Synthesis and characterisation of Pt-alloy oxygen reduction electrocatalysts for low temperature PEM fuel cells
- Authors: Mohamed, Rhiyaad
- Date: 2012
- Subjects: Electrochemical analysis , Proton exchange membrane fuel cells
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:10428 , http://hdl.handle.net/10948/d1018586
- Description: This dissertation the syntheses of Pt-based binary and ternary alloy electrocatalysts using the transition metals of Co and Ni are presented. These electrocatalysts were synthesised by an impregnation-reduction procedure at high temperature whereby Pt supported on carbon, (Pt/C (40 percent), was impregnated with the various metal and mixtures thereof and reduced at high temperatures in a H2 atmosphere. The procedure was also designed in such a way so as to prevent the oxidation of the support material (carbon black) during the alloy formation. The resultant nanoparticles (9-12 nm) of Pt3Co/C, Pt3Ni/C and Pt3Co0.5Ni0.5/C were also subjected to a post treatment procedure by acid washing (denoted AW) to produce electrocatalysts of Pt3Co/C-AW, Pt3Ni/C-AW and Pt3Co0.5Ni0.5/C-AW to study the effect of acid treatment on these electrocatalysts. The synthesised electrocatalysts were then characterised by a number of physical and electrochemical techniques and compared to that of commercial Pt/C (Pt/C-JM, HiSpec 4000) as well as Pt/C catalysts (Pt/C-900 and Pt/C-900-AW) treated under the same conditions used for the alloy synthesis. The electrocatalysts were then used to fabricate MEAs that were loaded into commercial single test cells and characterised by means of polarisation curves and Electrochemical Impedance Spectroscopy (EIS). The extensive physical characterisation included Powder X-Ray Diffraction (PXRD) analysis, Transmission Electron Microscopy (TEM), elemental analysis by Energy Dispersive Spectroscopy (EDS) and metal loading by Thermo-Gravimetric Analysis (TGA). These studies showed that Pt-based alloy electrocatalysts were successfully synthesised with particle sizes ranging from 9 - 12 nm, within their respective atomic ratios and whereby no significant loss of carbon support occurred. This indicated that significant sintering or electrocatalyst particles occurred when compared to that of the starting Pt/C catalyst (3 – 4 nm). From the combined results of the physical characterisation procedures, it was also shown that leaching as a result of acid washing was catalyst dependent with Ni containing catalysts showing a significant degree of leaching compared to that of Co containing catalysts. Electrochemical characterisation in terms of Electrochemical Active Surface Area (ECSA) by Cyclic Voltammetry (CV) and ORR activity by Rotating Disc Electrode (RDE) analysis revealed that a significant decrease in the ECSA resulted from the increase in particle size and this had a major influence on the ORR activity. Furthermore it was found that a significant improvement in the ORR activity was achieved by the synthesis of Pt-based alloys. It was also found that catalytic properties of the acid washed electrocatalysts were substantially different from that of non-acid washed electrocatalysts. The experimental data confirmed that it was possibly to achieve better catalytic performance as compared to that of Pt/C at a lower material cost when Pt is alloyed with base transition metals. The trend observed from the ORR activity studies by RDE was successfully repeated in the in-situ fuel cell testing in terms of mass activity of the electrocatalysts. Of the electrocatalysts studied under „real‟ fuel cell conditions Pt/C-JM had the best performance compared to the others, with the ternary Pt3Co0.5Ni0.5/C showing better catalytic performance compared to the Pt3Co/C electrocatalyst. This was found to be due to a higher charge transfer resistance observed in Pt3Co/C as compared to that of Pt3Co0.5Ni0.5/C which was similar than that of the commercial Pt/C-JM catalyst with both Pt3Co/C and Pt3Co0.5Ni0.5/C-AW having similar but higher ohmic resistances than that of Pt/C-JM as determined by electrochemical impedance spectroscopy. The results showed that a great potential exist to improve the catalytic performance of low temperature PEM fuel electrocatalysts at a reduced cost as compared to that of pure Pt provided a method of controlling the particle size was established.
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- Date Issued: 2012
Evaluation and application of electroanalysis for the determination of antioxidants
- Authors: Ragubeer, Nasheen
- Date: 2007
- Subjects: Antioxidants , Nervous system -- Degeneration , Electrochemical analysis , Marine algae , Natural products , Marine metabolites , Sargassum , Legumes , Nuclear magnetic resonance
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:3922 , http://hdl.handle.net/10962/d1003981 , Antioxidants , Nervous system -- Degeneration , Electrochemical analysis , Marine algae , Natural products , Marine metabolites , Sargassum , Legumes , Nuclear magnetic resonance
- Description: The role of antioxidants in the prevention of neurodegenerative diseases has been well documented. The use of synthetic antioxidants has decreased due to the ssociation of these compounds with certain cancers. Thus, the search for novel natural antioxidants has gained much focus in research. Most common methods of determining antioxidant capacity are the radical generated assays and biological assays such as lipid peroxidation and the nitroblue tetrazolium assay. Electrochemical methods have been proposed for the determination of bio-active compounds such as antioxidants. The electrochemical methods of cyclic voltammetry and square wave voltammetry were evaluated for the determination of antioxidant capacity initially examining known antioxidants and then using plant extracts of Sutherlandia frutescens as a case study. The antioxidant properties determined by electrochemical methods were validated utilising the non-biological methods of the DPPH, TEAC, ferrozine and FC assay and biological pharmacological methods. The results indicated that Sutherlandia frutescens contains potent antioxidant compounds that are able to reduce lipid peroxidation. The electrochemical techniques of square wave voltammetry and cyclic voltammetry were applied for the screening of a large number of extracts of various algae for the detection of antioxidant compounds. The results indicated that electrochemistry can be used as a preliminary method for the rapid screening of a large number of crude samples for antioxidant compounds. Electrochemical methods were also evaluated as a method for guiding the isolation and purification of antioxidant metabolites in Sargassum elegans. Solvent partitioning and fractionation of the marine alga allowed for the purification of antioxidant compounds. At each step of purification electrochemical methods were utilized to determine which fractions contained the more potent antioxidant compounds and thus guide further purification. The purified antioxidant compounds were elucidated using NMR to determine the structure of the antioxidant compounds.
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- Date Issued: 2007
Electrochemical studies of gold bioaccumulation by yeast cell wall components
- Authors: Lack, Barbara Anne
- Date: 1999
- Subjects: Hydrometallurgy , Electrochemical analysis , Gold ores , Gold
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4311 , http://hdl.handle.net/10962/d1004969 , Hydrometallurgy , Electrochemical analysis , Gold ores , Gold
- Description: Gold, amongst other group 11 metals, was almost certainly one of the first three metals known to man. In addition to the economic importance of the metal, gold has a wide variety of applications in the medical, electrocatalytical and micro-electronics fields. However, the determination of gold ions in solution, with accuracy, precision, sensitivity and selectivity is still an interesting and much debated topic in analytical chemistry. A system whereby gold ions have been successfully detected employing an electrochemical technique, known as stripping voltammetry, has been developed. The electrochemical method was chosen over other available techniques for the sensitivity, particularly at low concentrations, and selectivity properties; notably in the presence of other metal ions. Under acidic conditions, the electrochemical technique was applied and the presence of gold(III), at a concentration of 2.53 x 10⁻⁵ mol dm⁻³ in a mine waste water sample, was detected. Biomass, in particular yeast and algal types, have been successfully employed in extracting low concentrations of gold ions from industrial effluents. The manipulation of the biological facility for mineral interaction, biohydrometallurgy, may yield numerous potential new technologies. South Africa in particular would benefit from this area of research, since the country is a major ore and metal refining country and if the output and the efficiency of the mines could be improved, even by a small percentage, the financial rewards would be vast. In this study, the application of adsorptive cathodic stripping voltammetry (AdCSV) of gold(III) in the presence of various Saccharomyces cerevisiae cell wall components, was investigated to determine which, if any, were involved specifically in the chemical binding of the gold ions. The chitin and mannan extracts showed the most promise with detection limits of 1.10 x 10⁻⁶ mol dm⁻³ and 9 x 10⁻⁹ mol dm⁻³, respectively; employing the AdCSV technique. A modification of the stripping voltammetry technique, Osteryoung square wave stripping voltammetry (OSWSV), provided the lowest detection limit, for gold(IIl) in the presence of mannan, of 1.70 x 10⁻¹¹ mol dm⁻³ ; utilising a modified carbon paste electrode. The detection of gold(III) has been shown to be dependent on the type of electrode employed, the electrolyte solution and the presence of interfering agents. The effect of copper(II) and silver(I) on the detection of the gold(III) in solution was investigated; whilst the silver(I) has shown no detrimental effects on gold (III) detection systems, copper(II) has indicated the possibility of forming an inter-metallic compound with the gold(III). However, mannan has shown to selectively and preferentially bind the gold(III) in the presence of a ten-fold excess of copper(II). Nuclear magnetic resonance (NMR) and infrared (IR) spectroscopy, as well as computer modelling techniques were employed to further investIgate the mannan-gold(III) interaction and proposed complex formed. The NMR, IR and computer modelling data are in agreement with the electrochemical data on proposing a mannan-gold(III) complex. The co-ordination site was established to be in the vicinity of the H-I and H-2 protons and the gold(III) adopts a square-planar geometry upon co-ordination. The benefits of the research are useful from a biological perspective (i. e. as more is known about the binding sites, microbiologists/biochemists may work on the optimisation of parameters for these sites or work could be furthered into the enhanced expression of the sites) and an industrial one. In addition to the' two major benefits, an improved understanding of gold and its chemistry would be achieved, which is advantageous for other fields of research as well.
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- Date Issued: 1999