Synthesis Pt-based Core-shell Nanocatalysts As Oxygen Reduction Reaction Catalyst For PEMFC With Different Reductant

The high cost and low stability of the traditional Pt/C catalyst have limited the practical application of proton exchange membrane fuel cell. In recent years, it is of great interest to find more active catalysts which have better performance, durability and lower cost than the widely used carbon-supported platinum catalysts. Pt with core shell structure is highlighted as a promising candidate. The catalytic properties for Pt based core shell catalyst for oxygen reduction is directly related to the synthetic method. The Ptbased core shell catalyst with different morphology, structure and composition can be realized by suitable synthesis method. Among them, chemical reduction synthesis has the potential to achieve mass production of Pt based core shell catalyst due to the diversity of reductant and reaction system.According to the characteristics of the different reductantsof hydrazine hydrate, ethanol and sodium borohydride, we designed different process route to sythesize Pt based core shell catalysts. ICP, X-ray diffraction（XRD） and high angle annular dark field scanning transmission electron microscopy（HAADF-STEM）, coupled with energy dispersive X-ray spectroscopy（EDX）, are used to characterize crystallite morphology and composition. The ORR activities of the nanoparticles are measured with a rotating ring disk electrode（RRDE） technique. The important results are as follows:（1）It is difficult to reduce CoCl2 to Co nanoparticles by hydrazine hydrate, because the brucite-like β-Co（OH）₂ will be produced first.（2）We synthesized Pt nanowires network with sodium citrate dispersed CoOH（CO₃）₀.5 nanorods as a template: CoOH（CO₃）₀.5 and K2PtCl6 were dispersed indeionized water and added into ethanol with vigorous stirring, changing the ratio of CoOH（CO₃）₀.5 to K2PtCl6 and deionized water to ethanol, we could gain a stable colloidal solution. The reaction solution was then heated at about70°C and after being heated for about 1 h, Pt nanowires network other than Pt based core shell catalyst were synthesized.（3）Well defined and highly disperse ¹0 nm Co@PtCo/C core-shell nanocatalysts and hollow Pt₃ Co nanospheres covered by Pt shell with a thickness about 0.8nm were synthesized successfully. It was confirmed by XRD,HAADF-STEM, and the EDX element mapping and linear scan results.（4） After acid treated, Pt₃ Co hollow sphere with ultra Pt shell was formed. The specific activity and mass activity to ORR of the given carbon supported Pt₃ Co hollow sphere nanocatalysts isabout 6 and 4 times than that of the conventional Pt/C catalysts respectively. After 5000 cycles of CV, the specific activity and mass activity ofthe Pt₃ Co hollow sphere with ultra Pt shell decreased to 88% and 80% of the original ones respectively while thespecific activity and mass activity of Pt/C decreased to 70% and 68% of the original ones. The carbon supported Pt₃ Co hollow nanospheres with ultra Pt shell show good catalytic activity and stability to ORR.