Microstructural Characterization of SrZrO3 Formation and the Influence to SOFC Performance

One of the major phenomena to shorten SOFC durability is the formation of insulated phases, such as SrZrO₃, between the cathode and the electrolyte. It is known that SrZrO₃ is formed and grown during sintering processes as well as during long-term operation. A systematical study is therefore desired to clarify the SrZrO₃formation mechanisms and their influence on electrochemical properties.

 In this study, four parameters are considered and systematically varied: sintering temperatures of cathode, electrolyte, and buffer layer, and buffer layer thickness. The cells tested consist of yttria stabilized zirconia (YSZ) as the electrolyte, Ni- yttria stabilized zirconia (Ni-YSZ) as the anode, La(Sr)Co(Fe)O₃（LSCF）as the cathode, and Ce_0.9Gd_0.1O₂ (GDC) as the buffer layer. The GDC layer was sintered at 1300^oC with 5μm in thickness, while the YSZ electrolyte was sintered at 1400^oC. Cathode sintering temperature was varied from (1) 1000^oC to (2) 1100^oC and (3) 1200^oC. The electrochemical performance of the cells was tested at 800^oC, 0.2A/cm² for 100h by feeding humidified fuel: H₂ (97 cc/min) and H₂O (3 cc/min). Microstructure of the three types of model cells after sintering and testing for 100h was observed in details by FE-SEM (Field Emission Scanning Electron Microscope) and STEM (Scanning Transmission Electron Microscope).

 Microstructural observation has been revealed (1) no SrZrO₃ formation during sintering LSCF at 1000^oC, and (2) a partial formation of SrZrO₃ between the cathode and the electrolyte after sintering LSCF at 1100^oC. (3) After the sintering of the LSCF cathode at 1200^oC, the interface was fully covered by the SrZrO₃ layer, clarified by the microstructural observation with EDX elementary mapping. Such SrZrO₃ formation led to a decrease in electrochemical performance. These results indicate the importance of the SrZrO₃ formation controlling the SOFC performance.