Oxygen reduction reaction (ORR) is an essential step for the operation of polymer electrolyte fuel cells (PEMFCs). It is kinetically so sluggish that the large amount of Pt-based electrocatalysts are used, which weaken the economic competitiveness of PEMFC systems compared to the other energy conversion system such as the internal combustion engine. In order to reduce the cost of electrocatalyst, several types of catalytic materials including nitrogen-containing carbon and Pd-base metal alloys have been proposed, while these have too inferior catalytic performance to be employed for the current PEMFC stack. Therefore, it would be practical to decrease the amount of Pt used for the electrode by improving the activity of Pt.
In this work, the carbon-supported core-shell nanoparticles with PdCu alloy core and Pt shell (PdxCuy/C@Pt) were prepared by the galvanic replacement between Cu on the heat-treated PdCu2/C and Pt ions. The effect of type of Pt precursor (K2PtCl6 and H2PtCl6) was investigated on the galvanic replacement, and hence on the catalytic performance of resultant catalysts (PdCu2/C@Pt-K and PdCu2/C@Pt-H). When K2PtCl6 was used as the Pt precursor, the galvanic replacement was impeded by the presence of copper oxide surface species, leading to the inferior catalytic performance. On the other hand, H2PtCl6 was effective precursor for the formation of Pt shell. It is believed that hydrogen ions released by dissolving H2PtCl6 can eliminate the copper oxide surface species, by which facilitate the galvanic reaction between Cu of inner surface and Pt ions. The effect of hydrogen ions on the galvanic replacement was demonstrated by adding HCl into K2PtCl6 precursor solution. The catalytic performance of PdCu2/C@Pt-H was better than that of a commercial Pt/C. The effect of heat-treatment temperature for PdCu2/C and the ratio of Pd to Pt was also studied systematically on the catalytic performance. Among the catalysts studied in this work, the PdxCuy/C@Pt with Pd:Pt=3:1, which was prepared by using PdCu2/C reduced at 500oC, showed the best ORR performance.