Impact of Ni on degradation of 8.5 mol% Y2O3-doped ZrO2 (8YDZ)

The efficiency of solid oxide fuel cells is influenced by the electrochemical performance of the anode (8YDZ/Ni composite). One criterion is the ionic conductivity of the 8YDZ. Pure 8YDZ degrades by 40% (950 C) within thousands of hours due to the decomposition on the nanoscale. Moreover, the degradation of 8YDZ in the presence of Ni proceeds 50 times faster. To clarify this, NiO thin films were deposited onto 8YDZ substrates by electron-beam evaporation. Sintering (1400 C, 5 h, air) facilitated the incorporation of Ni into the 8YDZ. The influence of annealing under reducing conditions (950 C) was studied by transmission electron microscopy. The decreased solubility of Ni in 8YDZ under reducing atmosphere led to the nucleation and growth of Ni crystallites within the 8YDZ. Typical chemical variations on the nanoscale after degradation were revealed by analytical TEM techniques (FEI Titan 80-300 microscope). The enhancement of the cation mobility and thus the accelerated decomposition of 8YDZ are explained by the electron capture of dissolved Ni under reducing atmosphere. The strain field due to the increase in size (rNi2+=69 pm, rNi0=125pm) leads to larger displacements of the surrounding ions, which effects the diffusivity of ions in the vicinity of the exsolving Ni.