Kinetics of oxygen reduction on porous mixed conducting $(La_{0.85}Sr_{0.15})_{0.9}MnO_3$ electrode by Ac-impedance analysis

Abstract

The present work is concerned with the oxygen reduction reaction on porous mixed conducting $(La_0.85Sr_0.15)_0.9MnO_3$ electrode by analysis of the ac-impedance spectra. The $(La_0.85Sr_0.15)_0.9MnO_3$ specimens with various porosities were coated on the 8 mol% $Y_2O_3$ -stabilised $ZrO_2$ by screen printing method, followed by sintering at 1623 K for 30 min in air. In order to attain a mixed conducting state of the electrode with high oxygen vacancy concentration, the electrode specimen was subjected to the cathodic polarisation of -0.9 V at 1273 K for 6 h, and then the ac-impedance spectra were determined at various temperatures and oxygen partial pressures. The measured ac-impedance spectrum clearly showed a straight line inclined at a constant angle of 45℃ to the real axis in the high frequency range, followed by an arc in the low frequency range, i.e. it exhibited the Gerischer behaviour. From the results, it was recognised that the oxygen reduction reaction on the porous mixed conducting $(La_0.85Sr_0.15)_0.9MnO_3$ electrode involves diffusion of oxygen vacancy through the electrode coupled with the exchange reaction between oxygen vacancy and gaseous oxygen at the electrode/gas interface. This means that the electrochemical active region for oxygen reduction is not restricted to the three phase boundaries (TPBs) among electrode, electrolyte and gas, but it is markedly extended from the TPBs to the electrode/gas interfaces. According to the theoretical impedance model based on the Gerischer behaviour, the extended active length was quantitatively estimated to be 0.7 to 1.1 ?m in value, depending on the temperature and the oxygen partial pressure. From the fact that the ac-impedance spectrum deviated more significantly from the Gerischer behaviour with increasing electrode porosity, on the other hand, it is suggested that the oxygen reduction reaction on the highly porous electrode proceeds simply via the charge transfer reaction at the TPBs without invo...