| Direct methanol fuel cells (DMFCs) have been recognized as the one of the ideal choice to deal with the problem of energy in future because its high efficient energy, rich in the source of methanol, the price of methanol is low, easy to store and transport, zero pollution, and so on.So far, acid electrolyte is widely used in DMFCs and the platinum is as the main anode catalysts. But the high price, low storing and easy to poison by CO limit the application of Pt. Moreover, the low kinetics of reaction of methanol and oxygen reaction in acid electrolyte is another key issue that limit the commercialization of acid DMFCs. Recently, with the development of anion exchange membrance, increasing attention has been paid to the search for alkaline DMFCs. Compared with the acide DMFCs, there are several significant advantage in alkaline DMFCs, such as the high reaction kinetics of methanol oxidation reaction and oxygen reduction, methanol crossover can be suppressed more effectively, lower prices of anion exchange membrane, the non-Pt metals can be employed as ctalysts, and so on.In this paper, Au was be used as the core of nanoparticles because its high electronegativity (X=2.54) and excellent catalytic performance in nanometer, the relative low electronegativity metal Pt (X=2.28), Pd (X=2.20) composed the shell of nanoparticles. Due to the high utilization of Pt, Pd and the evident electronic effect between Au and Pt, Pd, the Au@Pt, Au@Pd core-shell nanoparticles exhibited high catalytic performance. The Au@Pt, Au@Pd core-shell nanoparticles with different ratios were synthesized by ultraviolet irradiation method. Additionally, in order to gain the high deposition of metal nanoparticles, the surface of multiwalled carbon nanotubes (MWCNTs) was functionally modified with the thiophenol. Finally, Au@Pt/MWCNTs, Au@Pd/MWCNTs compositions used as anode catalyst in the alkaline DMFCs were prepared by assembling the Au@Pt, Au@Pd nanoparticles on the surface of functionalized MWCNTs. To the best of our knowledge, related researches were scarce reported. UV-Vis absorption spectra confirmed that the Au@Pt, Au@Pd had a core-shell structure rather than the mixture of Au and Pt, Pd nanparticles. Transmission electron microscopy (TEM) indicated that Au@Pt, Au@Pd nanoparticles with small size (-5nm) were high dispersed on the MWCNTs surface. X-ray diffraction (XRD) revealed that Au@Pt, Au@Pd nanoparticles and MWCNTs had good crystal structure and the calculated sizes were well consistent with the TEM results. X-ray photoelectron spectroscopy (XPS) suggested that the obvious electronic effect between Au and Pt, Pd could be observed, and resulted in the change of electronic structure of Pt, Pd in Au@Pt/MWCNTs, Au@Pd/MWCNTs catalysts. The element ratios of nanoparticles measured by XPS also supported the formation of core-shell structure from another side. Cyclic voltammograms was used to analyse the surface composition of nanoparticles, and the results further confirmed that the obtained bimetallic nanoparticles had a core-shell structure. The catalytic performance of catalysts was investigated with cyclic voltammograms and chronoamperometry. The Au@Pt/MWCNTs (1:1) catalyst exhibited the highest catalytic activity and best stability, and its peak current density is 4.24mA cm-2, which is 2.3 times and 3.5 times higher than that of Pt/MWCNTs and commercial Pt/C(JM) catalysts, respectively. The enhancement catalytic performance of the Au@Pt/MWCNTs (1:1) catalysts could be attributed to the changed electronic structure of Pt, which weakens the chemisorption of the CO intermediate and thus easy to remove the adsorbed CO. For Au@Pd/MWCNTs catalyst, the highest catalytic activity and best stability appeared with the ratio of Au:Pd=4:1. The peak current density of Au@Pd/MWCNTs (4:1) was 1.78 times higher than that of Pd/MWNCTs catalyst. The eletronic effect and bifunctional effect could be used to explain the high catalytic performance of Au@Pd/MWCNTs (4:1) catalyst. |
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