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This thesis has investigated several complex situations that may be encountered in electrochemical studies. Three main situations have been examined, they include the formation of polymer films on electrode surfaces during measurements, a novel nanocatalyst modified electrode surfaces, and organised carbon nanotube (CNT) structures on electrode surfaces. These have been utilised for different electrochemical applications owing to their dissimilar properties. Voltammetric techniques of cyclic voltammetry (CV), square wave voltammetry (SWV) and Fourier transformed large amplitude ac voltammetry (FTACV) have been utilised to examine these reactions. Chapter 3 reports the investigation of catechol oxidation and subsequent polymerisation through crosslinking with D-glucosamine or chitosan. Hydrogel can be formed on the electrode surface during the process, which changes the viscosity of the solution and thus affects the diffusion of chemical species. This process has been examined by several voltammetric techniques. A further examination of the chemical system has also been conducted using FTACV for the first time. Chapter 4 describes the preparation of carbon microsphere supported molybdenum disulfide. The material has been utilised as electrocatalysts for hydrogen evolution reaction (HER) in acidic media, and the performance tested by traditional linear sweep voltammetry (LSV) and advanced FTACV techniques. The FTACV technique has been used for the first time for HER processes. In addition, the synthesised particles have also been used for thermal catalytic decomposition of hydrogen sulfide, which shows a significant improvement in the conversion rate over conventional examples. Chapter 5 demonstrates the direct growth of vertically aligned CNT forests on a gold electrode. The electrochemical response of the fabricated electrode has also been examined with ferrocyanide as the redox species. Furthermore, the immobilisation of anthraquinone onto CNT forest has been attempted. The fabricated electrode was utilised as a pH sensor via CV and SWV, and both indicates a well correlated pH-potential relationship in the pH range of 2 to 12. The sensor has also been assessed by the FTACV technique.

This thesis describes optical microscopical and spectroscopical studies of 2D materials, including graphite/graphene and multilayer/single layer MoS2, under strong charge transfer doping. Under this conceptually unifying umbrella lie many aspects of materials behaviors unique to each of the systems. The strong chemical doping results from intercalation and surface adsorption, and changes the electronic properties of the host 2D materials drastically. Associated with the significant electronic change, aspects such as mass transport, surface reaction, and phase transformation are covered in the following chapters. The first chapter introduces representative members of the 2D materials family, graphene and molybdenum disulphide (MoS2). It briefly reviews the history, discovery and unique properties of each materials class. The other part of the introduction focuses on the main methods utilized in the study of these materials. A concise survey of Raman spectroscopy and optical reflectance contrast spectroscopy will be presented. The second chapter investigates the intercalation process of Li into bulk graphite. This is a revisit of an extensively studied subject, with a new set of experiments and theories. Here we show that the daunting technical difficulties of disentangling complex electrochemical systems can be cleanly addressed with optical methods with well defined samples. Measuring and understanding the intrinsic transport of Li in graphite electrodes has been a difficult task. The challenge is well recognized to stem from a multitude of simultaneous electrochemical processes as well as systematic heterogeneities in the sample. We distinguish the Li intercalation process in graphite from all other processes, combining optical reflectance microscopy and Raman spectroscopy. The heterogeneity problem is circumvented by using lithography to tailor a single crystal into a defined geometry. We apply two levels of theoretical models to interpret the intrinsic information revealed in our data. Concentration dependent diffusion coefficients are measured, in agreement with theoretical results. The effects of sample geometry and electrode reaction kinetics on the overall intercalation are elucidated. The third chapter presents the study of lithiation on single and few layer graphene. Raman spectroscopy reveals a high doping level similar in strength to that of the bulk intercalated compound. The optical reflectance imaging, however, shows a different observation from the bulk case. We directly visualize the surface film formation and associated strong doping. The lithiation in single and few layer graphene progresses differently from the bulk graphite, since certain stages of the intercalation compound cannot be sustained by a single or few layer sample. The realization of strong charge transfer doping in lithiated single and few layer graphene could lead to discoveries of interesting physics. The direct visualization of surface film formation could have important implications in the design of electrochemical energy storage systems. The fourth chapter explores the structural effect of strong charge transfer doping in bulk and multilayer MoS2 with optical methods. MoS2, as a representative material of the transition metal dichalcogenide family, possesses different structural polymorphs. Strong charge transfer doping induces a structural phase change, which goes from the usual thermodynamically stable semiconducting 2H phase into the metallic 1T/1T' phase. The metallic 1T/1T' structure can remain a metastable phase without the stabilization of intercalants. We optically induce the 1T/1T' to 2H phase change and measure the temperature dependent kinetics of the structural phase transformation with in situ Raman spectroscopy. We demonstrate a photolithography technique, which efficiently patterns in-plane coherent heterojunctions between 1T/1T' and 2H MoS2. The fifth and final chapter describes the study of the structural change in single layer MoS2. More spectroscopic methods are employed for characterization, such as photoluminescence spectroscopy and second harmonic generation. The results indicate that the structural change occurs in single layer MoS2 after reaction with n-butyl lithium. The structural change can be reversed by thermal and laser annealing, similar to the case of bulk and multilayer MoS2. The annealed MoS2 exhibits reduced crystallinity. Future directions to further this work are outlined in the last section.

In this thesis, the adsorption of single-wall carbon nanotubes (SWCNTs) at the liquid/liquid interface, and the subsequent electrochemical investigation of the electrical properties of the adsorbed nanotubes have been studied. Prior to the adsorption of the nanotube, the stability of dispersion of SWCNTs in non-aqueous solvents was assessed by determining the onset of aggregation of the SWCNTs when organic electrolyte was introduced. It was found that electrostatic repulsion between the SWCNTs contributes significantly to the stability of the SWCNTs in non-aqueous solvents. Similar result was also found when the aggregation kinetics of molybdenum disulphide (MoS2) dispersion in non-aqueous media was studied using the same organic electrolyte. The formation of nanomaterial-polymer composites by deliberate electrochemical oxidation of pyrrole and the sonochemical polymerisation of the organic solvent was also studied. Electrolyte addition was shown to be a promising way to separate the 2D material from the sonopolymer.

The production of cost-effective catalysts for the production of hydrogen by electrolysis of water is important for clean energy production. In this work we report on a study of molybdenum disulfide (MoS2) as catalyst for the hydrogen evolution reaction (HER). Nitrogen doped carbon nanotubes (NCNTs) directly synthesized onto carbon paper have been decorated with MoS2. The electrodes utilize the improved conductivity of the NCNTs and the carbon paper for electron transport, combined with the high catalytic activity of MoS2. The NCNTs were successfully decorated with co-axial nano-flakes of MoS2 by a single step solvothermal process using Dimethylformamide (DMF) and ammonium tetrathiomolybdate. MoS2 was also prepared with alternative methods for comparison. The effects of supporting MoS2 on NCNTs were studied by simulations with density functional theory (DFT). The most active adsorption sites for hydrogen on MoS2 were identified and were on the edges. The catalyst showed competitive activity with other earth-abun- dant catalysts with an onset potential of 170 mV and a small Tafel slope of 40 mV/dec. The improved catalytic activity of HER by having NCNTs as support was confirmed by DFT and experimental results.

In order to obtain diamond-like carbon (DLC) thin films with improved mechanical, tribological, thermal and corrosion properties for practical applications, the structure and properties of various DLC thin films including hydrogen-free DLC, hydrogenated DLC, and DLC-MoS2 composites synthesized under different conditions were investigated in this thesis. The research methodologies and the main results are summarized in following paragraphs. Hydrogen-free DLC thin films were synthesized by biased target ion beam deposition (BTIBD) method, while hydrogenated DLC thin films were deposited by ion beam deposition technique using a Kaufman-type ion source and an end-Hall ion source. DLC-MoS2 composite thin films were also synthesized using BTIBD technique in which MoS2 was produced by sputtering a MoS2 target while DLC was simultaneously deposited by ion beam deposition. The influence of processing parameters on the bonding structure, morphology and properties of the deposited films was investigated using atomic force microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, synchrotron based near edge X-ray absorption fine structure spectroscopy, X-ray diffraction, scanning electron microscopy, nanoindentation, ball-on-disk and corrosion testing. Finally, the influence of annealing temperature on the structure and properties of pure DLC and DLC-MoS2 composite films in ambient air and low pressure environments was studied. In the case of BTIBD method, hydrogen-free DLC thin films with exceptionally high smoothness and low friction coefficient were prepared by biased target sputtering of graphite target without additional ion bombardment either by negative bias of substrate or assisting ion source. For ion beam deposition technique with Kaufman ion source, the DLC thin films synthesized at ion energies of 300 eV showed the highest sp3 content and optimum properties. Regarding end-Hall ion source, the best properties achieved in DLC films synthesized at ion energies of 100 eV. Comparing with pure DLC and pure MoS2 films, the DLC-MoS2 films deposited at low biasing voltages showed better tribological properties including lower coefficient of friction and wear coefficient in ambient air environment. Also, comparing with pure DLC films, the DLC-MoS2 thin films showed a slower rate of graphitization and higher structure stability throughout the range of annealing temperatures, indicating a relatively higher thermal stability.

Lithium-sulfur (Li-S) batteries are faced with practical drawbacks of poor cycle life and low charge efficiency which hinder their advancements. Those drawbacks are primarily caused by the intrinsic issues of the cathodes (sulfur) and the anodes (Li metal). In attempt to resolve the issues found on the cathodes, this work discusses the method to prepare a binder-free three-dimensional carbon nanotubes-sulfur (3D CNTs-S) composite cathode by a facile and a scalable approach. Here, the 3D structure of CNTs serves as a conducting network to accommodate high loading amounts of active sulfur material. The efficient electron pathway and the short Li ions (Li+) diffusion length provided by the 3D CNTs offset the insulating properties of sulfur. As a result, high areal and specific capacities of 8.8 mAh cm−2 and 1068 mAh g−1, respectively, with the sulfur loading of 8.33 mg cm−2 are demonstrated; furthermore, the cells operated at a current density of 1.4 mA cm−2 (0.1 C) for up to 150 cycles. To address the issues existing on the anode part of Li-S batteries, this work also covers the novel approach to protect a Li metal anode with a thin layer of two-dimensional molybdenum disulfide (MoS2). With the protective layer of MoS2 preventing the growth of Li dendrites, stable Li electrodeposition is realized at the current density of 10 mA cm−2; also, the MoS2 protected anode demonstrates over 300% longer cycle life than the unprotected counterpart. Moreover, the MoS2 layer prevents polysulfides from corroding the anode while facilitating a reversible utilization of active materials without decomposing the electrolyte. Therefore, the MoS2 protected anode enables a stable cycle life of over 500 cycles at 0.5 C with the high sulfur loading amount of ~7 mg cm−2 (~67 wt% S content in cathode) under the low electrolyte/sulfur (E/S) ratio of 6 μL mg−1. This translates to the specific energy and power densities of ~550 Wh kg-1 and ~300 W kg−1, respectively. Additionally, such values far exceed the electrochemical performance of the current Li-ion batteries. Therefore, the synergetic effect of utilizing the 3D CNT-S cathode and the MoS2 protected Li anode will allow the Li-S batteries to become applicable for the transportation and the large-scale energy grid applications.

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A dissertation submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, in fulfilment of the requirements for the degree of Master of Science in Engineering. Johannesburg, 21 July 2016 / The use of supercapacitors for energy storage is an attractive approach considering their ability to deliver high levels of electrical power, unlimited charge/discharge cycles, green environmental protection and long operating lifetimes. Despite the satisfactory power density, supercapacitors are yet to match the energy densities of batteries and fuel cells, reducing the competitiveness as a revolutionary energy storage device. Therefore, the biggest challenge for supercapacitors is the trade-off between energy density and power density. This presents an opportunity to enhance the electrochemical capacitance and mechanical stability of an electrode. Previous attempts to get around the problem include developing porous nanostructured electrodes with extremely large effective areas. One of the emerging high-power supercapacitor electrode materials is molybdenum disulfide (MoS2), a member of the transition-metal dichalcogenides (TMDs). Its higher intrinsic fast ionic conductivity and higher theoretical capacity have attracted a lot of attention, particularly in supercapacitors. In addition to double-layer capacitance, diffusion of the ions into the MoS2 at slow scan rates gives rise to Faradaic capacitance. Analogous to Ru in RuO2, the Mo center atom displays a range of oxidation states from +2 to +6. This plays an important role in enhancing charge storage capabilities. However, the electronic conductivity of MoS2 is still lower compared to graphite, and the specific capacitance of MoS2 is still very limited when used alone for energy storage applications. As evident in several literature reports, there is a need to improve the capacitance of MoS2 with conductive materials such as carbon nanotubes (CNT), polyaniline (PANI), polypyrrole (PPy), and reduced graphene (r-GO). Carbon nanospheres (CNS) have, in the past, improved the conductivity of cathode material in Li-ion batteries, owing to their appealing electrical properties, chemical stability and high surface area. The main objective of this dissertation research is to develop nanocomposite materials based on molybdenum sulphide with carbon nanospheres for pseudocapacitors with simultaneously high power density and energy density at low production cost. The research was carried out in two phases, namely, (i) Symmetric pseudocapacitors based on molybdenum disulfide (MoS2)-modified carbon nanospheres: Correlating physico-chemistry and synergistic interaction on energy storage and (ii) The effects of morphology re-arrangements on the pseudocapacitive properties of mesoporous molybdenum disulfide (MoS2) nanoflakes. The physico-chemical properties of the MoS2 layered materials have been interrogated using the surface area analysis (BET), scanning electron microscopy (SEM), transmission electron microscopy (TEM), x-ray diffraction (XRD), Raman, fourier-transform infrared (FTIR) spectroscopy, and advanced electrochemistry including cyclic voltammetry (CV), galvanostatic cycling with potential limitation (GCPL), repetitive electrochemical cycling tests, and electrochemical impedance spectroscopy (EIS). In the first phase, Molybdenum disulfide-modified carbon nanospheres (MoS2/CNS) with two different morphologies (spherical and flower-like) have been synthesized using hydrothermal techniques and investigated as symmetric pseudocapacitors in aqueous electrolyte. The two different MoS2/CNS layered materials exhibit unique differences in morphology, surface areas, and structural parameters, which have been correlated with their electrochemical capacitive properties. The flower-like morphology (f-MoS2/CNS) shows lattice expansion (XRD), large surface area (BET analysis), and small-sized nanostructures (corroborated by the larger FWHM of the Raman and XRD data). As a contrast to the f-MoS2/CNS, the spherical morphology (s-MoS2/CNS) shows lattice contraction, small surface area with relatively large-sized nanostructures. The presence of CNS on the MoS2 structure leads to slight softening of the characteristic Raman bands (E12g and A1g modes) with larger FWHM. The MoS2 and its CNS-based composites have been tested in symmetric electrochemical capacitors in aqueous 1 M Na2SO4 solution. CNS improves the conductivity of the MoS2 and synergistically enhanced the electrochemical capacitive properties of the materials, especially the f-MoS2/CNS-based symmetric cells (most notably, in terms of capacitance retention). The maximum specific capacitance for f-MoS2/CNS-based pseudocapacitor show a maximum capacitance of 231 F g-1 with high energy density 26 Wh kg-1 and power density 6443 W kg-1. For the s-MoS2/CNS-based pseudocapcitor, the equivalent values are 108 F g-1, 7.4 Wh kg-1 and 3700 W kg-1. The high-performance of the f-MoS2/CNS is consistent with its physico-chemical properties as determined by the spectroscopic and microscopic data. In the second phase, Mesoporous molybdenum disulfide (MoS2) with different morphologies has been prepared via a hydrothermal method using different solvents, water or water/acetone mixtures. The MoS2 obtained with water alone gave graphene-like nanoflakes (g-MoS2) while the other with water/acetone (1:1 ratio) gave a hollow-like morphology (h-MoS2). Both materials are modified with carbon nanospheres as conductive materials and investigated as symmetric pseudocapacitors in aqueous electrolyte (1 M Na2SO4 solution). Interestingly, a simple change of synthesis solvents confers on the MoS2 materials different morphologies, surface areas, and structural parameters, correlated by electrochemical capacitive properties. The g-MoS2 exhibits higher surface area, higher capacitance parameters (specific capacitance of 183 F g-1, maximum energy density of 9.2 Wh kg-1 and power density of 2.9 kW kg-1) but less stable electrochemical cycling compared to the h-MoS2. These findings have opened doors for further exploration of the synergistic effects between MoS2 graphene-like sheets and CNS for energy storage. / MT2017

My work presents a novel approach to fabricate binder free three-dimensional carbon nanotubes/sulfur (3DCNTs/S) hybrid composite by a facile and scalable method increasing the loading amount from 1.86 to 8.33 mg/cm2 highest reported to date with excellent electrochemical performance exhibiting maximum specific energy of ~1233Wh/kg and specific power of ~476W/kg, with respect to the mass of the cathode. Such an excellent performance is attributed to the fact that 3DCNTs offers higher loading amount of sulfur, and confine polysulfide within the structure. In second part of the thesis, molybdenum disulfide (MoS2) is typically studied for three electrochemical energy storage devices including supercapacitors, Li-ion batteries, and hybrid Li-ion capacitors. The intrinsic sheet like morphology of MoS2 provides high surface area for double layer charge storage and a layered structure for efficient intercalation of H+/ Li+ ions. My work demonstrates the electrochemical analysis of MoS2 grown on different substrates including copper (conducting), and carbon nanotubes. MoS2 film on copper was investigated as a supercapacitor electrode in three electrode system exhibiting excellent volumetric capacitance of ~330F/cm3 along with high volumetric power and energy density in the range of 40-80 W/cm3 and 1.6-2.4 mWh/cm3, respectively. Furthermore, we have developed novel binder-free 3DCNTs/ MoS2 as an anode materials in half cell Li-ion batteries. The vertically oriented morphology of MoS2 offers high surface area and active electrochemical sites for efficient intercalation of Li+ ions and demonstrating excellent electrochemical performance with high specific capacity and cycling stability. This 3DCNTs/ MoS2 anode was coupled with high surface area southern yellow pine derived activated carbon (SYAC) cathode to obtain hybrid 3DCNTs/ MoS2 || SYAC Li-ion capacitor (LIC), which delivers large operating voltage window of 1-4.0V with excellent cycling stability exhibiting capacitance retention of ~80% after 5000 cycles.