Microstructure evolution of Ni-yttria stabilized zirconia electrodes for solid oxide electrolysis cells: experimental characterization and phase field modeling

The growing global demand for energy necessitates the widespread adoption of renewable energy sources. To address the intermittent characteristics of renewable energy sources, such as wind and solar energy, numerous energy conversion devices have been developed. Solid oxide cells (SOCs) have generated interest worldwide in the last decades due to the advantages of high efficiency and gas flexibility. Another attractive feature is that the same cell can be operated as either a solid oxide fuel cell (SOFC) for electricity generation or a solid oxide electrolysis cell (SOEC) for fuel production. Despite the numerous advantages of SOCs, the durability during long-term operation has become a major limiting factor for their large-scale commercialization, especially in SOEC mode. The microstructure evolution in the fuel electrode of a single cell has been observed to play a primary role in cell performance degradation. Therefore, quantifying the microstructure evolution during long term operation and understanding the mechanism behind is crucial for the improvement of cell performance and durability. This thesis investigates microstructure evolution of the Ni/yttria stabilized zirconia (YSZ) electrode under different operating conditions through experimental characterizations and phase-field modeling.

In order to clarify the mechanism of cell performance degradation, efforts are devoted to the following three aspects in this thesis:

➢ Microstructure evolution of the Ni/YSZ electrode in CO2 electrolysis. Extensive studies on microstructure evolution of Ni/YSZ in steam electrolysis have been reported, whereas for CO2 electrolysis the information is rather limited. We characterized Ni/YSZ electrode microstructure before and after 1000 hours CO₂ electrolysis. There Ni migrates also toward the support layer and the extent of Ni migration was found to be similar to the one in steam electrolysis. We conclude that the driving force for Ni migration is not necessarily the gradient in the partial pressure of Ni-containing gas species, but the gradient of Ni-YSZ interfacial energy as we and some other researchers speculated.

➢ Ni/YSZ electrode microstructure after being exposed to different steam electrolysis conditions (gas composition, current density, and operation duration) and from different positions along the steam/hydrogen flow direction. We further simulated the distribution of local Ni/YSZ electrode overpotential and oxygen partial pressure at the Ni-YSZ interface using an in-house developed multi-physics model. The obtained microstructure characteristics
correlate well with the electrode and cell electrochemical performance and also with the local and global operating conditions.

➢ A modeling framework coupling the multi-physics electrochemical model and the phase field microstructure evolution model. The effect of overpotential on the microstructure evolution of Ni/YSZ electrodes under OCV, in SOFC and in SOEC was investigated first. The reliability of the model framework was validated by comparing simulations with the experimental results. Subsequently, semi-real-time coupling of electrochemical properties with microstructural evolution was also tried out. Only minor difference was found between time-constant and time-adjusted spatial distribution. Considering the reduction in computational efficiency, it was then concluded that real-time coupling is necessary only in long term scale where significant microstructure changes are expected after each simulation time step.

This work demonstrates that a combination of experimental characterization and numerical modelling utilizing multi-physics and phase field provides important insights into the mechanisms of Ni redistribution in the Ni/YSZ electrode and the influencing parameters behind. Further development of the computational tool can hopefully lead to optimized Ni/YSZ electrode microstructure with enhanced performance and durability, thus promoting the commercialization of the SOEC technology in the long run.