Controllable Synthesis And Performances Investigation Of Pt-based Anode Electrocatalysts For Direct Methanol Fuel Cell

Direct methanol fuel cell (DMFC) is a positive reaearching technique due to the high specific energy, environmentally friendly and portable device. However, high cost and short life-span are two urgent issues in DMFC developmet. In this dissertation, the research is devoted to enhancing the electrocatalytic activity and stability of catalysts for DMFC electrocatalysts.Firstly, high-performance Pt/C catalysts were prepared by microwave-assisted polyol process. Compared to conventional heating, microwave heating deserves the merits: quick and uniform heating, suitable for mass production of catalyst and high reproducibility. To optimize the preparation process of microwave-assisted polyol method, different reaction conditions were deeply investigated. The results of electrochemical characteristizations demonstrate that the Pt/C catalyst prepared by microwave-assisted polyol process at the pH value of about 12 and heating time of 40 s exhibits the best catalytic activity for methanol electrooxidation with highest electrochemical surface area (EAS) and utilization. The reason is that glycolate is replaced by glycolic acid at the lower pH value, which is a poor stabilizer resulting in large particle size. But at much higher final pH value, the ionic strength of the glycolate anion is too strong to prevent the deposition of Pt prticles on the carbon support. The different heating times lead to the different reaction temperatures, which influence the separation between Pt nucleation and growth process. A higher temperature acceralates the reaction rate between metal salt and ethylene, suitable for separation of nucleation and growth process and small particles generation.However, very small catalyst particles are formed if the reaction temperature is too high, which easily stay in the micropores of carbon support, declining the utilization of Pt and decreasing of Pt/C catalyst performance for methanol electrooxidation. On the basis of the optimal preparation technique of Pt/C catalyst, the Pt/Mesoporous carbon and high loading Pt/C catalyst were prepared and show the outstanding performance for DMFC compared to commercial catalyst.To further improve the anti-poisoning ability of catalyst, PtRu/C catalysts were prepared by microwave-assisted polyol process. The optimum preparation conditions are at the pH value of about 8 and heating time of 50 s for methanol electrooxidation. The reason can be concluded that the PtRu/C catalyst prepared at the pH value of 8 has the homogeneous element composition almost near to atomic ratio of 1:1, as well as the suitable size and uniform dispersion. The suitable microwave heating time can increase the reaction temperature and decrease the reduction rate diference between PtCl₆^2-and Ru³⁺, and fit for the formation of uniform composition. The annealing treatment of PtRu/C catalyst demonstrates the Ru species mainly exist as hydrous RuO₂, which promote the methanol oxidation due to good corrosion resistance and electron and proton conductivity. Furthermore, the core-shell Ru@Pt-skin/C catalyst was prepared to investigate the effect of structure on the performance. Compared to PtRu/C, the surface of Ru@Pt-skin/C catalyst is rich in Pt, which can increase the contact interface of Pt and Ru to full play the bi-functional mechanism, and d-band center shift of Pt to strengthen the interaction between Pt and Ru. Morever, Ru@Pt-skin/C structure can effectively inhibit the Ru dissolution and loss to enhance the stability of catalyst.To further decrease the cost and increase the stabality of catalyst, novel structural PtPd/C catalysts were prepared by microwave-assisted polyol process. The increased anti-poisoning ability was analyzed from the point of reaction mechanism, and explored the application of novel structure PtPd/C on the fuel cells. Compared to PtPd/C alloy, PtPd/C nanocrystals alloy shows the significant CO- tolerance, inhibits the growth and agglomeration of catalyst and improves stability for methanol oxidation. The reason is that the PtPd/C nanocrystals are composed of mass truncated octahedron, which contains plenty of stepped atoms and danging band and promotes the break of the chemical bonds. Another novel hollow spherical PtPd/C alloy catalyst with lay-lay structure was prepared by microwave-assisted polyol process and electrostatic self-assembly one-step reductive route. The results show the sandwich PtPd/C with Pd covered by Pt layer avoids Pd oxidation and dissolution in working conditions and enhances electrocatalytic stability toward methanol electrooxidation. The mechanism for the unexpectedly high activity for methanol electrooxidation on the PtPd/C hollow sphere catalyst is that Pd can efficiently oxidize the poisoning small molecular HCOOH directly to CO₂, to change the path and mechanism of methanol electrooxidation. In order to further surpress the catalyst particles growth and agglomeration during the long working time, the PtPd/C nanowires catalyst were prepared by phase-transfer and microwave-assisted polyol process. Its interconnected dendritic networks structure can effectively inhibit the particles dissolution, growth and Ostwald ripening, also prevent the particles embedded in micopore channels of carbon support and increase the efficiency of catalyst.Finally, the catalyst support was investigated and the cheap rare earth oxide CeO₂ was used as a support due to its higher oxygen storage capacity, anti-corrosion and hydrophilicity. The Pt/CeO₂-C catalysts were prepared by facile and effective microwave-assisted ethylene glycol process. The addition of CeO₂ into carbon black can improve the hydrophilicity and wettability of carbon to abtain the Pt deposition on the contact interface of CeO₂ and carbon black smoothly, achieving the contact between Pt and CeO₂ and avoiding the addition of other surfactant. The research also demonstates that Pt/CeO₂-C catalyst with a CeO₂ weight ratio of 20 wt.% exhibits the best catalytic activity for methanol electrooxidation, the reason are the enhanced poisoning-tolerance for CO_ads and CeO₂ nanoparticles as an anchor to block the coalescence of Pt particles. In order to weaken the side-effect resulting from the low electron conductivity and the un-attachment of Pt and CeO₂, Pt/C-CeO₂ catalyst were further investigated through a novel structural design by mesoporous CeO₂ coated with a certain amount of carbon deriving from theβ-cyclodextrin carbonization, and then deposit the Pt nanoparticles by fast and facile microwave-assisted polyol process. The overall procedure does not require complicated steps. The results show that the Pt/C-CeO₂ catalyst exhibited a greatly higher catalytic activity and stability than Pt/CeO₂ and the optimal weight ratio of carbon in Pt/C-CeO₂ is 40 wt.%.