Effects of temperature and hydrogen concentration during reduction on deformation behavior of NiO- yttria stabilized zirconia used in solid oxide fuel cells

For commercialization of SOFCs, it is important to understand the deformation behavior of anode during reduction, which causes cell deformation and fracture. In this study, the factors of deformation and the effect of reduction conditions were investigated using NiO-YSZ. The samples with low Ni volume fraction shows expansion, while the samples with high Ni volume fraction shows shrinkage, indicating that the Ni volume fraction is one of the major factors. Furthermore, the SEM images show that this fact is due to the connectivity of Ni. Considering the results of previous studies, the boundary of Ni volume fraction between shrinkage and expansion is expected to be about 30%. We clarified that the expansion behavior is due to the release of the compressive residual stress on the YSZ side during the reduction. This is because the strain change in the NiO-YSZ during the reduction and the strain change calculated from the residual stress change on the YSZ side during the reduction match each other in both direction and amount. In addition, the reduction conditions do not affect the expansion mechanism of NiO-YSZ in the temperature of 700–900 °C and hydrogen concentration of 4%–15%.