Design And Preparation Of Multi-Metallic Nanostructured Fuel Cell Catalysts

Increasing awareness of the environment and limited energy rescources to reduce the reliance on fossil fuels make us search for a cheaper, cleaner, smaller and more efficient energy supply mode. Therefore, it is important to explore a feasible and novel fuel cell catalyst route by improving the chemical and physical properties of these materials against the drawbacks of commercialization due to high cost, sluggish kinetics and long-term stability of fuel cell catalysts. The main results can be summarized as follows:1. A general tenplated-directed electrochemical method has been developed to synthesize one dimensional nanoparticle tubes in nonaqueous solution (polar aprotic solvent, DMSO). The synthesis mechanism of Pt, Pd, Au and Ag tubular materials with uniform tube wall, high aspect ratio, and high quality has been analyzed and explained base on the physical and chemical properties of solvent medium. Especially, the Pd and Pt tubes have hierarchical structure, in which the tube is built by semi-aristate sphere of 40 nm that consist of nanoparticles with 3-6 nm.2. Oxide-supported noble metal catalysts have potential application in fuel cells. The developed electrochemical method has also been used to synthesize the ternary PdAuCu heterostructure catalysts for oxygen reduction reaction, in which the Cu component almost keep the same, while the Au/Pd component ratio increase continuously. The results indicate that the oxygen reduction activity is improved because the Cu component can be easier to be oxidized to CuO which demonstrates both adsorption sites and mediator function for O2 when Au component is added in PdCu system. Moreover, this catalyst demonstrates highly stability for H2O2 reduction due to the introduction of Au. Therefore, this material provides possibility for development of mixed gas/liquid oxidant fuel cells. The nonaqueous solution electrochemical method has been further developed to synthesize Pd/Au heterostructure nanoparticle tubes with controlled component and length by tuning the concentration ratio of Pd/Au and applied potential. The electrocatalytic activity for ethanol oxidation has also been studied and improved by controlling the component ratio and thus adjusting the interface area. The results indicate that the optimal activity and ability for ethanol oxidation was obtained when little amount of Au in Pd/Au system, where the electronic structure was highly modified. 3. The surface restructuring on metal surfaces is a very common phenomenon. The Pt-rich porous PtCu alloy catalysts have been obtained by annealing the AAO template-supported PtCu nanoparticle tube in reductive conditions. This material demonstrates high lattice strain owing to the smaller atomic radius in the bulk, which can be one of the driving forces for lage scale restructuring under potential cycling assisted by the adsorption/desorption of oxygenated species. The results indicate that increasing the thermal annealing temperature can increase the lattice ordering which will highly affect the restructuring. The electrocatalytic activity and stability increase with the increase of the restructuring ability. After 10,000 cycles of stability test and subsequently cleaning of the surface contaminants, the catalyst with higher restructuring ability has higher restorable ability. Based on these results, the rearranged surfaces of PtNi system show high surface active sites, high activity and high restorable ability than those of commercial Pt/C catalyst.4. A novel electrochemical surface treatment method has been developed to prepare Pt monolayer or Pt shell low Pt content electrocatalyst. A low Pt content PtPdCu is used to investigate the surface segregation and restructuring phenomena. After thermal annealing treatment, the Pt/Pd migrates to the surface and a mass of Cu stay in the bulk results in large lattice strain. The results indicate that the surface restructuring highly enhanced the surface roughness and thus improved electrochemical active surface area. Moreover, the solute Pt and Pd atoms respectively tend to migrate to the surface and core region due to surface segregation differences, whereas Cu atoms with almost no leaching remain in the core after potential cycling. Thus, the PtPdCu materials can inhibite the leaching of Cu. The surface Pt/Pd component ratio increases from 11 to 50 atom % with increasing the potential cycling. The continuous atomic ratio change between Pt and Pd on the surface is further monitored in situ by selecting formic acid as probe molecule. Therefore, the surface segregation was confirmed by both quantitative and qualitative methods. The Pt/Pd atomic ratio continuously increase with increasing the potential cycling number, which modified the surface electronic structure and positively shift 70 mV of the desorption of oxygenated species. Therefore, the adsorbate bond energy and activity relationship was established and perfectly demonstrate the"Sabatier principle".