【作者】 李淑彦；

【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)是一种能够将化学能直接转化为电能,且具有效率高、绿色环保及燃料选择范围广等优点的电化学装置,在能源问题日益严重的今天,SOFC凭借自身优势而受到研究者的广泛关注。随着对SOFC应用研究的深入,如何有效降低电池工作温度成为当前的研究热点。而开发在中低温下具有良好性能的阴极材料是降低电池工作温度,保持电池输出性能的有效方法之一,许多研究者在开发新型中低温阴极材料方面做了大量工作。本文围绕中低温固体氧化物燃料电池阴极材料的开发展开。首先,利用EDTA-柠檬酸联合络合法合成了Sm替代Ba0.5Sr0.5Co0.8Fe0.2O3(BSCF) A位中部分Ba、Sr的材料,(Ba0.5Sr0.5)1-xSmxCo0.8Fe0.2O3-δ(BSSCF;0.05≤x≤0.20),并对BSSCF的结构,热性能,电性能及电化学性能进行了系统研究。研究结果表明,BSSCF具有与BSCF相同的立方钙态矿结构。由于Sm3+离子的半径小于BSCF材料A位中Ba2+和Sr2+离子的半径,因此掺杂造成BSSCF的晶胞收缩。30~800°C温区,BSSCF的热膨胀系数为19.1-20.3×10-6K-1。Sm掺杂有效提高了材料的电导率,电导率提高主要得益于Sm掺杂而增加的载流子浓度。而且,电导率随着Sm掺入量的增加而增加,其中掺杂量为x=0.20样品的电导率在392°C时达到了150 S?cm-1。BSSCF在350~500°C温区存在明显的电导弛豫现象,根据电导弛豫数据计算得到的BSSCF氧扩散系数和扩散活化能随着Sm的掺杂量的增加而减小。热重测试结果表明, BSSCF在200°C和350°C温度附近分别出现明显的增重和失重,此现象与Co/Fe离子价态变化伴随的氧吸收和脱出有关,这种变价和氧活动对BSSCF的电导率及热膨胀性能具有十分重要的影响。在BSSCF的电化学性能方面,我们首先对能够影响电极性能的烧结条件进行分析,确定电极的最佳烧结条件为:1050°C烧结4h。随后,对以BSSCF为阴极的半电池进行电化学测试。实验结果表明,Sm掺杂有效提高了材料的电化学性能,以交流阻抗谱为例,Sm掺杂量为x=0.10的材料在400,450,500,550oC的总阻抗分别为17.51,7.55,2.98,1.54 ??cm2,而BSCF材料分别为41.06,18.59,4.57,1.82 ??cm2。使用BSSCF阴极的阳极支撑SDC薄膜燃料电池的单电池具有优良的输出性能,其中电池在500,550,600,650 oC的最大功率密度分别为268,442,681,820mW?cm-2。这一结果好于同等条件下测量的BSCF阴极材料。在同一温度下,BSSCF半电池和燃料电池单电池的阻抗谱之间存在明显的差异,造成差异的两个主要原因在于SDC电解质所处气氛环境的不同,以及在H2-O2燃料电池单电池条件下SDC薄膜所具有的电子-离子混和导电性。在A位直接Sm掺杂取得成功的基础上,将稀土离子La3+作为掺杂离子引入BSCF,制备出(Ba0.5Sr0.5)1-xLaxCo0.8Fe0.2O3-δ(BSLCF;0.05≤x≤0.20)材料。重点考察了La掺杂对材料性能的影响,并与Sm掺杂材料进行比较。结果表明,BSLCF具有立方钙态矿结构。由于四价Co/Fe离子热还原的加剧,BSLCF材料在400°C开始出现明显的失重现象。另外,La掺杂略微增大了材料的热膨胀系数,BSLCF在30~800°C温度区间的热膨胀系数为18.7-25.8×10-6 K-1。La掺杂对于提高BSLCF材料电导率效果明显,例如,x=0.20样品在392°C的电导率可以达到376 S?cm-1。利用电导弛豫方法确定了BSLCF的氧扩散系数和扩散活化能,结果显示,BSLCF的扩散系数和扩散活化能均随着掺杂量的增加而减小。交流阻抗的测量结果证明BSLCF的电化学性能也要优于纯的BSCF材料。随后对Nd3+掺杂的(Ba0.5Sr0.5) 1-xNdxCo0.8Fe0.2O3-δ(BSNCF; 0.05≤x≤0.20)材料的结构,热、电及电化学性能进行研究,以进一步确定稀土掺杂对于BSCF材料性能的影响。结果表明,BSNCF同样具有立方钙钛矿结构。材料中氧的行为受扩散和表面交换速率的限制,与样品的致密度、表面积有关。Nd掺杂在一定程度上改善了材料的热膨胀系数,BSNCF的热膨胀系数较BSCF略小,在30~800°C,为18.7-19.7×10-6 K-1。BSNCF材料电导率提高明显,其中x=0.20样品的电导率最大。另外,在400~450°C温度区间内,BSNCF材料出现明显的电导弛豫现象,且其氧扩散系数和扩散活化能均随Nd掺杂量的增加而减小。交流阻抗谱结果证明,BSNCF材料的电化学性能要好于BSCF,例如,在550°C时,BSCF的阻抗为1.82 ??cm2,而x=0.20的样品仅为0.69 ??cm2,这也要小于Sm、La掺杂材料。为揭示A位稀土掺杂材料的性能与掺杂元素之间的本质联系并找出规律,我们从各种掺杂稀土元素(Ln=La, Nd, Sm)基本性质出发,系统分析了稀土掺杂化合物的物性。重点考察其结构、热性能以及电性能的差异及其产生原因。结果显示,这几种稀土掺杂材料仍保持立方钙态矿结构。虽然,容限因子随掺杂元素和掺杂量的变化有微小变化,但数值仍接近1。滴定实验结果表明,在掺杂量相同的情况下,不同稀土掺杂材料中三、四价Co/Fe离子数量及氧空位数量不同,这是造成不同掺杂材料各项性能差异的主要原因。稀土掺杂并未明显减小材料的热膨胀系数,相同掺杂量条件下的不同稀土掺杂材料的热膨胀系数不同。A位稀土掺杂有效提高了材料的电导率。相同掺杂量条件下,三种稀土掺杂材料的电导率不同。本文以中低温阴极材料BSCF为基础,利用三价轻稀土元素La、Nd、Sm部分替代Ba和Sr,有效提高了材料的电导率和氧电极电化学性能,成功开发出性能优良的新型阴极材料体系(Ba0.5Sr0.5)1-xLnxCo0.8Fe0.2O3-δ(Ln=La,Nd,Sm),对各材料中与高温氧化/还原相关的高温物性规律及其机制进行了研究。更多还原

【Abstract】 Solid oxide fuel cell (SOFC) is one kind of chemical device, which can convert the chemical energy into electrical energy directly. Recently, the energy problem is becoming seriously, thus the SOFC is attracting more and more attentions because of its high-energy conversion efficiency, low pollution and fuel flexibility. How to lower down the operating temperature is the key point of the SOFC’s application research. Finding out a cathode with good characteristics at intermediate temperature is one effective approach to lower down the operating temperature of SOFC while to retain the high performances. Many researchers have done mass works in developing the new cathode materials.The objective of this paper is around the development of the cathode material for intermediate temperature solid oxide fuel cell (IT-SOFC). Firstly, the Sm-doped material (Ba0.5Sr0.5)1-xSmxCo0.8Fe0.2O3-δ(BSSCF;0.05≤x≤0.20) was prepared by EDTA-citric acid method, and then the structure, the thermal character, the electrical character as well as the electrochemistry character were researched. The results demonstrated that the BSSCF have a cubic perovskite-type structure as BSCF, and the Sm-doping caused the lattice shrinkage because of the smaller radius of Sm3+ ion. The thermal expansion coefficients (TECs) of BSSCF are 19.1-20.3×10-6 K-1 from 30~800°C. The Sm-doping improved the electrical conductivity of BSCF effectively. While the conductivity is increasing with the increasing of Sm content, and the x=0.20 sample have aσ=150 S?cm-1 at 392°C. The improvement of electrical conductivity is mainly benefit from the enhanced concentration of electronic charge carriers, which is caused by the Sm-doping. Besides, at 350~500°C, there is an obviously electrical conductivity relaxation phenomenon for BSSCF. The results showed that, the chemical diffusion coefficient ( D~ ) of oxygen and the diffusion active energy (Ea) of oxygen were reducing with the increasing of Sm content. In the Thermograimetic measurements (TG) of BSSCF, a weigh addition at 200°C and a weigh loss at 350°C were observed, which is associated with the valence change of Co and Fe followed by the absorption/loss of oxygen.On the electrochemistry aspect, the sinter condition of cathode, which can influence the properties, was researched. The best sinter condition is, at 1050°C for 4h. Then, the electrochemistry properties of BSSCF were carried out on a half-cell. The results demonstrated that, the Sm-doping improved the electrochemistry properties of BSSCF effectively. Take the AC-impedance for example, the impedance of x=0.20 sample are 17.51,7.55,2.98,1.54 ??cm2 at 400, 450, 500, 550 oC respectively, while the impedance of BSCF are 41.06,18.59,4.57,1.82 ??cm2. The BSSCF cathode also showed good properties in anode-support, SDC film single-cell. For example, the maximum power densities are respectively 268,442,681 and 820 mW?cm-2 at 500, 550, 600, 650°C. Furthermore, the AC-impedance of BSSCF for half-cell and single-cell at the same temperature are different. The reasons are the different work-atmosphere of SDC electrolyte film in half-cell and single-cell, and the mixed conductivity character of SDC electrolyte film.Base on the results of Sm-doped BSCF, the La3+ was used to substitute the Ba2+ and Sr2+ ion in BSCF, and the (Ba0.5Sr0.5)1-xLaxCo0.8Fe0.2O3-δ(BSLCF;0.05≤x≤0.20) was compounded. An emphasis was made on the effect of La-doping on the material’s properties, and the properties of BSLCF were compared with the Sm-doped materials. The results showed that, the BSLCF have a cubic perovskite-type structure. There is an obviously weight loss for BSLCF after 400°C, which is associated with the reduction of Co/Fe with quadrivalence. Furthermore, the La-doping increased the TECs slightly, and the TECs of BSLCF are about 18.7-25.8×10-6 K-1 from 30~800°C. The La-doping increased the electrical conductivity of BSLCF apparently. Compared with the Sm-doping sample, the improvement of BSLCF is more apparently e.g., the sample of x=0.2 demonstrated a conductivity ofσ=376 S ? cm-1 at 392°C. Furthermore, the chemical diffusion coefficient ( D~ ) and diffusion active energy (Ea) of BSLCF were researched by the electrical conductivity relaxation method. The results showed that D~ and Ea were reducing with the increasing of La content. The AC-impedance proved that the electrochemistry properties of BSLCF were better than that of the BSCF.In order to ascertain the influence of A-site rare-earth doping on the properties of the material further, we introduced the Nd into BSCF, and got the (Ba0.5Sr0.5) 1-xNaxCo0.8Fe0.2O3-δ(BSNCF; 0.05≤x≤0.20). The structure, thermal, electrical conductivity and electrochemical properties of BSNCF were researched. The results showed that, the BSNCF have a cubic perovskite-type structure. The behaviors of oxygen in the BSNCF are restricted by the diffusion and surface-exchange rate, and are also associated with the density and surface-measure of the sample. To a certain extent, the Nd-doping improved the TECs. The TECs of BSNCF are smaller than that of the BSCF, with the data of 18.7-19.7×10-6 K-1 from 30~800°C. The results of electrical conductivity showed that, the Nd-doping improved the conductivity of BSCF apparently, and the sample of x=0.20 has the highest conductivity. There is an obviously electrical conductivity relaxation phenomenon for BSNCF sample at 400~450°C. The diffusion coefficient and active energy of BSNCF are reducing with the increasing of Nd content. The electrochemistry properties of BSNCF are better than that of the BSCF, which are proved by the AC-impedance. For example, at 550°C, the impedance of BSCF is 1.82 ??cm2, while that of the BSNCF is 0.69 ??cm2.To reveal the intrinsic relation between the doping elements and the material’s properties, moreover find out the regular, we analyzed the rare-earth elements (Ln= Sm, La, Nd) doped compounds’properties from the view of the doping rare-earth element’s character and the experiment results. An emphasis was made on the difference in the structure, thermal, electrical character and the reasons. The analysis results revealed that, the rare-earth doping material still have a cubic perovskite-type structure. Although the Tolerance Factor changed a little with the varying of the doping element and content, while the value is still around 1. The results of titration experiment showed that, under the same doping content, the amount of Co3+/Co4+, Fe4+/Fe3+ and the oxygen vacancy are different, which caused the difference in the properties of different element doped material. The rare-earth doping have not reduce the TECs, moreover, under the same doping content, the TEC of the different element doped material are different. The A-site rare-earth doping increased the electrical conductivity of BSCF apparently. Under the same doping content, the conductivity of different rare- earth doped material is different.This paper is based on the important cathode material of IT-SOFC BSCF, the rare-earth element La, Nd, Sm were used to substitute the Ba2+ and Sr2+ ion in BSCF, which improved the electrical conductivity and electrochemistry properties. A new type of cathode materials (Ba0.5Sr0.5)1-xLnxCo0.8Fe0.2O3-δ(Ln=La,Nd,Sm) were developed successfully. The high-temperature physical properties and their principles associated with the high-temperature oxidation/reduction of these materials were also researched.更多还原