Origin of Compositional Instability for a-Site/B-Site Rich La0.6Sr0.4Co0.2Fe0.8O3−D (LSCF6428) Under Oxygen Potential Gradient
Authors: WANG Xingwei , Miyazaki Takamichi , Yashiro Keiji , Hashimoto Shin-ichi , Kawada Tatsuya 
Published: February 2020

Currently, LaxSr1−xCoyFe1−yO3−δ (LSCF) is under intensive research as a potential air electrode material for SOFC and SOEC. As a mixed ionic and electronic conductor (MIEC) with high catalytic activity, LSCF shows high electrode performance.【1
】However, the stability of LSCF as air electrode has yet to be thoroughly verified experimentally. One of the major concerns for LSCF is the surface segregation of Sr.【2
】 This has been discussed in terms of the mismatch of the ionic radius of Sr and La.【3-5
】Another mechanism that can lead compositional instability is the kinetic demixing or kinetic decomposition. Previous work showed that the severe segregation of cobalt oxides appeared on the high-pressure side of LSCF placed under an oxygen potential gradient. The segregation tended to appear on the grain boundary lines【6
】. However, segregation was not always found in the same way, and the detailed mechanism for the segregation is not clear.

In this article, the aim is to uncover the mechanism of decomposition of LSCF under oxygen potential gradient. For this purpose, we compared the A-site and B-site rich LSCF6428 materials in the surface microstructure and microchemistry arising from the cation segregation.

The A-site or B-site 2% rich LSCF6428 were prepared using oxides: Co2O3, La2O3, Fe2O3 and SrO. La2O3is easy to absorb water, so it’s firstly put in the furnace at 1273k for 2 hours and then all the powder was carefully weighed in the ratio of 2% of commercial powder LSCF6428 and mixed them using a ball mill. The mixed powders were calcined at 1273 K for 10 h in air. The calcined powders were pressed into a cylindrical shape under the hydrostatic pressure of 200MPa and sintered at 1573K for 4 h. The density of the sintered pellets was higher than 97%. After the sintering, the materials were confirmed as the perovskite single phase without any obvious impurity phase by an X-ray diffraction analysis. The specimen was polished, and then a pellet was placed between two alumina tubes using gold rings as the sealing materials. Temperature was firstly brought to 1273 K, which is close to the melting temperature of gold (～1323 K), in order to seal via the pressurized gold ring. Then temperature was reduced down to the measurement temperature maintaining the gas tightness. Sintered pellets of LSCF6428 have been heat-treated with and without applying oxygen potential gradients. Their morphology and composition changes were investigated by Scanning Electron Microscope (SEM, JSM-7001F，JEOL ) and Energy Dispersive X-ray Spectroscope (EDS, INCA Energy250XT，OXFORD) and TEM.

As is shown in Fig. 1, a second phase can be easily observed on the oxidation surface of LSCF6428 membrane after the experiment under p(O2) gradients at 1273K, when 1 bar oxygen was introduced to the high p(O2) side, and 10−4 of oxygen was introduced into the low p(O2) side. For A-site rich sample, strontium oxide or related compounds were observed on the high p(O2) surface according to EDX analysis. This is different from the results of B-site rich sample, which was found with cobalt-rich precipitates on the high p(O2) surface. Meanwhile, materials were not homogeneous at the low p(O2) side, with Co oxide precipitates, and noticeable changes of the morphology happened. Co segregation was observed clearly on the grain boundary of cross section for the B-site rich sample after sintering at 1573K in the air. TEM test also show similar phenomenon using the high resolution image.

Reference:

1.Zehua Pan, Qinglin Liu, Lan Zhang, Xiongwen Zhang, and Siew Hwa Chana, Journal of The Electrochemical Society, 162 (12) F1316-F1323 (2015).

2. P. Iora, M. A. A. Taher, P. Chiesa, and N. P. Brandon, Int. J. Hydrogen Energy, 35, 12680 (2010).

3. W. Lee, J. W. Han, Y. Chen, Z. Cai, and B. Yildiz, J Am Chem Soc, 135, 7909 (2013).

4. H. Ding, A. V. Virkar, M. Liu, and F. Liu, Phys Chem Chem Phys, 15, 489 (2013).

5.Zhuhua Cai, Markus Kubicek, Jürgen Fleig, and Bilge Yildiz, Chem. Mater. 2012, 24, 1116−1127

6. Mi-Young Oh, Atsushi Unemoto Koji Amezawa, and Tatsuya Kawada, Journal of The Electrochemical Society, 159 (10) F659-F664 (2

Figure 1