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The Electrical Conductive Properties Of The Solid Oxide Fuel Cell Composites Impregnated Phase

Posted on:2017-01-05Degree:DoctorType:Dissertation
Country:ChinaCandidate:J W JuFull Text:PDF
GTID:1221330485953571Subject:Materials science
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Solid oxide fuel cells (SOFCs) are high efficient, environment friendly energy conversion devices working at high temperature. With the development of SOFCs, lower SOFCs working temperature becomes the trend. However, lower operating temperature will inevitably lower the electrode catalytic activity as well as the electrolyte conductivity. Consequently, the concerns are how to obtain the high performance electrode/electrolyte materials/structure at low opearating temperature. The electrodes fabricated by conventional mixing & sintering method may results in problems including the CTE mismatch and the chemical incompatibility between the electrode and the electrolyte, which limits the combination choices of electrodes and electrolyte. The impregnated electrodes, made via wet/soft chemical methods, can be a valid electrode structure to solve such problems. For one kind of electrochemical material, the conductivity is its fundmental property. We choose the frequently-used impregnating materials as the objects, simulate the structures of the porous electrodes and study their conductivities and electrochemical performances systematicly.In the 1st chapter, we briefly introduce the working principles and the key materials of solid oxide fuel cells. We compare the electrochemical performances and the stability of the impregnated electrodes with the conventional electrodes, propose the contents and the objects of this thesis.In the 2nd chapter, we investigate the ionic conductivity of nano-sized samaria-doped ceria (SDC), which is often deposited in the electrodes of solid oxide fuel cells to enhance their electrochemical performance byextending the three-phase boundary (TPB) length. The SDC nano-particles are fabricated via an ion impregnation/infiltration method using porous ceria as the backbone. The apparent conductivity, which is determined with electrochemical impedance spectroscopy, increases with SDC loading and reaches 8.40×0-4 S cm-1 at 700℃ for the loading of 25.1 wt.%. A model is developed to calculate the conductivity of the impregnated phase, which has a porosity of 50.4%. The nano-sized SDC conductivity at 700J/ψ is 9.82 ×10-3 S cm-1, lower than 2.09 ×10-2 S cm-1 for the bulk SDC prepared from the same precursor. Considering the Bruggeman factor, the conductivity of a dense impregnated SDC is estimated to be 5.88×10-2 S cm-1, higher than the bulk material. The impedance for the impregnated SDC is characterized by much smaller grain-boundary contribution than the grain-interior, which is quite different with the bulk SDC.In the 3rd chapter, Strontium-doped lanthanum manganite (LSM) nanoparticles are deposited onto porous yttria-stabilized zirconia frameworks via an ion impregnation/infiltration process. The apparent conductivity of the impregnated LSM nanostructure is investigated regarding the fabricating parameters including LSM loading, heat treatment temperature, heating rate, and annealing at 750J/ψfor 400 hr. Besides, the conductivity, the intrinsic conductivity as well as Bruggeman factor of the impregnated LSM is estimated from the apparent conductivity using the analytical model for the three-dimensional impregnate network. The conductivity increases with LSM loading while the interfacial polarization resistance exhibits the lowest value at an optimal loading of about 5 vol.%, which corresponds to the largest three phase boundary as predicted using the numerical infiltration methodology. For the 9.38 vol.% LSM impregnate sample, at 700 J/ψ, its apparent conductivity is 0.909 S cm-1. At the optimal loading, the area specific ohmic resistance of the impregnated LSM is about 0.032 Ω cm2 at 700J/ψ for a typical impregnated cathode of 30 μm thick. It is only 5.5% of the cathode interfacial polarization resistance and 3.3% of the total resistance for a single cell consisting of a Ni-YSZ support, a 10 μm thick electrolyte and a 30μm thick cathode, demonstrating that the ohmic resistance is negligible in the LSM impregnated cathode for SOFCs.In the 4th chapter, perovskite structure Lao.6Sro.4Coo.;Feo.803-6 (LSCF) nano-particles are deposited onto porous yttria-stabilized zirconia frameworks via an ion impregnation/infiltration process. The apparent conductivity of the impregnated LSCF nano-structure is investigated regarding the fabricating parameters including LSCF loading, heat-treatment temperature, heating rate, and annealing at 750J/ψfor 400 hr. The conductivity and the intrinsic conductivity of the impregnated LSCF is estimated from the apparent conductivity using the conductive model for the impregnated network. The conductivity increases with LSCF loading while the interfacial polarization resistance exhibits the lowest value at an optimal loading of about 5 vol.%, which corresponds to the largest three-phase boundary as predicted using the numerical infiltration methodology. For the 9.94 vol.%LSCF impregnate sample, at 700 J/ψ, its apparent conductivity is 1.38 S cm-1.At the optimal loading, the area specific ohmic resistance of the impregnated LSCF is about 0.027Ωcm2 at 700J/ψ for a typical impregnated cathode of 30 μm thick. It is about 20% of the cathode interfacial polarization resistance and 3.5% of the total resistance for a single cell consisting of a Ni-YSZ support, a 10 μm thick electrolyte and a 30μm thick cathode.Factually, both the porous YSZ and ceria substrates own electrical conductivity. Though the value is relatively small when the impregnaing amounts of SDC/LSM/LSCF are large, the existing conductivity does impact the estimation of the conductivity of the impegnated phase. Considering the alumina is one kind of inert, insulating ceramic materials, besides, the generally employed alumina powders cannot be fabricated into porous substrates with regular microstructure characteristics, in the 5th chapter, high purity porous alumina supports with adequate porosity are fabricated using ball shape alumina raw powders without pore former addition. The effects of sintering temperature on porosity, pore size, tortuosity, thermal conductivity and mechanical/bending strength are investigated. The elevated sintering temperature decreases the porosity and the pore size whereas increases the thermal conductivity and the mechanical strength of the supports. After the raw powders coated with boehmite, the porosity and the pore size decrease further while the thermal conductivity and the mechanical strength increase further. With boehmite coating, when the sintering temperature is 1450 J/ψ, the support with a porosity of 47.7%, an average pore diameter of 2.28 μm, a thermal conductivity of 3.194 W m-1 K-1, and a mechanical strength of 10.24 MPa is obtained. Due to its high purity, the fabricated alumina supports are expected to have good chemical resistance in acid and alkali media. The supports can be considered potentially useful in many applications such as heat-insulating structure, electrode in fuel cells and catalyst supports, etc.
Keywords/Search Tags:Solid oxide fuel cell, Impregnation/infiltration, Nano-particles, Conductivity, Ball-shape alumina
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