Understanding Thermal Cycling Degradation Mechanism of Solid Oxide Electrolysis Stack

Thermal cycling stability is critical for the practical application of solid oxide electrolysis cells (SOECs) when facing frequent start-up and shutdown operations, accompanied by thermo-mechanical stresses and thus interfacial degradation. To develop deeper insight into the degradation mechanism of SOEC stacks during thermal cycling, a 3-cell SOEC stack was subjected to 5 thermal cycles (400–750 °C). The stack exhibited noticeable degradation phenomenon during the initial 3 thermal cycles, whereas the degradation rate decreased in the subsequent thermal cycles. Electrochemical impedance spectroscopy reveals that applying a forward polarization current density of 0.200 A·cm −2 after each thermal cycle can partially recover the increased impedance and thus alleviate the degradation caused by thermal cycling. The forward polarization is beneficial for mitigating the migration of Ni toward the electrolyte observed during electrolysis mode. Furthermore, nickel redox cycling can improve the porosity and facilitate gas diffusion of the hydrogen electrode, whereas excessive cycling may promote Ni detachment, thereby reducing the triple-phase boundary length and consequent performance degradation. The oxidation in the central area of the current collector is the direct cause of increased interfacial resistance in the stack.