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Satoshi Ichikawa Institute for NanoScience Design, Osaka University, Osaka 560-8531, Japan

Satoshi Seino and Takashi Nakagawa Graduate School of Engineering, Osaka University, Osaka 565-0871, Japan

Hideo Daimonb) Hitachi Maxell, Ltd., Osaka 567-8567, Japan

Yuji Ohkubo, Junichiro Kugai, and Takao A. Yamamoto Graduate School of Engineering, Osaka University, Osaka 565-0871, Japan (Received 16 September 2011; accepted 3 February 2012)

A nanoparticle catalyst of PtRuAu/C was synthesized by including an Au precursor in the radiolytic process for preparing a PtRu/C catalyst. Their methanol oxidation activity and electrochemical durability were measured by linear sweep voltammetry before and after potential cycling treatment. PtRuAu/C had a significantly higher durability than PtRu/C while maintaining a comparable high activity. The morphology and substructure of the nanoparticles were investigated by energy-dispersive x-ray spectroscopy, x-ray diffraction, and x-ray absorption fine structure spectroscopy. Metallic nanoparticles with diameters of about 2 nm were obtained; they probably had Pt-core/PtRu-shell structures. Transmission electron microscopy observations after potential cycling revealed that 2-nm-diameter nanoparticles containing Au did not coarsen, whereas nanoparticles without Au coarsened significantly to 3.7 nm. Some crystal defaults were observed in the coarsened particles, implying that the coarsening was caused by Ostwald ripening. The Au addition to catalyst particles consisting of PtRu inhibits coarsening and consequently improves the electrochemical durability.

I. INTRODUCTION

Direct methanol fuel cells (DMFCs) are one of the most promising power sources for mobile devices in the near future.1 Their main advantages are the ease with which cartridges can be replaced and their high volumetric energy density, which is about 10 times greater than that of lithium-ion batteries. However, the high cost of Pt in the catalysts in DMFCs is a critical barrier to their commercial application. It is thus necessary to improve the catalytic activity, the durability, and the productivity of the synthesis process. PtRu nanoparticle catalysts have high activities for methanol oxidation,2 which is the anodic reaction in DMFCs. However, improved durability and synthesis productivity are still required to reduce costs. With the aim of developing highly durable catalysts, many researchers have investigated the degradation mechanism of Pt nanoa)

Address all correspondence to this author. e-mail: [email protected] b) Present address: Graduate School of Engineering, Doshisha University, 1-3 Tatara-Miyakodani, Kyotanabe-shi, Kyoto 610-0321, Japan. DOI: 10.1557/jmr.2012.65 J. Mater. Res., Vol. 27, No. 7, Apr 14, 2012

particle catalysts. In particular, coarsening of catalyst particles is a serious problem because it reduces the active surface area.3 Durability test with potential cycling to high potential [about 1.1 V versus normal hydrogen electrode (NHE)] is a standard technique.4 Potential cycling to high