Effect of Core Size on Activity and Durability of Pt Core-Shell Catalysts for PEFCs

Introduction For the reduction of Pt usage in polymer electrolyte fuel cells (PEFCs), it is important to improve the mass activity for oxygen reduction reaction (ORR). Core-shell catalysts, in which Pt monolayer is deposited on different metal cores, are one of the key techniques to improve the mass activity of Pt and thereby to reduce the Pt usage in PEFCs. Adzic et al. have reported that Pt/Pd/C core-shell catalysts have high specific activity for ORR, which is brought about by an electronic effect of the Pd core [1]. However, there have been few reports on the durability of the core-shell catalysts, which is important for practical use. In the present study, Pt/Au/C and Pt/Pd/C core-shell catalysts were prepared by under potential deposition (UPD) of Cu [2] using Au/C and Pd/C cores with different particle sizes. The activity for ORR and durability of the Pt/Au/C and Pt/Pd/C catalysts were investigated, and the effects of core size on the activity were elucidated. Two types of Au/C with Au particle size of 5 nm (Tanaka Kikinzoku Kogyo, 30 wt%) and 10-30 nm (BASF, 50 wt%), and three types of Pd/C with Pd particle sizes of 3.4, 4.8 and 11.5 nm (Ishihuku Metal Industry, 30 wt%) were used as core materials to elucidate the effects of core size on the activity and durability. The Au/C and Pd/C particles were loaded at 14.1 mgMetal cm on glassy carbon (GC) disk electrode (6 mmφ). The M/C (M = Au, Pd) electrode was immersed in deaerated 2 mM CuSO4 + 0.5 M H2SO4 solution, and the potential was polarized at 0.30 V (vs. RHE) for 10 min, at which CuUPD monolayer (ML) was deposited on M. Pt1ML/M/C catalyst was prepared by replacement of CuUPD atoms with Pt atoms in 5 mM K2PtCl6 solution under Ar atmosphere. These processes were repeated to obtain multi-layer PtxML/M/C (x = 1-5) catalysts. The mass activity was evaluated by hydrodynamic voltammetry using the RRDE technique from 1.0 to 0.1 V at 10 mV s in 0.1 M HClO4 at 25°C. Durability against platinum dissolution of the Pt/M/C catalysts was tested by square-wave potential cycling between 0.6 (3 s) and 1.0 V (3 s) at 60°C, while durability against carbon corrosion was tested by triangular-wave potential cycling at 0.5 V s between 1.0 and 1.5 V at 60°C. Figure 1 compares the mass activities of PtxML/Au/C (x = 1-2) and PtxML/Pd/C catalysts (x = 0-2) at 0.9 V. Pt1ML/Au/C (5 and 10-30 nm) and Pt1ML/Pd11.5 nm/C catalysts had by ca. 5-fold and 6-fold higher mass activity, respectively, than that of a standard Pt2.8 nm/C catalyst (TKK, TEC10E50E). This may be due to a high utilization of Pt atoms in core-shell structure as well as an electronic effect of the core materials (especially for Pd core), an effect of Pt-Pt interatomic distance, and so on. The mass activity decreased with an increase in Pt loading (i.e., the number of Pt atomic layers). The mass activity was independent of the core size for PtxML/Au/C, whereas the larger core gives a higher mass activity for PtxML/Pd/C. This is probably because Pt coverage is not complete for the smaller Pd core. Figure 2 shows the changes in normalized electrochemical surface area (ECSA) during the durability tests against Pt dissolution for Pt/Au/C and Pt/Pd/C. Opposite effects of core size on the durability were observed: Pt1ML/Au5nm/C (smaller core) and Pt1ML/Pd11.5nm/C (larger core) showed high durability comparable with that of Pt2.8 nm/C. However, the larger Au core (10-30 nm) and the smaller Pd core (3.4 nm) give inferior durability. These facts indicate that Pt dissolution into the core material (for Au cores) and the dissolution of the core material (for Pd cores) are important degradation mechanisms as well as Pt dissolution into the solution.