A Microstructural perspective of air-electrode delamination in the symmetric YSZ-LSM cells under long-term electrolysis operation

Abstract

Electrolysis is crucial in the renewable energy field, as it facilitates the storage of surplus electricity in the form of pure hydrogen. Recently, high-temperature solid oxide electrolysis cells (SOECs) have gained traction as a promising next-generation technology [1]. However, achieving continuous operation is a challenge due to the rapid delamination and degradation of the air electrode, which is not observed during the operation of solid oxide fuel cells. Delamination often leads to instantaneous catastrophic failure, making it challenging to obtain systematic information after complete failure. Therefore, most proposed mechanisms, such as pressure buildup caused by oxygen gas generation [2], formation of secondary phases [3], and cation migration [4], lack convincing evidence. In our study, we systematically observed the structural degradation that occurs during long-term electrolysis operation, particularly at the air-electrode interface of symmetric cells composed of yttria-stabilized zirconia (YSZ) electrolyte and strontium-doped lanthanum manganite (La0.8Sr0.2MO3, LSM) electrodes, using transmission electron microscope (TEM). We examined the microstructure of a cell that was operated for 200 hours at a commonly used current density of 1.5 A/cm2. During the operation, a sharp increase in the cell voltage curve was observed at multiple intervals, indicating a rapid decline in the electrochemical performance. Subsequently, both the ohmic and polarization resistance increased, indicating the delamination of the air electrode. However, the scanning electron microscopy (SEM) observations revealed that the air electrode was not entirely delamination during the operation. Upon examination through transmission electron microscopy (TEM), it was apparent that significant dislocations and a refined interface formed in the electrolyte at the air-electrode interface as a result of plastic deformation. This was speculated to be caused by the accumulation of oxygen ions, which obstructed the diffusion pathway of oxygen ions due to the dissociation of the interface bonding. Based on the density functional theory (DFT) calculation, it is suggested that the accumulation of oxygen ions can cause a compressive structural evolution in the YSZ electrolyte, ultimately leading to a direct degradation of the electrolyte’s electrochemical properties.