Kinetics of mixed ion migration and charge transfer-controlled oxygen reduction on $(La_{0.85}Sr_{0.15})_{0.9}MnO_3$ -YSZ and $La_{1-x}Sr_xCo_{0.2}Fe_{0.8}O_{3-\delta}$ electrodes for SOFC

Abstract

The present work deals with oxygen reduction at $(La_{0.85}Sr_{0.15})_{0.9}MnO_3$ (LSM) - YSZ and $La_{1-x}Sr_xCo_{0.2}Fe_{0.8}O_{3-\delta}$ (LSCF) Electrodes for SOFC using ac-impedance spectroscopy and potentiostatic current transient technique. Chapter III involves the effects of secondary phase and electrode thickness on the oxygen reduction reaction on dense LSM - YSZ composite electrodes by analyses of the ac-impedance spectra and cathodic potentiostatic current transients (PCTs). From the analyses of X-ray diffraction (XRD) patterns, scanning electron microscopic (SEM) images and energy dispersive spectroscopy (EDS) mappings, it was found that the sintered electrode shows a dense structure and the amount of the secondary phase, $La_2Zr_2O_7$, increases with increasing sintering time. From the analyses of the ac-impedance spectra and cathodic PCTs, it was recognised that overall oxygen reduction reaction on the dense composite electrode proceeds under the mixed migration and charge-transfer control. Furthermore, based upon the fact that the ratio of the ion migration resistance $R_i$ to the charge transfer resistance $R_{ct}$ decreases with decreasing thickness of the electrode and increasing amount of the secondary phase, it was conceivable that the charge transfer kinetics and migration reaction rate is predominantly impeded by the secondary phase and the electrode thickness, respectively. From the above results, it is concluded which reaction mainly contributes to overall oxygen reduction is crucially determined by the electrode thickness as well as the amount of the secondary phase. In chapter IV, the oxygen reduction mechanism of dense mixed-conducting LSCF electrodes with various Sr contents was investigated by employing ac-impedance spectroscopy and PCT technique. From the analyses of the ac-impedance spectra and the cathodic PCTs measured on the electrodes as functions of electrode thickness $\textit{l}$ and oxygen partial pressure $P_...