Study On Electrocatalytic Characteristics Of Pt-based Alloy Nanostructured Catalysts For Methanol Oxidation

Direct methanol fuel cell ?DMFC? has many advantages such as high energy conversionefficiency, high energy density and low or zero emissions. As a new type of proton exchangemembrane fuel cell, it can be used in all aspects of production and life. However, thedevelopment of the promising applications of the DMFC has been hampered up to now due toseveral technical disadvantages. One is the high cost of the catalyst Pt, another is that Pt is easilypoisoned by the intermediated species formed in the methanol oxidation process, which wouldlead to a decreased catalytic activity and stability. Therefore, decrease in the cost of the fuel cellsand enhancement in the catalytic activity and CO tolerance of catalyst Pt are of vital importanceto the development of commercialization of fuel cells. In the present work, a series of alloyedPt_mAg/C catalysts with different composition ?m being the Pt/Ag atomic ratio, m=0.03-1.0? wereprepared by the co-reduction of K₂PtCl₆and AgNO₃precursors with NaBH₄as the reducingagent. Ultraviolet-visible spectroscopy ?UV-Vis?, X-ray diffraction ?XRD? and transmissionelectron microscopy ?TEM? techniques have been used in the characterization of the samples.The electrocatalytic performance of the alloyed Pt_mAg/C catalysts for methanol electro-oxidationreaction ?MOR? both in acidic and alkaline electrolytes was investigated. The influences such ascomposition, electrolyte and chemistry of the supports on the catalytic performance of thecatalysts were also investigated. The specific contents are as follows:1. The electrocatalytic performance of the alloyed Pt_mAg/C catalysts for MOR in alkalineelectrolyte was investigated. Results indicated that the catalytic activity and CO tolerance of Ptwere improved remarkably by the co-presence of Ag. A volcano relationship between the activityand m was found for the Pt_mAg/C catalysts, and both the highest mass-specific activity ?MSA?and intrinsic activity ?IA? of Pt were reached at m=0.5. Interestingly, we also found that thecatalytic activities of the Pt_mAg/C catalysts strongly depended on the high potential limit of CV.When the Pt_mAg/C catalysts were cycled up to0.5V ?vs. SCE?, which induced significant redoxprocess of Ag, they showed ca.50times higher catalytic activity than those without beingsubjected to the Ag redox process ?up to0.1V?. Our study clearly demonstrated that the Agoxide played a crucial role in promoting the catalysis of Pt for MOR in alkaline electrolyte.2. The electrocatalytic performance of the alloyed Pt_mAg/C catalysts for MOR in acidicelectrolyte was investigated. These Pt_mAg/C catalysts after electrochemical dealloying treat_mentin H₂SO₄electrolyte showed enhanced catalytic performance for MOR and CO tolerance ascompared with the pure Pt/C catalyst. In addition, the Pt_mAg/C catalysts with a relatively high m?m?0.5? showed higher mass-specific activity ?MSA? and intrinsic acitivty ?IA? than the Pt_mAg/Ccatalysts with low m ?m?0.1?. The MSA and IA of Pt in the Pt_0.5Ag/C catalyst, which showed the highest activity among the series of Pt_mAg/C catalysts, were ca.2.0and3.6times that of Pt inPt_0.05Ag/C catalyst, respectively.3. The electrocatalytic performance of the Pt_0.5Ag/C samples supported on three differentcarbon supports ?Vulcan XC-72, carbon nanotube and grapheme? for MOR in acidic electrolytewas investigated. It was found that the catalytic performance of the catalysts was stronglydependent on the support. Pt_0.5Ag/graphene catalyst showed the highest activity and stabilityamong the three catalysts, which may be due to the highest dispersion of the metal nanoparticleson the graphene.