Methanol-Tolerant Heterogeneous PdCo@PdPt/C Electrocatalyst for the Oxygen Reduction Reaction J. Yang 1 , C. H. Cheng 1 , W. Zhou 1,2 , J. Y. Lee 1,3 *, and Z. Liu 2 1 Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore 2 Present address: Engineering School, Temasek Polytechnic, 21 Tampines Avenue 1, Singapore 529757, Singapore 3 Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, Singapore Received December 10, 2009; accepted May 05, 2010 1 Introduction The direct methanol fuel cell (DMFC) technology is still beset with a number of materials-related issues. One of them is the low activity of Pt-based cathode catalysts in the oxygen reduction reaction (ORR), which is exacerbated by the pres- ence of methanol crossover from the anode through the poly- mer electrolyte membrane (PEM) [1–6]. The improvement of both the activity and selectivity (i.e. fuel tolerance) of the Pt-based catalysts in ORR and the reduction of the Pt content in the catalysts to lower the catalyst cost are current objectives in the continuing development of the DMFC technology. Recent experimental results have shown that appropri- ately engineered nanoscale surfaces of the catalytic metals could increase catalytic activity and catalyst stability in the fuel cell reactions [7–13]. For example, Stamenkovic et al. [12] showed that a single crystal Pt 3 Ni(111) surface with a Pt monolayer and a Ni-enriched sublayer was 10 times more active than Pt(111) in ORR; and 90 times more active than the current state-of-art Pt/C catalysts. The authors attributed the enhancement to changes in the electronic structure (down- shifted Pt d-band centre) and the rearrangement of the sur- face atoms in the near-surface region [10, 13]. A more accessi- ble approach than the ultrahigh-vacuum (UHV) technique [14] used by these authors to fabricate nanoscale engineered surfaces is by means of a core–shell nanoparticle architecture. Core–shell nanoparticles are traditionally prepared in the presence of organic capping agents in order to achieve parti- cle size control and uniformity of size distribution. However, the capping agent is often difficult to remove resulting in a decrease of catalytic activity [15]. While the capping agent problem may be alleviated by using surface metal galvanic – [ * ] Corresponding author, cheleejy@nus.edu.sg Abstract We have prepared carbon-supported nanoparticles with the heterogeneous structure of a PdPt shell on a PdCo core which are effective for the oxygen reduction reaction (ORR) in the presence of methanol. The preparation was based on the galvanic replacement reaction between PdCo/C nano- particles and PtCl 4 2– , a method of general utility which can be extended to the preparation of other core-shell electroca- talysts. The heterogeneous PdCo-core and PtPd-shell archi- tecture was confirmed by multiple techniques including high resolution transmission electron microscopy, energy dispersive X-ray spectroscopy, powder X-ray diffraction and X-ray photoelectron spectroscopy. The activity of the PdCo@PdPt/C catalyst in ORR was evaluated in acidic solu- tions both with and without methanol (0.1 M). The results showed four to sixfold increases in activity over a standard Pt/C catalyst with no apparent loss of catalyst stability. It is inferred that the strain effect from the lattice mismatch between the shell and core components is the major contri- butor for the enhancement of ORR activity and selectivity. Keywords: Direct Methanol Fuel Cells, Heterogeneous Structure, Oxygen Reduction Reaction, Palladium, Platinum FUEL CELLS 10, 2010, No. 6, 907–913 © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 907 ORIGINAL RESEARCH PAPER DOI: 10.1002/fuce.200900205