Browsing by Author "Khotseng, L."

Now showing 1 - 7 of 7
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    Advanced materials on the basis of nanostructured catalysed magnesium hydride for hydrogen storage
    (University of the Western Cape, 2019) Goh, Jonathan Teik Ean; Lototskyy, Mykhaylo; Yartys, V; Khotseng, L.
    Magnesium hydride has long been regarded as a promising candidate for lightweight hydrogen storage applications, owing to reasonably high theoretical capacity (7.6 wt. %). It is burdened by slow absorption/desorption kinetics which has been the target for improvement of many research groups over the years. Nanostructured MgH2 prepared by high energy reactive ball milling (HRBM) of Mg under hydrogen atmosphere with the addition of V or Ti results in modified MgH2 that demonstrates superior hydrogenation/dehydrogenation kinetics without a crippling compromise in storage capacity. Mg – FeV nanocomposites prepared via ball milling of Mg and FeV raw materials demonstrated up to 96.4% of the theoretical storage capacity and comparable kinetics to Mg - V prepared via the same method using pure refined V (which is far costlier than FeV). In both cases, the hydrogenation/dehydrogenation kinetics was much improved than pure Mg alone, as evidenced by faster hydrogenation times. In terms of cyclic stability, Mg – 10FeV demonstrated improvement over pure Mg with final absorption and desorption capacities of 4.93 ± 0.02 wt. % and 4.82 ± 0.02 wt. % respectively over 30 cycles. When compared against Mg – V, Mg – FeV showed slightly inferior improvements, attributed to incomplete hydrogenation of V in the presence of Fe. However, they share similar crystalline BCC, BCT – V2H and FCC - VH phases with the size of less than 10 nm and demonstrated the same behaviour at high temperatures; at temperatures approaching 400 °C, particle sintering became an issue for both nanocomposites resulting in a drop in absorption capacity even in the first cycle. The further inclusion of carbonaceous species showed several effects, one of which was an improvement in hydrogen uptake speed as well as kinetics for the addition of 5 wt. % activated carbon. For the sample with 5 wt. % graphite, the appearance of an initial incubation period of up to 60 minutes was noted, presumably corresponding to the duration of time when the carbon was sheared and crushed before hydrogenation commences.
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    Advanced oxygen reduction reaction catalysts/material for direct methanol fuel cell (dmfc) application
    (University of the Western Cape, 2014) Motsoeneng, Rapelang Gloria; Khotseng, L.; Modibedi, R.M.
    Fuel cells are widely considered to be efficient and non-polluting power source offering much higher energy density. This study is aimed at developing oxygen reduction reactions (ORR) catalysts with reduced platinum (Pt) loading. In order to achieve this aim, monometallic Pd and Pt nanostructured catalysts were electrodeposited on a substrate (carbon paper) by surface limited redox replacement using electrochemical atomic layer deposition (ECALD) technique. Pd:Pt bimetallic nanocatalysts were also deposited on carbon paper. Pd:Pt ratios were (1:1, 2.1 and 3:1). The prepared mono and bimetallic catalysts were characterized using electrochemical methods for the ORR in acid electrolyte. The electrochemical characterization of these catalysts includes: Cyclic Voltammetry (CV) and linear sweep voltammetry (LSV). The physical characterization includes: scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) for Morphology and elemental composition, respectively. The deposition of copper (Cu) on carbon paper was done by applying a potential of -0.05 V at 60s, 90s and 120s. 8x cycles of Pt or Pd showed better electrochemical activity towards hydrogen oxidation reaction. Multiples of eight were used in this work to deposit Pt: Pd binary catalyst. Cyclic voltammetry showed high electroactive surface area for Pt24Pd24/Carbon-paper while LSV showed high current density and positive onset potential. HRSEM also displayed small particle size compared to other Pt:Pd ratios.
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    Characterization of platinum-group metal nanophase electrocatalysts employed in the direct methanol fuel cell and solid-polymer electrolyte electrolyser
    (University of the Western Cape, 2005) Williams, Mario; Linkov, V.; Khotseng, L.; Dept. of Chemistry; Faculty of Science
    Characterization of nanophase electrocatalysts, which are an essential part in the direct methanol fuel cell (DMFC) and solid-polymer electrolyte (SPE) electrolyser, have been studied in this work. Their nanoparticulate size raises significant challenges in the analytical techniques used in their structural and chemical characterization. Hence, the applicability of analytical protocols for the qualitative and quantitative characterization of structural and chemical properties of nanophase platinum and platinum-ruthenium electrocatalysts was investigated. Also, fabricated carbon-supported platinum, platinum-ruthenium, iridium oxide, and mesoporous silica-templated platinum electrocatalysts were screened on the basis of their electrocatalytic activity. A set of structural and chemical parameters influencing the performance of nanophase electrocatalysts was identified. Parameters included crystallinity, particle size, particle size distribution, agglomeration, aggregation, surface area, thermal stability, chemical speciation, electrocatalytic activity, and electrochemically-active surface area. A large range of analytical tools were employed in characterizing the electrocatalysts of interest. High accuracy and precision in the quantitative and qualitative structural characterization of nanophase electrocatalysts, collected by x-ray diffractometry and transmission electron microscopy, was demonstrated. Selected-area electron diffraction was limited to a rapid qualitative evaluation of electrocatalyst polycrystallinity and crystal symmetry. Scanning electron microscopy was limited to the qualitative evaluation of the agglomeration state of supported electrocatalysts. High-performance particle sizing was unable to resolve the particle size of the electrocatalyst from that of the support and was therefore employed in the quantitative investigation of aggregate size and size distribution in supported electrocatalysts. The technique produced high precision data illustrating the reproducibility of the aggregate size data. N2-physisorption produced surface area and pore size distribution data of high quality, but was unable to determine surface areas specific to the metal phase in supported electrocatalysts. The technique was deemed inconsistent in the accurate determination of average pore size. The resolution of scanning electrochemical microscopy and proton-induced x-ray emission spectroscopy (SECM) did not allow for an investigation of characteristics at the nanoscale. Quantitative chemical information was difficult to extract from SECM maps and the technique was limited to the qualitative characterization of surface topography. Thermogravimetry was suitable for the qualitative investigation of the thermal stability of the nanophase electrocatalysts of interest. In this study, temperature-programmed reduction was able to qualitatively speciate the surface chemical state and investigate the strength of the metal-support interaction in supported nanophase electrocatalysts. Cyclic voltammetry and linear-sweep voltammetry were employed in the electrochemical characterization of nanophase electrocatalysts and both qualitative and quantitative information were obtained. The techniques were able to discriminate between various commercial and fabricated electrocatalysts and identify new highly-active materials. Preparation variables could be critically evaluated for the fabrication of cost-effective highly-active nanophase electrocatalysts. Certain techniques were deemed to be highly applicable in discriminating between high and low activity nanophase electrocatalysts based on their structural and chemical properties. The electrocatalyst characterization strategy and methodology was developed and will be implemented for future characterization of nanophase electrocatalysts.
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    Electrochemical Characterization of Platinum based anode catalysts for Polymer Exchange Membrane Fuel Cell
    (University of the Western Cape, 2008) Gcilitshana, Oko Unathi; Khotseng, L.; Pasupathi, S.; Dept. of Chemistry; Faculty of Science
    In this study, the main objective was to investigate the tolerance of platinum based binary anode catalysts for CO poisoning from 10ppm up to1000ppm and to identify the best anode catalysts for PEMFCs that tolerates the CO fed with reformed hydrogen.
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    Electrochemical energy conversion using metal hydrides hydrogen storage materials
    (University of the Western Cape, 2010) Jonas, Ncumisa Prudence; Khotseng, L.; Lototskyy, Mykhaylo; Dept. of Chemistry; Faculty of Science
    Metal hydrides hydrogen storage materials have the ability to reversibly absorb and release large amounts of hydrogen at low temperature and pressure. In this study, metal hydride materials employed as negative electrodes in Ni-MH batteries are investigated. Attention is on AB5 alloys due to their intermediate thermodynamic properties. However, AB5 alloys a have tendency of forming oxide film on their surface which inhibits hydrogendissociation and penetration into interstitial sites leading to reduced capacity. To redeem this, the materials were micro-encapsulated by electroless deposition with immersion in Pd and Pt baths. PGMs were found to increase activation, electrochemical activity and H2 sorption kinetics of the MH alloys. Between the two catalysts the one which displayed better performance was chosen. The materials were characterized by X-ray difractommetry, and the alloys presented hexagonal CaCu 5–type structure of symmetry P6/mmm. No extra phases were found, all the modified electrodes displayed the same behavior as the parent material. No shift or change in peaks which corresponded to Pd or Pt were observed. Scanning Electron Microscopy showed surface morphology of the materials modified with Pd and Pt particles, the effect of using different reducing agents (i.e ., N2H4 and NaH2PO2), and alloys functionalized with γ-aminosopropyltrietheosilane solution prior to Pd deposition. From all the surface modified alloys, Pt and Pd particles were observed on the surface of the AB5 alloys. Surface modification without pre-functionalization had non-uniform coatings, but the pre- functionalized exhibited more uniform coatings. Energy dispersive X-ray Spectroscopy and Atomic Absorption Spectroscopy determined loading of the Pt and Pd on the surface of all the alloys, and the results were as follows: EDS ( Pt 13.41and Pd 31.08wt%), AAS (Pt 0.11 and Pd 0.78wt%). Checking effect of using different reducing agents N2H4 and NaH2PO2 for electroless Pd plating the results were as follows: EDS (AB5_N2H4_Pd- 7.57 and AB 5_NaH2PO2_Pd- 31.08wt%), AAS (AB5_N2H4_Pd- 11.27 and AB5_NaH2PO2_Pd- 0.78wt%). For the AB5 alloyspre-unctionalized with γ-APTES, the results were: EDS (10.24wt%) and AAS (0.34wt%). Electrochemical characterization was carried out by charge/discharge cycling controlled via potential to test the AB5 alloy. Overpotential for unmodified, Pt and Pd modified electrodes were-1.1V, -1.24V, and -1.60V, respectively. Both modified electrodes showed discharge overpotentials at lower values implying higher specific power for the battery in comparison with the unmodified electrodes. However, Pd electrode exhibited higher specific power than Pt. To check the effect of the reducing agent the results were as follows: AB5_ N2H4_Pd (0.4V) and AB5_NaH2PO2_Pd (-0.2V), sodium hypophosphite based alloy showing lower overpotential values, implying it had higher specific power than hydrazine based bath. Alloy pre-functionalized with γ-APTES, the overpotential was (0.28V), which was higher than -0.2V of the alloy without pre-functionalization, which means pre-functionalization with γ-APTES did not improve the performance of the alloy electrode. Polarization resistance of the electrodes was investigated with Electrochemical Impedance Spectroscopy. The unmodified alloy showed high resistance of 21.6884 while, both Pt and Pd modified electrodes exhibited decrease 14.7397 and 12.1061 respectively, showing increase in charge transfer for the modified electrodes. Investigating the effect of the reducing agent, the alloys exhibited the following results: (N2H497.8619 and NaH2PO212.1061 ) based bath. Alloy pre-functionalized with γ-APTES displayed the resistance of 9.3128. Cyclic Voltammetry was also used to study the electrochemical activity of the alloy electrodes. The voltammograms obtained displayed the anodic current peak at -0.64V to -0.65V for the Pt and Pd modified electrodes, respectively. Furthermore, the electrode which was not coated with Pt or Pd the current peak occurred at -0.59V. The Pd and Pt coated alloy electrodes represented lower discharge overpotentials, which are important to improve the battery performance. Similar results were also observed with alloy electrodes Pd modified using N2H4(-0.64V) and NaH2PO2(-0.65V). For the electrode modified with and without γ-APTES the over potentials were thesame (-0.65V). PGM deposition has shown to significantly improve activation and hydrogen sorption performance and increased the electro-catalytic activity of these alloy electrodes. Modified electrodes gave better performance than the unmodified electrodes. As a result, Pd was chosen as the better catalyst for the modification of AB5 alloy. Based on the results, it was concluded that Pd electroless plated using NaH2PO2 reducing agent had better performance than electroless plating using N2H4 as the reducing agent. Alloy electrode pre-functionalized with γ-APTES gave inconsistent results, and this phenomenon needs to be further investigated. In conclusion, the alloy modified with Pd employing NaH 2PO2 usased electroless plating bath exhibited consistent results, and was found to be suitable candidate for use in Ni-MH batteries
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    Electrochemical investigation of platinum nanoparticles supported on carbon nanotubes as cathode electrocatalysts for direct methanol fuel cell
    (University of the Western Cape, 2010) Ntlauzana, Asanda; Khotseng, L.; Ndungu, P.; Dept. of Chemistry; Faculty of Science
    The particles of the Pt metal were well dispersed on carbon nanotubes when EG was used and in isopropanol poor dispersion was observed and no further investigation was done on them. The platinum wt% on the supports observed from EDS was 21.8, 19.10 and 16.74wt% for Pt/EMWCNT, Pt/LPGCNT and Pt/ commercial CNT respectively. Pt/LPGMWCNT showed high electro-catalytic activity of 2.48 mA and active surface area of 76 m2/g, toward oxygen reduction, observed from cyclic voltammogram in iv sulfuric acid. Pt/LPGMWCNT also showed better tolerance toward methanol, however it was not highly active towards methanol, and hence the methanol oxidation peak current observed between 0.75 and 08 potential was the smallest. In this study a wide range of instruments was used to characterize the properties and behavior of Platinum nanoparticles on multi-wall carbon nanotubes. To add to the already mentioned, Scanning electrochemical microscopy (SEM), proton induced x-ray emission (PIXE), scanning electrochemical microscopy (SECM) and Brunauer-Emmett Tellar (BET) were also used.
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    Electrochemical study of electrode support material for direct methanol fuel cell applications
    (University of Western Cape, 2013) Bangisa, Andisiwe; Khotseng, L.
    This study focused on binary PtRu and PtSn electrocatalyst, synthesized using the polyol approach and supported on MWCNTs, TiO2 and MoO2 materials, after synthesis part of the resultant electrocatalyst was heat treated to improve alloying of the secondary metal to the primary platinum metal catalyst and also to enhance the stable distribution and uniform dispersion of the nanoparticles on the support material. Physical characterization of the supported catalyst was done using XRD, HRTEM, HRSEM and EDS for elemental analysis. For electrochemical characterization RDE-CV and RDE-LSV were employed. The homeprepared electro-catalysts were then compared to the Pt/C, PtRu/C and PtSn/C commercial electro-catalysts accordingly. XRD confirmed that the binary electro-catalyst for both the commercial and home-prepared display characteristic patterns similar to that of the standard Pt/C electro-catalyst, an indication that all catalysts have prevailed the Pt face-centred-cubic (fcc) crystal structure. Particle size and size distribution examined using HRTEM showed that Pt/C and PtSn/C was uniformly dispersed on the carbon support and that all electrocatalyst supported on MWCNTs showed small particle size known to enhance the activity of the catalyst. However, after heattreatment the particle size increased for all prepared supported electrocatalyst as was expected from literature. SEM micrographs showed that all electrocatalyst were decorated on the support material with agglomerates on some parts of the samples, agglomeration was more pronounced for catalysts supported on MoO2. The metal loading for the home- prepared electrocatalyst was examined using EDS and it was observed to be closer to that of the commercial catalysts. It was also observed that there were changes on the loading of the electrocatalysts after they were subjected to heat treatment and depending on the support material the metal loading of the catalyst was either more or less. This study found PtSn/C to be the most active commercial catalyst for methanol tolerant and oxygen reduction. For the home-prepared electrocatalyst supported on MWCNTs, PtSn/MWCNT-HT was found to be the most active catalyst while for catalyst supported on metal oxides PtSn/MoO2 was found to be more active than the rest of the Pt-based electro catalyst supported on metal oxides. Results showed that PtSn is more active than PtRu and could function as a methanol tolerant oxygen reduction electro-catalyst for the cathode of a direct methanol fuel cell. Furthermore, in terms of durability, the home-prepared electrocatalyst proved to be more durable than the commercial electro-catalyst supported on carbon black, with catalyst supported on MWCNTs showing more stability than other supported electro-catalyst. Multi-walled carbon nanotubes have therefore proven in this study to be the best supporting material for electro-catalyst as catalyst supported on them showed to be more stable than commercial catalyst supported on carbon black.