Study On Preparation And Performance Of Lanthanum Gallate Based Intermediate Temperature SOFC

This study prepared the lanthanum gallate, La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-δ(LSGM) electrolyte base of a large area with non-aqueous solvent tape-casting and conducted a sysmtematic investigation on the preparation technology, microstructure and electrochemical performance of the high performance electrodes, which layed a consolidate foundation for the assembly of the SOFC stacks.The preparation conditions of the LSGM powders by a solid state reaction method were investigated and the optimum sintering procedure was decided. The average size of LSGM powders was 3.04μm as obtained by being sintered at 900℃for 24 h and at 1300℃for 36 h. Then, the best slurry composition, which was decided by precipitation and viscocity tests, contained 80 g LSGM powders, 36 mL/24 mL butanone/ethanol as the solvent, 2 ml triethanolamine as the dipersant and 6.4 g polyvinyl butyral as the binder and 3.2 g polyethylene glycol/4 mL diethyl phthalate as the plasticizer.The smooth and homogeneous LSGM electrolyte base of 10 cm×10 cm in size was obtained by non-water solvent tape-casting and sintering the green tape at 1500℃for 6 h. The thus prepared LSGM electrolyte base possessed a relative density of 96%, a porosity of 0.38% and good thermal compatiblitty with the BCAS551 glass-ceramic sealing materials, which ensured the structural stability of the cell within the operating temperature. Electrochemical impedance spectrum test revealed that the conductivity of the LSGM electrolyte was primarily controlled by conductivity of the grain boundary at the low temperatures and the conductivity of the grain at the high temperatures. The conductivity activation energy was 0.91eV at low temperatures, which was higher than the conductivity activation energy of 0.62 eV at the temperature above 600℃.Moreover, the strategy of La³⁺ isoactivity in the anodic half cell of NiO-LDC anode/LDC interlayer/LSGM electrolyte was utilized to design and to evaluate the effectiveness of LDC as the buffer layer to restrain the reaction between the anode and the electrolyte. The LDC powders of 1.36μm with a narrow diameter distribution were prepared by oxalic acid coprecipitation method. A thin and dense LDC barrier layer of 10μm, which adhered strongly to the electrolyte, was successfully prepared by screen-printing method based on the systematic study of the slurry composition, screen-printing conditions and sintering procedure. The thus prepared barrier layer improved the chemical and thermal compatibility between the anode and the electrolyte, hampered the reation between the anode and the electrolyte, and facilitated the charge transfer process in the reducing atomophere.Furthermore, the anodic NiO-LDC composite powders mixed at the molecular level, which were later used to prepare the new NiO-LDC composite anode, were synthesized by the coprecipitation method. The conducting mechanism of the anode material depended on the NiO content strongly and the conductivity of the anode was 2260 S·cm^-1 with 60% NiO content. The anode with 60mass% NiO content possessed the best polarization performance, which was 200 mA·cm^-2 at the overpertantial of 0.1 V.SCF-LDC composite cathodes were prepared by the systemic investigation on the synthesis conditions of SrCo_0.8Fe_0.2O_3-δ (SCF) powders, sintering procedure of cathodes, thermal expansion and chemical compatibility of LDC, SCF and LSGM. It was found that the thermal expansion characteristic of the cathode was improved and triple-phane-boundaries were increased. Electrochemical performance of the double-layer SCF-LDC composite cathode with the interior layer doped 50mass% LDC was the best, which showed the polarization current density of 1.102 mA·cm^-2 at 800℃under the 0.1V overpotential and polarization resistance of 0.1480 ?·cm2. These properties were better than those of SCF60 cathode which was optimized in the monolayer composite cathode. The triple- layer SCF-LDC composite cathode with the respective SCF content of 50mass%,70mass% and 100mass% from the interior to the exterior layers was put forward and prepared, which not only improved the thermal compatibility between the cathode and the electrolyte, but also increased the three-phase boundaries and the total specific surface area. The triple-layer composite cathode had a current density of 1.32 A·cm^-2 under the 0.1 V overpotential. The electrochemical performance of the triple-layer composite cathode was studied by EIS and the reaction mechanism of the cathode was proposed. The results showed that cathode reaction process were composed of the diffusion, desorption and dissociation of oxygen, the electrochemical reduction of oxygen atoms at three-phane-boundaries and the diffusion of oxygen ions from three-phane- boundaries to the interface of cathode and electrolyte. The rate determining step was the diffusion, desorption and dissociation of oxygen.Finally, the thermal compatibility between all the components of the cell was studied and the cell assembled with the triple-layer SCF-LDC composite cathode has a maximum power density of 0.33 W·cm^-2 and an OCV of 1.073 V at 800℃.