Page 160 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

Cathodes 137

(i) Some composite cathodes exhibit good performance even at low
temperatures such as 800°C.
(ii) The main electrochemical active sites are located at the interface
between the electrolyte and the composite cathode, although three-
phase boundaries are spread throughout the cathode layer. The presence
of YSZ particles in the cathode layer makes it easier to have longer three-
phase boundaries at the interface.
(iii) The presence of YSZ in the cathode layer enhances oxygen permeability
through the cathode layer. However, the presence of YSZ does not
contribute to nitrogen removal from the electrochemically active sites
when air is used as oxidant.

Properties (ii) and (iii) strongly depend on the microstructure of the composite
cathodes making it difficult to quantitatively characterise such composite
electrodes and also to reproduce their electrode activities. Attempts to utilise
doped ceria instead of YSZ as a component oxide of composite cathodes have
shown some success, although it is hard to say which property dominates
in enhancing the cathode activity. Introducing a fine microstructure at the
LSM/YSZ interface by depositing a metal organic layer (several 10 nm thick)
of LSM, which spontaneously forms micropores in nm size on the passage of
current, provides fine three-phase boundaries which are stable during long
term operation [67].
Another option is to find alternative cathodes with higher catalytic activity at
lower temperatures, particularly cobaltite-based cathodes. As given in Table 5.2,
the high catalytic activity for oxygen incorporation reaction and the high oxide
ion conductivity of cobaltites make them superior to lanthanum manganite.
Furthermore, the driving force of reactions of LaCo03 with YSZ disappears below
11 73 K. This makes it attractive to use cobaltites at low temperatures. However,
there are several other issues regarding their use in YSZ electrolyte cells
including their high thermal expansion coefficient and high reactivity of dopant
oxide with YSZ. To reduce thermal expansion, alkaline earth substitution works
to some extent: however, it also enhances reactions with YSZ to form alkaline
earth zirconates. So far, no cobaltite cathode has been used successfully in YSZ
electrolyte cells below 800°C.
However, attempts have been made to use cobaltite cathodes with ceria-based
electrolytes. Compared with YSZ, ceria has less reactivity with perovskite
cathode materials; this is because of the less acidic nature of CeOz compared with
Zr02; La203 or SrO component can be regarded as basic oxides so that the
interaction with Zr02 or CeOz can be judged from their acidity. The same trend
can also be explained from other physicochemical properties: that is, compared
with Zr ions, Ce ions are too big to form the perovslrites with La or Sr. On this
basis, it is possible to use lanthanum strontium cobaltite-based cathode with
doped ceria electrolyte in intermediate temperature SOFCs. Even so, (La,Sr)Co03
still has a high thermal expansion coefficient (16-22 x lop6 ICp1) compared
with doped ceria (about 12 x lop6 ICp1). Consequently, (La,Sr)Co03 cathode is
not used even though the oxide ion conductivity is high. Steele and co-workers