Technological progress is determined to a great extent by developments of novel materials from new combinations of known substances with different dimensionality and functionality. We investigate the development of 3D ‘hybrid’ nanomaterials by utilizing graphene based systems coupled with transition metal oxides (e.g. manganese oxides MnO2 and Mn3O4). This lays the groundwork for high performance electrochemical electrodes for alternative energy owing to their higher specific capacitance, wide operational window and stability through charge-discharge cycling, environmental benignity, cost effective, easily processed, and reproducible in a larger scale. Thus far, very few people have investigated the potential of combining carbon sheets that can function as a supercapacitor in certain systems with transition metals that have faradaic properties to create electrochemical capacitors. Previous work by Wang et al. has focused on the structural combination of Mn3O4 and graphene based materials,1 and research by Jafta et al. studied the electrochemical properties of MnO2 with GO.2 We find that both physical and chemical attachment of manganese oxide on graphene allows for electrical interplay of the materials as indicated in electrochemical analysis and Raman spectroscopy. Attachment of the two materials is also characterized by scanning electron microscopy and X-ray diffraction.

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>Magister Scientiae - MSc / Spinel lithium manganese oxide (LiMn2O4), for its low cost, easy preparation and nontoxicity, is regarded as a promising cathode material for lithium-ion batteries. However, a key problem prohibiting it from large scale commercialization is its severe capacity fading during cycling. The improvement of electrochemical cycling stability is greatly attributed to the suppression of Jahn-Teller distortion (Robertson et al., 1997) at the surface of the spinel LiMn2O4 particles. These side reactions result in Mn2+ dissolution mainly at the surface of the cathode during cycling, therefore surface modification of the cathode is deemed an effective way to reduce side reactions. The utilization of a nanocomposite which comprises of metallic Cu and Au were of interest because their oxidation gives rise to a variety of catalytically active configurations which advances the electrochemical property of Li-ion battery. In this research study, an experimental strategy based on doping the LiMn2O4 with small amounts of Cu-Au nanocomposite cations for substituting the Mn3+ ions, responsible for disproportionation, was employed in order to increase conductivity, improve structural stability and cycle life during successive charge and discharge cycles. The spinel cathode material was synthesized by coprecipitation method from a reaction of lithium hydroxide and manganese acetate using 1:2 ratio. The Cu-Au nanocomposite was synthesized via a chemical reduction method using copper acetate and gold acetate in a 1:3 ratio. Powder samples of LiMxMn2O4 (M = Cu-Au nanocomposite) was prepared from a mixture of stoichiometric amounts of Cu-Au nanocomposite and LiMn2O4 precursor. The novel LiMxMn2O4 material has a larger surface area which increases the Li+ diffusion coefficient and reduces the volumetric changes and lattice stresses caused by repeated Li+ insertion and expulsion. Structural and morphological sample analysis revealed that the modified cathode material have good crystallinity and well dispersed particles. These results corroborated the electrochemical behaviour of LiMxMn2O4 examined by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The diffusion coefficients for LiMn2O4 and LiMxMn2-xO4 obtained are 1.90 x10-3 cm2 / s and 6.09 x10-3 cm2 / s respectively which proved that the Cu-Au nanocomposite with energy band gap of 2.28 eV, effectively improved the electrochemical property. The charge / discharge value obtained from integrating the area under the curve of the oxidation peak and reduction peak for LiMxMn2-xO4 was 263.16 and 153.61 mAh / g compared to 239.16 mAh / g and 120 mAh / g for LiMn2O4. It is demonstrated that the presence of Cu-Au nanocomposite reduced side reactions and effectively improved the electrochemical performance of LiMn2O4.

>Magister Scientiae - MSc / A study on carbon paste electrode (CPE) materials containing 1-methyl-3-octylimidazolium bis (trifluoromethylsulfonyl) imide [MOIM[Tƒ2N] – a hydrophobic room temperature ionic liquid (IL) - is reported. CPEs with (a) the IL as the only binder (ILCPE) and (b) 1:1 (v/v) IL: paraffin mixture as the binder (ILPCPE) were prepared, characterized, and applied to the electrodeposition of films of multivalent transition metal oxides (MV-TMO) from five precursor ions (Fe2+, Mn2+, Cu2+, Co2+, Ce4+) in aq. KCl. Cyclic voltammetry (CV) showed a potential window of +1.5 V to -1.8 V regardless of the electrode type, including the traditional paraffin CP electrode (PCPE). However, the IL increased the background current by 100-folds relative to paraffin. The electrochemical impedance spectroscopy (EIS) of ILPCPE in aq. KCl (0.1M) revealed two phase angle maxima in contrast with the single maxima for PCPE and ILCP. The study also included the CV and EIS investigation of the electrode kinetics of the Fe(CN)6 3-/4 redox system at these electrodes. The electrodeposition of Fe2+, Co2+, and Mn2+ possibly in the form of the MV-TMOs FexOy, CoxOy, and MnxOy, respectively, onto the electrodes was confirmed by the observation of new and stable cathodic and anodic peaks in a fresh precursor ion –free medium. CVs of H2O2 as a redox probe supported the same conclusions. Both ATR-FTIR spectra and SEM image of surface samples confirmed the formation of electrodeposited films. This study demonstrated that the use of this hydrophobic IL alone or in combination with paraffin as a binder gives viable alternative CPE materials with better performance for the electrodeposition of MV-TMOs films than the paraffin CPE. Thus, in combination with the easy preparation methods and physical “morpheability” in to any shape, these CPEs are potentially more useful in electrochemical technologies based on high surface-area MV-TMO films in general, and MnxOy films in particular.

The depletion of natural resources has long been a concern since the rapid increase in energy consumption in recent years. The gradual increase of pollution worldwide accompanied by energy generation process also started to post threats to the environment. With the evolution of technology and materials, power generation and energy storage with significant improved efficiency can be made possible, and further benefits the reduction of degree of pollution generated. In this research, synthesis, processing, characterization and application of nano materials towards energy generation and energy storage devices are studied. In chapter 2, superior corrosion resistance properties of HVOF thermal spray of TiC and TiB2 coatings on 304H stainless steel, 430, and P91 steels were reported. The coatings successfully served at a protection layer by limiting oxygen penetration, sulfur attack, and decreased the formation of pits and cracks on the substrates at 750°C for up to 800 hours. In chapter 3, continuous smooth TiC nanofibers were successfully synthesized by carbothermal reduction of electrospun titanium based nanofibers. XRD and HR-TEM analysis results indicated the synthesized nanofibers were composed of high purity TiC. Electric conductivity of a single fiber was in the 2.00×10^5 range. Symmetrical cyclic voltammetry curve further indicated good electrochemical properties of the fibers. In addition, the TiC nanofibers also exhibited excellent sintering properties over TiC or TiB2 nanoparticles. Studies on morphology and electrochemical properties of MnOx nanofiber and nanoparticles is reported in chapter 4. MnOx, MnOx/SnO2, and MnOx/CNT nanofibers synthesized using electrospinning method showed specific capacitance of 166.12 F/g, 182 F/g for, and 472 F/g at scan rate of 10mV/s. Analysis results also showed positive impact of conductivity and fiber morphology on the electrochemical properties of the fibers. morphology and electrochemical properties of the MnOx nanoparticles synthesized using solvents with different polarity with gelation pH of 8.5, 9.0 and 10.0 were also studied. Analysis results show the impact of particle sized and morphology on the electrochemical properties. Highest specific capacitance measured for the synthesized nanoparticles was 231.38F/g@10mV/s and 165.13F/g@10mV/s for methanol and mixture of methanol and propanol based MnOx respectively. Effect of solvent polarity of the manganese sol on MnOx formation and phase transformation temperature is also shown in the chapter.

<p> Cathodic electrodeposition method has been developed for the fabrication of manganese oxide films for application in electrochemical supercapacitors (ES). The manganese oxide films prepared from KMn04 and NaMn04 aqueous solution showed an increasing deposition yield with the deposition time. The deposition rate decreases with increasing the concentration of deposition precursor. The obtained films were characterized using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction analysis (XRD), thermogravimetric and differential thermal analysis (TGA/DTA). The SEM observations revealed uniform films of highly porous nanostructure on different substrates. The capacitive behavior of the deposits was investigated by cyclic voltammetry and chronopotentiometry method in 0.1M NaS04 aqueous solutions. As prepared deposits exhibited pseudocapacitive behavior in the potential window of 0-1.0 V versus SCE with excellent cyclability. A maximum specific capacitance (SC) of 353 Fig was obtained for the 45 μg/cm2 film deposited from KMn04 solution on stainless steel foil, at a scan rate of 2 m V /s in the 0.1 M Na2S04 solution. It was found that the SC decreased with increasing deposit thickness and scan rate. No significant effect was obtained on the films prepared from 20 mM KMn04 on stainless steel after heat treatment at various temperatures. The capacitance of as-prepared electrode did not change by changing the electrolyte from Na2S04 to K2S04 solutions. However, higher capacitance values were observed by using electrolyte with higher concentration. Different structures of manganese oxides were obtained when different deposition precursors were used. No significant difference in capacitive behavior was found between the films prepared from different deposition precursor. However it was concluded that conductivity of the film is key in determining the performance of the electrodes. The effect of substrates on the electrochemical behavior has also been investigated by using stainless steel and nickel foils. </p> / Thesis / Master of Applied Science (MASc)

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In this research, manganese oxide based nanoparticles were synthesized by sol-gel process. Methanol, ethanol, and propanol were used as alternative solvents during sol-gel process with manganese acetate as precursor for the preparation of pristine manganese oxide. Hybrid manganese oxide modified by additions of carbon nanotubes was further prepared. The effects of different solutions and heat treatment temperatures on the morphology, physical characteristics, and electrochemical properties of the manganese oxide based materials were investigated. Particle size of pristine manganese oxide samples prepared from methanol, ethanol, and propanol were compared by SEM and TEM image analysis. Smallest particle size was observed for manganese oxide prepared from propanol, with diameters range from 16 nm to 50nm. XRD results showed that the as-prepared manganese oxide based samples treated at calcination temperature of 300ºC and above were composed of Mn2O3 as dominant phase, with Mn3O4 as minor phase. Specific capacitance of manganese oxide prepared from methanol, ethanol, and propanol at scan rate of 10 mV/s measured using two electrode systems were 88.3, 66.0, and 104.8 F/g, respectively and that for the hybrid sample was 140.5 F/g. Results from electrochemical impedance spectroscopy (EIS) also showed superior electrochemical properties of the hybrid sample over pristine manganese oxide samples. It is evident that the addition of carbon nanotubes not only improved the specific capacitance but also the overall electrochemical properties of the manganese oxide supercapacitor.

<p> Electrochemical supercapacitors (ES) are urgently needed as components in many advanced power systems. The development of advanced ES is expected to enable radical innovation in the area of hybrid vehicles and electronic devices. Nanostructured manganese oxides in amorphous or various crystalline forms have been found to be promising electrode materials for ES. The use of composite electrodes of manganese oxide with carbon nanotubes is being proposed to improve the overall electrochemical performance of the ES.</p> <p> Electrodeposition methods have been developed for the fabrication of manganese oxide films with/without carbon nanotubes for applications in ES. Electrolytic deposition of manganese oxides was found to be possible using Mn2+ and Mn7+ species, co-deposition of multi wall carbon nanotubes (MWNT) and manganese oxide using cathodic electrosynthesis was successfully achieved.</p> <p> Novel chemical process has been developed for the synthesis of nano-size manganese oxide particles. Electrophoretic deposition of the nano-size manganese oxide particles was able to be performed in both aqueous and non-aqueous solutions. Electrophoretic co-deposition of the nano-size manganese oxide particles with carbon nanotubes was successfully achieved.</p> <p> The mechanisms and kinetics of all the deposition methods are discussed. Charge storage properties of the films prepared by different deposition methods are investigated and compared.</p> / Thesis / Master of Applied Science (MASc)

Chapter 1 Dynamic Processes Occurring at the Cr^III_aq – Manganite (γ-MnOOH) Interface: Simultaneous Adsorption, Microprecipitation, Oxidation/Reduction and Dissolution The complex interaction between Cr^III_aq and manganite (γ-MnOOH) was systematically studied at room temperature over a pH range of 3 to 6, and within a concentration range of 10⁻⁴ to 10⁻² M CrOH²⁺_aq. Solution compositional changes during batch reactions were characterized by ICP and UVvis. The manganites were characterized before and after reaction with XPS, SEM, high-resolution FESEM, and EDS analysis. Fluid-cell AFM was used to follow these metal-mineral interactions in situ. The reactions are characterized by 1) sorption of Cr^III and the surface-catalyzed microprecipitation of Cr^III-hydroxy hydrate on manganite surfaces, 2) the acidic dissolution of the manganite, and 3) the simultaneous reductive dissolution of manganite coupled with the oxidation of Cr^III_aq to highly toxic Cr^VI_aq. Cr^III-hydroxy hydrate was shown to precipitate on the manganite surface while still undersaturated in bulk solution. The rate of manganite dissolution increased with decreasing pH due both to faster acid-promoted and Mn-reduction- promoted dissolution. Due to direct redox coupling with Mn reduction, Cr oxidation was most rapid in the lower pH range. Neither Mn^II nor Cr^VI were ever detected on manganite surfaces, even at the maximum rate of their generation. At the highest pHs of this study, Cr^III_aq was effectively removed from solution to form Cr^III-hydroxy hydrate on manganite surfaces and in the bulk solution, and manganite dissolution and Cr^VI_aq generation were minimized. All interface reactions described above were heterogeneous across the manganite surfaces. This heterogeneity is a direct result of the heterogeneous semiconducting nature of natural manganite crystals, and is also an expression of the proximity effect, whereby redox processes on semiconducting surfaces are not limited to next nearest neighbor sites. Chapter 2 Comparison of the Reactivity of Various Mn-Oxides with Cr^III_aq: Microscopic and Spectroscopic Observations of Dissolution, Cr-sorption and Cr and Mn Redox Interactions The interaction between Cr^III_aq and seven different Mn-oxides (6 monomineralic, 1 synthetic) have been observed in pH ~4.4 HNO₃ and pH ~4.4 ~10⁴ M Cr^III_aq solutions. For each mineral-solution interaction, the aqueous chemical concentrations (e.g. [Mn]_aq, [Cr]_aq, [Cr^VI_aq]) were measured with time. Reacted samples were examined by XPS to determine if, and to what extent, the surface chemical states of Cr, Mn and O had changed. Microscopic observations of the reacted surfaces were obtained using AFM and high-resolution, low-voltage FESEM. The solubility of the Mn-oxides in the acidic, non-Cr bearing solutions varied inversely with the average Mn valence, but did not show systematic behavior with respect to the mineral structure type (e.g. tunnel, layer, framework). This trend was interpreted as resulting from the relative ability of an adsorbed proton to polarize surface Mn-O bonds, with the polarizability being in the order Mn²⁺-O > Mn³⁺-O > Mn⁴⁺-O. For samples reacted with Cr^III_aq, the rate and extent of reductive dissolution was always greater than for acidic dissolution during the initial time period. The measured ratios of the [Mn]_aq : [Cr^VI]_aq were approximately in agreement with the values expected from the proposed stoichiometric reactions. Cr-uptake was observed to occur in undersaturated solutions as a result of adsorption, absorption and surface catalyzed precipitation. The chromium as detected by XPS was predominately Cr^III, however pyrolusite contained both Cr^III and Cr^VI. Previous studies have implicated a chromium surface precipitate to be responsible for the cessation of the Cr^III_aq oxidation reaction. Our surface sensitive FESEM and AFM observations tend to suggest that Cr-uptake is by isolated site binding, very small (<30 nm) surface clusters or monolayer scale films. Cr-uptake was followed by slow Cr-release on several of the solids (particularly the layered solids) after a substantial portion of the total aqueous Cr had been converted to Cr^VI_aq. The oxidizing ability of the different Mn-oxides for Cr^III_aq is evaluated with regards to the energy level of the redox couple (i.e. the redox potential) as compared with the Fermi energy level of the Mn-oxide. Although these energies were calculated rather than directly measured, the results indicate that electrons originating from adsorbed Cr^III ions may be transferred into the conduction band or more likely, into available surface states. The presence of an initial limited quantity of electron accepting surface states likely explains the observation of a rapid initial Cr^III-oxidation followed by much slower oxidation. The Mn-oxides that exhibited the greatest and longest lasting Cr^III-oxidizing power were the Mn-oxides containing Mn⁺, and in particular those containing Mn³⁺ and Mn⁺. It is believed that the combined presence of a reducible Mn ion (e.g. Mn³⁺) and a highly soluble Mn⁺ ion facilitates a sustained Cr^III-oxidation reaction because fresh surface is exposed during the reaction. / Ph. D.