Page 267 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

244 High Tewiperature Solid Oxide Fuel Cells: Fundamentals, Design and Applications

"0
of the electrolyte <-TiyT->
Figure 9.4 Schematic of a porous MIEC electrode with possible reaction pathways and involved species for
the oxygen reduction reaction for SOFC application (adsorbedoxygen species O,(ad): 02,ad. Oad, O-ad, 02-,d).

level). In such composite materials used as electrodes, the TPB exists through the
thickness of the electrode, and the electrochemical reaction is spread into the
electrode, and not just restricted to the physically distinct electrolyte/electrode
interface. In a single phase MIEC electrode, the electrochemical reaction can
similarly occur over some distance into the electrode.
There are advantages and shortcomings to both approaches. If a single-phase
MIEC material is used, in principle, the electrochemical reaction can occur over
the entire porous surface. The potential disadvantage, however, is that careful
manipulation of defect chemistry is required to ensure that both ionic and
electronic conductivities are sufficiently high. This is often difficult to achieve,
especially over a wide range of oxygen partial pressures and temperatures. If a
two-phase MIEC material is used, it is necessary to ensure that both phases are
contiguous, while at the same time exhibiting a high TPB; that is, one phase
should not completely coat the other phase. This requires a careful control over
the microstructure. The advantage over single-phase MIEC materials, however,
is that an ability to mix two different materials allows flexibility in the choice of
materials so that transport properties of the two phases can be separately
optimised. In essence, by using two separate phases for the ionic and electronic
transport, greater flexibility is achieved by decoupling the functions. A host of
different materials for the ionic conducting part, such as YSZ, doped ceria,
stabilised Bi203, LSGM, etc., can be used; and a host of electrocatalysts, such as
LSM, Sr-doped LaFe03 (LSF), Sr-doped LaCo03 (LSC), etc., can be used. The use of
LSC or LSF in composite electrodes is expected to be beneficial as these materials
are themselves MIEC, albeit with much larger electronic conductivity compared
to ionic conductivity, as they offer additional pathways for the transport of
oxygen ions.
Theoretical aspects of porous MIEC electrodes, both using single-phase and
two-phase materials, have been analysed by many authors [18,2 7,30-341.
While the particulars of the models vary from model to model, general features of
the porous MIEC electrodes can be summarised as follows: (1) Gaseous species