DOI: 10.1002/chem.201000458 Structural and Electronic Effects of Carbon-Supported Pt x Pd 1x Nanoparticles on the Electrocatalytic Activity of the Oxygen-Reduction Reaction and on Methanol Tolerance Shih-Hong Chang, [a] Wei-Nien Su, [a] Min-Hsin Yeh, [a] Chun-Jern Pan, [a] Kuan-Li Yu, [b] Din-Goa Liu, [b] Jyh-Fu Lee, [b] and Bing-Joe Hwang* [a, b] Introduction The commercialization of fuel cells, either hydrogen or methanol fed, as clean-energy conversion devices largely de- pends on the development of stable and reliable electrocata- lysts. Voltage losses result from poor electrochemical kinet- ics at both the anodes and cathodes of H 2 - and CH 3 OH-fed fuel cells. [1–4] The situation is even worse if methanol crosses over from the anode to the cathode side. [5, 6] The competition between oxygen reduction and methanol oxidation reactions on traditional Pt electrocatalysts causes unfavorable overpo- tential for the oxygen reduction reaction (ORR), and the cathode efficiency decreases. [7–9] A further main factor re- sponsible for the kinetic limitations associated with the cath- ode electrocatalyst in direct methanol fuel cells (DMFCs) originates from the high energy barrier of dioxygen dissocia- tion and from the chemisorption of various oxygenated in- termediates at the active site of the electrocatalyst. [10] These issues are believed to be responsible for the reduced effi- ciency of DMFCs. One ambitious approach to solving the above-mentioned problems is to develop Pt-based bimetallic electrocatalysts with enhanced functionality to achieve superior ORR activi- ty and methanol tolerance. Recently, significant progress has been made towards understanding the role of surface chemistry and electronic structures of Pt-based alloy surfa- ces [11–13] and nanoparticles. [14] The correlation between the d- Abstract: We report a systematic inves- tigation on the structural and electronic effects of carbon-supported Pt x Pd 1x bi- metallic nanoparticles on the oxygen reduction reaction (ORR) and metha- nol oxidation reaction (MOR) in acid electrolyte. Pt x Pd 1x /C nanocatalysts with various Pt/Pd atomic ratios (x = 0.25, 0.5, and 0.75) were synthesized by using a borohydride-reduction method. Rotating-disk electrode measurements revealed that the Pt 3 Pd 1 /C nanocatalyst has a synergistic effect on the ORR, showing 50 % enhancement, and an an- tagonistic effect on the MOR, showing 90 % reduction, relative to JM 20 Pt/C on a mass basis. The extent of alloying and Pt d-band vacancies of the Pt x Pd 1x /C nanocatalysts were explored by extended X-ray absorption fine- structure spectroscopy (EXAFS) and X-ray absorption near-edge structure spectroscopy (XANES). The structure– activity relationship indicates that ORR activity and methanol tolerance of the nanocatalysts strongly depend on their extent of alloying and d-band vacancies. The optimal composition for enhanced ORR activity is Pt 3 Pd 1 /C, with high extent of alloying and low Pt d-band vacancies, owing to favorable OO scission and inhibited formation of oxygenated intermediates. MOR ac- tivity also shows structure dependence. For example, Pt 1 Pd 3 /C with Pt richcore Pd richshell structure possesses lower MOR activity than the Pt 3 Pd 1 /C nanocatalyst with random alloy struc- ture. Herein, extent of alloying and d- band vacancies reveal new insights into the synergistic and antagonistic effects of the Pt x Pd 1x /C nanocatalysts on sur- face reactivity. Keywords: fuel cells · nanoparticles · oxygen reduction reaction · palladium · platinum [a] Dr. S.-H. Chang, Dr. W.-N. Su, Dr. M.-H. Yeh, Dr.C.-J. Pan, Prof. B.-J. Hwang Department of Chemical Engineering National Taiwan University of Science and Technology #43, Sec.4, Keelung Rd., Taipei, 10607, Taiwan (R.O.C.) E-mail : bjh@mail.ntust.edu.tw [b] Dr. K.-L. Yu, D.-G. Liu, Dr.J.-F. Lee, Prof. B.-J. Hwang National Synchrotron Radiation Research Center 101 Hsin-Ann Road, Hsinchu Science Park Hsinchu 30076, Taiwan (R.O.C.) E-mail : jflee@nsrrc.org.tw Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201000458.  2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2010, 16, 11064 – 11071 11064