Page 173 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

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

6.2 Requirements for an Anode

The role of an anode in a solid oxide fuel cell is to provide the sites for the fuel gas
to react with the oxide ions delivered by the electrolyte, within a structure which
also facilitates the necessary charge neutralisation by its electronic conductivity.
These functional considerations together with the operating environment of the
anode are the key factors in the materials selection for the anode. It must
evidently be refractory, given the high cell operating temperature to be sustained
over a commercially useful lifetime, and be compliant with thermal cycling to
ambient temperature. In addition, the equilibrium between fuel gas and
oxidation products within the anode compartment results in an oxygen partial
pressure which is very low, but variable over several orders of magnitude
depending on the precise reactant and product conditions. Chemical and
physical stability despite such variations is essential since certain metallic
components of the anode could suffer corrosion by fuel oxidation products, while
electrical properties and lattice geometry of oxide components of the anode could
change by variation of stoichiometry.
For simplicity and reliability of operation, including start-up and shut-down as
well as tolerance of transients, redox stability is a further desirable attribute of an
anode material to permit brief excursions to high oxygen concentrations, even to
air, without irreversible loss of structural coherence and electrochemical
functionality. Stability implies the maintenance of structural integrity of the
anode itself over the whole temperature range to which it is exposed, from
the fabrication temperature through normal operating conditions, to the
repeated cycling down to ambient temperature.
Throughout these ranges of temperature and gas environment there should
also be maintained the necessary compatibility with the other materials with
which the anode comes into contact, specifically the electrolyte, the interconnect
and any relevant structural components. Physical compatibility requires a
match of thermomechanical properties such as thermal expansion coefficient
and an absence of phase-change effects which could generate stresses
during temperature variations. For chemical compatibility there should be no
solid-state contact reaction, interdiffusion of constituent elements of those
materials or formation of reaction product layers which would increase resistive
losses or otherwise interfere with anode functionality, despite the extremes of
temperature. After assembly into a series connected stack, of course the same
applies to the anode-interconnect interface. Compatibility must extend also to
the behaviour of the material towards the ambient gases including corrosion or
poisoning by trace impurities such as sulphur.
Obviously the anode material should not only be an adequate electronic
conductor, but also electrocatalytically active such that a rapid charge exchange
can be established. Resistive and overpotential losses are thereby minimised.
However, the catalytic behaviour of anode materials should not extend to the
promotion of unwanted side reactions, hydrocarbon pyrolysis followed by
deposition of carbon being an example. The electrochemical reaction takes place
in the region (Figure 6.1) where oxygen ions available from the electrolyte can