Because oxygen reduction reaction (ORR) at the cathode of fuel cells is kinetically slow and its mechanism is complicated, it is necessary to use Pt-based noble metal catalysts for delivering desired power density. This weakens the economical feasibility of fuel cell systems as a widespread power sources. There have been substantial efforts for cutting down on the cost of ORR catalyst, which includes the design of nonprecious catalyst as well as Pt-based catalyst with high performance. Considering the low activities and durabilities of nonprecious catalysts, however, it is rather practical to increase ORR performance of Pt-based catalysts. In this study, several methodological strategies are applied to design Pt-based catalysts with high ORR performance.
PtCux@Pt/C core-shell catalysts were synthesized by alloying Pt and Cu, followed by a galvanic replacement reaction between Pt ions and Cu on the surface of Pt-Cu alloys. The surface morphology and elemental compositions of the PtCux@Pt/C catalysts were significantly influenced by the initial ratio of Cu to Pt in the PtCux precursors and showed different electronic structures depending on the ratio of surface to bulk compositions. The d-band center of Pt was affected by the formation of porous structure, consequently, altering the catalytic performance. As a result, it can be seen that the morphology and atomic ratio of the particle surface have a significant influence on the catalytic activity. Among the catalysts studied, the PtCu7@Pt/C catalyst showed the best performance. The high durability of the PtCu7@Pt/C catalyst was attributed to the increase in the dissolution potential of the Pt surface layers on the Cu-enriched subsurface layer. In addition, owing to the surface protection by the stable Pt shell layers, the Cu dissolution from the subsurface regions was also considerably retarded.
A similar strategy applied for the preparation of PtCux@Pt/C was employed for the design of (PtAux)1Cu5@Pt/C catalysts in which the catalyst precursor, (PtAux)1Cu5/C-HT, was prepared by alloying Pt, Au, and Cu with a controlled composition. Owing to high standard potential, Au in the sublayer of the surface can enhance stability and activity of catalysts. The prepared catalysts showed a porous core-shell structure with a Pt-enriched surface and their physical properties were affected by the composition of (PtAux)1Cu5/C-HT. The (PtAux)1Cu5@Pt/C catalysts, regardless of their compositions, showed the superior catalytic performances to that of a commercial Pt catalyst. Among the catalysts examined, (PtAu0.1)1Cu5/C@Pt showed the best ORR activity. It was believed that the Au present in the catalyst particles prevents from dissolving Pt on the surface, which responsible for the improvement of (PtAu0.1)1Cu5/C@Pt in durability.
The last approach to improve the ORR performance of Pt-based catalysts is to design Pt-Ni nanoparticles with hollow structures. In this study, unlike conventional methods for the preparation of Pt hollow nanoparticles, carbon-supported Pt-Ni hollow nanoparticles were prepared by an one-pot and one-step process where Pt(NH3)4Cl2 was employed as a Pt precursor. Owing to a relatively low reduction potential of Pt precursor containing amine ligands, Ni nanoparticles are first formed on carbon and Pt is then reduced on the surface of Ni particles by a catalytic action of Ni. The hollow interiors are finally evolved by the Kirkendall effect, yielding carbon-supported Pt-Ni hollow nanoparticles (PtNix/C-H) with a controlled composition. Prepared PtNix/C-H catalysts were observed to have the nonporous shells consisting of Pt-Ni alloy. Due to a hollow-induced surface strain, PtNix/C-H catalysts showed superior ORR performance to that of a commercial Pt/C.
Ammonia is an ideal fuel for fuel cells since it can be easily delivered in a liquid state and oxidized to a non-toxic product such as nitrogen. However, its oxidation compared to hydrogen oxidation is kinetically so sluggish that the design of electrocatalyst for ammonia oxidation reaction (AOR) would be essential for the development of ammonia fuel cells. Compared to other oxidation reaction for fuel cells, ammonia oxidation is not only catalytic element-specific but also highly site-specific reaction. Among the elements tested so far, Pt (100) is reported to be best for AOR. In this study, Pt cube was prepared without using the surfactants. Instead, acetylacetone (acac) was added to a precursor solution for controlling the growth rate of specific crystalline plane. On the other hand, Pt nanoparticles with octahedral shape were formed without using acac. Pt cubes prepared by using acac showed the best AOR performance in both half and unit cells, while Pt cubes prepared by using PVP as a stabilizer was found to have a negligible AOR activity.