Fabrication And Characterization Of Anode Substrates For Tubular Solid Oxide Oxide Fuel Cells

Solid oxide fuel cell (SOFC) is a new energy conversion device which has been held the highest hopes because of their high efficiency, little pollution and flexibility in the choice of fuels, especially at the time when the current energy structure is not reasonable and the environment pollution becomes more and more serious. As a new promising technique, it will fulfill the increasing need of electricity, improve the current energy structure, and impact the whole world environment actively in the near future. With the development of SOFCs and its commercialization implementation, there are two main issues for SOFCs so far that are material technique and high cost problems related to high temperature operation (~ 1000 oC) as well as the limit for hydrogen fuels with respect to efficiency, storage and transportation, etc. It is crucial, therefore, to reduce the cost and use hydrocarbon as the fuel for SOFCs. Looking for the efficient and cost-effective fabrication techniques and decreasing the thickness of electrolyte, developing novel electrolyte with higher ionic conductivity and new electrodes with higher performance are the major approaches to lower the cost. For direct utilization of hydrocarbon, it is necessary to fabricate the highly catalytic anode with the capability to avoid carbon deposition by modifying and optimizing the electrode microstructure or developing new materials.This thesis aims to lower the cost and study the microstructure and electrochemicial performance of cells. In addition, the anode substrates prepared by the gel-casting technique have a large range of porosity, which make for the modification of the anode. The performance of the modified cell with hydrogen and methane as fuels is studied.Chapter 1 reviews the working principle, materials for SOFC, especially main points of anode developments with methane as fuel. Proposal on the thesis work is also presented in this chapter.Preparation of key component materials and cells is basal technique in developing SOFCs. Accurate and efficient characterization is critical in selecting suitable materials as well as overall evaluation of the performance of SOFC. Chapter 2 gives detail information about gel-casting technique, co-firing process and characterization technique. In our work, the ceramic powders and organic materials were made into stable suspension with high solid loading. Monomers polymerized into three-dimensional structure and made the suspension into green body at 80 oC. The gelcasting process is more cost-effective and flexible for fabricating complex three-dimensional ceramic parts, and also easy to scale up. The anode substrate which is uniform has enough strength. Effect of graphite addition on the porosity and sintering shrinkage of the substrate was studied. The single cell was fabricated by gel-casting, dip-coating and slurry-coating process. Firing process was 1150 oC(anode substrate)-1200 oC(anode substrate and interlayer)-1350 oC(anode and electrolyte co-firing). Characterization techniques are Ac impedance, laser granularity distributing analysis, scanning electron microscope (SEM) technique etc.In chapter 3, effect of SDC on the performance of cell with hydrogen fuel was studied. Porosity of anode substrate before reduction was about 30% with 15% graphite addition, and the optimum SDC loading is 273 mg cm-3 after 6 repeat times of the impregnation cycle in our experiments. The peak power density of the cell was increased by about 60% and the area specific resistance (ASR) decreased by about 47% at 700 oC, compared with the unmodified cells. Performance of cell was tested at 700 oC, 750 oC and 800 oC. Results indicated that OCV(open current voltage) of cell reached nearly to the theory value. Electrode attached to the electrolyte well, and the electrolyte was dense enough. In addition, the morphology of the resultant SDC particles was studied in detail.It is generally accepted that using hydrocarbon fuels will make SOFC more flexible and efficient than other fuel cell variants. Which fulfils the development of the energy structure. Chapter 4 introduced that the long-term performance of the cell with impregnation of SDC was stable when it was operated on humidified methane at 700 oC. Overmuch carbon deposition was effectively suppressed. Somewhat deposited carbon didn't destroy the anode substrate, and improved the performance of the cell.