| Solid oxide fuel cells(SOFCs) is an energy conversion device which produces electricity by electrochemical combination a fuel and an oxidant with high efficiency and cleanness characteristics.As a new 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 exist two main issues for SOFCs so far that are material and technique problems related to high temperature operation(～1000℃) as well as the limit for hydrogen fuels with respect to efficiency,storage and transportation,etc.It is crucial,therefore,to reduce the operation temperature and use hydrocarbon as the fuels for SOFCs.Decreasing the thickness of electrolyte, developing novel electrolyte with higher ionic conductivity and new electrodes with higher performance are the major approaches to lower operating temperature to 500-800℃.For direct utilization of hydrocarbon,it is necessary to fabricate the highly catalytic anode with the capability to resistant to carbon deposition by modifying and optimizing the electrode microstructure,and developing the novel electrode materials.This thesis aims to lower the operation temperature and directly use hydrocarbon as the fuels for SOFCs.On the one hand,we focus on the fabrication of the electrolytes with high sinterability as well as high ionic conductivity for intermediate/ low temperature SOFCs.On the basis of investigating the electrical properties of the electrolyte materials in depth,we prepared and characterized the single cells using it as the electrolyte membrane via single and cost-effective approach,and achieved the good performance at 500-600℃.As one of the cores of this thesis,on the other hand, we optimized theoretically and experimentally the ceria-modified anodes by modeling for anode geometric microstructure and conducting the corresponding experiments. Moreover,successful direct utilization of hydrocarbon on the optimized single cells with these anodes was demonstrated.Finally,a novel anode without ionic conductivity was developed which showed performance not lower than that of the conventional composite anode with mixed electron-ion conduction.The single cells with this anode exhibited more stable performance and open circuit voltage,compared with those with the conventional anode. In chapter 1,the SOFC principle was briefly introduced at first.The key component materials for SOFCs were reviewed,especially on the latest progress for the electrolyte materials and the anode materials related to hydrocarbon fuels.Based on highlighting the present development status and direction for SOFCs,proposal on the thesis work was thus presented in this chapter.In chapter 2,samaria-doped ceria powders were prepared via an optimized carbonate coprecipitation method.Besides studying systematically the sinterability and electrical properties,the single cells with these powders as the electrolyte were fabricated and characterized.The main achievements are summarized as follows:1) Nano-sized Ce0.8Sm0.2O1.9(SDC) powders were prepared via a carbonate coprecipitation method with more dilute solution and lower process temperature, compared with those reported in the literatures with similar methods.SDC powders consisted of spherical particles possessed high specific area and high sinterability,which could reach 98%of the theoretical density when they were sintered at 1100℃for 5 h.In addition,SDC ceramics derived from these powders had high ionic conductivity(the total conductivity was 0.022 S cm-1 at 600℃) and the low activation energy for conduction(0.66 eV),hence SDC powders suggested that it was a potential electrolyte for intermediate/low temperature SOFCs.2) The single cells with SDC powders as the electrolyte were fabricated via a co-pressing and co-firing technique.The peak power density was 400 mW cm-2 at 600℃using humidified H2 as the fuel.Considering the fact that the thickness of the electrolyte was～75μm,the cell showed comparable performance with that usually fabricated in our laboratory,in which the electrolyte powder was derived from glycine-nitrate process(GNP) and the thickness of the electrolyte was～40μm.Furthermore,the cells demonstrated good stability for power generation.3) The contributions of grain interior and grain boundary were resolved from the total resistance in AC impedance spectra via ZsimpWin software and brick-layer model,and their electrical properties were investigated,respectively.Analysis results showed thatⅰ) the total conductivity and the activation energy for it lied on the top level of SDC powders reported via different preparation methods.The effect of sintering temperature on the total conductivity was significant and the maximum values were achieved with those sintered at 1300℃for 5 h;ⅱ) high total conductivity should be associated with the small contribution of the grain boundary,ⅲ) the motion enthalpy for the grain interior decreased while the association enthalpy increased with increasing the sintering temperature up to 1300℃,which might be possibly originated from the increase in lattice parameters with the temperature.In chapter 3,an anode microstructure modification process was demonstrated in both theory and experiments by building up an anode geometric micro model, fabricating the anode with doped ceria impregnated nickel framework as well as characterizing the single cells related to this type of anode.By this process,the triple-phase boundary(TPB) in the anode could be dramatically increased,resulting in the improvement of the cell performance.A series of peak power density values were obtained with designing and modulating the microstructure parameters.Moreover, due to the effective covering and modification from SDC particles on the surface of nickel particles,the contact between nickel and hydrocarbon is blocked so that carbon deposition would be suppressed to maximum extent.Combining the electrochemical oxidation catalytic activity of SDC themselves,the single cells with the modified anode were experimentally demonstrated to be stable operated with direct utilization of hydrocarbon fuels.The main achievements are summarized as follows:1) Based on random packing sphere principle,coordination number method as well as percolation theory,an anode geometric micro model was developed where ion-conducting-phase particles modified electron-conducting-phase particles.With the assumption of mono-layer covering,the TPB length increased with the coverage of modified particles in the porosity range of 0.30-0.53.The elevated porosity increased the surface of the particles as the framework for modification, enabling the maximum monolayer modification loading to increase,resulted in the increase in the maximum TPB length.When exceeded the maximum monolayer modification loading,multilayer covering occurred and it will decrease the TPB length by blocking gas diffusion.In sum,the TPB length could be optimized by improving microstructure parameters such as the porosity and the loading.2) SDC modified anodes were fabricated via an ion impregnation method where nickel was as electron conducting framework and SDC as ionic conductor.In addition,the single cells were fabricated with SDC modified anode.The cell performance was characterized with humidified H2 as the fuel at 600℃.Due to the consistence of the cathode fabricating process for all single cells,the peak power density could be employed to evaluate the anodic performance.The experimental results were in good agreement with model prediction.The highest peak power densities of the cells whose anode prepared with 10,20 and 30 wt.% pore former(the porosities were 0.31,0.42 and 0.54 respectively) were 571,631 and 723 mW cm-3 respectively at 600℃,corresponding to SDC loading of 508, 564 and 648 mg cm-3.The modified method provided some insight into the optimization of the electrode,and accelerated the proceeding for lowering operation temperature in SOFCs.3) The optimized single cells with SDC modified anodes were investigated when hydrocarbons were used as the fuels.In pure methane,the cells exhibited stable power generation compared with that with the conventional anode.The peak power densities were 177,379 and 653 mW cm-2 at 550,600 and 650℃, respectively.OCV showed enhanced steady in humidified methane than dry methane.When heavy hydrocarbon iso-octane was applied as the fuel,the cells with SDC modified anodes showed comparable performance and higher OCVs compared those with Ru as the anode catalyst reported in the literature using the same fuels.The power density decreased slightly over 240 h as discharged under constant voltage of 0.5 V.The peak power densities were 397,369 and 346 mW cm-2 after 39,120 and 232 h operation.The interfacial polarization resistances were unvaried during this period,suggested that the SDC modified anode was stable for the direct utilization of octane,and slight degeneration in the performance was associated mainly with the loss of OCV,which was possibly derived from a little carbon deposition within the SDC-coated anode where the level is so low that it almost has no effect to cause severe damage to the anode. The fuel composition and operation conditions had remarkably influence on the cell durability,implied that the stability of the fuel cells could be further improved by varying the iso-octane/O2 ratio and conducting a continuous operation.In chapter 4,a high performance Ni/Sm2O3 anode was developed for intermediate/low temperature SOFCs.Unlike the conventional Ni/SDC anode,this anode was considered to be non-ionic conductive due to the negligible ionic conductivity of Sm2O3.This meant TPB in the anode should be constricted to the physical interface between the electrolyte and anode so that the anode bulk possessed far small TPB.Even so,the single cells with Ni/Sm2O3 anodes showed peak power density of 542 mW cm-2 at 600℃,comparable to,if not higher than those with the Ni-SDC anodes when the same cathodes and electrolytes were applied.XRD results and EDX analysis demonstrated Sm2O3 did not react with Ni and there is no obvious solid-state diffusion occurred between ceria and samaria at the electrolyte/anode interface,suggested that the high performance in Ni/Sm2O3 anode was possibly due to the catalytic property of Sm2O3 as well as the unique microstructure and particle morphology in the anode.In addition,compared with that with Ni/SDC anode, Ni/Sm2O3 anode exhibited stable OCV with H2 and the capability to directly use hydrocarbons as the fuel to some extent.
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