Carbon Supported Platinum Bimetallic Nanoparticle Electrocatalysts For Oxygen Reduction Electro-catalytic

Direct methanol fuel cell (DMFC) is regarded as a promising micro power source for various portable devices due to its high energy density and high enegy efficiency. However, the commercial viability of a DMFC is still hindered by its relatively low power density and limited lifetime. There are many factors that influence the lifetime of a DMFC. Among them, the long-term stability of both anode and cathode catalysts is an important factor. Thus, it is necessary to develop nanosized electrocatalysts with high activity and good stability for a DMFC.In this dissertation, Au clusters modified carbon-supported Pt nanoparticle electrocatalysts were prepared via two different routes. The effects of composition, structure and particle size of Pt-Au bimetallic electrocatalysts on their activity and stability towards the oxygen reduction reaction (ORR) were investigated and compared with that of commercially available E-TEK or Johnson-Matthey catalyst. The obtained main results are as follows:Carbon-supported Pt-Au bimetallic electrocatalysts with different Pt/Au atomic ratios were firstly prepared via a carbonyl complex route. Transmission electron microscopy measurements indicate that metallic Au cluters decorated on the surface of Pt nanoparticles, rather than on the support. As-prepared Pt-Au bimetallic catalysts exhibit a fcc structure. No obvious change in particle size for these bimetallic catalysts was observed after the catalysts were heat-treated at different temperatures. As an example, the mean particle size of the Pt-Au/C catalyst after heat treatment at 600℃is ca. 2.7nm, only showing a slight increase as compared to as-prepared catalyst with a mean particle size of ca. 2.1 nm. Thus, it is clear that the Pt-Au bimetallic catalysts exhibited significantly enhanced thermal stability. Eleccatalytic measurements indicate that the home-made Pt-Au/C catalysts showed slightly enhanced mass activity for the ORR in comparison to E-TEK Pt/C catalyst. Among diffenent Pt-Au catalysts, the Pt-Au catalyst with an atimic ratio of 9:1 exhibits a better perficmance. Such an enhanced catalytic acitivty could be due to the small particle size of the Pt-Au/C catalysts and the electronic effects induced by Au cluters. Moreover, the Pt-Au/C bimetallic catalysts showed significantly enhanced electrochemical stability for the ORR.Generally, Pt-based electrocatalysts should be at a high metal loading level on an adequate support in order to reduce the inner electrical resistance of the catalytic layer in a DMFC. Therefore, Au modified Pt/C at a weight loading of 60% (from Johnson Matthey) was prepared using methanol as reducing agent. X-ray diffraction results showed that the mean particle size of such a catalyst only increased a little after heat treatment at elevated temperatures, again indicative of a good thermal stability of the Pt-Au bimetallic catalysts. Whereas electrochemical measurements demonstrate that the bimetallic catalysts exhibited a better stability towards the ORR than Pt/C catalyst.Obviously, Au clusters decorated carbon-supported Pt nanoparticle electrocatalysts exhibit both good thermal stability and enhanced electrochemical durability towards the ORR. Thus, such a Pt-Au bimetallic electrocatalyst may be the more promising cathode catalyst for a DMFC.