Page 178 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

Anodes 155

eliminating microfissuring which contributes to electrode ageing [ 71. Nickel
oxide of grain size around 1 pm is now used, while the zirconia component often
contains a proportion of coarse powder 2 5 pm or larger [ 111 to form the anode
structural skeleton and inhibit the nickel aggregation, mixed with 0.5 pm fine
powder to promote sintering. The powder mixtures are applied to form the
electrode layer on the YSZ electrolyte substrate, 150 pm or thicker, which also
provides the structural rigidity of the cell.
A similar development strategy applies to the more recent anode-supported
cells, now beginning to find favour for operation at 800°C or lower. This lower
temperature relaxes materials specifications throughout the system, permitting
the use of lower cost metallic structural and interconnect components.
Another benefit is diminished thermomechanical stress and reaction at
these lower temperatures, significantly improving durability. On the other hand,
lower power output is unacceptable, so for YSZ electrolyte, its higher resistivity
at temperatures under 800°C must be compensated by reduction in its thickness
to 10 pm. Recent cells therefore tend to use the anode, not just as a functional
component, but also structurally as the load and stress-bearing support for a thin
electrolyte. As will be later noted, this structural cermet, up to 1 mm thick, can
serve not only as the site of oxidation of a reformate composed principally of
hydrogen and carbon monoxide, but also for the preliminary hydrocarbon fuel
processing reactions. At lower temperatures also, the thermal activation of the
oxidation reactions is significantly diminished, implying increased polarisation
and giving added importance to considerations of electrocatalysis at the
anode [12]. In this case graded anode structures are often advisable, with a
high-porosity large-grain substrate bearing a finer-structured electrocatalytically
active functional layer to contact with the electrolyte [ 131. The micrograph in
Figure 6.3 shows materials of an anode-supported cell [14].

Figure 6.3 Micrograph of anode-supported thin electrolyte cell structure. On either side of the dense
electrolyte is afine-structured ‘functional layer’for the electrocatalytic promotion of the electrode reactions.
The full thickness of the cathode layer is imaged, but onlg a small section of the anode substrate (bottom).
(Reproduced bycourtesyof the Research Centre, Jiilich (FZJ).)