Electrochemical Characterisation of Carbon-Supported Ru@Pt Core-Shell Catalyst for the Direct-Methanol Fuel Cell
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The world is in an energy transition period, going from predominantly fossil fuel energy sources to more renewable sources. This is especially happening in the transportation sector, where oil is the dominant source. Large CO2-emissions and fewer and more expensive oil-fields makes a transition necessary. The methanol economy is one of many models suggested to replace the oil based economy. Based on methanol as the cornerstone, methanol should work as an energy storage medium from renewable sources, where energy can be drawn from. One way to transform the chemical energy in methanol into electrical energy is by the use of the direct-methanol fuel cell (DMFC). Oxidising methanol directly on the anode, while reducing oxygen on the cathode, the DMFC has already demonstrated its potential. Though not delivering the power needed for the transportation sector, the DMFC has shown potential to replace batteries in portable electronic devices. Key elements in a fuel cell are the electrocatalysts, ensuring higher kinetics of the electrochemical reaction taking place. For DMFC, PtRu and Pt are the preferred catalysts for the anode and cathode, respectively. Since PtRu suffers from low structural stability under operation, and Pt suffers under the presence of methanol which diffuses through the membrane, new catalysts are needed. The Ru@Pt core-shell structured catalyst has shown potential for oxidising CO efficiently, a key intermediate and poison for the methanol oxidation reaction. With a higher stability than PtRu, and a lower onset potential for the CO oxidation compared to Pt, which is believed to be the rate-determining step (rds) for the methanol oxidation reaction (MOR), Ru@Pt may be a promising replacement for PtRu. Though its kinetics for the MOR has shown to be not as good as expected, despite of its good CO oxidation abilities. Therefore, more investigation has been needed to try to create an understanding for this behaviour. This thesis has therefore been focusing on creating an understanding of the MOR on the Ru@Pt and comparing it with the better documented Pt and PtRu. In addition, Ru@Pt was also tested as a possible replacement for Pt as cathode in the DMFC, where its combined methanol tolerance and oxygen reduction reaction (ORR) activity was investigated. Also, a new way of measuring directly the formaldehyde and formic acid species evolved from the MOR in an acidic solution was tested. As being among some of the most important species from the MOR, a knowledge of the relative amounts of those two species evolving from different catalysts is important, making a simple way of measuring them directly desirable. The PZTC-value for Ru@Pt in 0.1 M HClO4-solution was measured to be ca. 0.138 V vs. RHE. Its dependency of the potential of zero total charge (PZTC) value on the activity of the CO oxidation, ORR, and the MOR was investigated. A correlation in the order RuPt < Ru@Pt < Pt for the PZTC-value was obtained, with the same order for the ORR-activity and the opposite order for the CO oxidation. Based on correlations between the change in the PZTC-value (ΔPZTC) and the change in free energy (ΔG), which were obtained by different correlations found in the literature, predicted a shift in the rds for the MOR from OH-adsorption to CO oxidation when going from Pt to Ru@Pt. Also, predictions of the difference between the onset potentials for Pt and Ru@Pt for the CO oxidation and the ORR were made, which correlated well with experimental results. The methanol tolerance for Ru@Pt was found to be better than Pt, but its larger overpotential for the ORR ensures that Pt overall is the best cathodic electrocatalyst for a DMFC. A method to measure formaldehyde and formic acid by-products from the MOR in an acidic solution was demonstrated. The measurement were performed with the use of a rotating ring-disc electrode (RRDE) with an inserted Pd-ring. According to literature, keeping the Pt-loading above 10 wt% will ensure no formaldehyde formation, making it possible to exclusively measure the formic acid formation directly. Measurements with Pt, PtRu, and Ru@Pt showed only minor deviations between them, with a formic acid formation around 4% of the total product formation, which is one fifth of the reported formic acid formation by DEMS-measurement.