Page 159 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

136 High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and Applications

csmrenh Alan* Temperature, K

Figure 5.12 Performance ofa cell with a thinfilm of EVD YSZ electrolyte on A-site-deficient cathode [8]:
(a) the cathode overpotential for air and pure oxygen; (b) the reciprocal interface resistivity as afunction
of temperature.

In addition to A-site deficiency, improvement in cathode performance can also
be obtained by using composite cathodes consisting of LSM and YSZ [10,5 7-59].
Since oxide ion conductivity is small in lanthanum manganite, enlargement of
the electrochemical reaction sites can be expected from introduction of an oxide
ion path (through YSZ) in the cathode layer. This composite cathode is also
effective in minimising the formation of La2Zr207. During the fabrication of the
composite cathode, some La2Zr207 may form shifting the composition of LSM to
where no further formation of La2Zr207 occurs. When such a composite cathode
is placed on a YSZ electrolyte, no La2Zr207 is formed at the interface of LSM
particles in the composite cathode and the YSZ electrolyte. This helps in
obtaining a LSIvl/YSZ interface with long three-phase boundaries.

5.3.4 Cathodes for Intermediate Temperature SOFCs
At lower temperatures, the catalytic activity of oxide cathodes for oxygen
reduction decreases and the selection of cathodes becomes critical for obtaining
high performance in SOFCs. There are several strategies for utilising oxide
cathodes successfully in the intermediate temperature (600-800°C) region. One
option is to use the same materials as those for 1000°C cell operation as in Figure
5.12, which indicates little temperature dependence. Attempts have also been
made to use a composite (LSM + YSZ) cathode for intermediate temperature
SOFCs with some success. Even though the interfacial resistivity of the cathode in
Figure 5.12 shows very small temperature dependence, typical composite
cathodes do exhibit some temperature dependence and lowering of activity with
decreasing temperature. Although it is well recognised that it is difficult to obtain
reproducibility of cell performance with composite cathodes, their main features
can be summarised as follows: