Mixed Diffusion and Charge-Transfer-Controlled Oxygen Reduction on Dense La[sub 1−x]Sr[sub x]Co[sub 0.2]Fe[sub 0.8]O[sub 3−δ] Electrodes with Various Sr Contents

Abstract

The oxygen reduction mechanism of dense mixed-conducting La1−xSrxCo0.2Fe0.8O3−δLa1−xSrxCo0.2Fe0.8O3−δ (LSCF) electrodes with various Sr contents was investigated by employing ac-impedance spectroscopy and potentiostatic current transient (PCT) technique. From the analyses of the ac-impedance spectra and the cathodic PCTs measured on the electrodes as functions of electrode thickness ll and oxygen partial pressure pO2pO2 , it is confirmed that the overall oxygen reduction reaction mainly proceeds by oxygen vacancy diffusion through the electrode coupled with the charge-transfer reaction on the electrode surface. Furthermore, as Sr content increases, the decrease in the diffusion resistance RdRd due to higher diffusivity of oxygen vacancy D̃VOD̃VO is more predominant than that in the charge-transfer resistance RctRct due to the larger electrochemically active area AeaAea . The smaller values of RdRd and RctRct with increasing Sr content lead to the higher initial current IiniIini , higher steady-state current IstIst , and shortened time to reach steady-state current tsttst in the cathodic PCTs. From the analyses of the impedance spectra combined with the cathodic PCTs measured on the electrodes cathodically polarized for various times, it is suggested that the decomposition of SrOSrO on the electrode surface by the cathodic polarization enhances AeaAea , resulting in the kinetic facilitation of the charge-transfer reaction at the electrode/gas interface.