Rate limiting steps of the porous La0.6Sr0.4Co0.8Fe0.2O3-δ electrode material

Abstract

The electrode reaction of porous La0.6Sr0.4Co0.8Fe0.2O3-δ films deposited onto Ce0.9Gd0.1O1.95 (CGO) was investigated by impedance spectroscopy within the temperature and oxygen partial pressure (pO2) ranges of 500 ≤ T ≤ 700 °C and 10- 4 < pO2 < 1 atm, respectively, using Ar and He as gas carriers. The electrochemical impedance spectroscopy (EIS) measurements reveal a high frequency (HF) and a low frequency (LF) regions in the Nyquist plane. The high frequency (HF) region was fitted with a Warburg-type impedance element, and the low frequency (LF) region was reproduced with a resistance in parallel to a constant phase element. Both, the slight dependence of the polarization resistance (RW) and the small variation of the apex frequency (fv) of the HF Warburg-type element, on pO2, suggest that this contribution corresponds to the oxygen diffusion in the bulk of the La0.6Sr0.4Co0.8Fe0.2O3-δ electrode material. The variation of the polarization resistance of the LF region (Rrcpe) with pO2 indicates that as T increases, the limiting step evolves from dissociative oxygen adsorption to oxygen gas diffusion in the pores of the mixed ionic/electronic conductor (MIEC) electrode.