A COMPOSITE ELECTROLYTE FOR AN SOFC CONSISTING OF A CERIA SHEET AND A ZIRCONIA FILM DEPOSITED BY THE SOLs-GEL METHOD
- PDF / 1,676,153 Bytes
- 7 Pages / 414.72 x 648 pts Page_size
- 63 Downloads / 165 Views
for most of the power generation loss of an SOFC at such low temperature, and the ionic conductivity decreases when the operating temperature is reduced. Gd or Sm doped ceria (GDC or SDC) are candidates as the electrolyte material for low temperature operating SOFCs, because of their high ionic conductivity [4]. However, ceria-based materials have several problems originating from their instability in a reduced atmosphere. The ionic transport number for ceria is less than 1.0 [4]. This means that the electrolyte short circuits electrically and this reduces the electromotive force and results in a drop in the open circuit voltage from the value expected by the following Nernst equation. 7= f1o+ (RT/nF). ln[(Po, in anode)/(Po, in cathode)]
Moreover, the thermal expansion coefficients of ceria in a reduced atmosphere are larger than those in an oxidized atmosphere. This produces stress in the electrolyte and in the interfaces with the cathode or anode [5]. These problems can be overcome, if the ceria electrolyte surface in a reduced atmosphere (anode side) is covered with a film made of zirconia-based material [6]. In such a case, the film should be as thin as possible in order to retain the low ohmic loss in the electrolyte. Therefore, we prepared our electrolyte thin films by the sol-gel method, because it can provide films of submicron order thickness. In addition, the coating apparatus of the sol-gel method is much simpler than other commonly used methods such as electrochemical vapor deposition or 185 Mat. Res. Soc. Symp. Proc. Vol. 496 ©1998 Materials Research Society
vacuum processes including rf sputtering. We selected zirconia doped with Sc3' Al 3" as the film material to cover the ceria, because it exhibits the highest ionic conductivity among zirconia doped with rare-earth ions [7]. In this study, we fabricated a single cell with a composite electrolyte composed of a ceria sheet and zirconia sol-gel film to investigate the potentialities of this kind of electrolyte. EXPERIMENT The sol-gel film samples were prepared by the conventional sol-gel method [8]. We used an alkoxide of Zr, Al and a water solution of Sc(N0 2)3 mixed in alcohol. The 0.7 pmthick films, which consisted of ten individually spin-coated layers, were deposited on a sintered alumina substrate for characterization and on GDC sheets. Each layer was dried at 150'C and calcined at 400'C before the next layer was deposited. Once this process was complete, the films were annealed in air for two hours at 800'C-1600'C for crystallization. An X-ray diffractometer was used to investigate the crystal structure at room temperature. We investigated the morphology of the films and the interfaces between the electrolytes and the
electrode by examining cross-sections with a scanning electron microscope (SEM). We evaluated the ionic conductivity of the samples with the DC four terminal method. Here, the film samples were about 0.8 pm x 0.3 cm x 1.8 cm. Platinum wires were attached to the samples with platinum paste by firing at 800°C. A constant current of
Data Loading...
