Insights into Ni migration in long-term tested Ni-yttria stabilized zirconia electrodes by three-dimensional reconstruction aided phase-field simulations

Nickel (Ni) migration is a crucial factor in the degradation of Ni-yttria-stabilized zirconia (YSZ) electrodes during operation. However, current hypotheses to explain the Ni migration of Ni-YSZ electrodes remain inconsistent. In this paper, three-dimensional (3D) phase-field simulations based on 3D reconstructed microstructures were utilized to understand Ni migration in long-term tested solid oxide fuel and electrolysis cells (SOFCs and SOECs). Phase-field simulations on two representative sub-volumes under open-circuit voltage (OCV) conditions revealed the impact of microstructural heterogeneity on Ni migration. Further simulations under SOEC and SOFC modes demonstrated the combined effect of this heterogeneity and contact angle gradients on Ni migration. A comparative analysis of the evolution of the microstructure, phase volume fraction, and volumetric center for two sub-volumes in OCV, SOEC, and SOFC modes revealed distinct migration behaviors depending on sub-volume size and operating mode. Ni migration was more pronounced in the small sub-volume under SOEC mode, but greater in the corresponding electrolyte and active electrode region of the large sub-volume under SOFC mode. These differences stem from the coupled effect of driving forces from microstructural heterogeneity and contact angle gradients. With further experimental validation, the microstructure of Ni-YSZ electrodes may be quantitatively described, even enabling lifetime predictions by integrating machine learning.