, Manganese Oxide, Pollutant removal, Fuel cells, Electrochemistry

Major Advisor

Dr. Steven L. Suib

Associate Advisor

Dr. S. Pamir Alpay

Associate Advisor

Dr. Pu-Xian Gao

Associate Advisor

Dr. Alfredo Angeles-Boza

Associate Advisor

Dr. Fatma Selampinar

Field of Study

Materials Science

Degree

Doctor of Philosophy

Open Access

Campus Access

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 > OMS-2-6h > OMS-2-8h > 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.

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