A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the Degree of Doctor of Philosophy. Johannesburg, 2015 / Bimetallic platinum nanoparticles were synthesized for application as anode
catalysts for low temperature fuel cells such as direct methanol fuel cells (DMFCs).
Two distinct synthesis procedures were used; namely conventional synthesis with
post-synthesis heat treatment, and secondly polyol microwave-irradiation without
further heat-treatment. The aim was to synthesize interesting and novel bimetallic
nanostructures and relate their shape and morphologies to their methanol oxidation
reaction (MOR) activities and their CO tolerance.
Due to the high cost of the conventional synthesis processes as well as their use of
harmful solvents, microwave-irradiation was explored as a possible synthesis
procedure. It is a greener and more environmentally friendly approach with
possibilities of mass production of the nanoparticles. For both the synthesis
procedures, the reducing agent, the precursor salts, surfactants, pH of the solution
and molar ratios were varied to determine the effect on the shape, size and
ultimately the electrocatalytic activities of the Pt-Co and Pt-Ni nanoparticles.
For the conventional synthesis procedure, the main parameter of comparison was
the strength of the reducing agents, where NaBH4 and N2H4 were used under the
same reaction conditions. In this study, the strength of the reducing agent affected
the properties of the Pt-Co and Pt-Ni nanoparticles, such that, the stronger the
reducing agent, the higher the degree of alloying and the more electrocatalytically
active the materials. The drawback in the conventional synthesis was however low
current outputs, in the microamps range, which necessitates a need to explore other
synthesis procedures.
Microwave-irradiation was thus used as an alternative synthesis procedure in an
attempt to produce more active bimetallic platinum nanoparticles. Different reaction
parameters were changed in this process to optimize the synthesis process, namely
the pH of the solution, the amount of surfactant and the Pt-Ni molar ratio. In
changing the reaction parameters, there was an observed change in the structure of
the nanoparticles, with an average size in the order of 5 nm and different MOR
activities. Furthermore, it was found that the activity was highest for the optimum
amount of PVP and NaOH concentration of 500 mg and 1.0 M NaOH. In general, the
MW synthesized nanoparticles achieved current values in the microamps to amps
range, making it a more attractive synthesis procedure compared to the conventional
method.
The CO tolerance of the materials is an important aspect, as one of the main
drawbacks of the commercial application of fuel cells is the propensity of Pt to get
poisoned by CO during the methanol dissociation process. Therefore CO stripping
measurements were performed on the MW-irradiated catalysts. The catalysts
produced in this work showed good resistance towards CO. In general, the
behaviours of the catalysts were dependent on the amount of surfactant and the
molar ratio of the starting solution. The mechanism of CO tolerance in this case was
determined as the bifunctional model, where the Ni-oxide and Ni-hydroxide species
donate O to the electrooxidation of CO to CO2. In conclusion, the study of
microwave-irradiated bimetallic nanoparticles performed here, resulted in highly
active catalysts, which are even more active than commercial Pt/C nanoparticles.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/18525 |
| Date | January 2015 |
| Creators | Mathe, Ntombizodwa Ruth |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf, application/pdf |
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