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edfas.org ELECTRONIC DEV ICE FA I LURE ANALYSIS | VOLUME 23 NO . 2 22 TEM STUDY OF OXYGEN PARTIAL PRESSURE EFFECT ON EARLY LSM-YSZ SURFACE INTERACTIONS IN SOLID OXIDE FUEL CELLS H.J. Wang 1 , M. Chen 2 , Y.L. Liu 2 , L. Theil Kuhn 2 , and J.R. Bowen 3 1 NICE America Research Institute, Mountain View, Calif. 2 Department of Energy Conversion and Storage, Technical University of Denmark, Kgs. Lyngby 3 Xnovo Technology ApS, Denmark jason.wang@nicenergy.com EDFAAO (2021) 2:22-32 1537-0755/$19.00 ©ASM International ® INTRODUCTION Commercializing solid oxide fuel cell (SOFC) tech- nology requires long-term durability of the cells. [1,2] For anode supported SOFCs, long-term stability studies have revealed that (LaSr)MnO 3 -yttria-stabilized zirconia (LSM-YSZ) cathode degradation is the major contribution to the total cell degradation when the cells are subjected to severe testing conditions: intermediate temperature around 750°C with high current load of ~0.75 A/cm 2 . [3,4] Moreover, under these operating conditions, the cathode degradation rate is reported to exhibit a strong depen- dence on the oxygen partial pressure of the cathode gas, being significantly higher in air than inoxygen. WithSOFCs becoming an industrially relevant newenergy technology, better understanding concerning the microstructural degradation mechanisms of the cathode and how it is affected by oxygen partial pressure at relevant operating temperatures is required. Often severe cathode degradation in anode supported SOFCs is proposed to be caused by the formation of a zirconate phase (La 2 Zr 2 O 7 or LZO), which could reduce the active triple-phase boundary (TPB) sites and the efficiency of oxygen reduction reactions. [5] By far, most studies on LSM-YSZ interactions in SOFCs were carried out using model cells, [6-9] whichwere commonly subjected toharsher testing conditions (over 1000°C and high current) so that more prompt responses of LSM-YSZ interactions or more thorough zirconate phase formations were achieved in a relatively short time. However, in these studies critical information concerning early phase nucleation and their associated atomic diffusion mechanisms is not available due to the testing conditions. In this study, transmission electron microscopy (TEM) samples are acquired from technological cells, which are designed to undergo long-term duration tests at inter- mediate testing temperatures. In this case the cation interdiffusion mechanisms and the resulting LSM-YSZ interactions, as well as their impact on cathode degrada- tions, are allowed tobe examinedduring the early reaction stage of the LSM-YSZ materials. The focus of the present TEM study is directed toward themicrostructural and chemical characterizations in the vicinity of the cathode/electrolyte interface. Specifically, the local areas where the LSM grains are in contact with the YSZ electrolyte are primarily the feature of interest. EXPERIMENTAL PROCEDURE The SOFCs studied in this work are comprised of a Ni-8YSZ(Y 0.16 Zr 0.84 O 1.92 ) cermet anode, an 8YSZ electrolyte, and an LSM(La 0.75 Sr 0.25 Mn 1.05 O 3±δ )-8YSZ cathode. The cells were producedwith the procedures described inHagen et al. [10] The two cells selected for this study were fabricated identically. They were subjected to 1500 hours testing at 750°C under a high current load of 0.75 Acm -2 with air and oxygen as the cathode gas, respectively, and exhibited significantly different degradation behaviors. [4] The two tested cells were then polished to expose a cross section of the interface for subsequent TEM sample preparation. An untested as-fabricated cell was also included as a ref- erence cathode/electrolyte interface. The TEM thin foils across the cathode/electrolyte interface were prepared using focused ion beam (FIB) milling and standard in-situ lift-out in a Zeiss CrossBeam1540XBFIB-scanning electron microscope (SEM). A probe of 10 pA beamcurrent at 30 kV acceleration voltage was used for the final polishing. All