Page 166 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

Cathodes 143

5.6 Summary
The most important properties of cathodes are their catalytic activity for oxygen
reduction and their compatibility with the electrolyte (including thermal
expansion match and chemical non-reactivity). From the viewpoint of catalytic
activity, many perovskites have been considered and investigated. Although the
lanthanum manganite-based materials are not the best because of their low
oxide ion diffusivity and resulting limited electrochemical activity, these are the
most common perovslrites that are used with YSZ electrolyte at 1000°C because
of their superior chemical stability. For lower cell operation temperatures also,
even though chemical reactivity at the cathode/electrolyte interface is less
important, composite cathodes made from LSM/YSZ are used down to 700°C.
Less severe conditions for the electrolyte/electrode chemical reactions at
temperatures below 800°C make it attractive to use more catalytically active
perovskites as cathodes. Sr- and Co-doped lanthanum ferrite (La,Sr)(Co,Fe)03
has been widely investigated for intermediate temperature SOFCs. Recent
attempts have focused on (La,Sr)Fe03 since it has lower area specific resistivity;
however. fundamental phase relations and related high-temperature behaviour
associated with this material still need clarification. Interactions with the
interconnect can also be significant because poisoning of the cathode can occur,
for example by Cr contamination. The cathode performance depends
substantially on its surface area, porosity and microstructure, and therefore the
processing method used is very important in determining cathode performance.

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