Research Of Intermediate-temperature Solid Oxide Fuel Cell With LaGaE₃Electrolyte

Solid oxide fuel cell（SOFC）, a energy conversion device with all-solid-statestructure, high effiency, low cost and low-pollution, has attracted considerableattention. The traditinal SOFCs usually use yttria-stabilized zirconia （YSZ） as theelectrolyte. In order to obtain high efficiency, SOFCs with YSZ electrolyte must beoperated at high temperature up to800oC-1000oC. However, operation at such hightemperatures brings serious troubles, such as limited electrode materials, poorlong-term stability. Thus reducing temperature of operation is imperative, and theintermediate-temperature SOFCs emerge as the times require. There are twoquestions needing to be considered, the one is using the intermediate-temperatureelectrolyte with higher ionic conductivity, and the other is optimizing the stractureof anode.Strontium and magnesium-doped lanthanum gallate （LSGM）exhibits goodperformance at intermediate-temperature, such as high oxygen-ionic conductivity,high mechanical strength, good stability, and so on. However, the chemicalcompatibility is so bad between LSGM electrolyte and traditional Ni-based anode,due to the insulation phase produced at high temperature. In this thesis,impregnation technology was attempted to prepare the SOFCs based on LSGMelectrolyte, and then measurements of output performance, stability, ac impedancespectroscopy, micro-structure of the signal cell were conducted.We have synthetized the La_0.9Sr_0.1Ga_0.8Mg_0.2O₃-δpowers by solid-state reactionmethod. The pure phase LSGM can be obtained at1400oC, and the average grainsize of final powders is around0.12μm, which is appropriate for the deposition ofLSGM electrolyte film. The average coefficient of thermal expansion is10.88×10-6K-1, which is closed to that of YSZ. The material start to sinter at1050oC,and the rate of sintering reaches to the higest at1350oC. The shrinkage is about13.8%when the temperature reachs to1400oC. The result of conductivity shows thatthe ionic conductivity of LSGM reaches0.08S cm-1at800oC.The LSGM-based SOFCs with nanometer Ni/YSZ anode andBa_0.5Sr_0.5Co_0.8Fe_0.2O₃-δ（BSCF） cathode were prepared successfully usingimpregnation technology, and then the electrochemical were studied. The resultsshow that the distribution of pore produced by flour in the anode is not uniform,which can lead to the sintering of Ni in the process of test. On the other hand, thepores produced by NiO show a uniform microstructure. The introduction of urea toimpregnation liquid can improve the stability of singel cells. We also find thatreducing the operatering temperature is a feasible approach to improve the cell’s stability.Because of the poor electrochemical activity of Ni/YSZ anode at intermediatetemperature, we chose the nano-structured meter Ni/LSGM anode to improve theelectrochemical perfomance. The SOFCs with LSGM electrolyte, Ni/LSGM anodeand BSCF cathod were fabricated and tested. The results show that reducing thethickness of electrolyte can improve the performance of signle cells, but the ohmicresistance is not the key factor of limiting the output of cell. So reduction ofpolarization resistance of electrodes is necessary to further performanceimprovement. LSGM sintered at1250oC and flour were mixed with a mass ratio of5:2.5as anode, which finally sintered at1400oC in order to get densify electrolyte.When the mass fraction of Ni is5.7%, anode shows the state of breakover, and theSOFC exhibits good output performance. However, the anode’s stability is poorduring the testing procedure. When the mass fraction reach to7.5%, the outputperformance show little better, but the stability exhibits obvious improvement.When the pore-forming material is tapioca, there are many closed pores in anode,which restrict the amout of Ni. On the other hand, the large size makes it difficultyto prepare dense electrolyte film. In order to increase the porosity, LSGM powderused to prepare anode was calcined at1350oC, the SOFCs using this powder asporous anode and possessing20%of Ni reveal the highest power density,875mW cm^-2at650oC. Dry-presing method has been successfully used to prepareLSGM-based SOFC, which provides an open-circuit voltage of1.07V and maxmumpower density of523mW cm^-2at650oC。...