【作者】 赵飞；

【导师】 夏长荣； 陈仿林；

【作者基本信息】 中国科学技术大学 ， 材料学， 2009， 博士

【摘要】 固体氧化物燃料电池（SOFC）是一种高效、环保的发电装置。传统的SOFC在高温下工作。高温操作能促进快的反应动力学,减少了对贵金属催化剂的需求,还能让碳氢化合物燃料内重整,废热适合再利用。然而,从经济的角度来讲,SOFC在目前还不能够和已有的发电技术相竞争,主要是由于高温（>800℃）操作带来的诸多问题,包括电池组件的高温氧化、腐蚀、化学扩散和反应造成的性能衰减等。一个降低成本的方法就是降低工作温度到700℃以下。然而,降低SOFC的工作温度至500-700℃这个中温范围,仍旧是SOFC发展中的一个挑战。SOFC的整个电化学性能会随着工作温度的下降而下降,这主要是由于电极的极化电阻增加以及电解质电导率下降造成的。来自电解质上的问题在一定程度上,已经通过使用新型电解质材料和电解质薄膜化技术解决了。因此,关注的焦点从电解质转向了电极。因为电极在SOFC中表现出较高比例的电压损失。阴极成为电极发展关注的中心,这主要是因为氧还原反应在SOFC低温工作时更加难被激活发生反应。因此,开发高电化学催化活性的阴极对于中温SOFC至关重要。本博士学位论文的目的就是开发新的阴极,使其具有高的催化活性和长久的使用寿命。论文第一章,大致介绍了SOFC的工作原理和关键材料如电解质,阴极,阳极和连接材料。此外,还简单介绍了SOFC的现状和发展趋势,基于SOFC中温化的趋势,确立了本论文的研究目标:即开发新型的混合离子电子导体（MIEC）阴极材料和具有新颖微结构的复合阴极。论文第二章,为了理解氧还原过程,简要介绍了SOFC阴极氧还原反应中的三种反应路径。由于MIEC是最具潜力的阴极材料,所以有必要深入理解多孔MIEC中的氧还原过程。通过系统地分析和讨论连续理论模型,建立了对MIEC电极的氧还原过程清晰的认知图像。论文第三章,考察了La_2-xSr_xCo_0.8Ni_0.2O_4+δ（LSCN,x=0,0.4,0.8,1.2,1.6）-Ce_0.9Gd_0.1O_1.95（GDC）作为中、低温固体氧化物燃料电池阴极的可行性。K₂NiF₄型氧化物La₂Co_0.8Ni_0.2O_4+δ是一种超氧化学计量的氧化物,这类氧化物在450℃到650℃这一温度范围内具有高的氧扩散和氧表面交换系数,根据第二章中提到的连续理论模型的结论来看,它是一种潜在的MIEC阴极材料。我们做了一系列的实验表征了这类复合电极。其主要结果如下:1)对称电池的交流阻抗谱表明在一系列的LSCN-GDC复合电极中,La_1.2Sr_0.8Co_0.8Ni_0.2O_4+δ基电极有着最小的界面极化阻抗,即600℃电极没有活化时的阻抗为1.36Ωcm²。2)当单电池在200mAcm^-2的电流下通过30分钟后,可以观察到明显的活化效应。3)以La_1.2Sr_0.8Co_0.8Ni_0.2O_4+δ-GDC为阴极的电池在600℃的输出功率为350mWcm^-2,且在36小时的0.5V恒压放电测试中,输出功率没有明显下降,十分稳定。这些结果表明,La_2-xSr_xCo_0.8Ni_0.2O_4+δ是潜在的适合在中低温操作的阴极材料。进一步的工作将集中在改善LSCN-GDC的电化学活性上,即通过改变复合电极中GDC的含量,以及通过降低制备温度来增大表面积,还可以优化Sr的掺杂量来提高电极的性能。论文第四章,开发了一种高稳定的中温SOFC电极,这种电极是La_0.6Sr_0.4Co_3-δ（LSC）基的浸渍复合电极。这种复合电极的制备过程是:先通过丝网印刷和高温共烧工艺制备阴极的多孔骨架(Ce_0.8Sm_0.2O_1.9,SDC),然后以离子浸渍为手段在多孔骨架中负载高活性的阴极颗粒（LSC）。该电极的特征是:电极骨架完好地和电解质相连接,而细小的LSC颗粒均匀地附着在SDC骨架的内表面。其中,完好连接的结构为氧离子传输提供了通道（氧离子从阴极经过多孔骨架SDC传输到电解质基底上）,LSC颗粒之间的接触连接为电子的传输提供了通道（电子从集电极经过LSC传输到反应活性位）。对该电极的优化和电化学性能表征的主要结果如下:1) LSC的浸渍含量及LSC浸渍颗粒的焙烧温度均对浸渍电极的性能有明显的影响。在一定的操作温度下,浸渍电极的性能随着LSC的浸渍含量的变化而变化,浸渍含量有一个最佳值,它对应于电极的最低极化阻抗。在浸渍量低于最佳浸渍含量时,电极阻抗随着浸渍含量的增加而减小。在浸渍量高于最佳浸渍含量时,电极阻抗随着浸渍含量的增加而增加。浸渍颗粒LSC的焙烧温度也对电极性能产生影响。在浸渍颗粒完全成相的前提下,升高焙烧温度使得电极阻抗增加。对于LSC浸渍的颗粒而言,最佳的焙烧温度是800℃焙烧2小时。2)复合电极的电化学性能通过电化学阻抗谱来表征。尽管LSC和SDC的热膨胀系数差别很大,但是结果却证明了这种浸渍阴极具有高的热抗震性。该浸渍电极在20次500-800℃的热循环过程和10次从室温到800℃的热循环过程测试结束后,界面电阻没有增加。并且这种电极的长期稳定性同样令人兴奋,在600℃恒温72天的长期测试中,浸渍电极的阻抗基本恒定不变。因此,浸渍的LSC-SDC电极有高的热抗震性和很好的长期稳定性。3)考察了以浸渍LSC-SDC为阴极的单电池性能。以浸渍含量为55%,焙烧温度为800℃焙烧2小时的电极作为单电池的阴极。浸渍阴极和单电池均表现出极好的性能。单电池表现出高的长期性能,在650℃,电池经过0.5V恒压放电89小时后,电池的最大输出功率达到0.815 W cm^-2。而在四次热循环测试后,同样650℃测得的功率下降了20%,这主要是由于阴极和集电层Ag的分层造成的。目前的研究已经表明LSC浸渍电极表现出相当好的电化学性能、高的抗热循环或抗热震荡性、低的界面阻抗值。这些结果表明我们成功地开发出了一种适合中温工作的、可靠的、高性能的电极。值得一提的是:共烧制备的多孔骨架结构为浸渍阴极提供了通用的模板,可以通过改变浸渍的电子导电相材料和浸渍材料的形貌来制备多种高性能的复合阴极。论文第五章,成功制备了具有纳米网络状结构的复合阴极。这种复合阴极是以离子浸渍为手段,通过简单控制沉积下来的硝酸盐前驱物的粒子焙烧的升温速率来改变在多孔骨架中负载的阴极颗粒的形貌。其主要结果如下:1)具有纳米网络结构的阴极是由不到50nm的氧化物纳米颗粒相互连接形成纳米丝构成的。小颗粒表现出大的表面积和高的孔隙率,并形成了通畅的离子和电子传导路径,因此表现出相当低的界面极化阻抗。2)改变焙烧温度被发现是最有效的制备高性能纳米网络状电池的方法。与所有关于SSC文献报道的阴极阻抗相比,纳米网状SSC.SDC浸渍阴极表现出最低的界面极化电阻。500℃时界面极化阻抗为0.21Ωcm²,600℃时界面极化阻抗为0.052Ωcm²。单电池的性能也是SSC中最高的,在500℃时最大功率输出为0.44 Wcm^-2。尽管长期稳定的机理和纳米网络状形成的机理都还没有被进一步确定,但我们的结果却暗示了一个新的能显著改善低温SOFC性能的方向。更多还原

【Abstract】 Solid oxide fuel cells （SOFCs） are a forward looking technology for a highly efficient, environmental friendly power generation. The traditional SOFCs are operated at high temperature. The high operating temperature promotes rapid reaction kinetics, eliminates the need of precious metal catalysts, allows internal reforming of hydrocarbon fuels, and produces high quality byproduct heat suitable for co-generation However, SOFCs are currently not economically competitive to the existing power generation technologies due to problems associated mainly with high temperature （>800℃） operations, including performance degradation of cell components due to high temperature oxidation, corrosion, chemical interdiffusion and reaction. One approach to cost reduction is lowering the SOFC operating temperature to below 700℃,However, reducing the operation temperature of SOFCs to the intermediate temperature range of 500-700℃is still a challenge in the SOFC development. The overall electrochemical performance of an SOFC will decrease with the reduction in the operating temperature due to increased polarization resistances of the electrode reactions and decreased electrolyte conductivity. The problem coming from electrolyte has been solved to a certain extent by using new electrolyte materials or adopting film technique. Therefore, the shift in emphasis has been driven from the electrolyte to the electrodes where the electrodes show a higher percentage of the voltage loss for IT-SOFCs. The cathode has been the center of the focus in the electrode development largely because oxygen reduction is the more difficult reaction to activate in SOFCs operated at reduced temperatures. Consequently, development of cathode with high electrocatalytic activity becomes critical for IT-SOFCs.This p.h. D thesis aims to develop new cathodes with high catalytical activity and long life-time.In chapter 1, the principle of SOFCs and key component materials such as electrolytes, cathodes, anodes and connector, were generally introduced. In addition, the situation and the trend of SOFCs’ development were also briefly reviewed. Based on the trend in intermediate-temperature operation for SOFCs, development of new mixed ionic and electronic conductors （MIECs） and composite cathodes with novel microstructures was selected as the object of this thesis.In chapter 2, three reaction paths were briefly introduced to understand the oxygen reduction process. Since the MIECs are the most potential cathodes, it is needed to get an insight into the oxygen reduction process in porous MIEC.An image of oxygen reduction process in MIEC was set up through the systematic analysis and discussion on a continuum model.In chapter 3, composites consisting of La_2-xSr_xCo_0.8Ni_0.2O_4+δ （LSCN, x=0,0.4, 0.8, 1.2, 1.6） and Ce_0.9Gd_0.1O_1.95 （GDC） have been investigated as the cathodes for low-and-intermediate temperature solid oxide fuel cells （SOFCs）.K₂NiF₄ structured La₂Co_0.8Ni_0.2O_4+δ is an oxygen overstoichiometric oxide with high oxygen diffusion and oxygen surface exchange coefficients in the temperature range from 450 to 650℃, resulting in a potential MIEC cathode material according to the conclusion drawn from the continuum model which is mentioned in chapter 2. A series of experiments were conducted to characterize the composites. The main achievements are summarized as follows:1) AC impedance spectroscopy on symmetric cells indicated that among the series of LSCN-GDC composites, La_1.2Sr_0.8Co_0.8Ni_0.2O_4+δbased electrode had the lowest interfacial polarization resistance, which was 1.36Ωcm² at 600℃when the electrode was not activated.2) Significant activation effect was observed with a single cell when current treatment was performed at 200mAcm^-2 within 30min. The single cell with La_1.2Sr_0.8Co_0.8Ni_0.2O_4+δ-GDC as the cathode generated a power density up to 350mWcm^-2 at 600℃.3) In addition, the performance was pretty stable when a constant output voltage of 0.5 V was set for 36 h.These results suggest that La_2-xSr_xCo_0.8Ni_0.2O_4+δ could be promising materials as the cathodes for SOFCs that operated at low-and-intermediate temperatures. Further work will focus on improving the electrochemical activity of the LSCN-GDC composites by changing the GDC content, by decreasing the fabrication temperature leading to greater surface area, and by optimizing the amount of Sr-dopant.In chapter 4, a highly stable electrode based on La_0.6Sr_0.4Co_3-δ （LSC） was developed for intermediate temperature solid oxide fuel cells （IT-SOFCs）. The electrode was prepared by impregnating LSC into a porous samaria-doped ceria (SDC, Sm_0.2Ce_0.8O_1.9) frame, which was deposited to an SDC electrolyte using screen-printing and co-firing techniques. The electrode frame （SDC） was well-connected with SDC electrolyte, and the fine LSC particles coated the SDCframe. This well-connected structure makes a pathway for oxygen ion transport between the cathode （through the SDC frame） and the electrolyte （the SDC substrate） since SDC is an excellent oxide conductor. Connect among LSC particles results in a pathway for electron transport from electron collector to the reactive sites. Optimization of the LSC impregnated SDC composite was performed and its electrochemical properties were characterized. The main achievements are summarized as follows:1) Both the loading of LSC and the firing temperature have significant influence on the performance of the LSC-impregnated SDC composite electrode. At a certain operating temperature, the lowest interfacial resistance of the electrode corresponded to an optimal loading. Lower or higher the loading resulted in a larger resistance. And it is the same case in the firing temperature. The optimal firing temperature which is also the lowest phase-forming temperature is 800℃for 2h.2) The electrochemical properties of the composite electrode were investigated by impedance spectroscopy. High stability upon thermal cycle was demonstrated for this composite electrode although LSC and SDC have significant difference in thermal expansion. After 20 times of 500-to-800℃thermal cycles and 10 times of room-temperature-to-800℃thermal cycles, no increase in area specific resistance （ASR） was observed for such electrodes. And the long-term stability is also exciting. The resistance of the impregnated electrode kept constant when it was held at 600℃for 72 days. So the LSC impregnated SDC composite electrode had high resistance to thermal shock/cycles and good long-term stability despite significant TEC mismatch exists between LSC catalyst and SDC electrolyte.3) The performance of the cell with the LSC-impregnated SDC as cathode was investigated. With 55wt% LSC loading and firing at 800℃for 2h, the impregnated cathode and single cell showed excellent performance. The cell showed the peak power output of 0.815 W cm^-2 at 650℃after running with a constant voltage output of 0.5V for 89h. After four thermal cycling tests, the cell performance measured at 650℃reduced about 20% mainly due to the delamination between the cathode and Ag current collecting layer.The present study has demonstrated that the LSC-impregnated electrode shows remarkable performance. The high resistance to thermal cycles and thermal shock has been achieved. In addition, very low ASR has been achieved. These results imply that a reliable electrode with high thermal resistance and high performance has beendeveloped for IT-SOFCs. It should be noted that the well-connected SDC frame can be used as a general template which can be impregnated by other electronic conductors （with/witout special microstructures） to achieve more composite electrodes with high performance.In chapter 5, nano-network structured Sm_0.5Sr_0.5CoO_3-δcathodes for low-temperature SOFCs have been successfully fabricated by simply increasing the rate to heat the precursor nitrates deposited from a well-developed ion-impregnation process. The main achievements are summarized as follows:1) The cathodes are consisted of oxide nanowires formed from the nanobeads of less than 50 nm in diameter thus exhibiting large surface area and high porosity, forming straight path for ion and electron conduction, and consequently showing remarkably low interfacial polarization resistances.2) Change in the firing rate has found to be a highly effective approach to the fabrication of high-performance nano-network electrodes for low temperature SOFCs, producing the lowest interfacial polarization resistances （0.21Ωcm² at 500℃and 0.052Ωcm² at 600℃） ever reported for the SSC cathode materials. An anode supported cell with 10-μm-thick SDC electrolyte demonstrated a peak power density of 0.44 Wcm^-2 at 500℃,which is also the highest ever reported for the SSC electrodes.3) Durability test showed that the cathode performance increased with the operating time probably due to the cathode microstructure evolution to higher porosity to optimize the gas diffusion and well-connected SSC nanowires to strengthen ionic and electronic conducting path.Although the long-term stability and formation mechanism of the nano-network electrodes are yet to be further determined, the results indicate a new direction to significantly improve the performance of low temperature SOFCs.更多还原