Microstructure and anodic properties of Ni/YSZ cermets in solid oxide fuel cells

The anodic performances of Ni/YSZ cermets were studied in relation to their microstructure. For the preparation of cermets, two different sets of powder mixtures (NiO and YSZ) were used with a variation in the Ni loading. For the first-type cermets, the slurry of NiO (average particle size = 12.5 mu m) and YSZ (average particle size = 0.21 mu m) mixture was silk-printed on YSZ disks, followed by a heat-treatment for electrode adhesion and reduction with H-2. Before the slurry preparation for the second-type cermets, however, the particle size of both NiO and YSZ was adjusted to be ca. 3 mu m by a heat-treatment (at 1350 degrees C for 5 h) of the mixture and subsequent ball-milling. The SEM photographs indicated that in the resulting former electrode, Ni channel formation was not facilitated enough that anodic activity was poor due to low population of the reaction sites. On the other hand, in the latter type electrode, Ni and YSZ particles of a uniform size were evenly distributed on the YSZ disk surface and the particles were well-connected to each other. This desirable microstructure greatly improved the anodic activities. Among the second-type cermet electrodes of different compositions (25-65 vol% of Ni), the best properties were observed with those of even population (near to 40-50 vol% Ni). It is likely that in these cermets the similar-sized starting powders and near to equal population for each component provide the richest reaction sites with a well-developed channel structure and also a high stability against Ni sintering.