Carbon-supported Ru @ Pt Core-shell Catalyst for Low Temperature Fuel Cells
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A novel synthesis route for the manufacturing of Ru@Pt core-shell nanoparticles (NPs) originating from the University of Maryland, USA, was reproduced and adapted for electrochemical purposes in our laboratories at NTNU. The main objectives of this work were to electrochemically investigate the catalytic and structural properties of the nanosized Ru@Pt core-shell structures in the aqueous solution, test it as both a potential anode and cathode catalyst in a single cell, and ultimately to perform a test in a real fuel cell system. Consequently, the scope of the thesis covers several topics related to the operation conditions and reactions within low temperature fuel cell systems, such as hydrogen oxidation reaction (HOR), methanol oxidation reaction (MOR), oxygen reduction reaction (ORR), Pt poisoning with carbon monoxide, and catalyst’s stability. Prior to the study of the Ru@Pt structure, pure/uncoated Ru cores were investigated. Their amorphous structure and substantial activity toward CO-oxidation was revealed. The effect of the heat treatment on the structure and the activity was also examined. After the successive attempts of coating the Ru NPs with thin Pt shell electrochemical methods such like cyclic voltammetry and CO-stripping voltammetry were employed as a diagnostic tool for the determination of surface composition of various PtRu NPs. A series of electrochemical experiments developed in our group confirmed that the surface of the Ru@Pt particles does not contain significant amounts of ruthenium. The observed cathodic shift by 200 mV of the main CO stripping peak of the Ru@Pt with respect to that of Pt is consistent with what has been found at other catalysts with a Pt skin but of a different architecture. This significant shift and thus improvement in the activity towards CO electrooxidation is explained as a net effect of a d-band center downshift within the Ru-affected-Pt shell atoms and their consecutive relaxation. An additional small separate stripping peak at a potential corresponding to the oxidation of adsorbed CO at Pt/C is interpreted as being due to either separate Pt particles in the catalyst or thick Pt shells at a minor fraction of the Ru@Pt particles. Electrooxidation of methanol was subsequently studied at different Pt/PtRu surfaces. Similarly to the trend observed in the activity towards electrooxidation of CO, it was found the Ru-affected-Pt surface displays considerably lower onset potential during the oxidation of methanol with respect to the unaffected/pure Pt. The reaction has been studied in static and dynamic conditions along with differential electrochemical mass spectroscopy (DEMS). The observed changes in the activity are discussed in terms of negative d-band shifts at the core-shell catalysts leading to a weaker adsorption of both CO and oxygen-containing species Further the influence of a severe Accelerated Tailoring Protocol on the same set of Pt/PtRu catalysts was investigated. Rapid (1 V s-1) cycling within 0.6 - 1 V potential range resulted in modification in the core-shell structure, mainly due to the leakage of the Ru from the core to the electrolyte. As a result structures with thicker Pt shells were produced. It was concluded the thickening of the shell (from original 1.5 ML to few MLs) weakens the effect of the Ru substrate and thus reduces the original downshift of the d-band center. Interestingly, and in accordance with theoretical calculations, such an effect improves catalytic activity towards oxygen reduction reaction (ORR) but hinders it towards CO and methanol oxidation. The effect of the magnitude of the dband shift on these reactions is discussed. In the later stage the CO-stripping voltammetry and electrooxidation of methanol at Ru@Pt core-shell catalyst was studied at elevated temperature and pressure. It was found that the previously observed promotional effect and an increase in catalytic activity of Ru@Pt/C core-shell catalysts as compared to pure Pt/C catalyst depends strongly on temperature and concentration of methanol in the electrolyte. Moreover, the study revealed that increasing the temperature from 25 °C to 120 °C results in a negative shift of the methanol oxidation onset potential by 200 mV for both Pt and Ru@Pt catalyst. It therefore appears thermal activation of water adsorption enhances the oxidation of adsorbed intermediates and thus dictates the methanol oxidation onset potential. Finally, DMFC experiments showed only small performance differences between Ru@Pt/C and Pt/C anode electrocatalysts, suggesting the necessity of render possible the formation of surface oxygen species at lower electrode potentials.