Effect of Alloy Composition on Electrocatalytic Activity of PdAu Core/Pt Shell Nanoparticle Catalysts for Oxygen Reduction Reaction
Authors: Kuwahara Taiki , Chiku Masanobu , Higuchi Eiji , Inoue Hiroshi 
Published: February 2020

Introduction

Reducing the Pt consumption in electrocatalyst for polymer electrolyte fuel cells or improving mass activity of Pt (MAPt) for oxygen reduction reaction (ORR) is significant. The core/shell catalysts whose shell is a Pt monolayer can greatly improve the MAPt 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 MAPt 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 MAPt 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 (Pd100-xAux/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 Pd100-xAux/CB was 30 wt. %. A monolayer of Pt shell was deposited on the PdAu NPs by Cu-underpotential deposition and then galvanic replacement (Pt/Pd100-xAux/CB, x=5, 10, 20).1, 3, 4 For Pd100-xAux/CB, the core size was evaluated by transmission electron microscope (TEM). To evaluate electrochemical properties of the Pd100-xAux/CB and Pt/Pd100-xAux/CB catalysts, a Nafion-coated GC disk electrode was prepared according to the previous procedure.1 The ORR activity for the Pt/Pd100-xAux/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/Pd100-xAux/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 Pd100-xAux/CB. The nearest neighbor interatomic distance was evaluated with Vegard law. From the XRD patterns of Pd100-xAux/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 Pd100-xAux/CB was increased with the Au content, but it deviated from the Vegard law. From TEM, mean particle size of Pd100-xAux/CB was ca. 4.2 nm irrespective of the Au content. Fig. 2 shows the MAPt and specific activity of Pt (SAPt) at 0.9 V vs. RHE for the Pt/Pd100-xAux/CB electrodes. Fig. 2 exhibited there was a volcano relationship between MAPt or SAPt and the changed due to Au content of Pt/Pd100-xAux/CB, suggesting that the surface Pt-Pt distance could be optimized by controlling the alloy composition. Pt/Pd90Au10/CB exhibited the highest MAPtin 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).

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Figure 1