The work in this thesis addresses the synthesis and characterization of porous carbon substrates, and their electrochemical and fuel cell evaluation. The approach involves using porous carbon materials of different pore characteristics as electrocatalyst materials for use as cathode catalyst substrates in direct methanol fuel cells (DMFC). In this work, a porous carbon, known as carbonaceous Celatom or C-Celatom, was prepared by template synthesis using a widely abundant, inexpensive macroporous silica structure diatomaceous earth (Celatom FW-80). Ordered mesoporous carbon CMK-3 was also produced by template synthesis of mesoporous silica SBA-15. Scanning electron microscopy (SEM) and x-ray diffraction (XRD) were used to confirm the synthesis of the desired carbon structures. Three different platinum deposition techniques were investigated for electrocatalyst synthesis, an incipient wetness technique, as ethylene glycol reduction technique, and an alkoxide reduction technique. Transmission electron microscopy (TEM) and SEM analysis of the catalysts formed using the incipient wetness and ethylene glycol techniques showed that the synthesized catalysts were not suitable for fuel cell use. Optimization of the alkoxide reduction technique resulted in a deposition technique that resulted in a well-dispersed catalyst with small, uniform particle sizes (2.1-3.1 nm). The synthesized electrocatalysts were evaluated electrochemically and found to have high electrochemically active surface areas (ESA) of 33.38 m2 g-1 for Pt/Vulcan XC-72, 22.45 m2 g-1 for Pt/CMK-3 and 20.51 m2 g-1 for Pt/C-Celatom. The oxygen reduction (ORR) activity was evaluated by linear sweep voltammetry(LSV). The Pt/C-Celatom exhibited the greatest activity towards the oxygen reduction reaction, and the greatest number of active sites for the ORR. Assessment of the material by electrochemical impedance spectroscopy (EIS) also showed that an MEA with C-Celatom as the cathode catalyst has the lowest combines charge transfer and mass transport resistance. Single cell DMFC testing was carried out with each of the experimental substrates. The synthesized catalysts demonstrated high performance over a range of temperatures and feed molarity concentrations. The C-Celatom MEA exhibited the greatest power output of the synthesized catalysts for low molarity operation, with peak power densities of 25.8 and 32.6 mW cm-2 with 0.5M and 1M feed respectively.

A promising energy harvesting technique involves the use of thermal nanofluids, capable of collecting solar UV/visible radiation and storing it as latent heat in phase change materials (PCM), i.e. molten salts. Carbon-based materials are very interesting candidates for UV/visible light absorption and heat transfer in the nanofluid. The crucial point of the development is the stabilisation of dispersed carbon in a suspension. This works is an investigation on the preparation of stabilized suspensions of carbon black, mesophase carbon beads or exfoliated graphite, in aqueous solutions. To achieve this goal, chemical modifications of the surface functional groups of the carbon grains have been attempted. Carbon has many allotropes in which it can exist such as diamond, graphite and amorphous carbon. None of these forms a stable suspension in water without a proper surface treatment. The first priority of this study was to identify possible surface treatments that would modify the surface complex on the carbon materials. Once the treatments had been identified, the focus of the study moved to selecting the most effective treatment based on its dispersion properties. The temperature ranges under which the carbon material remained in a suspension were measured. Finally, the effect of the surface area of the treated carbon material on the stability of the carbon material suspension was investigated. The characterisation techniques that contributed to achieving these objectives were observations with the naked eye, mass spectrometer measurements, thermogravimetric analysis, BET and scanning electron microscope images. The mass spectrometer and scanning electron microscope provided information on the modification of the surface complexes to gain an understanding of the effect of the treatment on the surface of carbon materials. The stability of the carbon and water suspension was measured by using the mass spectrometer and doing a thermogravimetric analysis. The BET results indicated the size of the specific surface area. The size of the surface area could assist in understanding of the stability of the carbon and water suspension. Following an analysis of the results, the researcher reached the following conclusions: First, acid treatments with a concentration of 1 molar for the duration of a minimum of 4 days modify the surface complexes of the carbon black. Second, the treatment with the most stable dispersion properties is the potassium permanganate and nitric acid treatment, which is the only treatment that formed a stable suspension. Carbon materials with a smaller surface area than the treated carbon black does not form a stable suspension, even though they have similar surface complexes. The stable dispersion remains stable only up to 250 ºC. Higher temperatures cause the carbon material to start decomposing, when the carbon surface complexes are broken and released. / Dissertation (MEng)--University of Pretoria, 2018. / Chemical Engineering / MEng / Unrestricted

Carbon/molecule/Cu/Au molecular junctions were fabricated on 4-inch silicon wafers using microfabrication techniques common in commercial semiconductor manufacturing. Electron-beam deposited carbon films are introduced as substrates, and the junctions exhibited high yield and excellent reproducibility. Current density-voltage characteristics of the devices were area scaling, weakly dependent on temperature and exponentially on molecular film thickness, and quantitatively similar to those of devices made with other techniques reported previously in our group, which contained pyrolyzed photoresist films as substrates. Furthermore, the test of cycle life and thermal stability reveals that the devices can survive at least under several millions of potential cycles at room temperature in air, and elevated temperature up to 150 C in vacuum for >40 hours. Parallel fabrication, thermal stability, and high yield are required for practical applications of molecular electronics, and the reported results provide important steps toward integration of molecular electronic devices with commercial processes and devices.

No description available.

Due to its unique characteristics, carbon fiber is one of the leading materials for light weight, high strength and stiffness applications in composite materials. The development of carbon fibers approaching theoretical strengths and stiffness is a continuing process which has led to improved mechanical and physical properties over the recent years. Improvements in carbon fiber properties are directly dependent on the quality of the precursor fiber. Research and development of PAN precursor fiber requires extensive experimentation to determine how processing conditions affect the structure and properties of the precursor fibers. Therefore, it is the goal of this thesis to analyze the results of varying coagulation rates on fiber shape, density and porosity, to determine the effect of cross-sectional shape, density, and fiber diameter on the tensile strength of the fiber, and to investigate the most effective method for the reduction of fiber diameter. Results indicate a low temperature, high solvent concentration coagulating bath leads to a rounder cross section with lower void content. Reduction in fiber diameter was found to increase tensile strength while increased molecular orientation experienced during high draw down ratios led to an increase in fiber modulus.

Carbon fiber is one of the leading materials for high strength and modulus, and light weight applications. Improvements in carbon fiber properties are directly dependent on all aspects of manufacture, especially the process of stabilization. Therefore, it is the goal of this thesis to study the effects of the temperature and strain profile of the stabilization process, and the resulting carbon fiber tensile properties. In addition, the precursor fibers used were spun under two different draw ratios, to study the effects of the spinning parameters. Results indicated through DMA studies that completeness of stabilization reactions can be gauged by the peak and leveling of induced stress while fibers are stabilized in isostrain conditions. Through this method, carbon fiber tensile properties were maintained from the prior methods, but saved significant time for processing. Stress vs. strain tests throughout the stabilization process created a baseline for understanding the maximum capable strain on fibers throughout the stabilization process. Lastly, this information was summarized, combined, and basic mechanical engineering principles discussed for a continuous stabilization furnace with strain control, so that further research into the stabilization process can incorporate carbon fibers made with in situ stretch control.

This study involves the investigation of silicon-carbon systems using ab initio techniques. It was motivated by the search for off-50:50 alloys and a way to quantify the strengths of 2D silicon-carbon materials. The study also predicts some under-reported properties for three previously proposed hypothetical allotropes of carbon. Preferably stable off-50:50 structures are identified from a set of trial structures for silicon-rich and carbon-rich candidates and their conditions of stability and physical properties are identified. A two-dimensional equation of state is introduced and applied to analyze the relative strengths of various 2D silicon-carbon materials. Of the possible off-50:50 alloy combinations and candidate structures considered, only the pyrite-FeS2, glitter-SiC2 and t-BC2 structures for SiC2 are elastically and dynamically stable. Analysis of the instability of Si2C reveals that it seems likely that carbon rich alloys are more favorable to their silicon-rich counterparts due to the smaller size of the carbon atoms and the more compact carbon-carbon bonds which result in less distorted bonding that is less metallic. The stiffness of the silicon dicarbide structures rank, in increasing order with 3C-SiC included for comparison, as glitter --> pyrite --> 3C-SiC --> t-SiC2. The moduli values for t-SiC2 are very comparable to 3C-SiC since for both materials, all atoms are four-fold coordinated with t-SiC2 having similar but slightly distorted, strong covalent tetrahedral bonding. The pyrite and glitter structures exhibit metallic character whereas t-SiC2 is a semi-conductor. Not only has this work demonstrated that, in principle, off-50:50 alloys of carbon and silicon are plausible, it has also provided information on how the strength and elastic properties of these materials are effected by increased silicon content. This has filled in a significant lack of knowledge about these bulk systems. For 2D systems, an equation of state is proposed that equates in-plane pressure with a change in surface area. It extracts the layer modulus as one of its fit parameters, which measures a material's resilience to hydrostatic stretching and predicts the material's intrinsic strength. Graphene is the most resilient to stretching with the highest intrinsic strength of all structures considered followed by SiC. Buckled Si is the least resilient with the lowest strength. An off-50:50 planar alloy, called silagraphene, differs elastically from SiC but has a comparable strength due to the similarity of their layer modulus. The novel 2D equation of state presented here opens up new ways to study and compare the strength properties of mono or multi-layered 2D materials, especially how their resilience to isotropic stretching responds to in-plane pressure. / Thesis (PhD)--University of Pretoria, 2013. / Physics / unrestricted

Carbonaceous materials, especially in micro and nanoscale, are useful in optical, energy storage, electronic, and biomedical devices or technologies. Techniques have been developed for preparation and modification of the carbonaceous materials, while it is still challenging to tailor the properties of carbonaceous materials effectively and economically. Laser is a powerful tool in academic and industrial laboratories, which also plays important roles in the preparation and modification of high-performance carbonaceous nanomaterials. In this study low-cost hydrothermal synthesis, high-temperature annealing, and Laser ablation (LAL) methods are developed to prepare functionalized carbon nanomaterials and modify their electrochemical and optical properties. Sub-micro hollow carbon spheres are synthesized via hydrothermal carbonization and high-temperature activation without any templates. Good capacitive properties are obtained after activation. The electrochemical properties of the activated carbon spheres depend on the media of the activation. The capacitance of the activated carbon spheres significantly increases with the addition of water as an activation agent. Carbon dots (CDs) are synthesized via a facile and economic hydrothermal (HTC) process using both small-molecule sugar (Xylose) and ground soybean residuals as precursors. The photoluminescence (PL) properties of the as-prepared and further-treated CDs are systematically studied. For the xylose-synthesized CDs, the initial green PL emission disappears after high-temperature treatment at 850 ℃ for 2 h. With further LAL treatment in NH4OH solutions, the PL emission is re-acquired, and a blue shift in emission is observed. Thus, the LAL is found to be an effective method to modify the CDs and their PL properties. For the nitrogen-doped soybean waste-derived carbon particles, they show strong blue emissions, which essentially disappear after 850 ℃ annealing for 2 hours in an argon flow. Then, PL appears again after laser ablation in a 10% NH4OH solution. The conversion from the blue emission to no emission and then back to blue emission again implies the effect of the functional groups on the PL properties of the CDs.

Graphite and graphitic materials underpin a number of modern technologies such as electrodes for energy storage and conversion systems. Due to their aromatic honeycomb-type lattice and layered structure, these carbons host a rich variety of foreign elements in their interstices. Whether possessing a tubular morphology - that enables the encapsulation of inorganic compounds, or a planar texture - where anions and molecules can intercalate, the chemical analysis of graphite and graphitic materials is often confronted with the need to disintegrate the carbon matrix to quantify target elements, most often metals. However, the resilience of the sp2-hybridized carbon lattice to chemical attacks is an obstacle to its facile solubilization, a necessary step to perform some of the most common elemental analysis measurements. Over the years, a range of alternative approaches have sprung out to address this issue such as the combustion of the carbon matrix followed by the acid dissolution of its ash product. Unfortunately, none of these represents a viable method that can be applied to batteries, in great part because of the different components that make up the carbon-based electrodes. In this dissertation, a new protocol has been developed to digest graphitic materials aiming to access their elemental composition in bulk scale. The approach is based on the use of molten alkaline salts to promote the oxidation of the carbon lattice and leach out metals into a dilute acid solution. As a model sample, given the existence of standards with a matching matrix, single-walled carbon nanotubes were examined. After being subjected to the alkaline oxidation (a.k.a. fusion), they were solubilized and analyzed with Inductively Coupled Plasma-Optical Emission Spectroscopy, a widely popular tool for elemental analysis of metals. Structural analysis ensued to understand the interaction of the molten salts with the nanotubes. After evaluating the applicability of the protocol to other carbons, a more complex system was investigated, namely the carbon-based anode of an intercalation-type potassium ion battery. In this process, a direct way to quantify the mass of the alkali metal was discovered, one which makes use of complementary chemical and structural analytical tools.

Esta tesis doctoral se centra en el desarrollo de diferentes métodos electroquímicos y químicos para la funcionalización de materiales de carbono nanoestructurados, obteniendo materiales electródicos funcionales para aplicaciones bioelectroquímicas. En este sentido, las propiedades en superficie influyen directamente en la interacción entre el elemento bioreceptor o biocatalizador con el electrodo. Por tal motivo, se puede mejorar la cinética de transferencia de electrones, la inmovilización, orientación y distribución del bioelemento en el electrodo, mejorando el rendimiento del dispositivo bioelectroquímico, a partir de la química superficial del material empleado. El uso de diferentes funcionalidades de nitrógeno y fósforo generados electroquímicamente sobre los materiales de carbono nanoestructurados proporciona una plataforma para la síntesis controlada, para mejorar la actividad catalítica de biocatalizadores. Además, el proceso electroquímico de funcionalización ha demostrado ser una ruta interesante para preparar bioelectrodos en un solo paso, con bajo consumo de elementos enzimáticos y un rendimiento sobresaliente a los métodos convencionales actuales. Por otro lado, el uso de materiales de carbono dopados en N cuaternarios como elemento transductor en la síntesis de un biosensor enzimático de glucosa libre de metales proporciona, en condiciones aeróbicas, un biosensor de alta sensibilidad para la detección de glucosa en orina y bebidas azucaradas comerciales, con bajo efecto de los interferentes y alta estabilidad. Finalmente, la modificación de los nanotubos de carbono con nanopartículas de oro proporciona un material de electrodo transductor escalable y estable en el que tiene lugar la inmovilización de anticuerpos a través de interacciones Au-8. La formación de antígeno-anticuerpo complejo provoca efectos estéricos que producen un impedimento para la transferencia de electrones proveniente de una sonda redox activa hacia la superficie del electrodo, lo que facilita la detección del analito.