Verlag des Forschungszentrums Jülich
JUEL-3561
Erning, Johann Wilhelm
Untersuchungen zur Sauerstoffreduktion an Kathoden für Hochtemperatur-Brennstoffzellen
145 S., 1998
Lanthan-Strontium-Manganite perowskites are the most widespread materials in
use for Solid Oxide Fuel Cell cathodes. The electrode reaction taking place,
i.e. the reduction of oxygen supplied by air, was investigated by
electrochemical means to obtain further knowledge about the electrode
processes. The high activation energy of this reaction (200 kJ/mol), preventing
lower operation temperatures of the SOFC, was the starting point for the
investigation. Quasi steady state current voltage measurements and impedance
spectroscopy were performed in a three electrode configuration. The electrodes
were of circular shape with a diameter of 10 mm. The preparation was made by
screen printing as well as Wet Powder Spraying onto plates made of
Yttria-stabilized Zirconia. Perowskite powders of varying chemical and
stoichiometric composition were used. To obtain higher power densities an!d,
more important, lower apparent activation energies, catalytic layers were added
at the interface electrode/electrolyte. Additionally, a less complex system, a
model electrode/electrolyte setup made from single-crystal YSZ as electrolyte
and gold in liquid and solid state as electrode was developed to create a
better defined system. This setup was used to investigate the behaviour of the
electrode/electrolyte interface. Reliable, reproducible results could be
obtained using either setup.
The experimental conditions i.e. oxygen partial pressure, temperature and
overpotential were varied in order to determine the kinetic properties of the
electrodes. Apparent activation energies, pre-exponential factors, apparent
charge-transfer coefficients and electrochemical orders of reaction were
calculated from the current-voltage data in order to propose possible reaction
steps. The catalytic layer made of palladium lowered the apparent activation
energy to about 138 kJ/mol, but lowered the apparent pre-exponential factor as
well, thus resulting in current densities one order of magnitude higher than
without catalyst. By using a mixture of platinum and palladium, the current
densities obtained were even higher, caused by a higher pre-exponential factor.
Several electrodes showed a charge-transfer reaction determined behaviour for
small cathodic overpotentials (<100 mV).! For these potentials the behaviour
of the electrodes with additional catalytic layers was dominated by the
influence of the catalyst. The apparent electrochemical reaction orders for
intermediate temperatures were calculated in the region between 0.4 and 0.8
giving evidence for dissociative adsorption of oxygen. The analysis of the
charge-transfer coefficient [[alpha]] and its temperature dependence showed
negative values for the entropic part [alpha]c.
Impedance data gave further evidence for the proposed reaction steps but it was
not possible to correlate the time constants with singular reaction steps. All
results indicate a complex reaction mechanism involving several
rate-determining steps.
The use of the model electrode/electrolyte setup made it possible to isolate
several reaction steps which are depending on the geometry of the electrode.
The combination of all results gave evidence for the formulation of possible
reaction mechanisms which were verified by using a complex simulation program
which simultaneously fits current-voltage and impedance measurements using a
model based on the kinetic analysis of an assumed reaction mechanism. The
activation energies computed by the simulation program for single reaction
steps are similar to those calculated from specific potential regions for the
current-potential measurements. Thus the assumption of potential regimes in
which specific, different reactions are rate-determining is affirmed.
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