| Future energy concerns demand a transition from fossil fuels to new energy sources that are more environmentally benign and renewable. A promising route for accomplishing this goal is to use fuel cells to convert the chemical energy of fuel directly into electricity. In this sense, fue cells have received much attention because of several advantages, including high conversion efficiency, low pollution, and high power density, for a wide range of applications. The Pt catalysts are the most popular and effective electrocatalysts for both the anode and the cathode of fuel cell, however, Pt catalysts usually suffer from several disadvantages blocking the commercialization of fuel cells. To enhance the activity of catalysts and lower the use of noble metals, it is highly desirable to load catalysts on the surface of suitable supporting materials. Graphene, a recently discovered carbon nanomaterial with carbon atoms tightly packed into a two dimensional honeycomb lattice, possesses many novel and unique physical and chemical properties because of its unusual monolayer atomic structure. Because of its novel properties such as large specific surface area, high electrical conductivities, graphene has found potential applications in the field of fuel cells. In this paper, we synthesized a series of metal-graphene nanocatalysts, the electrocatalytic characteristics of the nanocatalysts were studied by voltammetry with organic molecular (glucose, methanol) oxidation reaction and the oxygen reduction reaction as model reactions. The main results are as follows:1. A gold nanoparticles (Au NPs)-graphene nanocomposite (Au-graphene nanocomposite) was prepared by electrochemically depositing Au NPs on the surface of graphene sheets, and characterized by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray powder diffraction (XRD), and electrochemical methods. The morphology and size of the Au NPs could be easily controlled by adjusting the electrodeposition time and the concentration of precursor (AuCl4-). The electrocatalytic activities of the nanocomposites toward oxygen reduction and glucose oxidation were investigated by cyclic voltammetry. The results indicated that the nanocomposites had a higher catalytic activity than the Au NPs or graphene alone, indicative the synergistic effect of graphene and Au NPs. Therefore, this study has provided a general route for fabrication of graphene-based noble metal nanomaterials composite, which could be potential utility to fuel cells and bioelectroanalytical chemistry.2. Bimetallic catalysts have proven superior to single metal catalysts in this respect. This chapter reports the preparation, characterization, and electrocatalytic characteristics of a new bimetallic nanocatalyst. The catalyst, Pt-Au-graphene, was synthesized by electrodeposition of Pt-Au nano structures on the surface of graphene sheets, and characterized by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray powder diffraction (XRD), and voltammetry. The morphology and composition of the nanocatalyst can be easily controlled by adjusting the molar ratio between Pt and Au precursors. The electrocatalytic characteristics of the nanocatalysts for oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) were systematically investigated by cyclic voltammetry. The Pt-Au-graphene catalysts exhibits higher catalytic activity than Au-graphene and Pt-graphene catalysts for both the ORR and MOR, and the highest activity is obtained at a Pt/Au molar ratio of2:1. Moreover, graphene can significantly enhance the long-term stability of the nanocatalyst toward MOR by effectively removing the accumulated carbonaceous species formed in the oxidation of methanol from the surface of the catalyst. Therefore, this chapter has demonstrated that a higher performance of ORR and MOR could be realized at the Pt-Au-graphene electrocatalyst while Pt utilization also could be greatly diminished. This method may open a general approach for the morphology-controlled synthesis of bimetallic Pt-M nanocatalysts, which can be expected to have promising applications in fuel cells.3. The structure, composition, and support material significantly affect the catalytic characteristics of Pt-based nanocatalysts. Fine control of the structural and compositional features is highly favorable for the creation of new Pt-based nanocatalysts with enhanced catalytic performance and improved Pt utilization. This chapter reports on a systematic and comparative study of the effects of structure, composition, and carbon support properties on the electrocatalytic activity and stability of Pt-Ni bimetallic catalysts for methanol oxidation, particularly the promoting effect of Ni on Pt. Graphene-supported Pt-Ni alloy nanocatalysts were prepared by a facile, one-step chemical reduction of graphene oxide and the precursors of Ni2+and PtCl62-. The nanocatalysts were characterized by transmission electron microscopy (TEM), ultraviolet-visible spectrophotometry (UV-vis), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). The electrocatalytic characteristics of the nanocatalysts were studied by voltammetry with methanol oxidation as a model reaction to evaluatethe effects of the structure, surface composition, and electronic characteristics of thecatalyst on the electrochemical activity. The catalyst with a Pt/Ni molar ratio of1:1exhibited the highest electrocatalytic activity for the methanol oxidation reaction withgreatly lowered Pt utilization. The mechanism of the promoting effect of Ni on Pt isexplained based on the modification of the electronic characteristics of the surface Ptatoms (Pt4f) by Ni atoms due to the shift in the electron transfer from Ni to Pt and thesynergistic roles of Pt and nickel hydroxides on the surfaces of the catalysts. Theeffects of the different carbon supports (i.e., graphene, single-walled carbonnanotubes, and Vulcan XC-72carbon) on the electrocatalytic characteristics of thenanocatalysts are investigated by Raman and XPS experiments. The resultsdemonstrate that the graphene-supported Pt-Ni catalyst has the highestelectrocatalytic activity of the three carbon materials due to abundantoxygen-containing groups on the graphene surface, which can remove the poisonedintermediates and improve the electrocatalytic activity of the catalysts.4. Size and shaped-controlled synthesis of nanocatalysts has attracted muchattention because their catalytic performance is highly dependent on the surface areas,surface atomic structures, and shapes. A novel graphene-supported hollow Pt-Nialloy nanostructure was first fabricated by galvanic displacement. The resulted hollowPt-Ni-graphene nanostructures were characterized by transmission electronmicroscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and X-raydiffraction (XRD). In order to investigate the growth mechanism, the formationprocess of the Pt-Ni nanostructures were monitored by TEM, and the effects of themolar ratio of the precursors (the ratio of the K2PtCl6to NiCl2·6H2O) and cappingagent were also investigated by TEM. The electrocatalytic characteristics of thehollow Pt-Ni-graphene nanocatalysts were studied with methanol oxidation as amodel reaction, and the results indicating that the resulted hollow Pt-Ni-graphenenanocatalysts exhibit more excellent electrocatalytic performance for methanol thanthe solid Pt-Ni-graphene, and commercial Pt/C catalysts, which might be due to thehollow structures of the nanocatalysts and the high electrical conductivity and largespecific area of the graphene sheets... |
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