4500

2. Alternate Electrolytes

There are several electrolytes, which have a considerable higher ionic conductivity than YSZ. The most well known and studied electrolyte is doped Ce02. A problem with Ce02 is that it is partially reduced to Ce^ on the anode side, which induces electronic conduction. This in turn will reduce the open circuit voltage and thus reduce efficiency. However, Prof. Riess"'has shown that a small amount of electronic conductivity can be tolerated, provided the fuel cell is operated close to its maximum power output. Therefore, a project to study the use of Ce02 electrolytes has been conducted at Ceramatec. Earlier problem with deterioration of performance with time were overcome and stable performance in single cells in excess of ten thousand hours has been demonstrated. Ceria electrolytes appear to be very compatible with the selected electrode materials, resulting in very flat I-V curves. Thus, the performance of Ceria based fuel cells at 700-800°C is similar to that of Zr 02 based fuel cells at 1000°C, when operated at 0.6V. This is shown in Figure 2JEfficiency losses due to electronic conductivity are small at 700C or lower.

At the University of Texas at Austin, Prof. Goodenough and his associates have studied the ionic conductivity of doped La Ga03. When using both cation and anion substitution, a composition with an ionic conductivity of 0.08-1.0 S/cm2at 800C was obtained. No electronic conductivity was present even at anode environments (H2 - 3% H20). Single cell experiments showed an OCV near theoretical and a maximum power density of about 350 mW/cm2. This is much lower than theoretically possible, mainly due to a large anode overpotentiaLWoik on improved electrolyte/electrode compatibility is in progress.

3. Single Component Solid Oxide Fuel Cells

The highest degree of compatibility between electrolyte and electrodes is obtained when all are made of the same base material, i.e., Zr02. To achieve the required electronic conductivity and electro catalytic properties, appropriate dopants must be found. The problem is being studied by Prof. W. Worell at the University of Pennsylvania. At present Tb doped Zr02 is being studied for the cathode and Ti doped Zr02 for the anode. Figure 3 shows that the performance loss at 820°C of a cell with a Tb doped ziiconia is considerably lower than that of a conventional LSM cathode. Performance of the Ti02 doped zirconia anode was about the same as that on Zr202 - Ni Cermets, which have already a very low overpotential. However, it is expected that debonding between anode and electrolyte will be improved. Scaleup of the technology is planned for next year.

References

1. I. Riess, "The Use of Mixed Ionic/Electronic Conductors in Fuel Cells," Solid State Ionics, 52(1992) 127-134.

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Figure 1. Performance of Thin Film SOFC at 800°C.

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Figure 1. Performance of Thin Film SOFC at 800°C.

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