Formation of Thin Films of Solid Electrolytes by Plasma Spraying and Their Application to Solid Electrolyte Fuel Cells

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A solid electrolytic fuel cell (SOFC) is surperior to phosphoric acid fuel cells and molten carbonate fuel cell (MOFC) in such points that the electrolytes do not consume due to evaporation during operation and thickness of the elecrolyte films can be made thinner. Therefore, for setup of SOFC a unit cell can be compactly stacked and des*igned from a small scale to a large one. However, operation temperature is too high at 800 to 1000ﾟC because of low electric conductivity. In order to overwhelm this disadvantage, the electrolyte films should be made as thin as possible. Proton-conductive electrolytes will be used in future because of many advantages for cell reactions. The authors investigated formation of thin proton-conduetive electrolytic films by using the plasma-spray method and evaluated the films and they were applied to SOFC. SrCe_<0.95> Yb_<0.05>O_3-alpha was firstly employed as a proton-conductive electrolyte. Its sprayed films showed the proton conductivity. However, they gave lower specific electric conductivity by one or two ordersand lower mechanical strength. It was found that mixing of SrZrO_3 to SrCe_<0.95>Yb_<0.05>O_<3-> alpha improved mechanical strength without reducing electric conductivity. When plasma sprayed SrCe_<0.95>Yb_<0.05>O_<3->alpha was used as a solid electrolyte in a H_2-O_2SOFC and was characterized. The cell generated around 0.5 V as EMF at 1000ﾟC which was about a half of the theoretical EMF and its discharging current was found to be 0.3 mA/cm^2. This seems to be because H_2 gas penetrated through the sprayed electrolyte film. Denser films should be produced for improvement of the fuel cell performance.