Page 189 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

166 High Temperaturr Solid Oxide Fuel Cells: Fundamentals, Design and Applications

already mentioned [ 301 through this distributed reforming over the anode [41]
(Figure 6.12). However, it is now evident that such a direct oxidation process,
without reforming, requires an innovation in anode materials, diverging from
the established nickel-zirconia option.

CO+ H,O- CO,+ H,
CH, + EH,O
\ I

Figure 6.12 Schematic of distributed processing of humidijed methane with internally generated
reaction products.

Spacil's alternative transition metals for the cermet, iron and cobalt, though
less active for the pyrolysis of hydrocarbons, do not have the corrosion resistance
of nickel in a high-steam environment. Considering silver and copper, their
oxides either decompose or melt at temperatures below the requirements for
cermet sintering: neither are the metals refractory (Ag m.p. 962°C: Cu m.p.
1083°C). However, in catalytic technology the advantages of copper composites
with ceria are recognised [42]. Partial reduction of copper oxide when exposed to
fuel at elevated temperature, and the resulting redox properties, permit
exchange of oxygen between the lattice and the gas phase, with availability for
surface reactions. A copper-ceria composite anode [43,44] is a recent promising
initiative. The difficulty of sintering a copper composite was avoided by forming a
porous zirconia skeleton on a dense electrolyte substrate of the same material,
then introduction of copper and cerium as their nitrate salts in solution, followed
by drying and pyrolysis, similar to a procedure already demonstrated for anode
and cathode catalysts [45]. Co-insertion of the two cations is possible since
copper does not form a solid solution in ceria so the two phases remain separate
as required for functionality of the electrode. In Figure 6.13 the reported
performance of cells using this zirconia-supported copper-ceria composite is
presented. Obviously the power density with methane fuel is significantly lower
than that with hydrogen, but the synergetic catalysis by ceria is evident from the
negligible power density with copper alone in the zirconia matrix. Figure 6.14
presents evidence of the stability of the composite anode, in contrast to a nickel
cermet where the fuel cell operation is suppressed irreversibly within 30 min by
the carbon accumulation. The ceria-copper system is now being further
investigated for the direct oxidation of higher hydrocarbons [43].