The geometry dependence of the polarization resistance of Sr-doped LaMnO₃ microelectrodes on yttria-stabilized zirconia

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.