Electronic Theses and Dissertations (PhDs)
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Item Preparation of nitrogen-doped multiwalled carbon nanotubes anchored 2D platinum dichalcogenides for application as hydrogen evolution reaction catalysts(University of the Witwatersrand, Johannesburg, 2024-09) Mxakaza, Lineo Florence; Moloto, Nosipho; Tetana, ZikhonaThe alkaline hydrogen evolution reaction (HER) (H2O + 2e − → H2 + 2OH−) is fast gaining traction as a sustainable hydrogen gas generation route but suffers from slow reaction kinetics because of the additional water dissociation step and large reaction overpotential. As such, the current state-of-the-art acidic medium Pt and Ru catalysts suffer from considerable loss of catalytic activity in an alkaline medium. We propose the development and use of platinum metal dichalcogenides for alkaline HER. Platinum dichalcogenides are 2D materials that offer the advantage of more exposed catalytic sites, show dramatic chalcogen-dependent electronic properties, and have a band gap (0.24 eV - 1.8 eV for PtS2 and PtSe2) thus extending the use of these materials to light-stimulated photo-electrochemical (PEC) HER. As such, PtS2 is reported to be a semiconductor, PtSe2 is semi-conductive/semi-metallic depending on the number of layers, and PtTe2 is metallic. The Pt-chalcogen covalent bond intensifies down the chalcogen group. Additionally, the interlayer interactions in Pt dichalcogenides are covalent, and just like the Pt-chalcogen bond, intensify as the chalcogen atom changes from sulphur to selenium to tellurium. This behaviour of Pt dichalcogenides results from the Pt bonding d orbitals and the chalcogen bonding p orbitals that are relatively close in energy than in other TMDs, and the difference in the energy becomes smaller and smaller down the chalcogen group. Herein, we report on the synthesis of PtSe2 and PtTe2 using the colloidal synthesis method for the first time and then applying them as electrocatalysts in alkaline HER. As mentioned, developing 2D materials results in band gap development, particularly in PtS2 and PtSe2. Following this, PtSe2 was explored as a photocathode in light-induced photo-electrochemical HER. Generally, semiconductors are poor electron transporters and one of the major requirements for an efficient PEC cathode is solar absorption, charge generation, and efficient charge separation. The charge separation properties of PtSe2 were improved by supporting this material on highly conductive, mechanically, and thermally stable nitrogen-doped multi-walled carbon nanotubes (N-MWCNTs). In Chapter 3, we report on the effect of varying selenium precursors from elemental selenium, sodium selenite to selenourea on the colloidal synthesis of PtSe2 in a mixture of oleylamine and oleic acid at 320 ℃. All the reactions resulted in the formation of PtSe2 although PtSe2 prepared from selenourea is amorphous, evidenced by relatively broader XRD peaks and a smaller crystallite size. HER activity of the three PtSe2 catalysts was evaluated in 1 M KOH at a scan rate of 5 mV/s and PtSe2 prepared from selenium exhibited the earliest onset potential of 46 mV, overpotential of 162 mV, and a smaller Tafel slope of 112 mVdec-1. This material exhibits the smallest resistance to electron transport and a high electrochemical surface area. We then explored the effect of altering tellurium precursor from elemental tellurium to tellurium tetrachloride, and sodium tellurite. Unlike the PtSe2 synthesis, different platinum tellurite phases, PtTe2, PtTe, and the mixed phase PtTe: PtTe2 were produced from Te, PtCl4, and sodium tellurite, respectively. Of the three, PtTe2 exhibited the highest alkaline HER activity with an onset potential of 29 mV, an overpotential of 107 mV, and a Tafel slope of 79 mVdec-1. In the same chapter, we compared the catalytic activity of PtSe2 (prepared from Se) and PtTe2 (prepared from Te) catalysts. We determined that PtTe2 has a high surface roughness and electrochemical surface, leading to relatively higher activity than PtSe2. However, PtTe2 is metallic and therefore does not have a band gap, which implies that it cannot be employed in light-stimulated catalysis reactions. In Chapter 4, we explored the use of PtSe2 as a light-stimulated PEC alkaline HER catalyst. We used in situ colloidal synthesis to grow PtSe2 on the walls of N-MWCNTs to improve the overall electron transport properties of PtSe2. PtSe2 anchored on N-MWCNTs was also studied in the dark and under illumination using 1 sun (100 mW/cm2) to determine the influence of light on the HER catalytic activity of the hybrid materials. This study demonstrates that the light-stimulated HER activity of PtSe2 improves when minimal amounts of N-MWCNTs are incorporated in the PtSe2 sample matrix. This then leads to employing these materials as photocathodes in PEC HER.Item Microwave-assisted synthesis of palladium-based ferroalloy electrocatalysts for application in alkaline direct alcohol fuel cells(University of the Witwatersrand, Johannesburg, 2024-11) Ramashala, Kanyane Nonhlanhla Eugenia; Billing, Caren; Modibedi, R. Mmalewane; Ozoemena, Kenneth IkechukwuThis research work describes the study of Pd-based ferro-electrocatalysts for application towards direct ethanol fuel cells (DEFCs), direct ethylene glycol fuel cells (DEGFCs), direct glycerol fuel cells (DGFCs) and oxygen reduction reaction (ORR) operated in a basic environment. The initial part of the research was to explore the Pd-based monometallic and bimetallic (Pd/C and PdFe/C) by utilising varied methods such as the conventional sodium borohydride (NaBH4) and microwave-assisted technique (MW) towards the oxidation of glycerol (gly), intending to choose the best method viable for these catalysts. This study revealed that MW techniques tuned the physicochemical properties of Pd/C and PdFe/C by augmenting their crystallinity and defect. These led to improved electrocatalytic activities towards glycerol oxidation reaction (GOR) over NaBH4 technique. MW process as a powerful tool was further used in the entire study to synthesise bimetallic and trimetallic electrocatalysts in ethanol (EtOH), ethylene glycol (EG) and glycerol (Gly) oxidation reaction in an alkaline environment. The synthesised bimetallic catalysts studied in this research work were (PdFe/C, PdCo/C, and PdMn/C) at varied ratios of Pd: M (Pd2M/C (2:1) and PdM/C (1:1)). Amongst them all, Pd2Fe/C and PdFe/C were observed to be the most favourable catalysts towards all the alcohols, with the excellent specific activity of about, for EtOH (11.59 and 4.15 mA cm-2), EG (9.82 and 5.51 mA cm-2) and Gly (8.94 and 4.73 mA cm-2), respectively. The satisfactory performance exhibited by the PdFe/C electrocatalyst prompted the exploration of the second 3d transition metal (PdFeMn/C and PdFeCo/C), intending to investigate the synergistic behaviour between the non-noble metals and Pd. The XRD confirmed that these electrocatalysts are in a crystalline nature with a decrease in d spacing (from 0.2247 nm, PdFe/C to 0.2236 nm (PdFeMn/C)) after the insertion of Mn into PdFe/C. This was supported by the TEM images obtained for the PdFeMn/C catalyst with a particle size of sub 10 nm. The comparison studies towards EtOH, EG and Gly were investigated for all the electrocatalysts and there was a remarkable observation, which is dissimilar from the theoretical studies (DFT). Density Functional Theory (DFT) revealed that PdFeCo performed better in terms of Gibbs free energy, binding energy, and energy band gap than PdFeMn; however, the experimental studies favoring the performance of PdFeMn. The PdFeMn/C delivered the best electrochemical activities, including a superior electrochemical active surface area (ECSA), larger current densities and mass activity response, and less susceptibility to poisoning and high conductivity as compared to PdFe/C and PdFeCo/C electrocatalysts. Furthermore, the PdFeMn/C electrocatalyst exhibited remarkable electrochemical properties during the ORR (basic medium). Ultimately, the best two anode electrocatalysts (PdFe/C & PdFeMn/C) were explored and tested for the proof-of-concept in the two-electrode configuration with the micro-3D printed cell. The PdFeMn/C delivered improved µ-ethylene glycol fuel cell, µ-glycerol fuel cell, and µ-ethanol fuel cell activities with respective to high voltage and power density of 33.27 mW cm-2, 11.00 mW cm-2 and 45,80 mW cm-2 respectively, operated at 100 mV / s. These electrocatalysts have demonstrated promising results in advancing ADAFCs.Item Application of oxidative enzymes in membrane systems for the bioremediation of triazines in wastewater(University of the Witwatersrand, Johannesburg, 2024-10) Lesaoana, Mahadi; Richards, Heidi L.; Brady, DeanThe prevalence of herbicidal pollutants present in various environmental matrices have become a global concern. The discharge and accumulation of s-triazine agrochemicals in effluents remains a major challenge, threatening the quality of freshwater resources. These are newly identified recalcitrant contaminants of concern (CECs) with complex structures, and inadvertent exposure poses deleterious ecological risks and human health-related adverse effects. Unfortunately, they have shown resistance to conventional treatment strategies, hence their persistence in wastewater treatment plant (WWTP) effluents and water bodies. Therefore, there is an urgent need for the exploration of alternative technologies for the effective eradication of such contaminants from water samples. The bioconversion of such micropollutants using oxidative enzymes like laccase is a promising research avenue, providing a sustainable, economically and ecologically benign strategy. The current research examined the potential of a hybrid biocatalytic membrane system to degrade common s-triazine agrochemical herbicides in aqueous solutions. Specifically, the use of Novoprime base 268 laccase coupled with hollow fibre polyethersulfone (PES) membranes was investigated for the bioremediation of atrazine (ATZ), ametryn (AMT), simazine (SMZ) , prometon (PMT) and terbuthylazine (TERB) in wastewater. In batch-mode reactions, major operating parameters (i.e. pH and temperature profiles, enzyme dosage and contact time) were varied for the laccase-assisted catalysis of s-triazine compounds. Optimised conditions provided highest removal efficiencies (> 88.9%) at pH 5.0, combined with a temperature of 25°C and 1.0 mg L-1 solution concentration after 24h reaction time. Through the addition of redox mediators viz. 2,2-azino-bis-3-ethylbenzothiazoline-6-sulphonic acid (ABTS), violuric acid (VA), vanillin (VA), syringaldehyde (SRA) and acetosyringone (ASR) recalcitrant triazine degradation was enhanced by 10 to 20 % at 1.50 mm. Subsequently, the performance of a standalone continuous flow-mode membrane system was evaluated firstly, using a bed adsorption column only operated under various conditions. The efficiencies were compared to batch-mode enzymatic experiments. The adsorption of triazines by PES was only weakly influenced by pH, and the optimum removal was attained at pH 5.0 (5.0 mg L-1), 2.35 g bed mass (14.0 cm height) and 24h column operation time. The overall removal percentages were 72.6%, 75.2%, 71.4%, 67.4%, and 68.2% for ATZ, AMT, SMZ, PMT and TERB, respectively. Although the results indicated satisfactory performances by both systems, their performance is limited when used as separate units (continuous membrane vs laccase reactor). A biocatalytic membrane system was achieved by integrating laccase into the dynamic packed-bed membrane column. Relevant process control design parameters of the fixed-bed biocatalytic column were carefully evaluated and recorded an optimum of 93.2 % removal efficiency as observed at a feed flow rate 2.0 mL min-1, at a bed height of 14.0 cm using an atrazine influent concentration of 5.0 mg L-1. Equilibrium dynamics of the breakthrough modelling were best fitted by Thomas model. Results attained demonstrated selectivity for triazines in matrix-matched real river water samples with remarkable recyclability after six successive operational cycles. This reflects the potential workability of the integrated system for extended enzymatic reactions evaluated under robust experimental conditions. As a benchmarking exercise, cost-analysis studies showed comparable projected scalability of our configuration at 1200 m3/d capacity at an estimated total cost of R7.036 mil.