| 總結: | <p>This thesis concerns the oxygen reduction reaction (ORR) in the proton exchange membrane fuel cell (PEMFC) and aims to understand the interaction of oxygen with platinum-cobalt (PtCo) catalyst nanoparticles. The techniques employed are aberrated scanning transmission electron microscopy (STEM), electron energy loss spectroscopy (EELS) and modelling through density functional theory (DFT). The comparison of certain features in the experimental and simulated EEL spectra allows for selective optimisation in catalyst design. It is imperative to optimise the design of Pt-based alloys because Pt catalysts are costly. There is also an added advantage of manifold increase in catalytic activity with PtCo alloys in particular.</p>
<p>In order to study the oxygen interaction, the focal point of study becomes the oxygen coverage on the surface of the PtCo alloys. This requires detailed characterisation at the nanoscale and as such, optical techniques are not ideal. Using HAADF STEM means high spatial resolution and Z-contrast, which is crucial to investigating light and heavy elements often seen in catalyst materials. However, due to the Z-dependence, elements as light as oxygen may not readily appear in the HAADF image of catalysts and therefore EELS becomes an important tool due to its large range of elemental sensitivity to chemical composition.</p>
<p>The most logical starting point to investigate the oxygen coverage on the PtCo alloys is to find a link between the oxidation state of the metals and the oxygen binding in the alloys. The next step is to correlate detailed features in the oxygen and metals EEL spectra and explain them through modelling. DFT modelling is not enough to fully explain these features and hence atomic multiplet theory (AMT) is another tool used to corroborate experiment, particularly in the calculation of Co spectra.</p>
<p>A methodology for the extraction of oxidation states using the white line ratio in the cobalt EEL signal (Co L2,3-edge) is developed. Specifically, the PtCo nanoparticles are separated into core and shell regions in order to highlight possible differences in the binding of oxygen on the surface compared to rest of the nanoparticle. In addition, the influence of size on the oxygen binding is also examined. The errors associated with this method as well as potential difficulties in its application are discussed. The experimental results are tied to observed features in the calculated Co and oxygen spectra from various environments using AMT and DFT.</p>
<p>Combining spectroscopy and modelling, it is shown that the understanding of the oxygen interaction with the PtCo alloys only increases efficiency of catalyst design. This advantage acts as a stepping stone for other studies with such nanoscale specimens like nanoparticles in the catalyst research field.</p>
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