Platinum-Based Nanocrystals and Their Use as Electrocatalysts toward Oxygen Reduction

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Xie, Minghao
Xia, Younan
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Platinum (Pt) is an intriguing catalytic material for a variety of reactions. One of the most important applications of Pt is in catalyzing the oxygen reduction reaction (ORR), a process key to the operation of fuel cells and metal-air batteries. However, its extremely low abundance in the earth crust and ever-increasing price have created a barrier to the large-scale use of this metal. One solution to this issue is to increase the atomic utilization efficiency (AUE) of Pt. This dissertation is focused on the development of synthetic strategies for the facile synthesis of Pt-based nanocrystals with well-controlled surface structures, compositions, and crystal structures, in an effort to optimize their catalytic performance toward ORR. Based on an oil-phase synthesis, I studied the nucleation and growth of Pt nanocrystals, revealing the roles played by oleic acid at different stages of the synthesis. Afterwards, my coworker and I designed a facile route to the production of Pt-Co truncated octahedral nanocrystals with tunable sizes and compositions. The nanocrystals delivered enhanced mass activity compared to those of commercial catalysts in both liquid half-cell and fuel cell tests. I also scaled up the production by in situ growth of Pt-Co nanocrystals on the surface of carbon supports. The effect of carbon supports on both synthetic and catalytic process were analyzed by comparing the electrochemical performance of the nanocrystals grown on different types of carbon supports. In the last project, I developed a strategy for the synthesis of Pt-Co@Pt octahedral nanocrystals featuring an intermetallic, face-centered tetragonal Pt-Co core and an ultrathin Pt shell, together with the dominance of {111} facets on the surface. When evaluated as a catalyst toward ORR, the nanocrystals delivered a mass activity which was 13.4 times higher than that of a commercial Pt/C catalyst. More significantly, the mass activity of the nanocrystals only dropped by 21% after 30,000 potential cycles, promising an outstanding catalyst with optimal performance for ORR.
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