School of Chemistry (ETDs)

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Browsing School of Chemistry (ETDs) by SDG "SDG-7: Affordable and clean energy"

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    Colloidal synthesis and characterization of molybdenum and tungsten-based phosphide electrocatalysts for hydrogen evolution reaction
    (2022) Nkabinde, Siyabonga Sipho; Moloto , Nosipho
    The production of hydrogen gas via hydrogen evolution reaction (HER) in acidic media has become an important area of research in light of the increasing demand for sustainable and environmentally friendly sources of energy. However, its large-scale production is currently being hindered by the lack of inexpensive and highly efficient non-noble electrocatalysts. Transition metal phosphides (TMPs) have transpired as favourable catalysts that can be prepared from cheap and readily available sources. Up to now, TMPs have been commonly prepared using solid-state and solid-gas reactions, which rely on the use of high temperatures and hence generate inhomogeneity in the prepared materials. Inhomogeneous materials are unattractive as catalysts because the correlation between a catalyst and its structural features cannot be systematically studied. For this reason, colloidal synthesis has emerged as a powerful method in the synthesis of TMPs as it allows for control over the resulting physical features (i.e. size, morphology, crystal phase, crystallinity etc.). The ability to tailor these physical properties provides room for improving the catalytic activity. By using the colloidal synthesis method, we have successfully prepared molybdenum and tungsten-based phosphide nanoparticles and studied the effect of their physical features on HER activity. In chapter 3, we report a facile colloidal synthesis method to produce an amorphous phase of molybdenum phosphide (MoP) by using trioctylphosphine (TOP) as a phosphorus source, molybdenum pentachloride (MoCl5) as a metal source and 1-octadecene (1-ODE) as a solvent/reducing agent. The use of the forementioned precursors promoted the formation of very small, shape controlled and well dispersed amorphous molybdenum phosphide (MoP) nanoparticles. Annealing (800 °C) of the amorphous MoP nanoparticles resulted in the formation of a crystalline MoP phase with a slightly bigger size but retained its dispersity and morphology upon exposure to high temperature. The amorphous and crystalline MoP phases were compared as HER electrocatalysts. HER results indicated that the amorphous MoP phase exhibited enhanced catalytic activity in hydrogen evolution reaction compared to the crystalline MoP phase. The high activity displayed by the amorphous MoP was attributed to the small sizes and the high density of unsaturated active sites characteristic of nanoparticles lacking long range crystalline order.
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    Development of eco-friendly building bricks derived from carbon nanotube-reinforced coal ash and low-density polyethylene waste materials
    (University of the Witwatersrand, Johannesburg, 2024) Makgabutlane, Boitumelo; Maubane-Nkadimeng, M.S.; Coville, N.J.
    This study reports on the incorporation of carbon nanotubes (CNTs) into the all-waste derived building bricks. The focus was on waste management and beneficiation of plastic waste and coal ash, which are generated in large volumes without sufficient recycling. The waste materials were characterized using a range of techniques to ascertain their properties for application. Multiwalled carbon nanotubes (MWCNTs) were synthesized using a facile floating chemical vapour deposition method (CVD) and their physicochemical properties were tested. Bricks with dimensions of 220 x 105 x 70mm were developed with an optimum 85:15 coal ash to plastic waste ratio respectively using a specialized reactor. The bricks were tested for compressive strength, split tensile strength, water absorption, strain, thermal stability and durability using oxygen permeability index, chloride conductivity index and water sorptivity index as indicators. Furthermore, environmental and financial sustainability and ecotoxicology were tested. At optimum conditions, high quality MWCNTs with a diameter of 83 nm, length of 414 μm and a carbon yield of 73% were obtained. The ID/IG ratio of 0.44, an oxidation temperature of 649 °C, a purity of 94% and surface area of 50.9 m2/g were achieved. Coal fly ash with a spherical shape, particle size of below 10 micron and a thermal stability of 680 °C was used as an aggregate for the bricks. The bricks (without CNTs) developed their maximum compressive strength of 11.9 MPa at 14 days. The incorporation of the CNTs improved the microstructure of the bricks by filling the voids and providing a bridging effect as reinforcement mechanisms. The optimum CNT loading of 0.05 wt.% produced bricks with a compressive strength of 22 MPa and tensile strength of 8.7 MPa, which exceeded the South African National Standards (SANS227:2007) requirements for building bricks by 450% and 625% respectively. The durability properties were improved as the CNT dosage was increased from 0-10 wt.%. The 0.05 wt.% bricks were categorized as “good” for all the durability indexes. The CNT containing bricks showed improved thermal stability and maintained their structural integrity. The chemical resistance also improved and the efflorescence was minimal on all the bricks. The utilization of waste in the bricks enabled resource conservation, reduced pollution and reduced cost compared to conventional bricks. When only considering the raw materials used, the cost of production per brick was $0.091. The ecotoxicology of the powdered brick samples was tested on Raphidocelis subcapitata (microalga) and Daphnia magna (aquatic organism) using leachates from neutral, acidic and basic mediums. Some heavy metals were leached above the threshold limit especially in acidic medium. The leachates were toxic to the test species at low concentrations and resulted in growth inhibition of the microalga and immobization of the aquatic organisms. The toxicity of the CNTs was inconclusive and dedicated tests are required to study their effect. With appropriate treatment of CFA, the waste derived CNT bricks have a great potential of being a sustainable alternative to the conventional bricks based on cost, properties and environmental impact
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    Magnetic enhancement of a high entropy spinel oxide electrocatalyst for rechargeable zinc-air batteries
    (University of the Witwatersrand, Johannesburg, 2024) Hechter, Ernst Heznz; Ozoemena, Kenneth
    The exploration of high entropy materials (HEMs) as electrocatalyst materials has only recently begun to accelerate. Similarly, the effect of magnetic fields on the oxygen evolution and reduction reactions has recently begun to attract great interest. In this work nanoparticles of the high entropy oxide (CuCoFeMnNi)3O4 were synthesized and supported on Vulcan carbon for use as a bifunctional OER/ORR catalyst in a rechargeable zinc-air battery (RZAB). The products were characterized to confirm and investigate the solid solution high entropy phase, and the electrochemistry was investigated with and without an external magnetic field. The HEMs demonstrated moderate intrinsic electrochemical properties, with overpotentials and current densities comparable to commercial platinum on carbon catalysts even at low loadings. Here is reported the most significant magnetic enhancement in RZAB power profile in literature at the time of writing, as well as improved RZAB stability and areal energy. This work offers insight into the mechanism of magnetic enhancement in the case of high entropy materials, and pioneers the use of combined strategies to achieve stable, cost-efficient and effective bifunctional OER/ORR electrocatalysis.
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    The microwave assisted synthesis of doped carbon dot/carbon nano-onion composites: A novel all-carbon counter electrode for dye-sensitized solar cell
    (University of the Witwatersrand, Johannesburg, 2023) Masemola, Khanyisile; Moloto, Nosipho; Maubane-Nkadimeng, Manoko S.; Coville, Neil J.
    Human society's development is heavily reliant on stable energy supply, and fossil energy sources have long been a very reliable energy source for this objective. However, being a non-renewable energy source, fossil fuel depletion is unavoidable and impending in this or future generations. To solve this issue, renewable energy, particularly solar energy, has received a lot of attention since it directly turns solar energy into electrical power with no environmental consequences. Various photovoltaic technologies based on organic, inorganic, and hybrid solar cells have been successfully manufactured to date. However, much study has been concentrated on organic solar cells for household and other commercial uses due to its inherent cheap module cost and ease of production. But dye-sensitized solar cells (DSSCs) have been reported to be the most efficient and simplest applied organic solar cell technology. In this study, carbon dot: onion-like carbon nanomaterial composites (Cdots: OLCNs) were synthesized for possible future application in electronic devices with particular attention to dye-sensitized solar cells. The nitrogen-doped carbon dots (NCdots) and functionalized onion-like carbon nanomaterials (OLCNs) were synthesized using a one-step hydrothermal microwave assisted irradiation method and flame pyrolysis method using liquid fuels, respectively. The as-synthesized OLCNs where purified and washed using an organic solvent n-pentane to obtain pristine OLCNs (p-OLCNs) which were further functionalized with N2 gas to obtained nitrogen-doped CNOs (N- OLCNs) and H2O2 to give oxygenated OLCNs (ox-OLCNs). For the synthesis of NCdots, various precursors (ethylenediamine, urea and fumaronitrile) were used to evaluate the effect of different nitrogen sources on the properties of these materials. Photoluminescence spectroscopy showed that the resulting NCdots exhibited the conventional excitation-dependency behavior. The NCdots which presented with the highest fluorescence quantum yield (made from ethylenediamine) were used to make the subsequent NCdot: OLCNs composites. The as-prepared p-/ox-/N-OLCNs all showed similar morphologies typical of chain-like carbon nanostructures, according to transmission and scanning electron microscopy studies, but with varying particle sizes of 42 nm, 125nm and 85 nm, respectively. The corresponding nanocomposites were used as counter electrode materials in DSSCs. The application of all the nanocomposites in the DSSCs resulted in cell efficiencies, current densities, open circuit voltages and fill factors that were lower than that of a conventional platinum (Pt) electrode. All nanocomposites tested presented with cell efficiencies <1%. Furthermore, the cells displayed some photovoltaic effect of minimal activity in the absence of light, under dark field conditions implying it is still a photovoltaic material. This photocurrent generated by the cell in the dark is suggested to be a dominant contributor to the low performance of the cells. However, what was remarkable was that this photovoltaic effect, primarily due to the thermal activity from the long lasting glow of the NCdots specifically, was found to be stable and efficient in response as infrared radiation even without being illuminated with light for 5 minutes. This suggests that the NCdots: OLCNs composites have potential application, possibly as efficient diodes rather than for use in DSSCs.