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Fabrication and Properties of Co-doped Lanthanum Gallate Electrolyte and Film

Author: HanYanHong
Tutor: HeTianMin
School: Jilin University
Course: Condensed Matter Physics
Keywords: Doped lanthanum gallate Solid electrolyte membrane Electrical properties
CLC: O484
Type: Master's thesis
Year: 2006
Downloads: 77
Quote: 1
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Abstract


Solid oxide fuel cells (SOFCs) are solid-state electrochemical devices that convertthe chemical energy of reaction directly into electrical energy. It possesses several distinctadvantages, such as environmental friendship, higher conversion efficiency and broadadaptive fuels, which is a new green energy developed in the world for 21 century.It was reported that LaGaO3 doped with Sr and Mg exhibited a highoxide ion conductivity. However, it was also found that the oxide ionconductivity was also improved by doping Co for Ga in LaGaO3-basedperovskites, although the hole conduction was detected at high oxygen partialpressures. A power density of 1.53w/cm2 at 800℃ was achieved by thecomposition, La0.8Sr0.2Ga0.8Mg0.115Co0.085O3 (LSGMC8585). In this paper, we usethe colloid deposition method to prepare La0.8Sr0.2Ga0.85Mg0.15O2.825 (LSGM)electrolyte membrane on a NiO/ Ce0.8Sm0.2O1.9 (SDC) anode support ,studyingthe effect of preparation conditions on properties ofLa0.8Sr0.2Ga0.8Mg0.115Co0.085O3 films on NiO/SDC anode substrate. In addition,we also investigated the structure and properties ofLa0.9Sr0.1Ga0.8Mg0.115Co0.085O3 electrolyte material .The incompletely crystallized LSGMC85 powder was used as thestarting material, the LSGMC85 films were prepared on NiO/SDC anodesupport using colloidal deposition method. X-ray diffraction, DC four-probemethod and scanning electron microscopy were employed to investigate theeffect of preparation conditions on the phase compositions, electricalproperties and surface morphology of LSGMC85 films. The results show thatthe calcining temperature of the anode substrate strongly affects the electricalproperties of LSGMC85 films. The LSGMC85 film sintered at 1400℃ for10h presents a higher electrical property and densification, and theconductivity of LSGMC85 film attains 0.97S/cm at 800℃. It is also foundthat Ni of anode substrate diffuses into the LSGMC85 films duringpreparation and introduces electronic conductance contribution, resulting inthe conductivity of LSGMC85 film on anode substrate increases obviously.The solid solubility of Sr-doping in A site is less than 6mol%. When thedoping content exceeds the limit,the second phase, (SrLa)Ga3O7 andSrLa(GaO4) are detected in the samples obtained. These impurity phasesdecrease the density and the conductivity of the samples. Furthermore, amongthe different compositions studied, La0.9Sr0.1Ga0.8Mg0.2O3-δ was identified asthe most promising oxide ion conducting material having potential as anattractive electrolyte material for SOFC applications. So we researched thestructure and properties of La0.9Sr0.1Ga0.8Mg0.115Co0.085O3 electrolyte. TheXRD results show that the structure of La0.9Sr0.1Ga0.8Mg0.115Co0.085O3 samplesform orthorhombic phase at the temperature of 1200℃. A extraordinaryevident endothermic peak appears at 1150℃ in DTA curve, this shows theperovskite phase is formed essentially. The sintering temperature ofLSGMC918 is approximately 1400℃. From ac impedance spectroscopy, thesamples were sintered at 1400℃, whose grain and grain boundary impedanceare higher than that at 1450℃. It shows that the conductivity in lowtemperature is controlled by the grain conduction. On the contrary, from 400℃, the higher frequencies arc was lower than the lower frequencies arc. Thegrain boundary controls the conductivity. Because the excessive sinteringphenomenon has taken place, the samples sintered at 1450℃,whose grain andgain boundary are higher than that at 1400 ℃ . The conductivity ofLa0.9Sr0.1Ga0.8Mg0.115Co0.085O3 samples sintered at 1400℃ for 6 hours isslightly higher than that at 1450℃, and attains 0.157S.cm-1 at 850℃. Thethermal expansion coefficient of the La0.9Sr0.1Ga0.8Mg0.115Co0.085O3 is 13.16×10-6K-1 from 30 to 1000℃..In order to enhance the current density and output power density, it isessential to reduce the thickness of the electrolytes. Therefore, somemembrane-forming technologies have been developed to prepare theelectrolyte membranes, for example, electrochemical vapor deposition (EVD),plasma spraying, sol-gel etc. Some of the technologies are not suitable forpreparing the electrolyte films on a large scale or the costs are expensive, thatlimit application in the SOFCs. To significantly reduce the cost of fabrication,we have studied colloidal deposition method for fabrication of dense ceramicmembranes on the porous substrates. The colloidal deposition method issimple, reproducible, and very cost-effective. We studied the performance ofelectrolyte films on the anode supports, and obtained some base data. Inaddition, to develop the superior solid oxide electrolyte materials, theLa0.9Sr0.1Ga0.8Mg0.115Co0.085O3-δ electrolyte with perovskite type wereexplored, and some good results were obtained. This means thatLa0.9Sr0.1Ga0.8Mg0.115Co0.085O3-δ may be is a kind of potential electrolytematerial for application in SOFCs.

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