Effect of Water Vapor and SOx in Air on the Cathodes of Solid Oxide Fuel Cells
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1041-R03-10
Effect of Water Vapor and SOx in Air on the Cathodes of Solid Oxide Fuel Cells Seon Hye Kim1, Toshihiro Ohshima1, Yusuke Shiratori1, Kohei Itoh1, and Kazunari Sasaki1,2 1
Faculty of Engineering, Kyushu University, Fukuoka, 819-0395, Japan
2
Hydrogen Technology Research Center, Kyushu University, Fukuoka, 819-0395, Japan
ABSTRACT The influence of chemical species, water vapor and SOx, included in air on the voltage drop of SOFCs was studied. La2O3 was formed on the surface of cathode and caused the significant voltage drop of cells under high water vapor concentrations at 800ยบ. The voltage drop of cells also occurred with SO2 in air but was recovered with supplying SO2-free air into the cells. It was considered that the cell voltage drop caused by SOx was due to the adsorption of sulfur and/or the formation of sulfur compounds on the surface of cathodes. INTRODUCTION Global warming is one of the serious environmental problems in these days and CO2 gas produced from the oxidation of fossil resources is a main cause of the warming. Solid oxide fuel cells (SOFCs) are the promising energy system which can reduce the emission of CO2 gas. SOFCs offer a clean, pollution-free technology to generate electricity with high efficiencies [1]. Therefore, there is a strong desire for the commercialization of SOFCs but there are many issues to be solved for commercialization. For the cathode of SOFCs, the ionization of oxygen on a cathode and the diffusion of oxygen ion in a cathode are very important, and the degradation mechanism of cathode during operation should be understood. It was reported that gaseous CrO3 vaporized from interconnects made of chromium-containing alloys damaged the performance of cathode because of the precipitation of nonconductive Cr2O3 particles at triple phase boundaries [2]. It is believed that continuous exposure of cathodes to ambient air containing SO2, NO2, H2S, H2O and O3 might causes cell performance degradation [3-4]. However, the influences of such minor constituents in air on the performance of cathodes have not been investigated yet intensively. Therefore, the aim of the present work is to study the influence of water vapor and SOx contained in air on the performance of SOFCs and to find the degradation mechanism of cathodes. Power generation characteristics of the cells were analyzed with measuring cell voltage at a constant current density under supplying the artificially-contaminated air with water vapor or
SO2 at various operation temperatures. The evolution of surface morphology and the change of chemical composition of the cathode surfaces due to poisoning were analyzed. EXPERIMENTAL DETAILS SOFCs with electrolyte-supported structure and anode-supported structure were used in this study. Composition of electrolyte with a diameter of 20 mm (Daiichi Kigenso Kagaku) was 10 mol% Sc2O3-1 mol%CeO2-89% ZrO2 (abbreviated by ScSZ). High purity 56% NiO-44% ScSZ powder mixtures were used for anodes. The cathode had a two-layer structure. Powder mixture of (La0.8Sr0.2)0.98MnO3 (LSM
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