Page 176 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

Anodes 153

6.4 Cermet Fabrication
The development of nickel-zirconia anodes leading to their present performance
in fuel cells has generally proceeded empirically since the first introduction of
this combination of materials by Spacil, in 1964 [l]. Though written almost
40 years ago, his text remains a strikingly modern presentation of the anode
specification, making reference to many of the concepts and procedures still
current in SOFC technology. The densification of a porous metallic structure
with time at temperature is the problem to which the metal-ceramic composite is
the answer. Intimate anode to electrolyte bonding is a further requirement
satisfied by the cermet, if necessary by adoption of a flux to aid sintering. A
minimum metal proportion in the cermet is necessary for continuity of electronic
conduction, while the zirconia particles are by preference also continuous. Spacil
had therefore recognised that the functionality of the ceramic in the composite
was essentially structural, to retain the dispersion of the metal particles and the
porosity of the anode during long-term operation. Structure and elemental
distribution in a typical anode cermet is shown in Figure 6.2 [6].

E

Figure 6.2 Scanning electron microscopy images of a Ni-YSZ cermet with elemental resolution, showing
(a) nickel distribution, (b) overall cermet morphology, and (e) zirconia structural skeleton [6].