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solid oxide fuel cells; lanthanum manganite; impedance; microelectrode; cathode
Abstract:
Impedance spectroscopic studies and I-V Measurements are
performed at Sr-doped LaMnO3 (LSM) microelectrodes in order to
elucidate the mechanism of the oxygen-reduction reaction on
yttria-stabilized zirconia. The geometry dependence of the
polarization resistance was investigated by systematic
variations of the microelectrode's size and thickness. The
relation between the resistance and the electrode geometry
turns out to be bias-dependent: in the cathodic regime and
close to equilibrium, the resistance is proportional to the
inverse electrode area. Moreover, measurements without bias
revealed an almost linear dependence of the resistance on the
electrode thickness, This suggests that the relevant oxygen
reduction path involves the transport of oxide ions through the
bulk of the LSM cathode. In the anodic regime, however, the
resistance becomes proportional to the inverse three-phase
boundary length and, hence, a mechanism involving the LSM
surface is most probable with a step close to the three-phase
boundary being rate limiting. Experiments performed on LSM
microelectrodes with thin alumina "discs" beneath the LSM to
partly block the oxygen ion transport through the bulk of the
electrode support this interpretation. (C) 2002 Elsevier
Science B.V. All rights reserved.
