Effects of impurities on microstructure in Ni/YSZ–YSZ half-cells for SOFC

Long-term properties of Ni/yttria-stabilized zirconia (YSZ) cermet anode+YSZ electrolyte pellet (d=7.4 mm) half-cells were evaluated experimentally. Two commercial NiO powders containing different levels of impurities were used for the anodes. The durability was evaluated at temperatures of 850 °C over 1500–1800 h in H2 with 1–3% H2O under an anodic load of 300 mA cm−2. The anodes containing, among others, SiO2 and Na2O at a concentration level of hundreds ppm degrade faster (within a period of 150–400 h) than those with a few tens ppm of SiO2. The impurity phase is characterized as a type of sodium silicate glass phase and is found to segregate and accumulate at the anode/electrolyte interface causing serious damage to the YSZ electrolyte in the vicinity of the interface. In the center region of the circular pellet, the YSZ grains are separated by silicate glass. The distribution of these impurities and the extent of structural damage are qualitatively compared to the profile of the H2O concentration.