Increasing Stability and Activity of Core-Shell Catalysts By Preferential Segregation of Oxide on Edges and Vertexes: Oxygen Reduction on Ti-Au@Pt/C

Abstract

The slow kinetics of the oxygen reduction reaction (ORR) is one of the key obstacles for the widespread commercialization of polymer electrolyte membrane fuel cells (PEMFCs), which has been established as one of the zero-emission power sources for the future [1-4]. Pt and its alloys with transition metals are frequently used as electrocatalysts for the cathodic oxygen reduction. However, the high cost and low utilization of Pt of the state-of-the-art Pt catalysts continues to humper its broad application in PEMFCs. This can be resolved using the Pt monolayer catalysts having low Pt loading, high utilization efficiency, as well as high activity and durability [5, 6]. We recently showed that Pt monolayer shell deposited on Pd or PdAu alloy core has higher activity and stability than pure Pt electrocatalysts [6]. However, total Pt group metal content is still too high for practical application. As a non-Pt group metal, Au is a good candidate for the core material since it is stable under the oxidizing conditions of the ORR. However, the lack of interest in the Au@Pt core-shell catalysts can be attributed to two impeding effects on ORR activity, that are, the strong bonding of OH and O to Pt due to the up-shifts of d-band center for Pt on Au core induced mainly by the strain effects [7], and the blocking of active sites of Pt due to the Au segregation onto the Pt surface [6, 8]. In this contribution we are reporting on a novel Au@Pt core-shell catalyst with the low-coordinated surface sites doped by Ti oxide, e.g. vertex and edges, while Pt occupies the surface facets, Ti-Au@Pt (Fig. 1). Its mass (3.0 A mg–1 Pt) and specific activities (1.32 mA cm–2 Pt) are about 13 and 5 times higher than the corresponding activities of the commercial Pt/C catalyst (0.22 A mg–1 Pt and 0.254 mA cm–2 Pt), respectively (Fig. 2). This mass activity is among the highest reported for the Pt based core-shell nanoparticles under similar testing conditions, while the durability tests show no activity loss after 10,000 potential cycles from 0.6 to 1.0 V (vs. RHE). By correlating electron energy-loss spectroscopy and X-ray photoelectron spectroscopy analyses with electrochemical measurements, we attribute the superior activity and durability of the new Ti-Au@Pt catalyst to its distinct microstructure. The enhanced ORR activity of Ti-Au@Pt/C compared to commercial Pt/C is due to a more effective hydrogenation of OHad on its Pt surface than commercial Pt/C catalyst. This can be attributed to the repulsive interaction between the OHad on Pt and the oxide species on a neighboring Ti. Improved durability can also be attributed to the presence of the Ti which prevents Au atoms from segregating onto the Pt surface and thus can largely retain the electrochemical surface area. This core-shell catalyst points to a new direction for the ORR and similar catalysts design and optimization. Acknowledgment Work at Brookhaven National Laboratory is supported by US Department of Energy, Division of Chemical Sciences, Geosciences and Biosciences Division, under the Contract No. DE-AC02-98CH10886.