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DEGRADATION MECHANISMS IN SOLID OXIDE ELECTROLYSIS ANODES: Cr POISONING AND CATION INTERDIFFUSION
of secondary phases that block the active sites where electro-catalysis occurs. At higher temperatures
of operation, interaction between the components of the cells becomes stronger (Adler, 2004), leading
to higher rates of degradation. The diffusion of Cr-containing species from the steel interconnects
used in the cells into the electrode microstructure (either through vapour phase or solid state diffusion)
and their reaction with the electrodes to form secondary phases has been identified as a major cause
for degradation when the cells are operated in the fuel cell mode (Matsuzaki, 2000, 2001; Fergus, 2007;
Stanislowski, 2007). Also, the segregation of cations in the electrodes of SOFCs is well documented.
It has been seen that certain electrodes such as LSCF, though offering higher power densities than LSM
electrode, do not possess long-term stability (Simner, 2006). This segregation can alter the properties
of the cell constituents and cannot only increase the ohmic resistance but such phenomena can
greatly affect the oxygen reduction reactions and the charge transfer mechanisms. As a result of this
degradation, the efficiency of the cells goes down with time. Similar degradation issues are relevant in
SOEC. A better understanding of the mechanisms of degradation can help us identify ways and
compositions that can counter the loss in cell performance. In this report, we summarise our results
obtained from the post-mortem analysis of the SOEC and identify a mechanism of degradation of the
oxygen cell of the cells.
The oxygen electrode of the cells under analyses in our project is a perovskite material
A 0.8 Sr 0.2 MnO 3 (element A is not disclosed because it is proprietary information). Scandia-stabilised
zirconia (SSZ) is used as the electrolyte. The cathode consists of a Ni-SSZ cermet. Also, a lanthanum
strontium cobaltite (La 0.8 Sr 0.2 CoO 3 , also known as LSC) is used as the bond layer.
Approach and techniques
The aim of our methodology is to employ a variety of spectroscopic techniques in an integrated
manner. Table 1 summarises the approach employed.
Table 1: Summary of our approach and the techniques used with their respective goals
Technique
Raman Spectroscopy
Nanoprobe Auger Electron Spectroscopy (NAES)
Focused Ion Beam (FIB)
Energy Dispersive X-ray Spectroscopy (EDX)/
Transmission Electron Microscopy (TEM)
Objective
Preliminary identification of secondary phases
formed on the surface of the bond layer
Electrode surface chemistry and microstructure and its
variation across the cross-section at a small scale (μm-nm)
Selective choice of the interface of
interest to prepare TEM samples from
High resolution identification of the chemical
composition and secondary structures formed
Results and discussion
Raman Spectroscopy was performed on the used cells. Data found in the literature (Chen, 2006; Hoang,
2003; Iliev, 2006; Li, 2003; Orlovskaya, 2005; Scheithauer, 1998) were used as references for Raman peaks
of the phases of interest found in our data. Figure 2 shows the air side of the one of the cells after
disassembly. We see dark and light lines that appear on the surface. This distinction occurs due to the
design of the corrugated flow channels to enable air flow. The LSC regions in contact with the flow
channels were comparatively lighter. We carried out Raman Spectroscopy on the dark and light
coloured regions on the air electrode side of this cell. The Raman results obtained clearly show that
the perovskite structure of the LSC has in fact decomposed and formed other phases. Also, the
presence of chromium is clearly manifest in the form of different Cr-containing compounds. This
chromium comes from the stainless steel interconnects. Figure 3 makes it clear that there is hardly
any structural difference between the light and dark regions. Both have the bond layer intact, even
though decomposed, and have not exposed the lower layers of the electrode. The colour contrast is
not distinguishable in terms of the phases present in these regions. As marked in Figure 3, the LSC
perovskite structure has decomposed into the cobalt oxide, chromium oxide and lanthanum
chromate phases, which have electronic conductivity that is an order or two lower than that of the
intial LSC phase.
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