(641b) Activation Mechanism and Nanostructuring of Solid Oxide Fuel Cell Cathodes

Perovskite structured oxides of
series La_1-xSr_xMnO_3-d (LSM) are widely
used as cathode for oxygen reduction reaction in a solid oxide fuel cell (SOFC).
To study the electrode mechanism, a geometrically well-defined model system was
developed using dense thin-film cathode. Polycrystalline nanostructured dense La_0.8Sr_0.2MnO_3-d
(LSM) thin-film cathodes with an average thickness of 600 nm were
fabricated on yttria stabilized zirconia (YSZ) or gadolinia doped ceria (CGO)
substrate electrolyte using spray pyrolysis. The polarization resistance of the
cathode was observed to be reduced on initial application of voltage or current
bias. In order to understand this initial activation, the reaction mechanism
was probed. This include altering the electrode surface by doping with La_0.6Sr_0.4FeO_3-d nanoparticles,
characterizing changes in bulk microstructure before and after application of
current and changing the nature of electrode-electrolyte interface by
introducing La₂Zr₂O₇ impurities. Two different
activation mechanisms were observed in the cathode. While applying current for
a short duration (5 min) the activation was linked to changes in the surface
activity, long term (16 hours) current application produced changes in bulk
microstructure.

For surface doping of electrode,
a reverse micelle (RM) based method was developed to synthesize highly active
electrocatalytic nanoparticles of composition La_xSr_1-xCo_yFe_1-yO_3-d.
The RM synthesis provides tight control over nanoparticle size and average
particles size was changed from 14 nm to 50 nm by changing the water:surfactant
ratio in the precursor RM solution. A noticeable difference in the sintering
behavior of these nanoparticles was observed. While the bulk samples of the
same perovskite materials sinter at high sintering temperatures, nanoparticles
sinter into a pure perovskite phase at temperature as low as 823 K. The unit
cell parameter (a=3.93 Å) for cubic perovskite nanoparticles was found to be
greater than the bulk material (a=3.89 Å) of similar composition and structure.