Structure And Properties Of LSCF Cathode Materials Prepared By Citrate Method

Perovskite complex oxide La1-xSrxCo1-yFeyO3 has received increasing attention in recent years due to their superior mixed electronic-ionic conducting properties, which make them promising candidate materials for many important applications, including cathodes for intermediate temperature solid oxide fuel cells, oxygen separation membranes, membrane reactors for syngas production and catalysts for oxidation of hydrocarbons. In this paper, La_1-xSr_xCo_0.2Fe_0.8O₃ (x=0.4, 0.6, 0.8) cathode materials are prepared by citrate method. The structure and properties are studied by modern testing technique.This paper is divided into two parts, one is about the preparation and characterization of the powder, the other is related to the sintered bodies. First of all, The results of XRD and SEM show that the synthesized samples at 900°C are single perovskite phase, no other phase is detected. The XPS analysis show that there were two different chemical conditions of O when Sr doped, corresponding to lattice oxygen and absorption oxygen, respectively. There are two chemical conditions for Fe and Co with Fe3+, Fe4+and Co3+, Co4+. It is ascribed to the substitution of La3+ by Sr2+ in LSCF that lead to the increases of the average radius of A-site ions and causes a charge imbalance and the charge compensation of oxygen vacancy. The XPS of sample La0.6Sr0.4Co0.2Fe0.8O3 under different atmosphere indicates that the atmosphere of inert gas is propitious to the increases of oxygen vacancy.The other discussed the microstructure and electric properties of the sintered bodies. The influences of the pH value of precursor solution and the sintering temperature on the phase structure of the sintering bodies are studied. The impacts of the pH value of precursor solution on the impedance characteristics of the sintered bodies are discussed in detail based on the AC impedance analysis. Experiment results show that the lowest impedance of the sintered body was realized when the pH value of precursor solution keeps at 4. The sintering temperature on the impedance characteristics of the sintered bodies are studied, during the sintering temperature, the contribution of the crystal grains to the impedance of the sintered body does not change very much, which means the crystal grains have almost the same impedance contribution. When the temperature rises to 1400°C, the contribution of the crystal boundary to the AC impedance declines obviously while that of the crystal grain keeps unchanged basically. Therefore, optimal sintering temperature can effectively control the impedance contribution of the crystal boundary and then reduce the impedance of the sintering bodies.The LSCF results of XRD shows the diffraction peak shifts toward high angle as Sr content increases. It is ascribed to the reason that the substitution of La3+ by Sr2+ in LSCF increases the average radius of A ions and causes a charge imbalance. In order to maintain electrical neutrality, some trivalent B ions oxidized to form B4+, which resulted in a decrease in the average radius of the B ions, as a result the volume of the unit cell decreases. The electrical conductivity increases to a maximum then decreases as temperature increases. The magnitude of the maximum conductivity increases and shifts to lower temperature with the increasing of Sr content. In this paper, the electrical conductivity reaches maximum value at x=0.4. Below the temperature corresponding to the maximum value, the electrical conductivity is found to follow the relationship for the small polaron hopping mechanism, charge compensation of oxygen vacancy dominates electrical conduction at high temperature, and oxygen vacancy acts as traps to catch carriers, resulting in the decrease of conductive carriers concentration and mobility. As a result, the electrical decreases.