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Barnaby Levin1 Andrew Dopilka1 Ethan Lawrence1 Peter Crozier1

1, Arizona State University, Tempe, Arizona, United States

Lanthanum strontium transition-metal oxide perovskites (LaxSr1-xMO3) are among the most promising materials for low temperature solid oxide fuel cell (SOFC) cathodes [1]. However, segregation of strontium to the surface of these perovskites has been observed to occur during synthesis, and at exposure elevated temperatures [2-4]. Surface strontium segregation affects the oxygen exchange at the perovskite-gas interface, which can alter the activity for oxygen electrocatalysis [5,6].
To deepen our understanding of the fundamental causes of strontium segregation and its impact on SOFC performance, we use aberration corrected transmission electron microscopy (TEM) to examine the chemical, crystallographic, and morphological changes that occur at the surface of lanthanum strontium cobaltite (La0.7Sr0.3CoO3) nanocubes upon exposure to oxidizing and reducing atmospheres at elevated temperatures.
We synthesize La0.7Sr0.3CoO3 nanocubes for TEM analysis using a molten salt synthesis method [7], with a mixture of sodium nitrate and sodium nitrite used as the solvent. Identical location ex-situ TEM, and in-situ TEM allow us to image individual nanocubes as they are exposed to different atmospheres. The high spatial resolution of TEM allows us to characterize and compare the changes occurring at the perovskite-gas interfaces for different La0.7Sr0.3CoO3 crystal facets, and in the bulk of the La0.7Sr0.3CoO3 nanocubes. Crystallographic and morphological changes that occur in the La0.7Sr0.3CoO3 nanocubes are explored using lattice resolution imaging in bright-field TEM and in annular dark-field (HAADF) scanning TEM (STEM), whilst chemical changes are explored with hyperspectral mapping using electron energy-loss spectroscopy (EELS).
Our results will provide important feedback to help guide the design of more durable perovskite cathodes for SOFCs.

References:
[1] Gao, Z., et al. Energy Environ. Sci. 9, 1602-1644 (2016)
[2] Crumlin, E. J. et al. Energy Environ. Sci. 5, 6081 (2012).
[3] Chen, Y. et al. Chem. Mater. 27, 5436–5450 (2015).
[4] Wang, H. & Barnett, S.A. ECS Transactions 78, 905-913, (2017).
[5] Feng, Z. et al. J. Phys. Chem. Lett. 5, 1027–1034 (2014).
[6] Rupp et al. J. Mater. Chem. A. 3, 22759-22769, (2015).
[7] Yang, J. et al. J. Alloys & Compounds 508, 301–308 (2010).

Acknowledgements: We gratefully acknowledge the support of NSF grant DMR-1308085 and ASU’s John M. Cowley Center for High Resolution Electron Microscopy.

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