Page 28 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

lntroduction to SOFCs 9

These electrodes are usually made from particulate materials which are partially
sintered to form porous conducting layers. Often, several layers are laid down
because this allows a gradient of properties ranging from nearly pure YSZ at the
electrolyte surface to almost pure electrode composition at the interconnect
contact, as illustrated in Figure 1.5 for a typical anode structure. In addition the
expansion coefficients can then be better matched across the layers.

YSZ

Nickel Three layer anode
Figure I. 5 Three-layer anode made by printing three inks ofdifferent composition onto YSZ[IZ].

Nickel is the main anode material used in SOFC anodes since 1964, largely
because of its known performance and economics. Unfortunately, nickel metal
does not adhere strongly to YSZ and flakes off unless it is mixed with zirconia.
This flaking is driven by the large difference in expansion coefficient between
metal and ceramic: YSZ expands at around 11 x 10P6/K whereas nickel expands
much more at 13.3 x 10Ph/K. By powder mixing 30 vol% nickel oxide with YSZ,
followed by firing at 1300°C to give a porous anode layer by reduction in
hydrogen, this mismatch can be reduced. The expansion coefficient of this ‘nickel
cermet’ anode is about 12.5 x 10P6/K, allowing much better adhesion to the
electrolyte. Sandwiching this anode cermet between two slightly different
compositions, one nearest the zirconia with less nickel, the other near the gas
stream with more nickel, can give excellent anode properties, both from the
catalytic and the electronic conduction points of view. The two main
requirements of the anode are to allow rapid, clean reactions with the fuel and to
provide good conduction to the interconnect.
The main problem with the nickel-based anodes is their propensity to coke,
that is to become coated with a carbon layer on reacting with hydrocarbon fuel.
This carbon layer has two deleterious effects: it can disrupt the anode by pushing
the nickel particles apart: and it can form a barrier at the nickel surface,
preventing gas reactions. Typically, if a hydrocarbon such as methane is fed
directly into an SOFC anode, then it may not remain functional after as little as
30 minutes as the coking proceeds. Additives to the Ni+YSZ cermet such as 5%
ceria or 1% molybdena can inhibit this process [19]. Alternatively, metals other
than nickel can be employed [20].
Cathodes present the main electrode issues in designing and operating SOFCs,
as described in Chapter 5. Since these operate in a highly oxidising environment,
it is not possible to use base metals and the use of noble metals is cost prohibitive.
Consequently, semiconducting oxides have been the most prominent candidates
since 1966 when doped lanthanum cobaltites began to be used, followed in