New insights on the microstructural degradation in LSM/YSZ solid oxide electrolysis cell using advanced transmission electron microscopy analysis.

Abstract

High-temperature solid oxide electrolysis cells have become a promising technology for the next generation due to their ability to store pure hydrogen and excess electricity from renewable energy sources [1]. However, these cells face challenges with long-term operation due to quick degradation and delamination of the air-electrode. This degradation can cause immediate failure, making it difficult to gather detailed information on the underlying mechanisms. Various mechanisms such as pressure build-up caused by oxygen gas generation [2], formation of secondary phases [3], and cation migration [4] have been proposed to explain the electrode delamination, but have not provided solid evidence regarding structural changes. This is because they only proposed changes in the topology information before and after operation in the microstructure perspective, without providing sufficient details about the underlying structural changes using scanning electron microscope (SEM) or transmission electron microscope (TEM) analysis. In our study, we embarked on a systematic investigation of the microstructure, specifically focusing on local strain and subgrain formation at the interface between air-electrode of a strontium-doped lanthanum manganite (La0.8Sr0.2MO3, LSM) and electrolyte of yttria-stabilized zirconia (YSZ) in a symmetric cell. To accomplish this, we utilized advanced transmission electron microscopy (TEM) analysis techniques, specifically employing a precession electron diffraction (PED) technique. Our examination of the interfacial microstructure of the cells operated at varying current densities yielded novel observations regarding previously undiscovered physical and chemical changes at the beginning of delamination. Specifically, significant structure changes, including nanopore formation, dislocation, and polygonization, were observed in the electrolyte near the interface. These structure changes were a result of localized strain occurring in specific regions where oxygen ions were unable to be released as oxygen gas. Through a strain mapping in TEM, we directly visualized the distribution of strains at the interface, demonstrating their sufficient capability to induce plastic deformation. Additionally, the orientation mapping enabled us to simultaneously observe the formation of subgrains as results of plastic deformation at locally strained regions. To validate our findings, we performed a density functional theory (DFT) calculation, which verified that the accumulation of oxygen ions is indeed triggers a compressive structural evolution in the YSZ electrolyte. Consequently, the injection of excess ions into the lattice resulted in the formation of dislocations and subgrains. This, in turn, initiated the formation of nanopores, ultimately resulting in crack propagations and the subsequent delamination of the electrode. In conclusion, our comprehensive study provided valuable insights into the mechanisms underlying delamination in the perspective of microstructure evolution. The use of advanced TEM techniques, including strain and orientation mapping, enabled us to visualize and understand the structural changes occurring at the interface.