Page 24 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

lntroduction to SOFCs 5

This process was rather like the ceramic capacitor process developed for the
electronic ceramics industry.
There were several benefits of this new device:

0 There was much less zirconia in it, about 2.8 g;
0 The thinner ceramic electrolyte gave much faster response:
0 Heaters and other circuits could readily be printed onto the flat sheets.

An immediate bonus of this technology was the possibility of producing linear
response sensors as opposed to the logarithmic response of the thimble type, so as
to match the electronic control system more easily. This was achieved by setting
the oxygen reference by using one of the sheets as an oxygen pump which could
then leak from the cathode compartment through a standard orifice.
Oxygen sensors are now widely used in food storage, in metal processing
and in flame controIIers, but the main market is automobiles. Zirconia
technology for sensors has been very successful in the marketplace, and it has
pushed forward the development of solid oxide fuel cell materials. The main
difference is that the power output of sensors is low so that partially stabilised
zirconia can be used. At higher power, fully stabilised zirconia must be used if
the electrolyte is to remain stable for long periods. The supply of this electrolyte
material is discussed next.

1.4 Zirconia Availability and Production
The main electrolyte material used in SOFCs at present is YSZ, as described more
fully in Chapter 4. Although many other oxide materials conduct oxide ions,
some rather better than zirconia, this material has a number of significant
attributes which make it ideal for this application, including abundance,
chemical stability, non-toxicity and economics. Against these one can mention
several drawbacks, including the high thermal expansion coefficient, and the
problems of joining and sealing the material.
Low-grade stabilised zirconia already commands a large market, especially in
refractories, pigment coatings and colours for pottery, but it is only recently that
technical-grade zirconias have been produced for applications such as thermal
barrier coatings on gas turbine components, hip joint implants and cutting
tools. Much of this technology has stemmed from the study of pure zirconia and
the effects of small amounts of dopants on the crystal structure and properties.
Large effects were seen in the early 197Os, pointing the way to substantial
applications of this material [7].
Figure 1.3 shows the trend in worldwide production levels of ionic conductor-
grade yttria-stabilised zirconia over time. It is evident that in 1970 there was
very small production at a rather high price. However, the introduction of the
zirconia lambda sensor to control the emissions of automobiles in the 1970s had
a large effect on the production rate, and price has dropped steadily since that
time. The price in 2000 was about $50 per kg in 50 kg lots but this is expected to