Study On Perovskite-Type Oxides For Cathode Materials Of Solid Oxide Fuel Cell

In the development of solid oxide fuel cell (SOFC) technology, one of the key problems is to improving the performance of electrodes at intermediate temperature (500℃-850℃). In this thesis, perovskite type oxides La1-xAxM1-yNyO3-δ(A=Sr, Ca; M and N=Fe, Mn, Co, Cu) were prepared and characterized to develop superior cathode materials for intermediate temperature solid oxide fuel cell (ITSOFC). The optimum conditions of preparation, the relation between structure and performance of the materials were investigated.Two methods were employed in material synthesis:EDTA complexing sol-gel method and glycine nitrite process. The materials including La1-xSrxCo1-yFeyO3-δ(LSCF), La0.8Sr0.2Co0.085CuxFe0.915-xO3-δ(LSCCuF), La0.8Sr0.2Co0.055FexMn0.95-xO3-δ(LSCFM) and La0.8Sr0.04Ca0.16Fe1-zCozO3-δ(LSCaCF) were studied.These materials were characterized by thermo gravimetric analysis (TG), differential thermal analysis (DTA), X-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microcopy (SEM), energy dispersive spectroscopy (EDS), temperature-programmed desorption of O2 (O2-TPD), X-ray photoelectron spectroscopy (XPS), characterization on the diameters of particles, the specific surface area mensuration, iodometric titration, electrochemical impedance spectra and direct current four-electrode techniques and so on.The optimum conditions of EDTA complexing sol-gel method are summarized as follow:the pH value of the solution should be adjusted to≥7; the mol ratio of metal ion to EDTA should be 1:1.1 to 1:1.2; the mol ratio of glycol to EDTA is 3:1; the concentration temperature should be 60℃-70℃; calcination temperature should be 700℃-800℃.All the powder samples prepared by EDTA complexing sol-gel method are perovskite-type compounds and no simple metal oxides or any other secondary phase exits in the structure. With the increase of Sr content, the main diffraction peaks of XRD move to large angles, which represent a distortion to small size of the cell. The powders are spherical shape and the powder can be identified to porous structure, with specific surface areas of about 11.6m2/g, total pore volume of 0.034cc/g and average pore diameter of 11.9nm. The particle is uniform with an average size of about 0.18μm by laser granularity analysis. Oxygen desorption from LSFC below 850℃was examined by O2-TPD and the result shows that there are three types of oxygen in the structure:adsorbed oxygen on surface, adsorbed oxygen in oxygen vacancy, lattice oxygen. The content of adsorbed oxygen on surface increase when Sr doped in A site and it seems that elements in B site have little influence to oxygen species. The La3d5/2 and O1s binding energy of the materials doped in both A and B site are lower than that only doped in A or B site, which indicate superior conductivity of double doped material.LSCF materials have good chemical compatibility with La1-xSrxGa1-yMgyO3-δ(LSGM) electrolyte.Pure perovskites are synthesized by both the two methods but morphology of powders have marked differences by comparison of EDTA complexing sol-gel method and glycine nitrite process. By EDTA complexing sol-gel method, the powders have small and uniform grain size. By glycine nitrite process, the powders have good porosity and large specific surface but more aggregate than by EDTA complexing sol-gel method. The remarkable dominance of GNP is the greatly shortened process.The La0.8Sr0.2Co0.085CuxFe0.915-xO3-δand La0.8Sr0.2Co0.05FexMn0.95-xO3-δpowders were prepared by the Glycine-Nitrate Process (GNP) method. The LSCCF-0.3 exhibits pure perovskite phase.The electrical conductivities of the La0.8Sr0.2Co0.085CuxFe0.915-xO3-δwere measured by a four point DC method. As the substitution amount of Cu increase, the electrical conductivity increases first and decreases later. The electrical conductivity of LSCCF-0.3 increases first and decreases later with the temperature increasing, and reach the maximum of 1809.47S/cm at 600℃. The electrical conductivity of LSCCF-0.3 in air is higher than in argon. There is no any second phase in the mixed powder of LSCCF-0.3 and 8YSZ after sintered at 800℃, but impurity phases form at 1000℃,1200℃. There is no any second phase in the mixed powder of LSCCF-0.3 and LSGM after sintering at 800℃, 1000℃and 1200℃initiate a small amount of La2SrFe2O7. By analysis, it illustrates that it is chemically compatible between the LSCCF-0.3 and LSGM.The La0.8Sr0.2Co0.05FexMn0.95-xO3-δpowders were measured by differential thermal analysis and thermogravimetry(TG-DTA) and the presintering temperature(1000℃, 2℃·min-1, 10h) was determined. The powder is pure perovskite phase by XRD analysis. It is found that the degree of oxygen non-stoichiometry (8) reduces with increasing x by the experiment of iodometric titration. The membranes have large density and no impurity-element is found through energy dispersion spectrometer (EDS). The conductivity measured by the four point DC method and the activation energy indicates that the conductive behavior of LSCFM agrees with the small-polaron hoping mechanism. The conductivity increases with the increasing temperature and reduces with increasing x, and besides, x=0 has the highest conductivity with 64.54S·cm-1 at 850℃.La1-x-ySrxCayFe1-zCozO3-δ(LSCaFC) fine powders have been synthesized by a glycine-nitrate process (GNP) method. The structure of La0.8Sr0.04Ca0.16CozFe1-zO3-δoxides were characterized to perovskite type oxides and the compositions are coincident with designed ratio. The oxygen non-stoichiometry values of La0.8Sr0.04Ca0.16CozFe1-zO3-δceramics increase with Co/Fe ration increase。The conductivity of LSCaFC materials increase with temperature increase and La0.8Sr0.04Ca0.16Co0.6Fe0.4O3-δmaterials have high conductivity of more than 100 S/cm from 550℃to 850℃. LSCaFC material have good chemical compatibility with LSGM electrolyte.The single cell with all perovskite materials was fabricated using LaCrO3-based, LaGaO3-based and LaFeO3-based materials as anode, electrolyte and cathode, respectively. The cathode films were prepared by two methodes:screen printing and co-sinter, spin coating and co-sinter. The maximum open circuit voltage of LSCrM | LSGM | LSCF (LSCrM50%) single cell is about 1.06V at 850℃using H2 and air as fuel and oxidant, respectively. The maximum open circuit voltage of LSCrMC (LSGM40%) | LSGM| LSCaFC (LSGM40%) and LSCrMC (CDC30%) | LSGM | LSCF(CDC30%) single cell is 0.914V and 0.93V respectively. The open circuit voltage value is similar to the theoretical electromotive force, shows the good cell sealing. But the maximum powder density is relatively small. The fabrication technique for single cell should be modified in the future.