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This dissertation describes the synthesis and characterization of nitrogen doped carbon nanotube (NCNT) supported nanoparticles for catalysis, specifically, the cathodic oxygen reduction reaction (ORR) in fuel cells. Strategies for synthesis of mono- and bimetallic nanoparticle catalysts through dendrimer based templating techniques and with the aid of metal organic chemical vapor deposition (MOCVD) precursors and efficient assembly protocols of the catalysts with the NCNTs are discussed in detail. Physicochemical properties of the NCNTs and NCNT supported catalysts were characterized using a host of tools including scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, x-ray photoelectron spectroscopy (XPS), thermo gravimetric analysis, BET surface area and pore size analysis and electrochemical techniques including cyclic voltammetry, chronocoulometry, chronoamperometry and rotating disk electrode voltammetry. Chapter 1 serves as a general introduction and provides a brief overview of challenges associated with the synthesis, characterization and utilization of graphitic carbons and graphitic carbon supported catalysts in heterogeneous catalysis. Chapter 2 provides an overview of the synthesis and characterization of systematically doped iron and nickel catalyzed NCNTs in an effort to understand the effect of nitrogen doping on ORR. Chapter 3 describes the use of NCNTs as supports for dendrimer templated nanoparticle catalysts for ORR. A facile synthetic strategy for the immersion based loading of catalysts onto NCNTs by spontaneous adsorption to achieve specific catalyst loadings is explored. Chapter 4 details the loading of monodisperse Pt, Pd and PtPd catalysts on the as synthesized NCNTs using MOCVD precursors. The MOCVD route offers promise for direct dispersion and activation of ORR catalysts on NCNT supports and eliminates a host of problems associated with traditional solvent based catalyst preparation schemes. Chapter 5 details future directions on a few topics of interest including efficient electrodeposition strategies for preparing NCNT supported catalysts, studies on PtCu catalysts for ORR and finally prospects of using NCNT supported catalysts in fuel cell applications.

In this research, multiwalled carbon nanotube (MWNT) and paper composite materials were fabricated by dropcasting aqueous dispersions containing MWNTs onto filter paper using vacuum filtration, a highly unidirectional drying technique. By varying the pore size of the paper backbone as well as the number of deposited MWNT layers, composites with distinct architectures and properties were created. This thesis provides numerous examples that show how the processing methodology used influences the location of the MWNTs, the amount of MWNTs deposited, and the interaction between the MWNTs and the paper backbone. These three factors work in tandem to form the structures and properties presented. Understanding how the structures and properties come about allows for the tailorability of these composites for different applications and devices. The pore size of the backbone material combined with the directionality of the drying methodology controlled the location of MWNT deposition. MWNT deposition occurred in three ways: on the paper surface only, within the paper material only, or combined surface and internal deposition. By varying the number of deposition steps, the properties of the composite could be altered in the location of deposition. Surface charge, dispersion concentration, paper pore size, drying methodology, MWNT length, the number of deposited MWNT layers, and post-processing techniques were all factors studied in this thesis which could successfully vary the interaction between the MWNTs and between the MWNT and paper materials and, ultimately, alter the properties of the composite. Regardless of the processing methodology employed and the starting materials used, structure and property evolutions in the composite materials were characterized using impedance spectroscopy, optical microscopy, scanning electron microscopy, and Current-AFM. Combining equivalent circuit fitting of the impedance data with the information obtained from the imaging techniques allowed for the elucidation of structural mechanisms which contribute to the electronic response measured for each composite. An overall equivalent circuit was built for each composite plane which could then be used to extract the electrical properties of the individual conduction mechanisms within the composite. In the in-plane, the electrical properties of the paper backbone, MWNT-MWNT junctions, MWNT bundles, and MWNT curved bundles could be determined. In the thru-plane, the electrical properties within the paper thickness, either paper-dominated or MWNT-dominated, could be measured. The resistance through the thickness of a bulk MWNT surface network could be also measured when the density of the MWNT network is sufficiently high.

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Numerous alternative energy sources are being researched for sustainable energy applications, but their overall benefit is still too costly for them to be considered viable. Commonly produced temperature gradients created by the environment, or are man-made, can be converted into useful energy by using thermoelectric materials. Inorganic semiconductors are the most commonly used thermoelectric materials, but have raised concerns due to toxicity issues, rarity of heavy elements used, and high fabrication temperatures. These concerns have led research efforts into electrically conductive polymer composites prepared in ambient conditions from aqueous solutions. By combining polymer latex with carbon nanotubes (CNT), electrical conductivity can resemble metals while thermal conductivity remains similar to polymers. Using different CNT stabilizers for these fully organic composites can tailor the thermoelectric properties and harvest thermal gradients from previously inconceivable places (e.g., body heat converted into a voltage). A semiconducting CNT stabilizer, meso-tetra(4-carboxyphenyl) porphine (TCPP), was used to investigate the influence stabilizers have on composite thermoelectric properties. As TCPP was compared to a similar system containing an insulating stabilizer, sodium deoxycholate (DOC), the multi-walled carbon nanotube (MWNT)-filled composites showed a 5x increase in the Seebeck coefficient (S). TCPP did not have a distinct effect on the electrical conductivity (σ), demonstrating the tailorability of S with this molecule. An intrinsically conductive polymer, poly(3,4-ethylenedioxythiophene) :poly(styrene sulfonate) (PEDOT:PSS), was used to stabilize highly conductive double-walled carbon nanotubes (DWNT) and demonstrate the promise of fully organic composites as thermoelectric materials. This combination of CNT and stabilizer produced metallic electrical conductivity (200,000 S m-1) and power factors (S2σ) within an order of magnitude of commonly used semiconductors (~400 μW m-1 K-2). Electrical conductivity was doubled by stabilizing single-walled carbon nanotubes (SWNT) with PEDOT:PSS in a thin film without the insulating polymer latex. To further demonstrate the tailorability of polymer composites, a dual stabilizer approach using semiconducting and intrinsically conductive stabilizers was used. This approach effectively provided the high electrical conductivity from PEDOT:PSS and the enhanced Seebeck coefficients of TCPP. By using multiple stabilizers for CNTs within the same composite, power factors among the highest reported for fully organic composites are achieved (~500 μW m-1 K-2). These water-based, flexible composites are becoming real competition as their conversion efficiencies, when normalized by density, are similar to commonly used semiconductors.

Owing to their exceptional mechanical and physical properties, carbon nanotubes seem to / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, 2006.