Improvement of SOFC Electrodes Using Mixed Ionic Electronic Conductors

Y. Matsuzaki and M. Hishinuma Fundamental Technology Research Laboratory Tokyo Gas Co., LTD.

16-25 Shibaura, 1-chome, Minato-ku Tokyo, 105 Japan

1. Introduction

Since the electrode reaction of SOFC is limited to the proximity of a triple phase boundary (TPB), the local current density at the electrode and electrolyte interface is larger than mean current density, which causes large ohmic and electrode polarization". This paper describes an application of mixed ionic-electronic conductors to reduce such polarization by means of (1) enhancing ionic conductivity of the electrolyte surface layer by coating a high ionic connductors, and (2) reducing the local current density by increasing the electrochemically active sites.

2. Experimental Procedure

Tetragonal yttria-stabilized zirconia (3 mole % Y203) sheets of 6 cm square and 100 |im thick were used as electrolyte. Sr-doped LaMn03 (LSM) was used for the air electrode material. Ce08Sm02O19 (SDC) was used as a coating material of the electrolyte surface of the air electrode side. To obtain the dense SDC layer, a Ce metallo-organic compound was added to the SDC powder and the mixture was milled by a ball mill. The slurry was then screen-printed on the electrolyte sheet before firing. The fuel electrode was prepared by using the PMSS process" which gave high dispersion of the fine YSZ particles in the cermet. Ni-YSZ cermet was typically used for the fuel electrode, but in some cases, Ce02 was deped to YSZ in the cermet to investigate the effect of it.

3. EffectofSDC layer on the cathode

The ionic conductivity of Figure 1 Cross sectional image or the SDC inter-layer SDC at 1000°C is 0.35 Scnr', prepared at the cathode/electrolyte interface, which is approximately six times

larger than that of 3YSZ2). To reduce the overpotential at the air electrode, the SDC layer was prepared at the cathode/electrolyte interface. As shown in the SEM image in Figure 1, the SDC layer is about 5 Jim thick and dense. Figure 2 shows the V-I characteristics of three types of single cells. Hydrogen was used as a fuel, and air as an oxidizer. Cell (a) had the Ni-YSZ anode and the LSM cathode. Although cell (b) had the same anode of the cell (a), it had modified cathode with the SDC layer. The SDC layer coating was found to improve cell performance. It was also found through half cell tests that the polarization (IR free) characteristics of the air electrodes were almost same, irrespective of the SDC layer. This suggests that the improvement in performance from cell (a) to cell (b) is due to the reduction in ohmic resistance.

Figure 3 shows the effect of the SDC layer on cell durability. The formation of the SDC layer at the cathode/electrolyteinterface is extremely effective in improving long-term stability. SEM observation of aged samples revealed that the air electrode without the SDC layer showed some change in microstructure at the cathode/electrolyte interface, while the air electrode with

1.2 i i i i i | i i i i | i i i i | i i i i | i i i i the SDC layer showed negligible change in the microstructure. This is because of the difference in the reactivity of YSZ and SDC to La/),. The ionic radius of La3* is larger than that of Zr4*, so that La3t can hardly dissolve in Zr02, thereby forms La2Zr2073)-4> which would causes significant loss in cell performance and change in microstructure. Ce4*, on the other hand, has a larger ionic radius than Zr4t. Ce02 can thus dissolve La3*", which are considered to work as a dopant as shown in the reaction formula given below, forming O vacancies.

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