Result And Discussion

Figure 1 shows the typical generation performance of the single cells with the active anode area of 200-450 cm2. The diameter and active length are 13 and 700 mm for the 200 cm2-cell, 16 and 700 mm for the 280 cm2-cell, and 21 and 900 mm for the 450 cm2-cell, respectively. The maximum power was 60 W at the current density of 0.6 A cm-2 for the 200 cm2-cell, 75 W at 0.49 A cm2 for the 280 cm2-cell, and 125 W at 0.47 A cm*2 for the 450 cm2-cell.

Figure 2 shows the long-term stability for the cell with the active area of 28 cm2 (the diameter and active length are 21 and 50 mm) at 0.3 A cm-2. In this test, three times of shut-down due to outage of the testing device had occurred but the degradation of the cell performance resulting from the thermal cycle to room temperature was not observed.

Long-term stability has been evaluated for the cell with the active anode area of 450 cm2 at a constant current density of 0.2 and 0.3 A cm-2. Figure 3 shows a typical result. The degradation rate was fairly low.

For the practical use of SOFC, the cell should be sustainable against operational thermal cycles or emergency outages. Thermal cycle test consisting often times cooling below 200°C and twenty times cooling to 600°C were carried out for the 450 cm2-cell. Figure 4 shows a typical result of generation performance before and after the thermal cycle test. The degradation from 1.5 to 3.0 % was observed for I-V characteristics, but the reason for this degradation is now under investigation.

Practical SOFC should be assembled into a module for increasing power output. Therefore it is important to develop the cell structure suitable to manufacturing the cell bundle. We have been developing an Integrated Tubular-Type (TIT) cell which has a monolithic structure consisting of parallel-connected three tubular cells. Figure 5 shows the appearance of the ITT celL A bundle with 9 series-connected ITT cells (the active area of each ITT cell was 840 cm2) was tested. Figure 6 shows I-V and I-P characteristics of the bundle. The open circuit voltage was ca. 8.63 V which was almost equal to the theoretical value. The maximum output power was 1020 W at 0.22 A cm"2. The mean generation performance of the bundle is about 15 % lower than the performance ejected from that of the single cell. Further investigation and development, for example control of the fuel gas flow through the channels of the cell bundle , are required to further improve the cell and bundle generation performance.

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