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CAUSES OF DEGRADATION IN A SOLID OXIDE ELECTROLYSIS STACK
Bond coat
Figure 4: SEM images and EDS maps of Cr diffusion into the
cross-section of an oxygen electrode of a SOEC (left) and SOFC (right)
SOEC
SOFC
O 2
Bond coat
Electrode 1
Electrode 2
Electrode 1
O 2
Electrolyte
Electrode 2
Electrolyte
35 μm
Cr Kα
35 μm Cr Kα
affect the activity of oxygen evolution. Conversely, in the SOFC, oxygen from air flows toward the
electrolyte interface where it is reduced and transferred into an oxygen vacancy in the electrolyte.
This carries with it any gaseous Cr emanating from the flow field. Consequently, Cr concentrates
deeper into the oxygen electrode poisoning its most active regions.
Under long-term operation, >1 500 h, we found that the oxygen electrode delaminated from the
electrolyte. This became evident in our examinations of the cell surface by mapping (Mawdsley, 2009).
Figure 5 shows evidence of an over-sintered region. Coupled with high evolution rates of oxygen, the
dense region would likely delaminate over time. Another hypothesis is that oxygen could be released
under the grain of a non-oxygen-ion-conducting electrode, causing the formation of defects that grow
into cracks. This is due to the inability of the electrode material to conduct ions away from under the
particle. Electrode delamination is a long-term effect and does not account for the large degradation
in early stages of SOEC stack operation.
Figure 5: SEM image of the interface between the oxygen electrode and the electrolyte
Oxygen
electrode
Oversintered
region
Electrolyte
10 μm
There has been some question concerning the stability of the electrolyte. In earlier studies, we
have found evidence of monoclinic ZrO 2 formation by Raman microspectroscopy. However, at this
point we have not determined whether or not this plays a significant role in stack degradation.
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