| The fuel cell has been paid more and more attention because of the shortage of fossil fuels and environmental pollution produced using fossil fuels. Among the fuel cells, people attaches more and more important to proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cell (DMFC). However, their commercial viability is still hindered by several factors, including the poor kinetics of the cathodic reaction and the high costs of Pt-based electrocatalysts. Even under the open-circuit condition, the overpotential for oxygen cathode in PEMFC is around 0.2-0.3V due to non-reversibility of oxygen reduction and to the "mixed potential" effect. The methanol crossover from the anode to the cathode in a DMFC through the polymer electrolyte leads to a mixed potential at the cathode. This effect causes a significant decrease in performance for oxygen reduction reaction and an increase in overpotential of ca. 0.2-0.3V. Aiming to increase catalytic activity of oxygen reduction reaction, to lower the cost of the catalysts and to enhance the methanol tolerance, it is very important to study the oxygen reduction reaction at present. At the same time, there is great room for improvement in oxygen reduction reaction kinetics.In this dissertation, the carbon-supported Pt (Pt/C) and bimetallic Pt-Co(Pt-Co/C) nanoparticle electrocatalysts were prepared via the carbonyl complex route. Their electrocatalytic activity for oxygen reduction was evaluated and compared with that of commercial E-Tek catalysts. The obtained results are shown as follows:The Pt/C catalysts with different Pt loadings were prepared via a Pt carbonyl cluster route. X-ray diffraction (XRD) and transmission electron microscopy (TEM) results indicated that the average particle size of Pt in the Pt/C catalysts with different Pt loadings is ca. 2~4 nm with a relatively narrow particle size distribution and a good dispersion. The average particle size of Pt in the Pt/C catalysts is very close to that of the commercial E-Tek catalyst. The oxygen reaction activity decreases with the increase in Pt loading when keeping Pt amount on the surface of the catalytic electrodes. The change in catalytic activity is probably ascribed to a decrease in Ptreal surface area.The Pt-Co/C catalysts with different Pt:Co atomic ratios were prepared via the carbonyl complex route. XRD data showed that as-prepared Pt-Co/C catalysts are basically with the single-phase disordered structure (solid solution) and that the lattice parameter decreases with the increase in Co amount within the catalysts. The TG and ICP analysis confirmed that atomic compositions of the as-synthesized catalysts are nearly the same as nominal values. It was observed from TEM images that the Pt-Co alloy nanoparticles in the Pt-Co/C catalysts is ca. 2-4 nm and possess the good dispersity, relatively narrow particle size distribution. As compared to the commercial E-Tek Pt/C or homemade Pt/C catalyst, the Pt-Co alloy catalysts with the different Pt/Co atomic ratios showed an enhanced electrocatalytic activity for the oxygen reduction reaction and an enhancement factor of about 1.2-2.2 was found. The enhanced electrocatalytic activity of as-prepared Pt-Co alloy catalysts is attributed to the high dispersion of the alloy catalysts, to their disordered structure, and to the favorable Pt-Pt mean interatomic distance caused by alloying. |
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