Preparation and Characterization of Metal Sulfide Electro-catalysts for PEM Fuel Cells
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Preparation and Characterization of Metal Sulfide Electro-catalysts for PEM Fuel Cells Hua Zhang1, Ysmael Verde-Gómez3 and Allan J. Jacobson1 Alejandra Ramirez2 and Russell R. Chianelli2 1 Department of Chemistry, University of Houston Houston, TX 77204 2 Materials Research and Technology Institute, University of Texas at El Paso, El Paso, TX 79968 3 Centro de Investigacion en Materiales Avanzados, Chihuahua, Chihuahua, 31109, Mexico. ABSTRACT Ruthenium sulfide samples were prepared by flowing pure hydrogen sulfide into an aqueous solution of ruthenium chloride followed by further sulfidation in hydrogen sulfide. The final products were characterized by X-ray diffraction and crystallite-sizes were estimated from line broadening. The specific surface areas of catalysts were measured using the multipoint BET method and compositions were determined by thermogravimetric analysis. Ruthenium sulfide loaded gas diffusion electrodes were fabricated by a spraying technique and their electrochemical behavior studied. The electrochemical oxidation of hydrogen was investigated in a three electrode cell using a ruthenium sulfide loaded gas diffusion electrode as the working electrode with humidified hydrogen containing small amounts of carbon monoxide. Results on the activity and the effects of carbon monoxide with reference to a standard platinum electrode measured at the same conditions show that ruthenium sulfide has a lower activity for hydrogen oxidation but is not susceptible to CO poisoning. INTRODUCTION Polymer Electrolyte Membrane Fuel Cells (PEMFCs) are increasingly used for many purposes, from power generation to portable applications and for electric vehicle applications. Platinum or other Pt alloys are used as electro-catalysts, on both anode and cathode sides. For transportation applications, the fuel source is hydrogen produced by reforming or partial oxidation of hydrocarbons. The reformate gases contain impurities (CO and S) which severely poison Pt electro-catalysts and which must, therefore, be reduced to low levels before entering the fuel cell. The development of an alternative CO tolerant electro-catalyst to platinum has been an important aim in order to reduce cost. The carbon monoxide in the reformate fuel must be reduced to 10 ppm by water gas shift for fuel cell operation, which increases the cost of fuel processing and complicates the overall fuel cell system. Several metal alloy fuel cell electrocatalysts with CO tolerance have been developed, such as Pt-Ru [1], Pt-Sn [2], Pt-Bi [3] Pt-Pd [4], and Pt-Mo [5]. These examples all use Pt as hydrogen catalyst and consequently it is of some interest to investigate the possibility of other types of efficient hydrogen and oxygen electrocatalysts with carbon monoxide and sulfur tolerance. Catalysts based on transition metal sulfides, for example, ReRuS, MoRuS [6] and RuSx [7] have been characterized and investigated as catalysts for molecular oxygen reduction in acid media, due to their selectivity in the presence of methanol. On the other hand, ruthe
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