Microstructural studies on degradation of interface between LSM-YSZ cathode and YSZ electrolyte in SOFCs

摘要

The changes in the cathode/electrolyte interface microstructure have been studied on anode-supportedtechnological solid oxide fuel cells (SOFCs) that were subjected to long-term (1500 h) testing at 750 °C underhigh electrical loading (a current density of 0.75 A/cm~2). These cells exhibit different cathode degradationrates depending on, among others, the composition of the cathode gas, being significantly smaller in oxygenthan in air. FE-SEM and high resolution analytical TEM were applied for characterization of the interface on asubmicron- and nano-scale. The interface degradation has been identified as the loss of LSM coverage andthe loss of three-phase-boundary (TPB) length. Firstly, the degradation is caused by the size reduction ofthe individual LSM/YSZ electrolyte contact points (areas) that are initially of 100-200 nm in diameter.Quantitative microstructure evaluation shows that in the cell tested in air this mechanism contributes to anestimated overall reduction in the LSM coverage and the TPB length by 50 and 30%, respectively. For the celltested in oxygen the corresponding values are 10 and 4%. Secondly, in the cell tested in air the LSM coverageand the TPB length appear to decrease further due to the more pronounced formation of insulating zirconatephases that are present locally and preferably in LSM/YSZ electrolyte contact areas. The effects of the cathodegas on the interface degradation are discussed considering the change of oxygen activity at the interface,possible changes in the Mn diffusion pattern as well as the LSM/YSZ reactivity. Finally, based onthermodynamic calculations a T-p(0_2) diagram predicting the safe and risky operation conditions in termsof the zirconate formation is presented and compared with the experimental observations.