Page 269 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

246 High Temperature Solid Oxide Fuel Cells: FUndQmentQk, Design and Applications

where d is the grain size of the ionic conductor (e.g. YSZ) in the composite MIEC
cathode, V; is the porosity in the cathode interlayer, and p; is the ionic resistivity
(inverse of ionic conductivity, 1/4) of the ionic conductor in the composite
cathode. The preceding equation is illustrative from the standpoint of assessing
the role of various parameters on the polarisation resistance of a composite
cathode in terms of the physically measurable parameters. In this equation,
has the same meaning as stated earlier. If the grain size of the ionic conductor
(e.g. YSZ) in the composite cathode, d, is -1 pm, the ionic resistivity, p; is -50
Qcm, and if the cathode interlayer porosity, V; is -0.25, and if R:t is -2 Qcm2,
then the effective charge transfer or polarisation resistance (using equation (24))
turns out to be -0.12 Qcm2 -that is a reduction by a factor of 16. The critical
distance, A,, similarly, depends upon various parameters, and has the form [2 71,

For the case of the LSM + YSZ composite cathode described above, and for the
selected values of the parameters, the magnitude of the critical cathode
interlayer thickness turns out to be -1 7 pm. If, on the other hand, the electrode
is made of porous LSM only, but of essentially the same microstructure, the
value of A, is on the order of 1 micron, assuming ionic resistivity of LSM to be
10,000 Qcm. This shows that if the ionic resistivity of the porous MIEC is very
high, the charge transfer reaction is essentially confined to the physically distinct
cathode/electrolyte interface. The corresponding polarisation resistance (as
determined using the general equation given by Tanner et a1 [2 71) or the effective
charge transfer resistance is essentially the same as R:t or -2 Qcm2 in this
illustration. That is, with single phase LSM, the reaction zone is confined to the
physically distinct cathode/electrolyte interface, and the polarisation resistance is
high since LSM is not an MIEC (or is an MIEC with very low ionic conductivity).
The preceding discussion and equations show that a fine cathode
microstructure is preferred. Fabrication of such cathodes requires careful control
of microstructure. It has been demonstrated that a typical high performance
cathode has a particle size (of the oxide ion conductor in the composite cathode)
on the order of a micron. At 800°C, the polarisation resistance less than about
-0.1 Qcm2 has been demonstrated with such cathodes. Figure 9.5 shows an SEM
micrograph of a typical anode-supported cell. Regions adjacent to the electrolyte
are the cathode and anode electrocatalytic layers of fine microstructure
to facilitate electrochemical cathodic and anodic reactions, respectively.
Regions next to these electrocatalytic layers have a coarse microstructure
and greater porosity to facilitate easier gas transport. These regions also exhibit
greater electronic conduction and serve as current collector regions.
A well-defined increase of the effective electrolyte surface area can also be
achieved by a structured electrolyte surface. Sintering separate 8YSZ particles
onto the electrolyte substrate and covering the increased surface area by an
electrochemically active thin porous film cathode via metal-organic-deposition