Shell Thickness Effect on ORR Activity of Au Core - Pt Shell Electro-Catalyst

1. Introduction

   Polymer electrolyte fuel cells (PEFC) have been expected as a power source of fuel cell vehicles (FCV). For the commercialization of PEFCs as FCV use, reduction of the Pt cost and to accelerate the slow oxygen reduction reaction (ORR) are inevitable. One of promising approaches enhancing ORR activity and reduction of Pt usage is to design electrocatalysts having monolayer amounts of Pt on the surface of suitable metal nanoparticles. R.R Adzic et al. reported that the mass activity of Pt monolayer core-shell catalyst is several times higher than that of Pt nanoparticles [1]. Recently, Pt-modified Au catalysts have been attracting much attention due to their high activity in ORR [2]. However the ORR mechanism of the core-shell catalysts has not been fully understood.

   This study examines Au core - Pt shell electrocatalysis. To understand the ORR mechanism of core-shell catalyst, it is essential to clarify the influence of the number of Pt layer on the ORR activity under the PEFC operation condition. For the investigation of ORR mechanism of Pt core-shell catalyst, in situ X-ray absorption spectroscopy (XAS) technique is applied. We prepared Pt_x/Au/C (x: Pt shell layer number, x= 1-5) core-shell catalysts and focus on the investigation of them to elucidate the dependence of ORR on their electronic structure.

    

2. Experimental

   The Au core - Pt shell catalyst was prepared by the way R. R. Adzic et al. have reported [3]. The ORR activity was measured with rotating disk electrode. In situ XAS measurements for Pt L_ΙΙΙ-edge and L_ΙΙ-edge of Pt_x/Au/C (x=1-5) catalysts were carried out by using synchrotron radiation at the beam lines BL01B1, Spring-8, Hyogo, Japan with an in-situthree-electrode cell. All of the measurements were performed by fluorescence method.

    

3. Results and Discussion

   Figure 1 shows cyclic voltammograms for Au/C and Pt_x/Au/C (x=1-5). For Pt₁/Au/C and Pt₂/Au/C, it was observed the Au oxide reduction peak. The Au oxide reduction peak decreased with increasing Pt layer number. The peak was almost disappeared for Pt₃/Au/C. Pt coverage was increased with increasing Pt layer number.

   Figure 2 shows the specific activities and the mass activities for Pt_x/Au/C (x=1-5). These activities are enhanced by core-shell technique. The specific activity and the mass activity decreased in increasing Pt layer number and approached that of Pt/C.

   The electronic structure of Pt_x/Au/C (x=1-5) was investigated by in situXAS measurements. The 5d-orbital vacancy was calculated according to the method by Mansour et al. [4], and results are summarized in Figure 3. With the increase in Pt layer number, the 5d-orbital vacancy decreased and closed to the bulk value. The change of 5d-orbital vacancy is one of factor to change ORR activity in Au core – Pt shell catalysts.

   Acknowledgement

   This study was supported by Japan New Energy and Industrial Technology Development Organization (NEDO).

   References

   1. J. Zhang, Y. Mo, M. B. Vukmirovic, R. Klie, K. Sasaki, R. R. Adzic, J. Phys. Chem. B, 108, 10955 (2004).

   2. J. Miomir, B. Vukmirovic, Y. Xu, M. Mavrikakis, R.R. Adzic, Angew. Chem. Int. Ed., 44, 2132 (2005).

   3. S. R. Brankovic, J. X. Wang, R. R. Adzic, Surf. Sci., 474, L173 (2001).

   4. A. N. Mansour, J. W. Cook, Jr., D. E. Sayers, J. Phys. Chem. , 88, 2330 (1984).