Degradation of solid oxide cell (SOC) stacks with the support of electrochemical impedance spectroscopy (EIS) and modelling

The degradation of solid oxide cell (SOC) stacks needs to be alleviated to accelerate its commercialization. Therefore, a time-dependent model is proposed for a SOC stack designed by Forschungszentrum Jülich GmbH for the fuel cell and the steam electrolysis modes. The model employs a macroscale modeling approach, computational fluid dynamics. The duration of the simulation is up to ~ 100 kh. The model can be used to attribute the causes of severe degradation and serve as guidance for future stack design. The modeling framework includes well-known degradation processes, such as metal interconnect oxidation, chromium poisoning, nickel agglomeration and migration, and manganese poisoning. The simulated degradation is supported by experimental data, such as voltage-time plots and electrochemical impedance spectroscopy. Simulations reveal that for the state-of-the-art SOC stack under the fuel cell operation, degradation is primarily attributed to nickel agglomeration and migration, and manganese poisoning. Additionally, a good protective coating is crucial for maintaining a low degradation rate. For the steam electrolysis mode, nickel migration reduces the thickness of the reactive fuel electrode by forming an unreactive layer, resulting in a high degradation rate. Based on these findings, an optimization of the stack design is carried out to reduce the degradation with the help of the model. Major modifications include using a different ionic conductor in the fuel electrode, increasing the thickness of the fuel electrode, and applying a metal interconnect with a low content of manganese. According to simulations, under the same operation conditions, the optimization helps decrease degradation rates by two-thirds and by seven-eighths for the fuel cell operation and the steam electrolysis operation respectively.