Synthesis Of Carbon Supported Bimetallic Pt-Ag Particles With Tunable Nanostructures And Their Superior Electro-Catalytic Activity Toward Oxygen Reduction Reaction

The ability to precisely control the chemical compositions and nanophase structures in bimetallic catalysts is the key for achieving the goal of improved performances in electrocatalysis for oxygen reduction reaction (ORR). In this thesis, we synthesized PtAg/C bimetallic nanoparticles (NPs) featured different nanostructures by simply regulating concentrations of H2PtCl6in aqueous solutions containing Ag NPs as the sacrificial templates via a galvanic replacement reaction and reduction reaction. Besides, we prepared Ag@Pt/C nanocatalysts with different shell thickness to further probe the most outstanding performance toward the oxygen reduction reaction. The morphology and phase structure of the Pt-Ag NPs supported on carbon were characterized by high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD). The employed electrochemical replacement reaction is responsible for these Pt-Ag NPs featured different structures that dominate the ORR activities. The electrocatalysis experiments in an acidic solution reveal a strong correlation between ORR activity and nanostructures of these synthesized catalysts. Furthermore, the alloying PtAg/C particles exhibit properties to glucose oxidation. To fully elucidate the factors contributing to the improved electrocatalytic activity toward ORR, the electronic structures of these Pt-Ag NPs were assessed by X-ray photoelectron spectroscopy. X-ray photoelectron spectroscopy (XPS) measurements demonstrate that nanophase structures of the catalysts caused their changes of electronic structures, which are highly associated with the trends in the ORR activity. The remarkably improved electrocatalytic activity for ORR is likely due to the counterbalance between two converse effects that revolve with the adsorption of active oxygen and desorption of the oxygenate intermediates generated by ORR on the bimetallic NPs surfaces as a result of the electronic interactions between Pt and Ag components. The origin of the enhancement for activities could be considered by the unique electronic structures in these bimetallic nanostructures, which can effectively achieve a counterbalance between two opposite effects that revolve with the adsorption of active oxygen and desorption of oxygenate intermediates during ORR on these catalysts with appropriate nanophase structures.