| Solid oxide fuel cell?SOFC?is an attractive energy conversion system because of its scalability in electricity generation applications,high electrical efficiency and low level of pollution produced during operation.Especially,it has provided an opportunity to utilize carbon-containing fuels?including natural gas,biogas,gasified biomass,etc.?owing to its high operating temperatures.Currently,Ni-yttria stabilized zirconia?Ni-YSZ?cermet is the most commonly used anode material for SOFCs from the view of a reasonable balance among the requirements.However,the problem that Ni catalyzes carbon deposition reactions,resulting in catastrophic fracture of the anode,has been always hindering the development of SOFC in practical use.To solve these problems,a clear knowledge on the conditions under which carbon deposition occurs and the mechanisms of the deactivation is necessary,so that proper methods can be applied to improve the Ni-based anode for SOFCs operating on carbon-containing fuels with high performance,which is of great theoretical significance in application of SOFCs.This thesis centers around the topic of conventional Ni-YSZ anode and the researches are carried out for the key questions about the deactivation mechanism analysis and preventive measures of Ni-based anode in SOFCs operating on carbon-containing fuels,aiming to speed up the commercialization process of SOFCs.Based on the mechanisms of carbon fiber growth caused by nickel and carbon containing gas interaction,considering the special structure of a nickel-based anode of SOFC,and according to some corresponding initial experimental results,carbon deposition is characterized which formed on Ni-YSZ anode operated on humidified methane at 650 °C and 800 °C,respectively.The relation of stress,arisen in the Ni-YSZ anode through the interaction of nickel with methane,with the interaction time is measured in situ to determine the exact step causing the deactivation.The result shows that the life time of the Ni-YSZ anode operated on methane is 14 min at 800 °C,les than that of the Ni-YSZ anode at 650 °C?68 min?.Both SEM and HRTEM measurement results suggest that a considerable amount of encapsulating carbon are observed on nickel particles when the anode is treated in methane at 800 °C,while that of the anode at 650 °C contains large number of carbon fibers and few encapsulating carbon.Based on the mechanisms of carbon deposition caused by nickel and carbon-containing gas interaction,considering the special structure of a Ni-YSZ anode,and according to the corresponding initial experimental results,it can conclude that the dynamic process of the interaction between nickel and carbon-containing gas determines the forms of carbon deposited on the anode,and instantaneous expansion of the bulk volume is the fundamental cause of anode deactivation when exposed to methane at 800 °C.To reduce the effects of the stress,arisen in the Ni-YSZ anode through the interaction of nickel with methane,the laminated anode structure is employed to enhance the strength of the anode skeleton.The tape-casting technique is developed to fabricate layered and uniform NiYSZ anode supported SOFCs,and the corresponding electrochemical performances are characterized using moist hydrogen and methane as fuels,respectively.The result reveals that with hydrogen as fuel,the single cell with layered Ni-YSZ anode shows maximum power densities of 413 mW?cm-2,749 mW?cm-2,952 mW?cm-2 at 700?,750? and 800?,respectively;while that with uniform Ni-YSZ anode shows maximum power densities of 450 mW?cm-2,767 mW?cm-2,1016 mW?cm-2 at 700?,750? and 800?,respectively.On the other hand,with methane as fuel,the single cell with layered Ni-YSZ anode shows maximum power densities of 360 mW?cm-2,615 mW?cm-2,890 m W?cm-2 at 700?,750? and 800?,respectively;while that with uniform Ni-YSZ anode shows maximum power densities of 421 mW?cm-2,668 mW?cm-2,928 mW?cm-2 at 700?,750? and 800?,respectively,which is a bit lower than the performances in hydrogen due to bigger mass and volume of methane molecule.The impedance spectra analysis indicates that the decrease of electrochemical performance of the single cells with layered Ni-YSZ anode is due to higher polarization resistance.However,the thermal expansion result shows that the linear expansions of the layered Ni-YSZ anode is 36.2 %,which is nearly six times of that of the uniform Ni-YSZ anode?211.2 %?,indicating that carbon deposition can be reduced within limits by improving the structure of Ni-YSZ anode.To further improve the performance of SOFCs with Ni-based anode and broaden its application scope,Ni1-xFex-YSZ anode supported SOFCs directly operated on carbon?DCSOFCs?is studied.SOFCs with YSZ electrolyte supported on composite anode of nickel and iron bimetal with YSZ are prepared,and the electrochemical performance is tested and analyzed using hydrogen and Fe-loaded activated carbon as fuel,respectively.Initial results show that the optimal doping amount of Fe is 10 mol% no matter which fuel is used,the corresponding SOFC reveals the highest power densities of 790 and 529 mW cm-2,and the polarization resistances of 0.091 and 0.173 W cm2 when operated on hydrogen and carbon at 800 °C,respectively.As a comparison,the SOFC with traditional Ni-YSZ anode only gives the maximum power densities of 576 and 456 mW cm-2,and the polarization resistances of 0.259 and 0.317 W cm2 under the same condition.The compositions and preparation conditions for the electrolyte and cathode,as well as the single cell fabrication steps,were identical for all the single cells;therefore,the cathodic polarization resistance and total ohmic resistance of the cells should be the same.In other words,the anodic polarization resistanceis the major factor in determining the electrochemical performance of SOFCs.In addition,relatively ideal discharging plateaus are obtained from the DC-SOFC with Ni0.9Fe0.1-YSZ anode operated under current densities of 0.1,0.2 and 0.4 A cm-2 while the voltages keep decreasing with operating time for the DC-SOFC without Fe in the anode.The DC-SOFC of x=0.1 is steadily operated under 0.1 A cm-2 for 15 h,revealing a stable voltage plateau of 0.86 V at 800 °C.Both electrochemical performance and SEM measurement results suggest that the superior performance of the DC-SOFCs with a proper amount of Fe addition into the anode is not only related to the increased porosity,but also the reduced activation energy of the anode reaction and the concentration polarization of the corresponding SOFCs.To enhance the strength of the YSZ framework,the effects of doping amount and particle size of alumina on the sintering behavior and electrical performance of YSZ are investigated through microstructure and impedance spectroscopy analysis.Initial results show that the conductivity of the pure YSZ is 0.045 S/cm while it increases to 0.067 S/cm for the specimen doped with 0.7 wt% raw Al2O3.Furthermore,the ionic conductivity of the specimens doped with 1 wt% 1400 °C-Al2O3 powder shows the highest value 0.071 S/cm,an increase of almost 58 %,compared to that of the pure YSZ.The result indicates that a proper amount of Al2O3 doping can effectively improve electrical performances of YSZ,and the enhancement is not only because of the improvement of sintering behavior but also the change of grain size.On the other hand,the optimum doping amount varies with the particle size of Al2O3.When raw Al2O3 powder?modal particle size: 0.28 mm?is used as dopant,the optimal doping amount for electrical and sintering improvement is 0.7 wt% and 3 wt%,respectively,while that with 1400 °C-calcined-Al2O3?0.45 mm?,1 wt% and 4 wt%.Single cells with 1 wt% 1400 °C-Al2O3-YSZ and 0.7 wt% Al2O3-YSZ electrolyte reveal the maximum power densitis of 218.9 mW cm-2 and 206.7 mW cm-2 at 800 °C,respectively;both are higher than the conventional cell with pure YSZ electrolyte(174.5 m W cm-2),and further confirms that the sintering behavior and electrical performance of YSZ can be improved by a proper amount of Al2O3 addition. |
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