Synthesis, Electrical and Electrochemical Behavior of Lanthanum Strontium Cobalt Ferrite for SOFC Cathode Application

Abstract

Mixed ionic and electronic conductive (MIEC) perovskites, particularly La1-xSrxCoyFe1-yO3-δ, have been widely explored to replace the conventionally used LaSrMnO3 (LSM) materials for intermediate temperature SOFC applications. The B-site Co rich compositions have a fairly good electrical conductivity but because of its poor thermal and chemical compatibilities the iron rich composition (La1-xSrxCo0.2Fe0.8O3-δ) has usually been a preferred destination. The La-Sr variations on A-site have however a set of scattered data that needs to be optimized. In addition to the MIEC character, cathodes need a microstructural tailoring to enhance the oxygen reduction reaction sites. For this the cathode particles must have a considerably smaller size with high exposed surface area. In the present work La1-xSrxFeO3- perovskites with x=0.0-1.0, have been synthesized. Electrical conductivity and thermochemical stability study as a function of x showed La0.6Sr0.4FeO3- as a potential candidate for cathode application. Iron rich La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) has been synthesized following solution combustion technique using citric acid and glycine as fuel. Different process parameters namely metal cation to fuel ratio (elemental stoichiometric coefficient), extra chelating agent and oxidant addition, and pH of the precursor solution has been optimized. It is seen that a fuel rich precursor solution favored the formation of high surface area (~30m2/g) LSCF powder. The effect of EDTA in reducing the calcination temperature and that of pH in powder homogenization helps in yielding fine powders. Infra-red spectroscopy (FTIR) study showed the purity of the well dispersed fine powder from the carbon residues. A correlation between the BET specific surface area, the crystallite size and SEM average particle size was established and was seen to be linked with the elemental stoichiometric coefficient. The influence of the processing parameters on the sintering, thermal expansion, thermal stability and electrical conductivity was studied. Thin electrolyte substrates were prepared from the in-house synthesized 8%-yttria stabilized zirconia (YSZ) and samarium doped ceria (SDC) powder. The chemical compatibility of LSCF powder with these electrolytes has also been studied and SDC was observed to be a better candidate in comparison to YSZ. Half electrochemical cells were prepared by screen printing the in-house prepared LSCF ink on YSZ and SDC substrates. For comparison half cells with SDC interlayer were also prepared with thin layer of LSCF electrode. Electrochemical study revealed that the polarization resistance of the half cells, apart from being influenced by the kind of electrolyte used and by the sintering temperature, are also influenced by the powder synthesis conditions. The improved electrochemical properties in case of LSCF electrode was attributed to its fineness at its powder synthesis level and the resulting microstructure. It has also been found that the electrode polarization of LSCF cathode is minimum with SDC electrolytes and maximum with YSZ electrolyte. The study also suggests the use of SDC interlayer on YSZ electrolyte as a potential solution for application of LSCF cathode with YSZ.