Internal Surface and Grain Boundary Degradation of La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ /Sm 0.2 Ce 0.8 O 2-δ Porous Air Electrode of Solid Oxide Fuel Cells Operated in Humidified Air

For the porous air electrode of solid oxide cells (SOCs) operated in either fuel cell or electrolysis mode, the humidity in the air can affect its nanostructure, impact its performance, and complicate its tolerance toward other contaminants. The internal surface, located underneath the external surface of the entire porous electrode, is an interconnected network of percolation pores that provide a surface area for electrochemical reactions. The internal surface is also where the humidity interacts directly with the electrode, inducing extrinsic degradation. Understanding the origin of such extrinsic degradation in nanostructures, especially the internal surface of the porous electrode, is indispensable for further enhancing the SOC performance through electrode structure engineering or optimization of its operating conditions. A commercial cell with La0.6Sr0.4Co0.2Fe0.8O3-δ/Sm0.2Ce0.8O2-δ (LSCF/SDC) cathodes is operated in 10% humidified air at 0.6 A/cm2 and 750 °C for 2571 h, while a contrasting baseline cell is operated in the same conditions but in dry air for 4610 h. The impedance of the cell operating in humidified air presents a more pronounced increase than that of the cell operating in dry air. Postoperation TEM analysis indicates that Sr segregation is not found for cells operated in dry air. For the cell operating in the humidified air, a conformal layer enriched with Sr and La is present on the internal surface of the active layer. (Co, Fe)Ox precipitates are formed at the LSCF/LSCF grain boundaries. Based on the resultant microstructure of the electrode upon operation in humidified air, the formation mechanisms of the surface layer and the (Co, Fe)Ox precipitates induced by moisture, as well as their impact on the electrochemical performance of the cells, are discussed and deduced.