【作者】 魏波；

【导师】 苏文辉； 吕喆；

【作者基本信息】 哈尔滨工业大学 ， 光学， 2008， 博士

【摘要】 固体氧化物燃料电池(SOFC)作为一种高效、无污染、可以直接将燃料的化学能转变成电能的发电方式正受到日益重视。目前SOFC的研究重点是把工作温度降低到500-800°C的中低温区,以加速产业化进程。阴极极化电阻随温度降低而迅速增大并成为限制电池性能的主要因素,故开发高性能的阴极材料一直是研究重点之一。由于可以使用比氢气更经济,来源更广泛的烃类、醇类等做燃料,含碳燃料的SOFC相关研究日益增多。另外,具有无密封特点的单气室SOFC在便携式电源方面展现出很好的优势,是新的研究亮点。针对目前SOFC研究的趋势和热点,我们选取了新型高性能阴极材料以及它们在单气室SOFC(甲烷为燃料)中的应用作为本论文的研究重点。所涉及阴极材料包括三种,即在传统La1-xSrxMnO3 (LSM)阴极基础上改进的纳米Sm0.2Ce0.8O1.9 (SDC)修饰LSM复合阴极,新报道的Ba1-xSrxCo0.8Fe0.2O3-δ(BSCF)阴极,以及我们首次提出作为SOFC阴极使用的无钴Ba0.5Sr0.5Zn0.2Fe0.8O3–δ(BSZF)材料。首先我们对改性后的LSM阴极在SC-SOFC中的应用进行研究。使用SDC浸渍后LSM阴极的单电池性能得到很大提高。750oC时的最大功率密度达到404mW?cm-2,电池内阻为1.6??cm2,分别是未浸渍电池的4倍和38.5%。浸渍后LSM阴极被许多被纳米级SDC颗粒包覆,新增大量电化学活性区,进而显著提高电池性能。使用阳极对阴极结构将阳极支撑单电池串联组成电池组,实际测试的两个单电池组成的电池组在700oC时最大功率为371mW。电极的极化电阻是限制单电池和电池组性能的主要原因,而多个电池组成的电池组性能更受到流场和容器限制明显。针对上面结构遇到的问题,我们发明了一种新型的星形结构电池组,具有流场均匀,抗震性能好,易于实现高功率输出等优点。使用4节串联的星形电池组在750°C的最大功率达到了421mW,可以成功驱动一个USB风扇。进一步放大后在便携式电源方面展现出很好的应用前景。Ba0.5Sr0.5Co0.8Fe0.2O3是一种高性能中低温SOFC阴极材料,在双室和单气室SOFC中都表现优异。本文对BaxSr1-xCo0.8Fe0.2O3系列样品性质进行系统研究。BSCF的成相经历了复杂的中间过程,得到纯相BSCF需要900°C或更高焙烧温度。对BSCF中Ba的含量对晶体结构、热重、热膨胀和电导率性质的影响研究发现,随着Ba含量的增加,BSCF失氧量减少,热膨胀系数降低(500-1000oC),电导率降低。此结果揭示了氧缺陷在其中所起的重要作用,晶格氧在400oC附近就十分活跃,导致大量的氧空位的生成和高价Co和Fe离子发生热还原,进而对BSCF样品性质产生很大影响。首次发现并解释了BSCF体系中存在的反常热膨胀现象。对BSCF的电导弛豫和尺寸弛豫进行了初步研究,样品在400-500°C的扩散系数为5.0×10-9-5.9×10-8 cm2?s-1。掺入SDC后复合阴极的电化学性质得到很大提高,且以掺入30wt%SDC时性能最好,适合作为中低温SOFC的阴极材料。使用该复合阴极,SDC夹层的YSZ薄膜电池在800°C,阴极气氛为空气和氧气时的最大功率密度分别达到了1090mW?cm-2和1200mW?cm-2。在单气室条件下,电池的开路电压较高,550°C时约为1V。在CH4:O2=1.5:1,电池炉温为650°C时的最大功率密度达到了240mW.cm-2。升高到700°C后电池性能由于电极选择催化性的下降而降低到224mW?cm-2。最后,我们首次将以往应用于透氧膜的无钴BSZF材料作为SOFC阴极材料进行了研究。结合碘滴定实验和热重研究发现BSZF中含有大量的氧空位,有利于氧还原反应。BSZF电导率在590°C达到最大值9.4S?cm-1,并出现半导体导电到金属型导电转变。电导率较低的原因是由于大量氧空位的存在和二价的Zn离子的掺入不会对小极化子跳跃导电产生贡献。BSZF阴极在SDC电解质上的最佳烧结条件为950°C焙烧4h。根据极化电阻和氧分压的关系分析了BSZF阴极反应机理,氧吸附/脱附过程为速度控制步骤。纯相BSZF阴极在650°C极化电阻为0.48??cm2,性能与LSCF相当。使用浸渍SDC纳米颗粒后减小到0.28??cm2,恒温活化24h后降低到0.21??cm2。Ni+SDC?SDC?BSZF单电池使用湿润H2作燃料和空气为氧化剂时,650°C,600°C和550°C时最大功率密度分别达到了392mW?cm-2,208mW?cm-2和107mW?cm-2。在600°C/0.6V电池稳定运行了22h,基本没有衰退。在单气室条件下,炉体温度为600°C的最大功率分别密度为202mW?cm-2,由于自放热效应使得电池性能与使用氢气作燃料相当。本论文中对三种新型的阴极材料进行了系统研究。使用浸渍后LSM阴极可以显著增强电池性能,提出星形结构电池组,可望在便携式领域应用。对BSCF和BSZF两种混合导体阴极的物性和电化学性质进行系统研究,在中低温和单气室SOFC中都表现较好,是有前景的阴极材料。更多还原

【Abstract】 Solid oxide fuel cell (SOFC) is a device that can directly convert chemical energy to electric energy with very high efficiency and low emissions. With the advantages of fuel flexible, all-solid-state and long-lifetime, much attention around the world has been paid on the research of SOFCs. Current research of SOFCs focuses on lowering the operation temperature range of 500-800°C to promote the commercialization process. However, cathodic overpotential increases considerably with temperature decreasing, and limits the cell performance critically, making the search of high performance cathode urgent. As the hydrocarbon fuels are more economical than hydrogen and are easily obtained, the related research increases in recent years. Furthermore, sealant-free single-chamber SOFC (SC-SOFC) has potential application in portable area, and thus becomes another hot research topic. Based on the research trends, the objective of this work is to investigate the high performance cathode materials and their applications in methane-fueled SC-SOFCs. The cathodes studied were nanosized Sm0.2Ce0.8O1.9 (SDC) modified La1-xSrxMnO3 (LSM), the newly-developed Ba1-xSrxCo0.8Fe0.2O3-δ(BSCF) perovskites and the novel Ba0.5Sr0.5Zn0.2Fe0.8O3–δ(BSZF) cathode that proposed by us firstly.The application of modified LSM cathode in SC-SOFC was firstly studied. With the SDC impregnated LSM cathode, the cell performance was improved significantly. At the furnace temperature of 750oC, the max power density and total cell resistance were 404mW?cm-2 and 1.6??cm-2, which respectively, were 4 times higher and 38.5% of the non-impregnated cell. The enhancement was attributed to the fact that the LSM grains were covered by many ionic conducting SDC nanophase, which generated amount of electrochemically active area and improved the electrochemical properies. The anode-facing-cathode micro-stack using anode-supported cells was proposed. A stack with two single cells generated 371mW at 700oC. Impedance spectra data confirmed that electrode polarization resistance was the primary reason limiting the output of both single cell and stacks. The performance of stacks with 3 or more cells was also limited by the flow field and the size of quartz tube. To solve the problem, another novel design with star-shape was developed, which enables the advantages of uniform flow field, highly shock-resistance and easily obtained higher output. The stack with 4-cells contacted in series generated higher output of 421mW at 750°C, which successfully powered a USB fan. Enlarged stacks are attractive alternatives to batteries in the micropower arena.Ba0.5Sr0.5Co0.8Fe0.2O3–δcathode has demonstrated high performance in both dual- and single-chamber cells. Our research focused on the properties of BaxSr1-xCo0.8Fe0.2O3–δperovskites that were synthesized by a combined EDTA-citric acid complexing method. The phase development of BSCF experienced complexing intermediate process and the baking temperature of≥900°C was need for the formation of pure-phase BSCF. The effect of Ba content on crystal structure, TG, thermal expansion and conductivity were measured. The results showed that the oxygen loss, TEC and conductivity decreased with the increasing of Ba content, indicating the important role of oxygen deficiency. Lattice oxygen began loss at about 400oC, resulting in amount of oxygen vacancy and the thermal reduction of high valance of Co and Fe ions. Abnormal expansion behavior was found for the first time and its mechanism was explained. The conductivity relaxation and size relaxation of BSCF were parimarily studied. The oxygen diffusion coefficients were 5.0×10-9-5.9×10-8 cm2?s-1 between 400°C and 500°C. Moreover, the BSCF55-SDC composite cathodes were studied. Our results showed that the electrochemical performance of BSCF was improved obviously with the addition of SDC electrolyte, and the optimal composition was found to be BSCF-30wt%SDC. Using this composite cathode, the SDC interlayered YSZ film cell generated max power densities of 1090mW?cm-2 and 1200W?cm-2, respectively, when the cathode was fed by air and oxygen flow. Electrode polarization resistance mainly limited the performances. In single-chamber mode, the OCV of the cell reached high value of about 1V at 550°C. At the furnace temperatures of 600°C, the cell output achieved 220mW?cm-2. But the output decreased to 224mW?cm-2 when temperature increased to 700°C, due to the imperfect selective of electrodes at higher temperature.Finally, the cobalt-free Ba0.5Sr0.5Zn0.2Fe0.8O3–δ(BSZF) perovskite, formerly as oxygen permeation membrane, was investigated as a novel cathode for SOFCs. After baking at 950°C for 5h, precursor turned to cubic perovskite powder completely. The room-temperature oxygen nonstoichiometry in BSZF, as determined by iodometric titration experiment, was as high as 0.412, indicating high concentration of vacancy existed which was preferred for oxygen reduction. The electrical conductivity was relatively low with a peak value of 9.4 S?cm-1 at about 590°C, which was mainly caused by the high concentration of oxygen vacancy and the doping of bivalent zinc that doesn’t contribute to small polaron conduction. The optimal firing condition of BSZF cathode on SDC was 950°C for 4h. The dependence of Rp with oxygen partial pressure indicated that the rate-limiting step for oxygen reduction was oxygen adsorption/desorption kinetics. The polarization resistance of pure BSZF at 650°C was 0.48??cm2, which is comparable to LSCF cathode. After SDC impregnation, the resistance decreased to 0.28??cm2, which was further activated to 0.21??cm2 after 24h operation. Using BSZF as the cathode, the wet hydrogen fueled Ni+SDC?SDC?BSZF cell exhibited peak power densities of 392mW?cm-2, 208mW?cm-2 and 626mW?cm?2 and 107mW?cm-2 at 650°C,600°C and 550°C respectively, using stationary air as oxidant. When using oxygen as oxidant, improved outputs of 626mW?cm-2 , 353mW?cm-2 and 173mW.cm-2 were obtained. In single-chamber condition, the cell exhibited peak power densities of 202mW?cm-2 and 173mW?cm-2, at the furnace temperatures of 600°C and 550°C respectively. Owning to the self-heating effect, the performances in single chamber mode were comparable to that in dual-chamber cell powered by hydrogen.In conclusion, three kinds of new cathodes were investigated. Impregnated LSM cathodes can enhance the cell performance obviously. A novel star-shaped micro-stack was proposed which can be applied in portable arena. The properties of mixed conducting BSCF and cobalt-free BSZF cathodes were systemically studied, which exhibited attractive performances for intermediate-to-low temperature SOFC and single-chamber SOFC.更多还原