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DEGRADATION MECHANISMS IN SOLID OXIDE ELECTROLYSIS ANODES: Cr POISONING AND CATION INTERDIFFUSION
Figure 5: Auger data from points 1, 2 and 3 in Figure 4, with fraction of various cations inset
The presence of La, Sr, Co, O and Cr is clearly manifest on the surfaces of this region. From Figure 5
it is clear that the three different points have significant differences in their local compositions and
chemical signature even though they are separated by only a few of microns from each other. The
ratio of La to Co is seen to vary roughly from a mere 0.65 to around 9. Ideally, this ratio of the
as-prepared LSC should be 0.8. Another point to note is the observation of “crystallite”-like structures
in the LSC bond layer. One such crystallite is visible in the SEM figure above (area 1). A noteworthy
observation is that these crystallites show a significantly higher Cr content than the rest of the
neighbouring microstructure, as is visible from the table inset in the spectra in Figure 5. These results
hint at the non-uniform segregation of the cations on LSC grains very drastically, that may have led to
phase instabilities.
Eventually, after the AES analysis, in order to investigate the structure and chemistry of the
individual grains at an even higher resolution, we employed TEM coupled with EDX. STEM at a high
resolution was carried out to study the distribution of the different cations across the grains and to
see if there was any relationship between them. Figure 6 shows the results of one such STEM analysis.
It shows the presence of Cr everywhere in the electrode. The interesting observation here to note is
that regions with high Cr content are accompanied by a high La content and a lower Co content and
vice versa, suggesting that Cr and La could be in a chemical form together but Co forms a different
chemical species, possibly Co 3 O 4 with oxygen. Nonetheless, the segregation of the cations is evident
and again backs the results obtained by AES confirming that the perovskite bond layer has in fact
dissociated severely even at such a high resolution.
Conclusions
Raman Spectroscopy results clearly showed that the LSC bond layer had degraded and the secondary
phases of individual lanthanum and cobalt oxides are formed. Results also show the presence of
chromium-containing phases (Cr 2 O 3 , LaCrO 3 ), thus clearly hinting at the chromium diffusion from the
steel interconnects into the anode bond layer. This is a major cause for the loss in performance of the
cells. To further investigate the surface chemistry and microstructure of the air electrode and the bond
layer, particularly across their cross-sections, we carried out scanning Auger Electron Spectroscopy
with nanoprobe capability. Our results pertain to the presence and extent of Cr in the bond layer and
the variations in the surface compositions of the bond layer. We studied different regions in the LSC
material to investigate the cross-sectional variation of chromium and the constituent cations. The
results show an average peak chromium fraction of approximately 0.07 on the surface of the LSC bond
layer. In certain spectra, we observed crystallite-like phases in the LSC region, corresponding to the
highest Cr content. The LSC layer exhibited local variation of chemical constituents on the surface of
its grains. La/Co ratio varies from 0.65 to 9, a significantly different range compared to the expected
bulk average value of 0.8. Thus, there is a large local variation on the LSC grain surfaces even when
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