Effect of Alloy Composition on Electrocatalytic Activity of PdAu Core/Pt Shell Nanoparticle Catalysts for Oxygen Reduction Reaction

Abstract

Introduction Reducing the Pt consumption in electrocatalyst for polymer electrolyte fuel cells or improving mass activity of Pt (MA Pt ) for oxygen reduction reaction (ORR) is significant. The core/shell catalysts whose shell is a Pt monolayer can greatly improve the MA Pt for ORR. Recently, we have succeeded in the preparation of Pd nanoparticles (NPs) loaded-carbon black catalysts (Pd/CB) with ca. 4.2 nm in mean size by using CO as a reducing agent in acetonitrile solution containing Pd(CH 3 COO) 2 . 1 The MA Pt at 0.9 V vs. RHE of Pd core/Pt shell catalyst using Pd/CB as a core (Pt/Pd/CB) was about 5 times as high as that of the commercial Pt/CB (Pt/CB-TKK, TEC10E50E). Wang et al. reported that the surface Pt-Pt distance strongly influenced ORR activity for core/shell catalysts which consisted of various core metals and a Pt monolayer. 2 So, the MA Pt of Pt/Pd/CB higher than Pt/CB-TKK can be ascribed to more appropriate surface Pt-Pt distance. The optimization of the surface Pt-Pt distance must be required for the maximization of ORR activity. It is known that the surface Pt-Pt distance of the Pt shell depend on the atomic radius of core metal. 3 In this study, we attempt to adjust alloy composition of PdAu alloy core NPs to optimize the surface Pt-Pt distance or maximize ORR activity of Pt/PdAu/CB catalysts. Experimental The PdAu alloy NPs were prepared by bubbling CO in acetonitrile solutions containing Pd(CH 3 COO) 2 and KAuCl 4 at 4 °C, and loaded on CB (Pd 100-x Au x /CB, x=5, 10, 20). Then the concentration of precursors was changed to 0.25~2 mM to control particle size of PdAu NPs. The content of PdAu NPs in the Pd 100-x Au x /CB was 30 wt. %. A monolayer of Pt shell was deposited on the PdAu NPs by Cu-underpotential deposition and then galvanic replacement (Pt/Pd 100-x Au x /CB, x=5, 10, 20). 1, 3, 4 For Pd 100-x Au x /CB, the core size was evaluated by transmission electron microscope (TEM). To evaluate electrochemical properties of the Pd 100-x Au x /CB and Pt/Pd 100-x Au x /CB catalysts, a Nafion-coated GC disk electrode was prepared according to the previous procedure. 1 The ORR activity for the Pt/Pd 100-x Au x /CB was evaluated in an O 2 -saturated 0.1 M HClO 4 aqueous solution at 25 ºC by rotating disk electrode method. Durability against Pt dissolution of the Pt/Pd 100-x Au x /CB was tested using square-wave potential cycling between 0.6 V for 3 s and 1.0 V for 3 s at 60 °C. 1 Results and Discussion Fig. 1 shows the relationship between the nearest neighbor interatomic distance and alloy composition of Pd 100-x Au x /CB. The nearest neighbor interatomic distance was evaluated with Vegard law. From the XRD patterns of Pd 100-x Au x /CB (Fig 1), the diffraction peak assigned to Pd(111) was shifted to lower angle when Au content was increased. The nearest neighbor interatomic distance of Pd 100-x Au x /CB was increased with the Au content, but it deviated from the Vegard law. From TEM, mean particle size of Pd 100-x Au x /CB was ca. 4.2 nm irrespective of the Au content. Fig. 2 shows the MA Pt and specific activity of Pt (SA Pt ) at 0.9 V vs. RHE for the Pt/Pd 100-x Au x /CB electrodes. Fig. 2 exhibited there was a volcano relationship between MA Pt or SA Pt and the changed due to Au content of Pt/Pd 100-x Au x /CB, suggesting that the surface Pt-Pt distance could be optimized by controlling the alloy composition. Pt/Pd 90 Au 10 /CB exhibited the highest MA Pt in this study, which was approximately 8 times as high as that of Pt/CB-TKK. Acknowledgement This work was supported by New Energy and Industrial Technology Development Organization (NEDO) through the industrial technology research grant program (08002049-0). References R. Sakai, M. Chiku, E. Higuchi, H. Inoue, ECS Trans. , 41 , 2211 (2011). X. Wang, Y. Orikasa, Y. Takesue, H. Inoue, M. Nakamura, T. Minato, N. Hoshi, Y. Uchimoto, J. Am. Chem. Soc., 135 , 5938 (2013) . J. Zhang, Y. Mo, M. B. Vukmirovic, R. Klie, K. Sasaki, R. R. Adzic, J. Phys. Chem . B, 108 , 10955 (2004) J. Zhang, M. B. Vukmirovic, Y. Xu, M. Mavrikakis, and Radoslav R. Adzic, Angew. Chem. Int. Ed ., 44 , 2132 (2005). Figure 1