| The use of methanol as energy carrier and its direct electrochemical oxidation in a direct methanol fuel cell (DMFC) represents an important challenge for the polymer electrolyte fuel cell technology, since the complete system without a reformer and further gas treatment steps.However, there are two main technique problems in direct methanol fuel cell. One is anode catalysts with low activity for methanol oxidation, the other is the methanol crossover through the polymer electrolyte leads to a mixed potential at the cathode, which results from the oxygen reduction reaction (ORR) and the methanol oxidation occurring simultaneously. This effect causes a negative potential shift at the cathode and a significant decrease of performance in a DMFC. For the latter, the problem should be solved by using electrolytes with lower methanol permeability or by developing new cathode catalysts. So a new type catalyst must have two characteristic, first, cathode catalysts in a DMFC should show a high methanol tolerance, that means the oxygen reduction will not be affected by the adsorption and oxidation of methanol. Secondly, the catalysts should show a higher exchange current density for oxygen reduction compared to Pt.There are some kind of methanol-tolerant catalysts. For example, Pt-M alloy, macrocyclic complexes, transition metal oxides, transition metal sulfides based on Chevrel phases, amorphous transition metal sulfide phases, etc. The prevailing view still considers carbon-supported Pt alloy to be the most efficient cathode atalysts for oxygen reduction. But there are two disadvantages: low methanol-tolerant and economic feasibility. Contrary, in recent years, amorphous transition metal sulfide phase catalysts have been proved that they have more superiorities in activity, methanol-tolerant, stability and the price. So transition metal sulfide phase catalysts were considered a substitute of Pt in the future.Ruthenium-chalcogenide clusters MxRuySe2 (M = Mo, W, Fe, Ni, 0.02xRuySez compounds have a core of ruthenium atoms, which is the center of oxygen reduction. Doped ligand element and ruthenium can formate M-Ru alloy, which change d-state of Ru or increase activity site. Because Pt have high activity site for oxygen reduction, doping Pt to Mo-Ru-Se,W-Ru-Se improve catalytic activity. So the Pt-M-Ru-Se (M = Mo,W) catalysts were synthesized by low temperature (139℃) method and characterized by XRD, EDS and XPS. The activities were measured by potentiodynamic and AC impedance techniques.The results show that:①The activity and methanol-tolerant on Mo-Ru-Se catalysts doped with platinum were greatly heighten, and the oxygen reduction reaction on them has higher current density and lower overpotential than that of platinum-free Mo-Ru-Se catalysts under the same conditions. The peak current density of platinum-free Mo-Ru-Se is 96 mA·mg-1 and the potential is -0.1V. The optimum of Pt loading is about 5 wt.%, and the peak current density is 213 mA·mg-1, approach 70 % that of Pt, and the potential is -0.05 V.②The oxygen reduction performance is greatly affected by W-Ru-Se catalysts doped with platinum. The current density is 140 mA·mg-1 and the potential is -0.1V in 0.5 mol·L-1 H2SO4. Under the same conditions, Pt-W-Ru-Se (5 wt.%Pt), the peak current density is 310 mA·mg-1, approach 2.2 times that of W-Ru-Se, 103 % that of Pt, and the potential is 0.15 V. In the presence of CH3OH, Pt-W-Ru-Se (5 wt.%Pt) is better than Pt in activity and methanol-tolerant, but comparing to W-Ru-Se, Pt-W-Ru-Se (5 wt.%Pt) is not markedly improved in methanol-tolerant.In short, the current densities have the relation of iPt-W-Ru-Se > iPt > iPt-Mo-Ru-Se> iMo-Ru-Se> iW-Ru-Se.③The structure of catalyst affect catalytic performance of oxygen reduction. In the synthesis process of catalyst, the shorter interval of adding Pt(CO)2 and other reactants, the better activity of catalyst and methanol resistance. |
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