This journal is c the Owner Societies 2012 Phys. Chem. Chem. Phys., 2012, 14, 9683–9695 9683 Cite this: Phys. Chem. Chem. Phys., 2012, 14, 9683–9695 Carbon-supported Pd–Co as cathode catalyst for APEMFCs and validation by DFT S. Maheswari, a S. Karthikeyan, b P. Murugan, b P. Sridhar* a and S. Pitchumani a Received 1st March 2012, Accepted 1st May 2012 DOI: 10.1039/c2cp40655a Carbon supported PdCo catalysts in varying atomic ratios of Pd to Co, namely 1 : 1, 2 : 1 and 3 : 1, were prepared. The oxygen reduction reaction (ORR) was studied on commercial carbon- supported Pd and carbon-supported PdCo nanocatalysts in aqueous 0.1 M KOH solution with and without methanol. The structure, dispersion, electrochemical characterization and surface area of PdCo/C were determined by X-ray diffraction (XRD), Transmission Electron Microscopy (TEM) and Cyclic Voltammetry (CV), respectively. The electrochemical activity for ORR was evaluated from Linear Sweep Voltammograms (LSV) obtained using a rotating ring disk electrode. The catalysts were evaluated for their electrocatalytic activity towards oxygen reduction reaction (ORR) in Alkaline Polymer Electrolyte Membrane Fuel Cells (APEMFCs). PdCo(3 : 1)/C gives higher performance (85 mW cm 2 ) than PdCo(1 : 1)/C, PdCo(2 : 1)/C and Pd/C. The maximum electrocatalytic activity for ORR in the presence of methanol was observed for PdCo(3 : 1)/C. First principles calculations within the framework of density functional theory were performed to understand the origin of its catalytic activity based on the energy of adsorption of an O 2 molecule on the cluster, structural variation and charge transfer mechanism. 1. Introduction Some of the most advanced fuel cells in terms of applications and commercialization are alkaline fuel cells (AFCs). They were used successfully in space programs in the mid 1960s. AFCs provide distinct advantages compared with polymer electrolyte membrane fuel cells (PEMFCs); in particular, the inherently faster kinetics of the oxygen reduction reaction (ORR) in AFCs allows the use of non-noble metal electro- catalysts. 1,2 The major operating constraints for liquid AFCs are carbonation and electrolyte leakage. An anion exchange membrane (AEM) can be used as an electrolyte instead of a conventional liquid electrolyte in an AFC to avoid the above-said problems. Replacement of liquid electrolytes in conventional AFCs with metal cation free AEMs that can transfer hydroxide ions (OH  ) can revitalize the AFC technology and impart a new momentum to it and are called anion exchange polymer electrolyte membrane fuel cells (APEMFCs). 3,4 In recent times, increasing attention has been focused on APEMFCs due to the potential usage of non-Pt catalysts, which account for the improved durability and faster ORR kinetics in alkaline medium than in acidic medium. 5 Conventional carbon-supported platinum-based materials so far have been the most active, efficient, relevant and successful electrocatalysts for electrochemical devices at the current technology stage in acidic medium. However, in alkaline electrolyte, a wide range of non-noble metals and their oxides are stable enough for practical applications due to lower corrosion than in acidic electrolyte. This allows the investigation of a large number of non-Pt (transition metal) catalysts, especially with regard to ORR and methanol tolerance (i.e., an oxygen reduction reaction selective electrocatalyst) characteristics in alkaline medium. 6 Very high ORR catalytic activity of Pd in alkaline solution compared with Pt has been reported 7–9 and the same has also been observed by us in our earlier study. 10 In recent years, Pd and Pd alloys have attracted greater attention as non-Pt catalysts because of the high proportion of surface to bulk atoms and about 50 times more abundance of Pd than Pt. 11 Recent developments in using Pd and Pd-based alloys as ORR catalysts have shown promise for their use in APEMFCs. 12 The main objective of Pd-based catalyst research is the development of synthesis methods resulting in high catalytic activity of Pd. Metal based alloys have been found to exhibit higher electrocatalytic activities than pure metal. 13 Bard et al. suggested that for Pd–M alloys, the metal M constitutes the site for breaking the O–O bonds. 14 Incidentally, the addition of a second metal such as Co, Fe to Pd decreased the metal particle size. So alloying of Pd with transition metals (Fe, Co, Cr, Ni, Ag, etc.) is of significance for improving the ORR activity due to the changes in Pd–Pd bond distance, modification of electron configuration and alteration a CSIR - Central Electrochemical Research Institute - Madras Unit, CSIR Complex, Chennai, 600 113, India. E-mail: psridhar@csircmc.res.in; Fax: +91-044-22542456; Tel: +91-044-22542068 b CSIR - Central Electrochemical Research Institute - Karaikudi, 630 006, India PCCP www.rsc.org/pccp PAPER