| In recent years,due to severe environmental pollution and climate change caused by the greenhouse effect,and people's environmental awareness has increased in various places,every country is striving to develop and promote zero-pollution power supply equipment.Solid fuel cell(SOFC)is a new type of energy technology with high energy conversion rate and less environmental pollution,which has attracted people's attention[1].The key components of the SOFC include anodes,cathodes,and electrolyte materials as well as connectors materials.Because the operating temperature of SOFC is usually in the temperature range of 500?-800?,under such a high temperature environment,the performance requirements of all parts of the battery are stricter.In order to meet the requirements,it is necessary to conduct in-depth research on the selection and preparation of the materials of the various parts of the battery and the structural reaction mechanism.LaCoO3 is a perovskite cathode material that has been developed in recent years Though much research has been done on its electronic structure,defects and doping modification,there are still some differences in the electronic structure,electrical properties and thermodynamic properties of the rhombohedral crystal.In this paper,the rhombohedral LaCoO3 cathode material was synthesized,and its macroscopic properties are predicted by the first principle calculation method based on density functional theory.From the electronic and atomic scale to explain the change of material performance,and we do some basic research work for the development of perovskite cathode materials,to discovery and provide theoretical basis for the design and application of new material and experimental basis.The paper mainly focuses on the development work of the cathode material that matches the thermal expansion coefficient of electrolyte materials commonly used.Pure phase LaCoO3 and B-site transition metal ion doped LaCo0.5Fe0.5O3,LaCo0,5Ni0.5O3 and LaCo0.5Mn0.5O3 powder samples were synthesized by sol-gel method,and the structure and morphology of the samples were characterized by XRD and SEM.At the same time,the electronic structure,phonon dispersion spectrum,phonon density and some thermodynamic properties of pure phase and doping system were calculated by first-principles calculations.Analysis of the diffraction data shown that the space group of all the samples is R-3c.Scanning electron microscopy showed that the grains of the doped system were all smaller than those of the pure phase system,and the LaCoO3,LaCo0.5Fe0.5O3 and LaCo0.5Ni0.5O3 crystals all exhibited regular form with good crystallinity.In the theoretical calculation,four different exchange-correlation functional are used to optimize the structure.By comparing the calculated results with the experimental data,it is concluded that the exchange-correlation functional of the GGA of the LaCoO3 system is more applicable.The electronic properties of LaCoO3 under different spin states have been calculated.It was found that LaCoO3 transforms from semiconductor to metal during the transition from low spin to high spin.On the basis of calculating the phonon dispersion spectrum,the thermodynamic properties such as heat capacity,entropy,and thermal expansion coefficient of LaCoO3 were calculated and found to be in agreement with the experimental data.In addition,the average thermal expansion of LaCoO3 calculated by us is 20x10-6K-1,which is obviously larger than that of common electrolytes.The same calculation method was used to calculate the electronic properties of the doping system,the phonon dispersion spectrum and the thermal expansion coefficient.Analytical calculations and experimental results show that the thermal expansion coefficient of the doped system is significantly lower than the thermal expansion coefficient of the pure LaCoO3 phase and matches with the commonly used electrolytes,achieving our original design intention. |
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