Nickel and tungsten combined with cerium oxide catalyst: A suitable SOFC anode for direct reforming of biogas

摘要

The study of the catalytic properties of anode materials is relevant for the development of Solid Oxide Fuel Cells (SOFCs) that can sustain direct internal conversion of the feeding fuel. Biogas is a mixture of methane and carbon dioxide with trace of other compounds. It is produced by anaerobic digestion of waste streams containing organic matter, such as waste-water, animal waste, food waste, or landfills. The interest in biogas utilization as fuel stems largely from its high methane content, renewable status, relatively low feed stock cost, and increasing resource potential (due to population growth)[l]. Biogas reforming is essentially carbon dioxide reforming of methane (also known dry reforming of methane). Indeed, the high operation temperatures of SOFCs make them ideal for direct internal reforming of biogas. Unfortunately, the state of the art SOFCs with Ni-YSZ anodes suffer from serious carbon deposition when operation on carbon-containing fuels, including biogas, because carbon formation on the anode materials is thermodynamically favored and well catalyzed by nickel [2]. Tremendous efforts were devoted to the development of non-nickel anodes with improved coking resistance. However, those materials displayed decreased electrochemical activity for hydrocarbon oxidation and reduced carbon deposition rate; as a result, relatively low power output was often observed. Thus, Ni-based cermets are still the best choice as materials for SOFC anodes due to their favorable electrochemical activity, high conductivity, easy fabrication and good thermal and chemical compatibility with the electrolyte. In this context, ceria-based materials show significantly higher ionic conductivity and enhanced direct oxidation of hydrocarbons due to the rapid lattice oxygen mobility [3]. Also, the addition of tungsten to Ni based catalysts exhibited an improved catalytic stability for dry reforming of methane [4]. On the basis of the above considerations, WNi alloys combined with ceria could be an efficient system suitable to be used as catalyst anode in a SOFC fed by biogas.