Hiroyuki Yoshida

Fabrication and Characterization of Ni-Doped Ceria Anode-Supported Cells Using Lanthanum Gallate-Based Electrolyte

Lanthanum gallate-based compounds (LSGM) are drawing attention as electrolyte materials of intermediate temperature SOFC that operates with 600 800oC (1). The ionic conductivity of LSGM at 800oC is comparable to that of yttria-stabilized zirconia (YSZ) at 1000oC. On the other hand, the attempt to apply YSZ to intermediate temperature operation SOFC is widely done. It is known that good performance can be obtained at even 800oC or lower because IR drop of the electrolyte is minimized by stacking and co-sintering of the NiO-YSZ anode sheet and YSZ electrolyte sheet (2,3). It is expected that the cell with higher performance can be obtained by thinning the electrolyte even when LSGM is used as an electrolyte. However, it has a problem that the reaction product having high electric resistance (LaNiO3based material) is formed by the reaction of Ni in the anode and La in the electrolyte during co-sintering of the Ni-containing anode sheet and the LSGM electrolyte sheet. Then, the investigation on controlling the diffusion of La was carried out by inserting the La-doped ceria (LDC) interlayer between the Ni-containing anode and the LSGM electrolyte (4). If the doped-ceria compound with higher ionic conductivity is used, the cell performance is expected to improve further, because the ionic conductivity of LDC is known to be relatively low among the rare-earth doped ceria compounds (5). In this study, therefore, the anode-supported cells with the doped-ceria compounds with higher ionic conductivity than LDC and with LSGM electrolyte were fabricated and the cell performances were measured. NiO-Ce0.8Sm0.2O1.9 (SDC) anode substrate sheet were prepared by doctor-blade technique. NiO and SDC powder mixture was ball-milled with organic binder and plasticizer to obtain the slurry. The slurry was cast and dried to be the seat. The typical thickness of the resulting sheets was 0.32 mm. The sheet was cut to be disks of 42 mm in diameter, and the slurry of the doped-ceria compound for the interlayer was screen-printed on one side of the disks (described as Sheet A). The slurry of LSGM (La0.9Sr0.1Ga0.8Mg0.2O2.85, prepared by solid-state reaction method of each oxide) was screen-printed on Sheet A, and co-sintered at 1400oC, which was described as Sample 1. On the other hand, Sheet A was sintered at 1500oC, and then LSGM slurry was screen-printed and sintered at 1400oC (Sample 2). The slurry of Sm0.5Sr0.5CoO3-δ (SSC) cathode was screenprinted on LSGM and sintered at 1100oC. The samples after cathode sintering of Sample 1 and 2 are described as Cell 1 and 2, respectively. Fig. 1 shows the SEM image of the cross-section of Cell 1. Good performance was expected as the electrolyte was dense in spite of the pores in the interlayer. However, the cell performance was lower than that of the standard LSGM cell. In order to confirm the chemical reaction of the electrolyte and the anode, the surface of the LSGM before cathode sintering was analyzed by Xray diffraction. The XRD pattern looked like that of LaNiO3 rather than that of LSGM, therefore, the highly resistive phase was formed by the interdiffusion of La and Ni. Then, instead of co-sintering, the sheet composed of the anode sheet and the interlayer was sintered at the higher temperature than that of Cell 1 before the screen-printing of the LSGM slurry. Fig. 2 shows the SEM image of the cross-section of Cell 2. The interlayer of Cell 2 was not so dense sintered even at 1500oC, and the performance of Cell 2 was lower than that of the standard cell. The doped-ceria powder of higher sinterability is demanded, and the improved cell is being fabricated. The result using the improved cell will be presented at the meeting.