Platinum based Catalysts for Methanol Fuel Cells: Metal Clusters and Carbon Supports
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In this thesis two objectives are pursued in parallel. The first is to reduce the Pt loading and thereby reducing the cost of the catalysts for anode application in direct methanol fuel cell. The second is to tune the surface properties of Pt and thereby enhance the activity in the methanol oxidation reaction. The former is achieved by reducing the Pt cluster size and thus increase the Pt surface area, or by using Ru@Pt core-shell structures to reduce the Pt loading but achieve similar Pt surface area. The surface properties of Pt are tuned through the Ptgraphite edge interface and by tailoring of the RuPt architecture in the shell of the Ru@Pt core-shell NCs. To achieve the above goals, a ex-situ polyol synthesis method was adopted. In this method, monometallic Pt cluster size as well as Ru@Pt cluster size are controlled in the colloid solution before deposition on the corresponding supports. One advantage of this method is that it is possible to study the influence of oxygen groups present on the carbon support surface on the Pt dispersion. It was found that the Pt dispersion was highly dependent on the amount of surface oxygen groups. The methanol oxidation reaction was investigated on Pt clusters with various sizes on carbon nanofibers and carbon black supports with different surface oxygen concentration, aiming at gaining a better understanding of the relationship between the catalyst properties and the electrochemical performance. It was found that CNF-supported Pt clusters showed better performance than carbon black-supported Pt clusters for all the Pt particle sizes. Furthermore, Pt supported on oxygen depleted CNF has better performance than the Pt supported on oxygen rich CNF. Due to the combined advantages of optimum Pt particle size, oxygen free surface and the unique properties of CNF, Pt supported on heat-treated CNF exhibited a mass activity about 2 times higher than for the commercial E-TEK catalyst. Another advantage of the ex-situ polyol method is that the RuPt shell composition in the Ru@Pt core-shell NCs can be tailored simply by controlling the pH of the synthesis medium during Pt deposition. The structural and surface properties of the catalysts were characterized with X-ray diffraction, transmission electron microscopy, CO stripping, and energy dispersive X-ray spectroscopy. It was found that a coreshell catalyst with alloyed shell was obtained at a pH of 5.8, whereas a monometallic Pt shell was obtained at a pH of 10. The methanol oxidation reaction was investigated for the carbon black supported Ru@Pt catalysts with different shell compositions. It was shown that the core-shell catalysts gave the highest steady-state current for methanol oxidation by a factor of 10 for alloyed shell and by a factor of 5 for the Pt shell compared to the pure Pt catalyst. It can be concluded that the improved catalytic performance of the core-shell catalysts can be attributed to the ligand effect in the Pt-rich shell and to a combination of the ligand effect and bi-functional character in the alloyed shell.