To understand the ceria promotion effect of Pt-CeO(2)/C catalysts on methanol oxidation, microstructural and metal-oxide interactions of Pt-CeO(2)/C catalysts with an atomic ratio of Pt/Ce between 0.14 and 1.4 were systematically examined using high-resolution transmission electron microscopy and electron energy loss spectroscopy (EELS). With an increasing Pt content in the catalysts, Pt particles gradually invaded into the ceria supports and decoration on Pt particles was observed. Simultaneously, the morphology of the supports was dramatically modified with nanocrystalline and amorphous ceria formed between and/or around the Pt particles. It reveals that the Pt-ceria interaction could take place in the catalysts and the influence of the interaction was enhanced with an increasing Pt/Ce ratio. The EELS study demonstrated that the strong Pt-ceria interaction was related to the redox reaction between Pt and ceria. Experimental results also suggested that the strong interaction between Pt and ceria could contribute to the promotion effect of ceria on the oxidation of methanol.
A Pt on nano‐sized CeO2 particles that in turn are supported on carbon black (CB) was synthesized using the co‐impregnation method. This potential anode material for fuel cell applications was synthesized in a stepwise process. The pure CeO2 was synthesized using an ammonium carbonate precipitation method, and the Pt particles dispersed on the CeO2 in such a way that a uniform dispersion with the CB was obtained (Pt–CeO2/CB). The electrochemical activity of the methanol (CH3OH) oxidation reaction on the Pt–CeO2/CB was investigated using cyclic voltammetry and chronoamperometry experimentation. The onset potential of CH3OH oxidation reaction on the Pt–CeO2/CB anode was shifted to a lower potential as compared with that on commercially available Pt–Ru/carbon (C) alloy anode. In addition, the activation energy of the Pt–CeO2/CB anode was much lower than that of the Pt–Ru/C alloy anode. Moreover, the current density of the Pt–CeO2/CB anode was much higher than that of the Pt–Ru/C alloy anode at temperatures between 28° and 60°C. These results suggest that the anode performance of the Pt–CeO2/CB anode at the operating temperature of typical fuel cells (80°C) is superior to that of the more usual Pt–Ru/C alloy anode. Importantly, the rare metal, Ru, is not required in the present anode material and the amount of Pt required is also significantly reduced. As a consequence, we report a promising candidate Pt–CeO2/CB composite anode for application in the development of direct methanol fuel cells.
The ionic conductivities and microstructures of Ce 1−x Yb x O 2−x/2 ͑x = 0.10, 0.15, 0.20, and 0.25͒ with average grain sizes in a range of 0.2-3.7 m were studied systematically. Nanosized domains were confirmed through detailed studies of transmission electron microscopy. The relationships of the domains, doping concentration, and grain size were determined, and their effects on the ionic conductivities were examined. It was concluded that the domains were formed via the segregation of Yb cations so that they could trap the oxygen vacancies and lower the conductivity. Furthermore, the development of the domains, which depends on the doping concentration and grain size, can affect the doping concentration and grain-size dependencies of the conductivity.Ceria doped with rare-earth elements has attracted much attention as an important candidate electrolyte material for intermediatetemperature solid oxide fuel cells ͑SOFCs͒. 1-3 The ionic conductivity of doped ceria is greatly influenced by doping concentration. It was expected that the ceria with higher doping concentration could exhibit higher conductivity. However, the concentration of dopant cations that corresponds to the maximum conductivity is usually about 10-20 atom %, which is much lower than the solubility of the dopants in ceria. 2-5 Some investigators suggested that this phenomenon was attributed to the clustering of dopant cations and their associated oxygen vacancies, 3,5,6 through which the mobility of oxygen vacancies can be decreased. With increasing the doping concentration, the density of the clusters increases, leading to a decrease in conductivity.In addition to the doping concentration, the grain size also influences the conduction behavior. This influence was originally studied in stabilized zirconia. 7-11 Verkerk et al. found that the grainboundary conductivity of Y-stabilized zirconia increased linearly with the grain size increasing from 0.3 m to 2-4 m and was constant for larger grain size, while the bulk conductivity was nearly independent of the grain size. 9 This phenomenon was explained by the space-charge model, 12 in which it was suggested that spacecharge layers were formed by the segregation of Y cations and the depletion of oxygen vacancies around the grain boundaries. Because the space-charge layers can increase the grain-boundary resistivity, the increase in the density of the grain boundaries with decreasing grain size can cause a decrease in the total conductivity. The similar grain-size dependency of conductivity was also observed in doped ceria, 4,13,14 which was also in good agreement with the space-charge model. 15 However, some exceptions were found as well. In 20 atom % Gd-doped ceria, the conductivity could increase as the grain size decreased from 3 to 0.7 m. 16 Recently, in 10-25 atom % Y-doped ceria, Ou et al. also observed that the conductivity increased with decreasing grain size when the grain size was smaller than 200-500 nm. 17 It is difficult to explain this phenomenon by the space-charge model.In recent years...
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