Results And Discussion

The major goals of anode development are not only to increase the initial electrochemical performance, but to increase their long-term stability. One mechanism of anode degradation is the coarsening of the microstructure due to sintering of fine nickel particles to large particles. This coarsening can be prevented by a continuous 8YSZ-skeleton or framework in the microstructure, which limits the space for the growth of coarse nickel particles. To achieve such 8YSZ-framework inside the anode, deagglomeration and stabilization, as well as homogenization of the initial powders in the spray suspension are of major importance. Best results were achieved by milling the initial powder (56% NiO with 22% 8YSZ as recieved and 22% 8YSZ pre-calcined at 1300°C for 2 hours ) together with the solvent and the dispersant for 48 hours in a ball mill. This powder preparation leads to a homogeneous distribution of nickel- and 8YSZ-particles in the microstructure. Table 2 documents the main manufacturing and microstrucural features of such an anode, the averaged current-potential-curve of which is given in Fig. 1. Due to the formation of a continous and fine 8YSZ-framework in the microstructure, the anode showed an enhanced long-term stability. A galvanostatic long-term test was performed for 300 hours (950°C, 0,2 A/cm2). The measurement started immediately after the anode had reached the working temperature. The increase of the overpotential was less than 9% per 1000 hours.

By optimizing powder preparation and suspension stabilization, powder particles with less than 1 pm could be successfully processed for cathode manufacture, so the required fine cathode microstructure could be achieved. Further optimization was performed concerning the content of electrolyte material in the cathode, the sintering conditions and the layer thickness. A detailed report about this optimization is given in (7). The manufacture and microstructure of the optimized cathode is characterized in Table 2. A mean overpotential of about -50 mV was measured at a current density of -0.8 A/cm2 at 950°C in air. The standard deviation, which was calculated from more than 30 measurements, was 16 mV or 32%. The electrochemical activity was stable for at least 100 hours at -0.2 A/cm*. The fineness and homogeneity of the microstructure is presented in Fig. 2.

Table 2 Manufacturing and microstructural features of WPS-Electrodes ( / bottom layer onto the electrolyte, 2 top layer)

Layer

Composition fwt.%]

Powder [Um]

Sintering (3h) [°C]

Thickness [Um]

[Jim]

CATHODE/

60% LSM 40% 8YSZ

0 0

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