Page 156 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

Cathodes 133

interdiffusion of manganese, lanthanum and strontium ions across the interface
into the YSZ electrolyte. Tricker and Stobbs [48] confirmed, by examining the
electrode/electrolyte interface with transmission electron microscopy (TEM),
that during high-temperature treatment, La2Zr207 is formed between LSM and
YSZ, and that after 24 h the LazZr207 formed at the interface moves towards
LSM, leading to narrowing of the interface with the YSZ. The latter fact is
important in understanding the chemical nature of the La2Zr207 formation [49].
To minimise reactions between the cathode and the electrolyte, in Japan, most
research efforts have focused on the A-site-deficient lanthanum manganite. In
the USA and Europe, however, efforts [50] have been made to seek alternative
cathodes, but with only limited success. Perhaps the most significant finding has
been the use of composite cathodes in contact with the YSZ electrolyte. These
composite cathodes minimise cathode/electrolyte interaction by mixing LSM and
YSZ powders and laying down a thin layer of this mixture on the electrolyte [lo].
Another step forward has been the use of an activation process to reduce the
polarisation loss at the electrode [5 11.
To check the thermodynamic predictions of the cathode/electrolyte
interaction, effects of the A-site deficiency in lanthanum manganite on the
(cathode performance were investigated by Dokiya et al. [ 71 using the non-A-site-
deficient and the A-site-deficient lanthanum manganites. It was found that the
overpotential is smaller when the A-site-deficient lanthanum manganite is
applied at temperatures below 1473 K. Figure 5.10 shows a cell performance
with the A-site-deficient lanthanum manganite cathode: the observed cathode
overpotential (open circles) is compared with the evaluated resistivity (dashed
line) originating from the electrolyte. The difference between two values gives
the overpotential from the cathode reaction, which is less than 30 mV at a
current density of 1400 mA cm-2.
Yoshida et al. made systematic investigations on the effect of using partially
stabilised zirconia (PSZ, Y203 content = 3 mol%) on the electrochemical
performance of cathodes with different dopants and their concentrations [ 521.
The overpotential of the LSM/PSZ is always and systematically higher than that
of the same LSM with fully stabilised YSZ. This is apparently due to the chemical
interaction between LSM and PSZ; that is, the tetragonal phase (on the surface of
the PSZ electrolyte) is transformed into the cubic phase after manganese
dissolution into the tetragonal phase [5 31. This increases the overpotential of the
LSM cathode.
Although the A-site-deficient LSM can avoid reaction with YSZ, its
performance as cathode depends sensitively on its heat treatment temperature
[7,54]. After heat treatments at lower temperatures below 1473 I<, these
cathodes exhibit excellent performance (see Figure 5.10, heat treated at 1423
K), whereas heat treatment above this temperature gives rise to an increase in
overpotential. Since no chemical reaction is expected as described above, this is
due to the diffusion of La and Mn and related sintering behaviour. The
sinterability is enhanced in the A-site-deficient LSM. It appears that diffusion
inside the A-site-deficient manganite can be accelerated by increasing the
number of oxide ion vacancies at high temperatures. In addition, Zr and Y