Palladium/Cobalt Coated on Multi- Walled Carbon Nanotubes as an Electro- catalyst for Oxygen Reduction Reaction in Passive Direct Methanol Fuel Cells H. Gharibi 1,2 *, F. Golmohammadi 2 , M. Kheirmand 3 1 Department of Material Science and Engineering, 122 S Campus Drive, University of Utah, Salt Lake City, UT 84112, United States 2 Faculty of Science, Department of Chemistry, Tarbiat Modares University, P.O. Box 14115-175, Tehran, Iran 3 Faculty of Science, Department of Chemistry, Yasouj University, Yasouj, Iran Received November 29, 2012; accepted July 01, 2013; published online October 16, 2013 1 Introduction Recently intense research has focused on alternative energy technologies that can reduce the dependence on fossil fuels and reduce the pollution from these fossil fuels. Fuel cells become the promising power sources due to their high- energy conversion efficiency and environmental affinity. In the last decade, great attention has been paid to the proton exchange membrane fuel cells (PEMFCs) fueled by H 2 or liq- uid fuels, such as low molecular weight alcohols [1, 2]. Com- pared with hydrogen-fed fuel cells, direct methanol fuel cells (DMFCs) are recognized to be a promising power source for portable electronic devices and electric vehicles [3, 4], due to their high energy conversion efficiency, low pollutant emis- sion, low operating temperature, and simplicity of handling and processing of liquid fuel [5]. The electroreduction of oxy- gen has been extensively studied owing to plays a major role in electrochemical energy conversion in fuel cells and metal- air batteries, and is equally important for corrosion processes. One of the challenging problems involving the oxygen reduc- tion reaction (ORR) is to find out an effective electrocatalyst to accelerate this reaction with low over-potential at low tem- perature. Numerous studies in the literature over the past few years have demonstrated that Pt based binary or multi- metallic alloy catalysts exhibit enhanced activity to ORR in comparison with pure Pt in acidic solution [6–12]. Balbuena et al. [13] suggested that enhancement of the catalytic activity is due to the presence of the secondary metal on the catalyst surface, where alloys of Pt with cobalt and chromium are Abstract This work reports the synthesis of Pd-based alloy electroca- talysts of Co supported on multi-walled carbon nanotubes (MWCNTs) and their evaluation as cathode materials in a passive direct methanol fuel cell (PDMFC). The X-ray dif- fraction (XRD) analysis showed well-defined reflections cor- responding to a face centered cubic phase of palladium. As compared to the Pd/MWCNT electrocatalyst, the bimetallic alloy electrocatalysts with the different Pd x Co atomic ratios showed highly enhanced mass activity (MA) for the oxygen reduction reaction (ORR); however, the significant enhance- ment in the specific activity (SA) by a factor of about 1.2–5.6 for the ORR was found on the Pd x Co alloy electrocatalysts in the presence and absence of methanol electrolyte solution. This enhancement SA in of the Pd-based electrocatalysts was correlated to the changes in the lattice parameter and Pd x Co surface composition. Surface area changes of Pd- based electrocatalysts supported on MWCNT were evalu- ated using an accelerated durability test (ADT). The results obtained using the ADT were correlated to the performance of the Pd-based electrocatalysts in the PDMFC. A better per- formance was obtained for the cell using Pd 3 Co/MWCNT (2.53 mW cm –2 ) compared to Pd/MWCNT (1.64 mW cm –2 ) and Pt/C-Electrochem (1.20 mW cm –2 ) as cathode in the PDMFC. In the presence and absence of methanol the impe- dance Bode spectra showed one time constant that asso- ciated to follow a four electron pathway. Keywords: Cathode Stability, Multi-Walled Carbon Nano- tubes, Oxygen Electroreduction, Pd x Co Bimetallic Nanopar- ticles, Passive Direct Methanol Fuel Cells (PDMFCs) – [ * ] Corresponding author, h.gharibi@uta.edu; gharibi@modares.ac.ir; h.gharibi@gmail.com FUEL CELLS 13, 2013, No. 6, 987–1004 © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 987 ORIGINAL RESEARCH PAPER DOI: 10.1002/fuce.201200220