(A) study on oxygen reduction mechanism of dense mixed-conducting $La_{1-x}Sr_xCo_{0.2}Fe_{0.8}O_{3-δ}$ electrodes with various Sr contents by analyses of ac-impedance spectra and potentiostatic current transients

Abstract

Oxygen reduction mechanism of dense mixed-conducting $La_{1-x}Sr_xCo_{0.2}Fe_{0.8}O_{3-δ}$ (LSCF) electrodes with various Sr contents was investigated by employing ac-impedance spectroscopy and potentiostatic current transient (PCT) technique. For this purpose, four kinds of dense LSCF electrodes with various Sr contents were prepared by controlling the molar ratio of Sr to La at the initial stage of powder mixing. From the analyses of the ac-impedance spectra and the cathodic PCTs measured on the dense LSCF electrodes as functions of electrode thickness $\lota_{th}$ and oxygen partial pressure $p_{O_2}$, it is confirmed that the overall oxygen reduction reaction at the electrode mainly proceeds by the oxygen vacancy diffusion through the electrode coupled with the charge transfer reaction on the electrode surface. The longer diffusion length with increasing l¬th more impedes the oxygen vacancy diffusion through the electrode and the larger electrochemical-active area $A_{ac}$ with increasing $p_{O_2}$ further promotes the charge transfer reaction on the electrode surface. Furthermore, as Sr content increases, the decrease in diffusion resistance $R_d$ due to higher diffusivity of oxygen vacancy $\~{D}_v_{\"{o}$ is more predominant than that in charge transfer resistance $R_{ct}$ due to larger $A_{ac}$. The larger values of $\~{D}_v_{\"{o}$ and $A_{ac}$ with increasing Sr content lead to the higher steady-state current $I_s$ and the more shortened time to reach steady-state current $t_s$ in the cathodic PCTs. From the analyses of the impedance spectra combined with the cathodic PCTs measured on the dense LSCF electrodes cathodically polarised for various times, it is suggested that the decomposition of SrO on the electrode surface by cathodic polarisation enhances $A_{ac}$, resulting the kinetic facilitation of charge transfer reaction at the electrode/gas interface.