12 Electrochemical and Solid-State Letters, 2 (1) 12-15 (1999) S1099-0062(98)08-029-8 CCC: $7.00 © The Electrochemical Society, Inc. Investigation of Enhanced CO Tolerance in Proton Exchange Membrane Fuel Cells by Carbon Supported PtMo Alloy Catalyst S. Mukerjee, a,*,z S. J. Lee, a,* E. A. Ticianelli, a,* J. McBreen, a,* B. N. Grgur, b N. M. Markovic, b P. N. Ross, b,* J. R. Giallombardo, c and E. S. De Castro c,* a Department of Applied Science, Brookhaven National Laboratory, Upton, New York 11973, USA b Materials Sciences Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720, USA c E-TEK, Incorporated, Natick, Massachusetts 01760, USA We report a two- to threefold enhancement of CO tolerance in a proton exchange membrane (PEM) fuel cell, exhibited by carbon supported nanocrystalline PtMo/C as compared to the current state of the art PtRu/C electrocatalysts. The bulk of these nanocrys- tals were comprised of Pt alloyed with Mo in the ratio 8.7:1.3 as shown by both X-ray diffraction and in situ extended X-ray absorp- tion fine structure measurements. Rotating disk electrode measurements and cyclic voltammetry in a PEM fuel cell indicate the onset of CO oxidation at potentials as low as 0.1 V. Further, the oxidation of CO exhibits two distinct peaks, indicating redox behav- ior involving oxyhydroxides of Mo. This is supported by in situ X-ray absorption near edge structure measurements at the Mo K edge. © 1999 The Electrochemical Society. S1099-0062(98)08-029-8. All rights reserved. Manuscript submitted August 10, 1998; revised manuscript received September 28, 1998. Available electronically October 30, 1998. CO tolerance in reformer-based low and medium temperature H 2 /O 2 proton exchange membrane fuel cells (PEMFC) is crucial for the viability of this technology for applications in transportation and portable power generation applications. The choice of an appropri- ate anode electrocatalyst with low susceptibility to CO poisoning and a high kinetic rate for hydrogen oxidation is therefore para- mount. The most commonly used anode electrocatalyst, Pt/C, is sus- ceptible to poisoning by CO, leading to high overpotentials. Alternate binary electrocatalysts containing a more oxidizable ele- ment with ability to activate oxygenated species at lower potentials and hence initiate CO oxidation on the surface with lower overpo- tentials have been the focus of several decades of research. Prior lit- erature is replete with investigations on alloys such as PtSn, 1-2 PtRh, 3 PtRu, 4-6 and Pt with oxygen adsorbing adatoms such as Ge, Sb, and Sn 7-8 etc. In recent years PtRu alloys have received renewed attention as promising candidates for CO oxidation in PEM fuel cells. 9-11 A recent report by Oetjen et al. 12 indicates a fourfold per- formance enhancement with highly dispersed PtRu as compared to Pt at 80°C with CO concentrations up to 250 ppm. Despite these improvements the overpotential with CO concentrations of 100 ppm at moderate temperatures such as 85°C is substantial, resulting in a loss of 270 mV at 1 A/cm 2 . 12 Recently, the electro-oxidation kinetics of H 2 , CO, and an H 2 /CO mixture were studied on smooth and well-characterized PtMo sur- faces in 0.5 M H 2 SO 4 at 60°C. 13,14 Grgur et al. proposed that the oxidation states of Mo surface atoms as well as the nature of Mo sur- face oxides are determining factors in the electrocatalysis of H 2 , CO, and H 2 /CO mixtures on these alloys. It was suggested that the oxy- hydroxide state of Mo [predominantly as MoO(OH) 2 ] is reactive for oxidative removal of CO, but this state can also reduce the availabil- ity of adjacent Pt surface atoms for the dissociative adsorption of molecular hydrogen. 15,16 This work also indicated that a surface composition of 20-25 atom % Mo was optimum for the rate of oxidative removal of CO and the rate of H 2 adsorption. 13-16 It was assumed that, unlike bulk alloys, there is no surface segregation of Pt on the small PtMo/C particles. Accordingly, a 4:1 (Pt:Mo) PtMo/C catalyst was selected. In this paper, we present measurements for electro-oxidation of H 2 , CO, and H 2 /CO mixtures on bimetallic PtMo-4:1 catalyst sup- ported on Vulcan carbon black (PtMo/C). First, we present kinetic measurements using a thin catalyst layer in the rotating disk config- uration (RDE) and, then, the cyclic voltammetry and the polarization data for an anode in a proton exchange membrane fuel cell. In addi- * Electrochemical Society Active Member. z E-mail: mukerjee@bnl480.das.bnl.gov tion, preliminary efforts to understand the role of Mo in these alloys using in situ X-ray absorption spectroscopy (XAS) are included. Experimental PtMo/C supported catalyst in the RDE configuration.—The PtMo/C bimetallic catalysts with a 30% loading, prepared by using a proprietary method, were obtained from E-TEK, Inc. (Natick, MA). The carbon black support was Vulcan XC-72. For comparison, high surface area supported (Vulcan XC 72) Pt/C and PtRu/C (50 atom % Ru) catalysts, each with a 20% loading, were also obtained from E-TEK. The preparation of PtMo/C supported catalyst into the RDE closely followed the method recently described by Schmidt et al. 17 A suspension of 2 mg of the PtMo/C in 1 mL of C 2 H 5 OH was redispersed ultrasonically for 15 min, and then 10 μL (6 μg/PtMo) was pipetted onto the polished surface of glassy carbon disk elec- trode (0.283 cm 2 ) imbedded into a Pine Instruments interchangeable arbor. After evaporation of alcohol at 354 K in a furnace under an argon atmosphere (ca. 5 min), the electrode surface was covered with 10 μL of Nafion (5% solution CH 3 OH/H 2 O) and returned to the furnace where the electrode was heated in an argon atmosphere at 374 K for ca. 10 min. The prepared electrode had a catalyst loading of ca. 6 μg of PtMo alloy/cm 2 , corresponding to a catalyst layer thickness close to 1 μm. 15 The RDE was immersed in the solution under potential control at ca. 0.05 V. All potentials in this paper are referred to the reversible hydrogen electrode (RHE) at the same tem- perature. The PtMo/C catalyst was characterized by X-ray diffraction (XRD) analysis by methods described elsewhere. 13,14 The results indicated a single metallic face-centered cubic (fcc) phase for PtMo/C with a lattice constant of 0.391 ± 0.005 nm. The surface area, as determined from line broadening, was close to 68 m 2 /g. The electrolyte (0.5 M H 2 SO 4 , Baker Ultrex), prepared with triply pyrodistilled water was thermostated at 333 K, in a standard three-compartment electrochemical cell. The reference electrode was a standard calomel electrode (SCE) separated by a bridge from the reference compartment. The CO/H 2 mixtures as well as pure H 2 and CO (6 N H 2 , 4 N CO) were purchased from Matheson; the puri- ty of the argon was 5N8 (Air Products). Data from the rotating disk electrode setup was acquired using a Pine Instruments bipotentiostat interfaced with an IBM PC using Lab View for Windows. X-ray absorption spectroscopy.—XAS measurements were con- ducted at beam line X11A at the National Synchrotron Light Source (NSLS), both at the Pt L (L 3 and L 2 ) and Mo K edges. Details of the beam line optics, monochromator and detuning, etc. are given else- where. 18 Electrodes for XAS measurements were prepared accord- ing to the methodology described elsewhere, 18 the catalyst loading on the electrode was ~10 mg of PtMo alloy/cm 2 . All electrodes were Downloaded 09 Mar 2011 to 129.10.187.55. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp