Electrical transport properties and defect structure in the strontium-iron-cobalt-oxygen system

The mixed conducting Sr-Fe-Co oxides are potential materials for use as solid-oxide fuel cells, batteries, sensors, oxygen permeable membranes and other electrochemical devices. The SrFeCo{dollar}rmsb{lcub}0.5{rcub}Osb{lcub}x{rcub}{dollar} sample has not only superior combined electronic and oxygen ionic conductivities but also structural stability under both oxidizing and reducing environments. Dense ceramic membranes made of this material can separate oxygen from air without external electrical circuitry. The use of this material would greatly improve the economics of producing fuels.; Structure of the SrFeCo{dollar}rmsb{lcub}0.5{rcub}Osb{lcub}x{rcub}{dollar} sample has been investigated by X-ray powder diffraction experiments, and was found to have layered structure which consists of perovskite like layers and rock-salt like layers. The perovskite like layers are formed from FeO{dollar}sb6{dollar} octahedra sharing four equatorial vertices, and the rock-salt like layers are formed from polyhedra with vertices and/or edge sharing. These two types of layers are joined through vertex-sharing to form the 3d structural framework, and Sr{dollar}sp{lcub}2+{rcub}{dollar} ions occupy interstices in the framework. The perovskite like layers make up the frame of the structure, while the rock-salt like layers are able to absorb oxygen under oxygen-rich environment and release oxygen under reducing environment, acting as oxygen reservoirs as well as the primary paths for oxygen transport.; The total and oxygen ionic conductivities of the SrFeCo{dollar}rmsb{lcub}0.5{rcub}Osb{lcub}x{rcub}{dollar} sample were investigated with conventional and electron-blocking four-probe methods. The electronic and ionic conductivities are 10 and 7 {dollar}rm S{lcub}cdot{rcub}cmsp{lcub}-1{rcub}{dollar} respectively, at 800{dollar}spcirc{dollar}C in air. The ionic transference number is {dollar}approx{dollar}0.4 and independent of temperature over the range of 600 to 1000{dollar}spcirc{dollar}C. The oxygen diffusion coefficient was measured with conductivity relaxation experiment. At 900{dollar}spcirc{dollar}C, the oxygen diffusion coefficient is {dollar}rm{lcub}approx{rcub}9times10sp{lcub}-7{rcub} cmsp2{lcub}cdot{rcub}secsp{lcub}-1{rcub}.{dollar} The reduced oxygen environment was achieved experimentally with a gas-tight electrochemical cell. The measured ionic conductivity decreases with decreasing oxygen partial pressure (pO{dollar}sb2),{dollar} while the total conductivity first decreases, reaches a minimum and then increases with increasing pO{dollar}sb2.{dollar} The oxygen permeability measured by using a gas-tight electrochemical cell agrees with the estimated values obtained from conductivity data. The ionic transference number as a function of pO{dollar}sb2{dollar} obtained from electromotive force experiments is consistent with that derived from the conductivity results under reduced pO{dollar}sb2.{dollar} A defect model has been proposed and the electrical transport properties of the Sr-Fe-Co-O system can be understood with the proposed defect model.