Correlation between defect structure and electrochemical properties of mixed conducting La_0.1Sr_0.9Co_0.8Fe_0.2O_3−δ

Abstract

The high catalytic properties of LSCF1982 arise from its defect structure. In this work, the oxygen nonstoichiometry (δ) of LSCF1982 was analyzed as a function of oxygen partial pressure (P_O2)$(P_{{\text{O}}_2})$ and temperature for the -6⩽log(P_O2/atm)⩽0$-6⩽\log (P_{{\text{O}}_2}/\text{atm})⩽0$ and 800 ⩽ T/°C ⩽ 1000 ranges. A defect structure model for LSCF1982 was presented, which fitted well with the experimental data for δ. The equilibrium constants of appropriate defect reactions were determined. Analysis of the defect structure of LSCF1982 suggested that the conduction mechanism of LSCF1982 is governed by hopping conduction and band conduction of p-type carriers, which was determined by the analysis of thermoelectric power. The characteristic membrane thickness (L_c), indicating the transition from predominantly bulk-diffusion controlled reaction to surface-exchange controlled reaction, had a value of 3.5 ± 0.9 × 10⁻² cm. The oxygen vacancy diffusivity was calculated from the relationship between oxygen flux and oxygen chemical potential gradient. The chemical expansion was measured as a function of P_O2$P_{{\text{O}}_2}$ and temperature in the 10^-3⩽P_O2/atm⩽0.21${10}^{-3}⩽P_{{\text{O}}_2}/\text{atm}⩽0.21$ and 800 ⩽ T/°C ⩽ 1000 ranges. The chemical expansion model based on the relative change in mean ionic radius was employed to compute the chemical expansion vs. δ, which indicated that the spin states of B-site transition metal ions are a mixture of high-spin and low-spin states, and made the transition from the low-spin to the high-spin state with an increase in δ and temperature.