Rate limiting steps of the porous La(0.6)Sr(0.4)Co(0.8)Fe(0.2)O(3-d) electrode material

The electrode reaction of porous La(0.6)Sr(0.4)Co(0.8)Fe(0.2)O(3-d) films deposited onto Ce(0.9)Gd(0.1)O(1.95) (CGO) was investigated by impedance spectroscopy within the temperature and oxygen partial pressure (pO(2)) ranges of 500 < = T < = 700 'C and 10(- 4) < pO(2) < 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 (R(W)) and the small variation of the apex frequency (f(v)) of the HF Warburg-type element, on pO(2), suggest that this contribution corresponds to the oxygen diffusion in the bulk of the La(0.6)Sr(0.4)Co(0.8)Fe(0.2)O(3-d) electrode material. The variation of the polarization resistance of the LF region (R(rcpe)) with pO(2) 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.