Fabrication And Property Of La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ A-Site Deficient SOFC Cathode Materials

In this paper, the properties of three kinds of AD-LSCF were investigated: La_0.6-zSr_0.4Co_0.2Fe_0.8O_3-δ, La_0.6-zSr_0.4-zCo_0.2Fe_0.8O_3-δ , (La_0.6-zSr_0.4)_1-zCo_0.2Fe_0.8O_3-δ, z=0.02, 0.05, 0.1. Studies were conducted detailedly from several aspects including powder preparation, ionic and electronic conductivity, cathode fabrication, composite cathode fabrication, two layers cathode fabrication. The results provided the foundation of experiment and theory for studying the low-intermediate temperature cathode materials.The powder of AD-LSCF materials was prepared by citrate Sol-Gel method. The results showed that ratio of chemometric number of citric acid to total metal cation content (abbreviated as C/Mⁿ⁺), pH of precursor solution, Sol temperature, calcining temprature were main contributing factors to the formation of the sol and gel as well as the crystalline structure and morphology of the calcined powder. Fine powder of AD-LSCF was obtained by optimizing the above-mentioned conditions.The electrical conductivity of AD-LSCF was measured by a DC four-terminal method. At the testing temperature ranging from 100 to 800℃, the electrical conductivity of AD-LSCF increased with increasing the temperature, and reached the maximum at about 400℃and then decreased. The electrical conductivity of AD-LSCF accords with the small polaron hopping mechanism. The electrical conductivity La_0.6-zSr_0.4Co_0.2Fe_0.8O_3-δ decreased along with elevating z values when the temperature was lower than 450℃. The electrical conductivity decreased in the order of L58SCF>LSCF>L50SCF>L55SCF when the temperature was over 450℃. The electrical conductivity of La_0.6-zSr_0.4-zCo_0.2Fe_0.8O_3-δ decreased along with elevating z values in the temperature range of 100～800℃. The electrical conductivity (La_0.6-zSr_0.4)_1-zCo_0.2Fe_0.8O_3-δ decreased along with elevating z values when the temperature was lower than 450℃. The electrical conductivity of (LS)90CF was highest at the temperature higher than 450℃. The electrical conductivity of L58SCF was highest in three kinds of AD-LSCF system.The ionic conductivity of AD-LSCF was studied by electron blocking electrode and AC impedance spectrum. Electron blocking electrode YSZ was fabricated by magnetron sputtering method, and calcined at 800℃for 6h. The SEM micrographs showed that the thickness of YSZ film was 2³Î¼m and the YSZ film was dense. The ionic conductivity was first obtained and measured from 400 to 800℃. The ionic conductivity increased slowly with increasing the temperature when lower than 600℃, and increased fast with increasing the temperature when higher than 600℃.The ionic conduction of the AD-LSCF materials is due to the creation of oxygen vacancies in themselves. At high temperature, the oxygen vacancies both participate in ionic transfer as carrier and supply path for oxygen ion. The ionic conductivity of L58SCF was the highest in three kinds of AD-LSCF system. L58SCF powder was used to study the fabrication and property of cathode, composite cathode and two-layer cathode. Several key fabrication parameters including selection of additives (binder and pore former), effect of coating thickness, sintering temperature, sintering time and removing organic materials temperature on the microstructure and electrochemical performance of cathode were investigated by scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS). Results firstly showed that the cathode possessed not only fine microstructure, sufficient porosity, ideal electrochemical property but also lower manufacturing costs when polyvinyl butyral (PVB) served as both binder and pore former in the cathode. No resistive phases SrZrO3 were detected by X-ray diffraction for L58SCF cathodes on YSZ by the optimization fabrication parameters. Based on the optimized parameters, the polarization resistances of the L58SCF cathode displayed the R_p values of 0.183Ω·cm² at 800℃, 0.33Ω·cm² at 750℃, 0.59Ω·cm² at 700℃and 1.37Ω·cm² at 650℃. GDC (Ce_0.8Gd_0.2O_1.9-δ) powder was prepared by an oxalate coprecipitation route. The L58SCF-GDC (40:60,50:50,60:40,70:30,80:20, by mass) composite cathodes were prepared firstly by mixing with Gd_0.2Ce_0.8O_2-δ powder.Impedance spectroscopy measurements showed that the addition of 40% GDC to L58SCF (L58SCF-GDC40) resulted in the lower polarization resistance (0.07Ω·cm² at 800℃, 0.11Ω·cm² at 750℃and 0.22Ω·cm² at 700℃) than other composite cathodes. No resistive phase, either La₂Zr₂O₇ or SrZrO₃, was detected by X-ray diffraction for the L58SCF–GDC40 cathode on YSZ. SEM and Surface backscattered images of the L58SCF-GDC40 showed that the particles of L58SCF-GDC40 were found to be continuous and uniform and to form the three-dimensional framework in the composite electrode. The interface of L58SCF-GDC cathode and YSZ electrolyte showed no detectable SrZrO₃ formation by Energy dispersive X-ray spectroscopy. The higher power density of the cell with L58SCF-GDC40 cathode was 264mW·cm^-2 at 800℃when H2 flow rate was 50ml·min^-1.The two-layer cathode was fabricated to improve the power density of the cell. The function layer near YSZ was L58SCF-GDC40, the current collecting layer L58SCF was coated on the function layer. The parameters of coating layers, porosity and the ratio of particle size were optimized for the function and current collecting layer. Based on the optimized parameters, the higher power density of the cell with two layers cathode was 359 mW·cm^-2 at 800℃when H2 flow rate was 50ml·min^-1.