A PtAu Nanoparticle Electrocatalyst for Methanol Electro- oxidation in Direct Methanol Fuel Cells Jong-Ho Choi, a Kyung-Won Park, b, * In-Su Park, c Keon Kim, d Jae-Suk Lee, a and Yung-Eun Sung c, * ,z a Department of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju 500-712, South Korea b Department of Chemical and Environmental Engineering, Soongsil University, Seoul 156-743, Korea c School of Chemical and Biological Engineering and Research Center for Energy Conversion and Storage, Seoul National University, Seoul 151-744, Korea d Division of Chemistry and Molecular Engineering, Korea University, Sungbuk-Ku, Seoul, 136-701, Korea PtAu alloy nanoparticle catalysts for use in direct methanol fuel cells were synthesized by reduction with NaBH 4 and freeze-drying and their electrocatalytic activities were examined for reactions of methanol, formaldehyde, and formic acid. The extent of alloy formation and average particle size were characterized by X-ray diffraction and transmission electron microscopy. X-ray photo- electron spectra confirmed that the surface state of Au in the PtAu alloy was exclusively metallic, while the Ru in the alloy has not only metallic characteristics but is also present in oxidized form. Based on electrochemical measurements, the PtAu catalyst showed a more enhanced activity than pure Pt for the oxidation of methanol, having a lower onset potential and a larger current density. The electrocatalytic activity of the PtAu catalyst was also enhanced in the oxidation of formic acid but not formaldehyde. This provides evidence for differences in the catalytic activity of PtAu in the oxidation of low molecular weight organic com- pounds. The origin of the enhanced catalytic activity of the PtAu catalysts is discussed from the standpoint of a modified methanol oxidation pathway in which formaldehyde, formic acid, and CO are produced as putative intermediates. © 2006 The Electrochemical Society. DOI: 10.1149/1.2224055 All rights reserved. Manuscript submitted May 4, 2005; revised manuscript received December 12, 2005. Available electronically July 26, 2006. Direct methanol fuel cells DMFCs have attracted considerable interest for use as a power source in portable electronic devices because they have a variety of merits, including low operating tem- peratures, ease of handling liquid fuel, and the high-energy density of methanol. 1-5 The excellent catalytic activity of Pt for methanol oxidation, especially at low temperatures, makes this metal electro- catalyst attractive for use as an anode in DMFCs. However, pure Pt is readily poisoned by carbon monoxide  CO, an intermediate that is produced during the electro-oxidation of methanol at low tem- peratures. Based on the bifunctional mechanism, the CO-poisoned Pt can be regenerated via the reaction of surface CO with oxygen species associated with an element such as Ru to yield CO 2 . In addition to the applicability range of this mechanism, electronic ef- fects ligand effects might also be involved in the enhancement by Ru. 6-16 Other species such as Bi, Os, Rh, W, Sn, Ni, and Mo have been reported to have a positive effect on catalytic activity with respect to methanol oxidation. 17-27 Although Ru and other elements are added to Pt to enhance the oxidation of CO, the rates of metha- nol oxidation are still too low to guarantee the commercialization of a DMFC. In order to design a new anode catalyst with an acceptable activity, it is essential to clarify the mechanism involved in the oxi- dation of methanol. Although the formation of various adsorbates, such as  CH x OH ad , - COH ad ,  -HCO ad ,  -COOH ad , linear bonded CO, and bridged CO, have been proposed as reactive inter- mediates in methanol oxidation from in situ infrared spectroscopy data, it is well known that formic acid  HCOOH and formaldehyde  HCHO are formed as intermediates and are dissolved in the solution. 28-33 The goal of this study was to design Pt-based alloy catalysts containing Au for use in methanol electro-oxidation. Au has long been known as being catalytically less active than other noble met- als. Since the discovery of the high catalytic activity of nanosized Au depending on cluster size and types of supports used, many recent studies have focused on the development of Au-based cata- lysts for use in low-temperature catalytic combustion, the partial oxidation of hydrocarbons, the hydrogenation of unsaturated hydro- carbons, and the reduction of nitrogen oxides. 34-38 It also has been reported that Au-based catalysts are potentially capable of being effectively employed in fuel cells, such as hydrogen purification and electrochemical oxidation reactions of organic species. 39-42 How- ever, the science of Au catalysis is still quite new and is currently undeveloped. In this article, PtAu alloy nanoparticle catalysts for enhancing the electro-oxidation of methanol were synthesized by a borohydride reduction method combined with freeze-drying. The structural prop- erties of the alloy catalysts were characterized using X-ray diffrac- tion XRD, transmission electron microscopy TEM, and X-ray photoelectron spectroscopy XPS, and the electrochemical charac- teristics of the catalysts with respect to methanol oxidation were evaluated. In order to identify the origin of the catalytic activity enhancement by PtAu nanoparticle catalysts in the electro-oxidation of methanol, their catalytic activities were also investigated assum- ing formaldehyde and formic acid are produced as reaction interme- diates. Experimental Unsupported Pt-based alloy nanoparticles were synthesized by a conventional borohydride reduction method using NaBH 4 Aldrich Chemical Co. combined with freeze-drying. H 2 PtCl 6 ·xH 2 O, RuCl 3 ·xH 2 O, and HAuCl 4 ·3H 2 O all from Aldrich Chemical Co. in the desired stoichiometry were completely dissolved in Millipore water  18 M cm. After several hours, the solutions were reduced by the addition of 0.2 M NaBH 4 . A threefold excess of reducing agent over the valences of the metal salts was used, which was sufficient for the complete reduction of the salts to the elemental state. After precipitation, the resulting materials were washed with Millipore water several times and dried by freeze-drying using liq- uid N 2 without the use of any heat-treatment. Structural analyses of pure metal and Pt-based alloy catalysts were carried out using a Rigaku X-ray diffractometer equipped with a Cu K source. To estimate the extent of alloy formation of the Pt-based alloy catalysts, the 111 peak was fitted using the Lorentzian/Gaussian function. The composition of the alloy cata- lysts was determined by Vegard’s law. 24 The size of the alloy cata- lysts was investigated by TEM on a JEOL instrument JEM- 2000FXII at a 200 kV accelerating potential. A specimen was prepared by ultrasonically suspending the particles in deionised wa- ter. A drop of the resulting suspension was deposited on a standard Cu grid covered with a carbon film  200 mesh and allowed to dry before being inserted into the microscope. In order to analyze and * Electrochemical Society Active Member. z E-mail: ysung@snu.ac.kr Journal of The Electrochemical Society, 153 10 A1812-A1817 2006 0013-4651/2006/15310/A1812/6/$20.00 © The Electrochemical Society A1812