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

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(CH₃COO)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(CH₃COO)₂ and KAuCl₄ at 4 °C, and loaded on CB  (Pd_100-xAu_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-xAu_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-xAu_x/CB, x=5, 10, 20).^{1, 3, 4} For Pd_100-xAu_x/CB, the core size was evaluated by transmission electron microscope (TEM). To evaluate electrochemical properties of the Pd_100-xAu_x/CB and Pt/Pd_100-xAu_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-xAu_x/CB was evaluated in an O₂-saturated 0.1 M HClO₄ aqueous solution at 25 ºC by rotating disk electrode method. Durability against Pt dissolution of the Pt/Pd_100-xAu_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-xAu_x/CB. The nearest neighbor interatomic distance was evaluated with Vegard law. From the XRD patterns of Pd_100-xAu_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-xAu_x/CB was increased with the Au content, but it deviated from the Vegard law. From TEM, mean particle size of Pd_100-xAu_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-xAu_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-xAu_x/CB, suggesting that the surface Pt-Pt distance could be optimized by controlling the alloy composition. Pt/Pd₉₀Au₁₀/CB exhibited the highest MA_Ptin 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

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