Page 175 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

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

As an alternative to a single-phase metallic or conductive ceramic electrode
material, the accepted compromise has been the use of a porous composite of
metal and ceramic, a ‘cermet’ (Figure 6.1). In the SOFC anode most commonly
used at present, the nickel-zirconia cermet, the primary role of the zirconia is
structural, to maintain the dispersion of the nickel phase and its porosity by
inhibition of the aggregation and grain growth of the metal and so to achieve an
adequate anode lifetime. The adhesion of the zirconia part of the cermet to the
electrolyte gives a structural ruggedness able to withstand the thermal stress due
to differential thermal expansion, which anyway is lowered in the composite by
the ceramic volume fraction. The provision of oxide ion conductivity
complementary to the electronic conductivity and electrocatalytic action of the
metal is a useful secondary role of the ceramic, enhancing the electrochemical
performance by delocalisation of the electrochemically active zone already
mentioned. However, even as a compromise material the nickel-zirconia cermet
does not fulfil all the requirements of an ideal anode. Fuel specification in
particular is an important parameter. Most literature results report on hydrogen
as fuel, but the commercial imperative requires hydrocarbons. These, however,
are rapidly pyrolysed on nickel surfaces at high temperature, depositing a dense
carbon which blocks the anode porosity and ultimately disrupts the structural
integrity of the cermet. There is, however, a compensating consideration.
Carbon monoxide, which is not tolerated by the platinum catalyst in
low-temperature polymer electrolyte fuel cells, is a perfectly acceptable SOFC
fuel, though slower to oxidise than hydrogen. Therefore the SOFC is robust with
respect to fuel specifications, and can be fuelled with gas mixtures rich in
hydrogen and carbon monoxide derived from hydrocarbons by partial oxidation
or by reforming reactions with steam or carbon dioxide. There still remains a
concern about impurities in the hydrocarbon fuels. Nickel at high temperature is
sensitive to sulphur compounds at concentrations even as low as 0.1 ppm. These
may occur at source in natural gas, for example, and thiophene and mercaptans
are systematically added to it as odorants for safety reasons. Fortunately the
sulphided nickel surface is not necessarily irrecoverable, and operation with a
clean sulphur-free fuel may restore performance. Nonetheless desulphurising
systems which either adsorb the contaminant on activated carbon, or react it
with zinc oxide to form a solid sulphide are regarded as essential for high-
temperature fuel cell operation. Dependent on the origin of the fuel, for example
from coal gasification, biomass pyrolysis or fermentation, other impurities may
. also occur, particularly ammonia and possibly hydrochloric acid. Tolerance over
2000 h for HC1 is somewhat better than that for HIS, though still in the low ppm
range. However, from all evidence, ammonia even in elevated concentrations
does not pose a problem: concentrations up to 5000 ppm have had no effect on
SOFC cell voltage over a 2 500 h test [5]. Inorganics may also be found, entrained
as dust, which can then react with the ceramic components of the cell giving, for
example, silicates. It is therefore evident that adequate cell performance can only
be achieved, and maintained, by careful fuel pretreatment. Despite these
compromises, however, the nickel-zirconia cermet has become the most
common anode in SOFC technology.