New insights into the fuel electrode degradation during solid oxide cells operation
KHAN M. 1, LENSER C. 1, DELLEN C. 1, SEBOLD D. 1, SOHN Y. 1, WESSEL E. 2, MENZLER N. 1
1 Institute of Energy and Climate Research (IEK-1), Forschungszentrum Juelich, Juelich, Germany; 2 Institute of Energy and Climate Research (IEK-2), Forschungszentrum Juelich, Juelich, Germany
The electrochemical performance of solid oxide cermet electrodes is expected to improve with increased ionic conductivity of the ion-conducting phase. We explored this concept coupled with the effects of microstructural and compositional changes in nickel-yttria-stabilized zirconia (Ni-YSZ) and nickel-scandia-ceria-stabilized zirconia (Ni-ScCeSZ) on their electrochemical performance. The electrodes with 56:44 (NiO:YSZ or ScCeSZ) ratio and different thicknesses were symmetrically screen printed on YSZ pellets and sintered at 1400 °C for 5 h. The electrochemical impedance spectroscopy (EIS) for the prepared cells was then evaluated between 800-600 °C in Ar-3%H2. It was found that the area-specific resistance (ASR) for Ni-ScCeSZ was much higher compared to the conventional Ni-YSZ under these conditions, even though the ionic-conductivity of ScCeSZ is greater than YSZ. Increasing the initial NiO content in NiO-ScCeSZ from 56 wt. % to 65 wt. % also did not improve its electrochemical performance relative to Ni-YSZ. The cells were then analyzed using scanning electron microscopy (SEM), which showed the formation of cracks and a contact loss between Ni and ScCeSZ particles after EIS test. No such cracking was observed in these cells before EIS test, demonstrating that the contact loss is associated with the reduction of Ni. For the conventional Ni-YSZ, all the particles were still intact, before and after EIS test, illustrating the presence of large number of triple phase boundaries and hence a better performance than Ni-ScCeSZ. The X-ray diffraction (XRD) analysis did not show any new phase formation or changes in the crystal structures of Ni(O) and ScCeSZ. The transmission electron microscopy (TEM), however, showed the presence of scandium (Sc)-rich phase at the NiO grain boundaries after sintering and provided some interesting insights into the diffusion-related degradation phenomenon responsible for the contact loss between Ni and ScCeSZ particles.