Page 210 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

lnterconnects 18 7

7.4 Protective Coatings and Contact Materials for Metallic
Alnterconnects

The metallic interconnects have two main disadvantages. The first is the release
of volatile Cr species. In atmospheres containing water vapour, the most volatile
specie is chromium acid, H2Cr('I)04 [37, 741, which is transported with the
oxidant gas through the cathode to the cathode/electrolyte interface, competes
with the oxygen molecules for the electrochemically active sites and blocks them
with Cr("') [74]. This results in an increase in cathode polarisation [75, 761.
After the initial blocking of the electrochemically active sites, the ongoing
transport and reduction of chromium species lead to decomposition of the
cathode perovskite material and the formation of spinels [76,77]. The chromium
transport in the cathode compartment needs to be minimised to overcome these
problems, either by using 'Cr getter' materials like La203 [3 51 in the cathode or
by applying protective coatings of lanthanum chromites [64, 651, lanthanum
manganites [47] or yttrium manganites [66] to the interconnect (Table 7.5). The
thermally sprayed manganite coatings have led to stable long-term performance
lasting about 12,000 h with a degradation rate in cell voltage of less than 1%/
1000 h [78].
Matsuzalti and Yasuda reported that Lao.6Sro.4Feo.8Coo.z03 cathodes are
much more stable against Cr poisoning than Lao.&o.l 5Mn03 or
Pro.6Sro.4Mn03 cathodes [ 791, especially when ceria-based solid electrolytes are
used, and no enrichment of chromium was found at the Lao.6Sro.4Feo.sCoo.203/
ceria interface (after 10 h). A possible explanation might be the different
overvoltages of the cathodes for oxygen and H2Cr(V1)04 reduction. While for the
manganites the reduction of the chromium oxyhydroxide is the energetically
preferred reaction, in the presence of the ferrites the reduction of oxygen
appears to require less activation energy. Although the exposure times
were short in their experiments, modification of cathodes may be a possible
alternative to avoid Cr poisoning and the use of protective coatings. The extent
of Cr evaporation from steels or other alloys and the effectiveness of
protective coatings can be estimated before stack assembly by transpiration
experiments [ 74,801. Such investigations are useful in new interconnect
material development and also for an understanding of the cell and stack
degradation rates.
The second major disadvantage of metallic interconnects is the formation of
oxide scales leading to significant ohmic losses. The interaction of the metallic
interconnect with the adjacent ceramic cell components and the resulting time-
dependent resistance of these material combinations is highly important. In the
case of Cr 5Fe 1Y203, the best contact material for the cathode was found to be
]LaCo03 [64] before protective layers of (La,Sr)Cr03 were applied to avoid Cr
evaporation. LaCo03 was also successfully used in combination with ferritic steel
1491, although the thermal expansion coefficient of the cobaltite is higher
[64,81] than the other SOFC materials [82]. Besides the lower contact resistance
(Figure 7.7), LaCo03 appears to react with the released chromium species to form
a La(Co,Cr)03 perovskite and therefore retains the Cr vapour.