【作者】 丁冬；

【导师】 夏长荣；

【作者基本信息】 中国科学技术大学 ， 材料学， 2008， 博士

【摘要】 固体氧化物燃料电池（SOFC）是一种高效、洁净的新型能源技术,有望在不久的将来满足世界快速增长的能源需求,改善能源结构,进而对全球环境产生巨大积极的影响。随着SOFC技术的不断开发和完善,商业化进程不断推进,面对着传统的SOFC在高温（1000℃左右）下运行带来了诸多材料和技术方面的问题,以及氢能燃料作为SOFC燃料的局限性,研究人员越来越认识到降低操作温度和使用碳氢化合物作为燃料的重要性。降低操作温度可以通过开发高电导率的电解质材料、降低电解质厚度和提高电极性能来实现,而通过电极结构的修饰、优化和开发新型电极材料,进而获得高催化活性和抗积碳的阳极,则是SOFC使用碳氢燃料的必由之路.本论文工作紧紧围绕这两大主题展开,一方面从制备高烧结活性和高电导率,适合作为中、低温SOFC的电解质材料入手,在深入研究该材料电性能的基础上,成功将其应用于SOFC,并通过低成本的工艺条件实现了电解质薄膜化,在中、低温下取得了较好的电池性能.另一方面,也是本论文工作的中心,则是通过构建阳极几何微结构模型,从理论和实验两方面对氧化铈修饰阳极进行了优化,并证实了优化后的单电池在碳氢化合物燃料中的成功运行。与此同时,开发了一种新型的不含离子导电相阳极,该阳极显示了不亚于传统电子-离子混合导电复合阳极的性能,以此的单电池在中、低温条件下也展示了较传统阳极单电池更稳定的性能和开路电压（OCV）。论文的第一章简单介绍了SOFC的操作原理,综述了SOFC各关键材料,重点讨论了SOFC的电解质和以碳氢燃料相关的阳极材料的最新研究进展,在概述了SOFC的发展现状和趋势的基础上,确立了本论文的研究目标和研究内容。第二章采用优化的碳酸盐共沉淀方法制备了氧化钐掺杂氧化铈（SDC）粉体,在系统研究该粉体的烧结性能和电性能的基础上,成功制备了以此作为电解质的单电池.主要结果如下:1)与文献中类似方法相比,采用较稀浓度的溶液和较低的温度,通过碳酸盐共沉淀方法制备了纳米尺寸的Ce_0.8Sm_0.2O_1.9（SDC）电解质粉体,实验结果显示这种由球形粒子组成的SDC粉体具有较高的比表面和较高的烧结活性,在1100℃烧结5小时能达到98%的致密度。而电导率测试结果证实由该粉体制备的SDC陶瓷样品具有较高的离子电导率(600℃总电导率为0.022 S cm^-1)和较低的电导活化能（0.66 eV）,是一种很有前景的中、低温SOFC电解质材料.2)采用共压共烧技术制备了以上述SDC粉体为电解质薄膜的单电池,并在500-600℃温度范围内以湿H₂为燃料得到了较好的电池性能,600℃最大功率密度达到400 mW cm^-2。考虑到电解质厚度为～75μm的事实,这种电池显示出与本实验室常用的GNP法制备的～40μm电解质的单电池相当的性能。同时单电池也显示出较好的长期稳定性。3)借助ZsimpWin软件,利用砖层模型,将阻抗谱中晶粒和晶界电阻从总电阻中解析出来,并分别研究了晶界和晶粒的电性质。实验结果显示总电导率和电导活化能处在目前不同方法制备的SDC粉体的最优水平,烧结温度对总电导率有影响,在1300℃取得最大电导率:较高的总电导率可能是来自于较小的晶界贡献;晶粒的电导迁移焓和缔合焓也随着烧结温度而改变,前者的减小和后者的增加均与品格常数随烧结温度增加而增加的事实相关。论文第三章首先通过构建阳极几何微结构模型以及掺杂氧化铈浸渍金属镍（Ni）阳极骨架的修饰阳极制备和相关单电池测试,从理论和实验两方面证实了一种阳极微结构修饰方法。该方法能够提高阳极三相线（TPB）,进而改善电池性能。并通过微结构参数设计和调整,取得了一系列单电池最高功率密度的优化结果。由于该阳极中SDC在Ni粒子表面的有效覆盖和修饰,隔断了Ni粒子与碳氢化合物的直接接触,能够最大程度的抑制积碳.同时SDC本身具备优异的电化学催化氧化能力,因此我们从设计和实验上都证实了碳氢化合物作为燃料在该阳极电池中的直接利用,主要结果如下:1)根据随机堆积球原理、粒子配位数方法和渗流理论,构建了离子导电相粒子修饰电子导体相粒子的阳极几何微结构模型。基于单层覆盖假设,模型计算显示,在0.30-0.53的孔隙率范围内,TPB长度随修饰粒子的覆盖度增加而增加;而孔隙率的增加提高了可供覆盖的骨架粒子表面,使得能够获得的最大单层修饰量增加,从而最大TPB增加.在超过最大修饰量以后,多层覆盖发生,阻碍了气体的传输,反而TPB降低。模型计算结果证明通过联合阳极衬底孔隙率和修饰量两个微结构参数,能够对TPB进行优化,在研究范围内获得最长的TPB。2)以Ni作为电子导电相粒子,SDC作为离子导电相粒子,采用离子浸渍法在NiO粒子骨架上进行了SDC粒子的浸渍制备了SDC修饰阳极,并以此阳极为支撑体,采用共压共烧工艺制备了SDC薄膜单电池。在600℃,H₂为燃料下表征了电池性能。由于阴极制备工艺的一致性,可以用电池的最大功率密度作为衡量阳极性能的指标。实验结果与模型预测一致,当阳极造孔剂量为10 Wt.%,20 Wt.%和30 Wt.%时（对应于还原前孔隙率0.31,0.42和0.54）,最大功率密度的最大值分别为571,631和723 mW cm^-2,对应的SDC浸渍量分别为508,564和648 mgcm^-3。这种方法为电极优化提供了一种思路,也加速了固体氧化物燃料电池的中、低温化进程。3)利用阳极优化的实验结果,以加入20 Wt.%造孔剂的阳极衬底,SDC浸渍量为564 mg cm^-3的SDC修饰阳极为主要对象,研究了对应单电池在直接碳氢化合物燃料中的应用.在低碳燃料纯CH₄中,与传统阳极电池相比,SDC修饰阳极电池显示出稳定的长期放电行为,550℃,600℃和650℃时电池最大功率密度分别为177,379和653 mW cm^-2。CH₄的湿润程度对电池OCV稳定性影响较大。在高碳液态燃料iso-octane中,SDC修饰阳极电池也显示出与文献报道中同种燃料下用Ru作为修饰剂的阳极单电池相当的性能和更高的OCV,在240小时以上的放电过程中没有明显的性能衰减。经39,120和232小时长期测试后的最大功率密度分别为397,369和346 mW cm^-2。阻抗谱显示在此期间电极界面极化电阻几乎不变,意味着最大功率密度稍微的下降主要来自于OCV的变化。这可能是由于阳极内部不足以破坏阳极微结构的少量积碳导致的。燃料组分和测试条件对电池稳定性有着较大影响,说明通过改善O₂/iso-octane比例以及维持电池在更长时间内放电能够获得更好的长期输出性能。第四章开发了一种高性能的中、低温Ni/Sm₂O₃阳极.与传统Ni/SDC阳极不同的是,由于Sm₂O₃可忽略的离子导电性,这种阳极可认为是非离子导电的。这必然意味着该阳极的TPB仅仅限于电解质/阳极的物理界面,从而导致阳极体内较低的TPB。即便如此,该阳极显示出不低于Ni/SDC阳极的性能,在600℃湿氢气为燃料下Ni/Sm₂O₃阳极对应的单电池最高功率密度分别为540 mW cm^-2,高于Ni/SDC阳极对应的单电池的471 mW cm^-2.XRD和电子能谱分析（EDX）结果显示Sm₂O₃并没在阳极体内与Ni反应,也没在电解质/阳极界面处明显扩散形成更高离子导电相的固溶体,说明阳极的高性能可能来自于Sm₂O₃本身较高的氧化催化能力和这种电极独特的微结构和粒子形貌.不仅如此,与Ni/SDC阳极相比,这种阳极也展示了氢气条件下较稳定的OCV和一定程度上在碳氢燃料中直接使用的能力.更多还原

【Abstract】 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 Ce_0.8Sm_0.2O_1.9（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.更多还原