【作者】 刘铭飞；

【导师】 孟广耀；

【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、便捷和对环境友好的能量转换装置,在世界范围内受到广泛关注和大力研发,正在走向实用化、产业化。高性能的新一代SOFC,主要是瞄准中温（500—800℃）应用,基于阳极支撑的薄膜化固体电解质,采用陶瓷膜制备技术,可更确切的称为陶瓷膜燃料电池（CMFC）。本实验室针对CMFC的关键材料、核心制备技术及电池构型设计等展开了广泛研究,取得了快速进展,进入了推动CMFC在中国实用化发展的阶段。本论文工作针对发展低成本的CMFC制造技术,从研发低成本阴极新材料开始,侧重提高CMFC的中低温性能,使单电池的输出功率密度进入国际先进行列,同时为发展便携式CMFC电源装置开发适宜的碳氢燃料,这些研究都取得了满意的结果。首先,本论文在第一章介绍了SOFC/CMFC的基本原理,目前国内外的研发现状和发展趋势,着重综述了电解质薄膜制备技术和国际上CMFC实用化所遇到的电极积碳和燃料选择问题。从CMFC产业化角度,探索研制了在中低温下适宜YSZ基电池的阴极材料（第二章）;发展了适于规模化生产的低成本制备技术——浆料浸渍法,成功实现了YSZ,Sm_0.2Ce_0.8O_1.95（SDC）及高温质子导体电解质薄膜化制备（第三章,第五章）,同时还采用同一技术有效的改善了电极/电解质界面,打破了国际公认的YSZ基CMFC难以中温应用的界限（第四章）;特别是实验证实了本实验室关于“甲醇燃料CMFC可能不会积碳”的预言,以液体甲醇直接为燃料的CMFC,在中温下取得了目前国际文献报道的最高的功率输出性能,并长期稳定（第六章）;另外,还对CMFC电池堆的关键材料——陶瓷连接材料及其薄膜化制备进行了研究（第七章）。论文所取得的主要成果和创新点可归纳如下:1.高性能阴极材料的研制SOFC的发展趋势是降低操作温度,实现中温化,但随着操作温度降低,电极尤其是阴极的电导率和催化活性急剧降低,特别是对基于YSZ电解质的SOFC,其阴极材料和界面极化问题尤为突出。传统的广泛应用的La_1-xSr_xMnO₃（LSM）阴极在高温具有令人满意的电化学性能,但随着温度的降低,其催化活性显著下降,界面极化电阻迅速增加,从而不适宜用作中温CMFC的阴极材料;近年来广泛研发的其它阴极材料如钙钛矿结构的钴酸盐复合氧化物虽然在中低温下表现很高的催化活性,但这类材料热膨胀系数比较高,另外,含钴的阴极不仅原料价格高,且易与YSZ反应生成高阻的锆酸盐第二相,如La₂Zr₂O₇,SrZrO₃等。因此,对于YSZ基的SOFC需要寻找可替代LSM的阴极材料。本论文尝试采用Pr-Nd混合稀土氧化物（这是稀土分离的中间产物,价格较便宜）却鶯SM中的La——（Pr-Nd）_1-ySr_yMnO₃（PNSM）作为SOFC的阴极材料。研究发现以甘氨酸—硝酸法合成的（Pr-Nd）₀₇Sr_0.3MnO₃（PN3SM）在800℃时电导率为216Scm^-1,仅略低于用纯Pr和Nd为原料合成的PNSM;其电导活化能低,约为10kJ.mol^-1,预示随温度的改变电池性能涨落会较平稳。XRD研究表明,PN3SM在电池制备的高温处理情况下不与YSZ反应,且其平均热膨胀系数为11.1×10^-6K^-1,与YSZ的10.8×10^-6K^-1十分接近,适宜用作SOFC的阴极材料。以PN3SM/YSZ为阴极的管状CMFC获得了相当高的电池功率输出,在850—700℃时最大输出功率密度为415—282 mWcm^-2,达到已产业化的美国Acumentrics公司的水平。而在相应温度下,界面极化电阻相当低,分别为0.14,0.21,0.31,0.42Ωcm²（电池在开路下测得）。高的输出功率和低的界面极化电阻表明Pr-Nd混合稀土取代的LSM非常适于YSZ基电解质。廉价混合轻稀土的应用将进一步降低CMFC的造价,也有利于我国稀土资源的高端应用开发。2.低成本电解质薄膜化制备技术的研发固体电解质层是CMFC最核心的部件,它的厚度和致密度将直接影响电池的功率输出。多孔电极支撑的薄膜化电解质构型是实现CMFC中温化、高性能的关键技术路线。电解质薄膜制备方法文献报道颇多,如化学气相淀积/电化学气相淀积、溅射法、流延法、丝网印刷法等,但由于成本及工艺复杂程度和难度等因素,真正能实现低成本、批量化和保证高电池性能的方法却不多。本论文发展了高可靠性的电解质薄膜制备技术——浆料浸渍法,成功的在平板和管状阳极支撑体及平板阴极支撑体上实现了电解质薄膜制备。实验表明,电解质的厚度可以通过控制浆料的固含量和浸渍浆料的体积来控制。通过扫描电镜分析和单电池开路电压测试,表明此方法制备的电解质薄膜（YSZ和SDC）均非常致密。进一步通过单电池性能测试表明制备的单电池性能达国际先进水平:YSZ基平板单电池（YSZ:15μm）在850,800,750和700℃时最大输出功率密度分别为:905,654,487和326mW/cm²;SDC基平板单电池（SDC:24μm）在650,600和550℃时分别达到了870,592和307 mW/cm²的最大功率密度;另外采用这种方法制备的PNSM/SDC阴极支撑的单电池,800℃最大输出功率为657mW cm^-2,即使在650℃也达166 mW cm^-2。这种设备要求简单、成本低、操作方便、成膜效率高的浆料浸渍法将成为发展我国CMFC产业的重要技术支撑之一。3.中温化YSZ基CMFC的研制YSZ基电解质由于其具有高的化学和结构稳定性,是SOFC产业化首选的电解质材料。围绕YSZ中温化应用国内外展开了大量研究,主要是通过发展薄膜化技术,可以获得10μm左右的电解质薄膜,使电解质的欧姆损失降低到可以忽略的程度,在800℃操作时,电池的功率密度高于500 mWcm^-2,甚至有接近2Wcm^-2的报道。然而随着温度降低,会出现显著的电极极化而使电池性能降低,难以满足实用化要求。为提高YSZ基电池在中低温下的输出性能,本论文提出制备梯度阳极修饰层和引入SDC活性层来分别改善阳极/电解质界面和阴极/电解质界面,从而大大提高电池的性能,实现了YSZ基SOFC的中温化。通过在阳极支撑体表面引入梯度组成的超细Ni/YSZ修饰层后,在600和650℃时,界面极化电阻比未加修饰层之前降低了42%和32%,而电池最大输出功率密度分别达到144和353 mW/cm²,分别提高了44%和56%;通过在YSZ电解质和阴极界面进一步引入SDC活性层后,使阴极层的氧离子传输特性及其与电解质层的密合性都得到显著改善,界面极化电阻进一步减小,600和650℃时分别为1.36和0.49Ωcm²,使电池性能进一步提高,最高功率密度达187 mW/cm²和443mW/cm²,分别提高了87%和95%,已能满足YSZ基CMFC中温化要求。4.高温质子导体电解质膜的制备及性能研究:钙钛矿结构的BaCeO₃基高温质子导体在中低温下具有相当高的电导率和十分低的电导活化能,是中低温CMFC的优良电解质材料。但由于这类质子导体材料的薄膜化困难和操作条件下（对CO₂和H₂O）化学稳定性差,因而在CMFC中应用研究的进展不是很大。基于近几年来对BaCeO₃材料的掺杂改性其化学稳定性大大改善,本论文创新的发展了一种原位反应合成技术并结合浆料浸渍法,从合成钙钛矿型复合氧化物电解质的组分氧化物、碳酸盐等原料出发,成功地在多孔阳极支撑体上直接合成了薄膜化电解质。采用这种新颖技术制备的Ba(Zr_0.1Ce_0.7Y_0.2)O₃（BYCZ）电解质薄膜（厚度20μm）,以Sm_0.5Sr_0.5CoO₃/BaY_0.2Ce_0.8O₃（SSC/BYC）为阴极,在700,650和600℃时的开路电压分别为1.010,1.032,1.052 V,十分接近电池的理论电动势,表明电解质膜非常致密。相应的最大输出功率密度达到582 mW/cm²、426 mW/cm²和311 mW/cm²。在国际上率先把BaCeO₃基质子导体燃料电池的性能数据提高到与氧离子导体电解质（如SDC）电池相当的高水平。另外,还采用这种方法制备了组分更加复杂的“六元复合氧化物”新材料:Ba(Ce_0.4Pr_0.1Zr_0.3Y_0.16Zn_0.04)（BYCZPZ）,以BYCZPZ为电解质的单电池在700,650和600℃时的开路电压分别为0.947,0.974,0.986V,对应的最大输出功率密度达到597 mW/cm²、427 mW/cm²和295 mW/cm²。长期性能测试表明,电池输出性能随时间延长会进一步提高,在600℃下0.5V放电时,经过240h测试,输出电流提高了约40%,其性能随时间而变优化的机制尚待进一步研究、查实。这些结果表明,这种新颖的技术不仅能实现电解质薄膜化,而且非常适于新材料体系的探索研究,为今后新型质子传导材料的探索和CMFC的低成本制造提供了一类极佳的新工艺路线。5.直接液体甲醇燃料的CMFC:与其他类型的燃料电池相比,SOFC/CMFC的最大优势是燃料适应性强,可以使用廉价易得的碳氢燃料。然而,迄今发展的CMFC使用的高性能镍基阳极对碳氢化合物的催化活性过高,因而若不加堆外重整而直接应用碳氢燃料往往引起阳极积碳,导致电池性能快速退化,近几年来国际上对这个问题虽经强力研究,但始终没有得到很好的解决;另一方面,CMFC技术首先市场化的应用方向应是小型化、便携式的分散电源,液体燃料应是最佳选择。本实验室针对这一问题,首次提出以发展氨燃料和甲醇燃料的技术路线。已率先突破了氨燃料CMFC,把电池性能提高了一个量级以上,而向世界展示了氨燃料CMFC的实用性。本论文主攻甲醇燃料CMFC,利用上述技术制备的高性能电池研究了直接液体甲醇为燃料的CMFC性能。在中温条件下:650,600和550℃,电池的最高功率密度分别为698,430和223 mW/cm²,完全达到实用化水平。而在550℃（约75h）和650℃（约80h）下0.5V放电的长期性能实验表明电池性能无衰减,表明甲醇在阳极不积碳,这一结果突破了CMFC产业化的一大瓶颈:直接液体（碳氢化合物）燃料CMFC实现了无需外重整的重大突破。6.高性能连接材料及其薄膜化研究:就SOFC/CMFC单电池而言,无论是关键材料还是相关制备技术,都已经达到了相当高的水平,但还必须通过连接材料把一系列的单电池串连起来形成电池堆,才能提供足够大的功率,满足用户的需要。因而连接材料性能及其薄膜化将决定电池堆的输出性能,是实现高性能CMFC产业化的关键因素。一般对于中温下应用的平板状SOFC可以使用廉价的金属基连接材料,但存在表面改性和保护涂层的难题,管状CMFC构型（本实验室近期主攻方向）则采用陶瓷基连接材料,具有兼顾中高温操作的长处。已经成功使用的陶瓷连接材料为钙钛矿型的掺杂铬酸镧陶瓷,但其存在烧结性能差和中低温下电导率低等缺点。近几年来,本实验室通过A位掺杂取代或添加CeO₂基电解质等方式显著的提高了这类陶瓷材料的电导率并大大降低了其烧结温度,在1400℃左右,烧结致密度可达97%以上。本工作通过对研究较多的La_0.7Ca_0.3CrO₃（LCC）进行B位Zn掺杂——La_0.7Ca_0.3Cr_0.95Zn_0.05O₃（LCCZ）,使烧结温度进一步降低,在1200℃能达95.2%的致密度,1350℃可达98%。在空气中和氢气中的电导率也得到显著提高,在800℃时分别达47.5和6.15 Scm-1,即使在500℃仍高达37.2和2.06Scm^-1。另外,该材料热膨胀系数为11.5×10^-6K^-1,可满足对连接材料要求。另外,通过浆料浸渍法成功的在NiO/YSZ阳极支撑体上制备了约40μm的LCCZ薄膜,与阳极支撑体结合紧密,但薄膜的致密化程度还有待进一步提高。更多还原

【Abstract】 As a promising energy conversion device,Solid Oxide Fuel Cell（SOFC）has attracted widespread attention due to its high efficiency,fuel flexibility and environmental benefits,and now it is on the way to practical application and commercialization.A new generation SOFC with high performance,mainly aims at the application for intermediate temperatures（500-800℃）,were characterized with the anode supported thin electrolyte and prepared mainly by the ceramic membrane fabricating techniques,so ceramic membrane fuel cell（CMFC）could express it more exactly.In recently years,our groups have put great effect on key material research, essential fabrication techniques and cells structure design to promote the commercialization of CMFC in China.My dissertation work was focused on the fabrication,improvement and characterization of intermediate temperature CMFC （IT-CMFC）.In the first chapter,this paper introduces the fundamental principles of SOFC/CMFC,the present research status and developing trend both in China and abroad,and pays attention to summarize the electrolyte membranes fabrication techniques,the problems about carbon coking on electrode and fuel choice for practical application of CMFC.On the base of analyzing the development of CMFC, we fabricate and investigate a new cathode material for YSZ-based cells for intermediate temperature application（in chapter 2）;developed a cost effective and mass production process to fabricate YSZ,SDC and high temperature proton conductor membranes（in chapter 3 and chapter 5）;reduce the operation temperature of YSZ based CMFC（in chapter 4）;try to directly fueled CMFC with liquid methanol （in chapter 6）and also studied on the ceramic interconnect materials.The main achievements and innovations in this paper are summarized as follows:1.Research on high-performance cathode materialsThe current development trend of SOFC is to lower the operation temperature. However,a major issue produced by the reduced operating temperature is the decrease in electrical conductivity and catalytic activity of the cathode materials.For the YSZ-based electrolyte,the problems of cathode materials and cathode polarization are extraordinarily serious.The traditional cathode material La_1-xSr_xMnO_3-δ（LSM） perovskite oxides show high electrochemical properties which have been widely investigated as a promising cathodes for high temperatures,however,the catalytic activity of LSM was decreased rapidly as the temperature decreases.Other cathode materials developed in recent years,such as perovskite-type cobaltite oxide compounds,though they exhibit high catalytic activity at intermediate temperatures, shows high thermal expansion coefficient.What’s more,the cobaltite cathode is very expensive and easy to react with YSZ to form the second phase of zirconate with high resistance,such as La₂Zr₂O₇,SrZrO₃ and so on.Therefore,YSZ based SOFC need to find alternative cathode materials for LSM.In this paper,we try to use Pr-Nd mixed （which is intermediate products of rare earth separation and is much cheaper）to substitute La in LSM-（Pr-Nd）_1-ySr_yMnO₃（PNSM）as the cathode material.The conductivity of（Pr-Nd）_0.7Sr_0.3MnO₃（PN3SM）,synthesized by Glycine-Nitrate Process,could reach 216 S/cm at 800℃,only slightly lower than the PNSM synthesized using the pure Pr and Nd.And the activation energy of conductivity is only about 10kJ/mol,indicating the performance fluctuation with the temperature change is smooth.The XRD data shows that PNS3M does not react with YSZ during the high-temperature processing of preparing the cells,and the average thermal expansion coefficient is 11.1×10^-6K^-1,which is very close to that of YSZ,which is 10.8×10^-6K^-1,apparently suitable cathode material for SOFCs.With PN3SM/YSZ as cathode,the tubular CMFC get very high power output,and the maximum power densities are 415-282 m W cm^-2at 850-700℃,respectively,which has reached the industrialization level of Acumentrics Corporation of America.At corresponding temperatures,the interfacial polarization resistances are 0.14,0.21,0.31,0.42Ωcm². The high power densities and low interface polarization resistances indicate that Pr-Nd mixed oxide substituted LSM is very suitable candidate cathode material for YSZ electrolyte based SOFC.Besides,the application of low-cost rare earth in CMFC will not only further reduce the cost,but also benefit for the development of China’s rare earth resources in the high-end application.2.Research on the cost effective process for electrolyte membranes fabricationThe solid electrolyte membrane is the core part in CMFC,for its thickness and density will directly influence the output power densities of the cell.Configuration of porous electrode supported thin electrolyte is the key technique route for CMFC to achieve high performance at intermediate temperature.Various techniques,such as chemical vapor deposition/electrochemical vapor deposition,sputtering,tape-casting, screen printing method,have been developed for electrolyte membranes fabrication. However,considered the high costs and technical complexity,there are few methods that can really achieve low-cost and high performance output.In our work,we developed a highly reliable technique of suspension coating process to prepare thin electrolyte membranes,through which dense electrolyte membranes were successfully fabricated on planar and tubular anode supports and planar cathode support. According to the results of SEM and single-cell open circuit voltage tests,the electrolyte films（YSZ and SDC）prepared by this method are very dense.Moreover, the single cells performance reach the international level:YSZ-based planar single cell（15μm）gets the maximum power density of 905,654,487 and 326 mW/cm2 at 850,800,750 and 700℃,respectively;while SDC based planar single cell（24μm）at 650,600 and 550℃,respectively reached 870,592 and 307 mW/cm²;and the maximum power density of the PNSM/SDC cathode supported single cell prepared using this method,achieved 657 mW/cm² at 800℃,even at 650℃,could still reach 166 mW/cm².Such suspension coating process,requiting simple devices,low cost, easy operation,and highly efficient will be one of the important technical supports for the development of CMFC industry in China.3.Research on IT-CMFC based on YSZYSZ is the preferred electrolyte material in the commercialization of SOFC for its high chemical and structural stability.In order to operate YSZ electrolyte based CMFC at intermediate temperatures,a large number of studies have been done to fabrication thin electrolyte film around 10μm and even less on porous anode support to reduce the electrolyte ohmic resistance to a negligible level.At 800℃,a higher power density of 500mW/cm² and even close to 2W/cm² has been reported.However, as temperatures decrease,there will be significant electrode polarization resulting in lower cell performances,which will not meet the practical requirements.In order to improve the performance of YSZ based cell at intermediate temperature,we introduced gradient anode transition layers and SDC function layer to improve the anode/electrolyte and cathode/electrolyte interface,respectively,thus greatly improved the cell performance at intermediate temperature.Through the introduction of a gradient composition of ultra-fine Ni/YSZ layer on the anode support,the interface polarization resistance reduced 42%and 32%than those without modification,at 600 and 650℃,respectively;while the maximum power densities were reached 144 and 353 mW/cm²,that are increased by 44%and 56%, correspondingly.By further introduction of SDC function layer to the YSZ electrolyte and cathode layer,the interface polarization resistance has been further reduced to 1.36 and 0.49Ωcm² 600 and 650℃,respectively.A fairly high maximum power densities of 187m W/cm² and 443mW/cm² were reached,showing an increase about 87%and 95%compared with the performance without any modification.The high performance achieved in our result could meet the practical requirements of IT-CMFC based on YSZ electrolyte.4.Research on proton conductor based CMFCFor their high proton conductivity and low conductivity energy activity,the BaCeO₃ based high temperature proton conductors have attracted wide attention and been applied as the electrolyte materials for IT-CMFC.However,the poor sinter ability and poor chemical stability under operation condition（to CO₂ and H₂O）have prevented the application of this kind of materials in CMFC.Recently,the chemical stability of BaCeO₃ based material has been greatly improved by Zr substitution, which shows potential application.In this work,we developed a situ reaction,by using the oxide and carbonates that be used to synthesize the electrolyte,combined with suspension coating process to prepare proton conductor,and dense electrolyte membranes have been successfully fabricated on porous anode supports.The Ba(Zr_0.1Ce_0.7Y_0.2)O₃（BYCZ）electrolyte membrane fabricated through this innovation technology（the thickness is 20μm）,with Sm_0.5Sr_0.5CoO₃/BaY_0.2Ce_0.8O₃（SSC/BYC）as the cathode,could reach the open circuit voltage of 1.010,1.032,1.052 V at 700, 650 and 600℃,respectively,which are very close to the EMF,indicating that the electrolyte membrane is very dense.The maximum power densities of 582,426 and 311 mW/cm² have been achieved correspondingly.Through membrane fabrication process,the cell performance of BaCeO₃ based CMFC was achieved as high as that of oxide electrolyte（for example,SDC）CMFC.What’s more,this process has applied to fabricate a more complex compound material（contain 6 metal components）: Ba(Ce_0.4Pr_0.1Zr_0.3Y_0.16Zn_0.04)（BYCZPZ）.With BYCZPZ electrolyte,the OCV were 0.947,0.974 and 0.986V,and the maximum power densities were 597,427 and 295 mW/cm²,respectively at 700,650 and 600℃.When the cell operated at a voltage of 0.5V at 600℃,the out-put current density increased 40%after 240h testing.However, the reason for such phenomenon still need to be further investigated.All the data achieved in our results indicated that this innovation technology not only could realize the thin electrolyte membrane fabrication,but also suitable for the exploration and research of new materials system,which provides a new technology route for the exploration of new proton conductor materials and low cost fabrication method for CMFC.5.Direct liquid methanol fueled CMFCCompared with other categories fuel cells,the outstanding advantage of SOFC/CMFC is its great fuel flexibility,because they could use cheap and easy get hydrocarbon fuels.However,CMFC with Ni-based anodes suffers a number of drawbacks while using hydrocarbon fuels due to the propensity of Ni to catalyze carbon formation,resulting in the rapid degradation of the cell performance.In recent years,the research efforts have been put on solving the coking problem,but no effective way has been found.At the other hand,the first-choice commercial application of CMFC technology is portable system,so the liquid state fuels would be a better choice.To solve the carbon coking problem,our group,for the first time,put forward the new energy route of fueled CMFC with ammonia and methanol.In the past three years,the cell performances ammonia fueled CMFC have been increased many times in our group and achieved practical use lever.My work was focused on directly fueled CMFC with methanol.With SDC electrolyte,maximum power densities of 698,430 and 223 mW/cm² have been achieved respectively at 650,600 and 550℃,by using methanol as fuel.To investigate the endurance of the cell with methanol as fuel,the cell was operated at a voltage of 0.5V at 550℃for about 75hrs, then at 600℃for 5hrs and finally at 650℃for more than 80hrs until the methanol was exhaust.No detectable decay in the cell current was observed within the operating duration.This result surely demonstrates no degradation of the cell performance,indicating no coking on anode.That result is a great break through for CMFC commercialization by directly using liquid hydrocarbon fuels6.High performance interconnect material study and membrane fabricationBoth key materials and fabrication techniques developed so far for single cell of SOFC/CMFC have achieved a rather high level.However,we need to use interconnect materials to connect the series of single cells to achieve higher output power density to meet the requirement for practice.The performance of the interconnect material would determine the output performance of the cell stack,which is the key material for CMFC commercialization.Though,the planar SOFC operated at intermediate temperatures could use much cheap alloy interconnect,the surface of alloy should be protected by a doped LaCrO₃ coating,which was difficult.For tubular SOFC,ceramic interconnect materials is the only choice.Now,the successfully used ceramic-based interconnect material is doped LaCrO₃,however,this kind of materials show poor sinter ability and low electric conductivity at intermediate temperatures.In recent years,we have devoted great effect to decrease its sintering temperature by using A-site substitution and/or adding doped CeO₂,and the bulk density could achieve as high as 97%of the theory density at around 1400℃.This work was aimed at further decreasing the sinter temperature of La_0.7Ca_0.3CrO₃（LCC）by doping Zn at B site:La_0.7Ca_0.3Cr_0.95Zn_0.05O₃（LCCZ）.In our result,the density could reach 95.2%at 1200℃and 98%at 1350℃.The electric conductivity both in air and hydrogen increased notably,which were 47.5 and 6.15 Scm^-1at 800℃,respectively,even at 500℃,there still as high as 37.2 and 2.06Scm^-1.What’s more,the thermal expansion coefficient of such material is 11.5×10^-6/K,which could meet the need for interconnect materials.Also we try to fabricate LCCZ membrane on NiO/YSZ anode supports by using suspension coating process.The LCCZ membrane with the thickness of about 40μm was adhered well with the porous anode support,however, much work should be done to make the interconnect membranes more dense.更多还原