【作者】 刘石明；

【导师】 肖建中； 钱晓良；

【作者基本信息】 华中科技大学 ， 材料学， 2007， 博士

【摘要】 锶、铁掺杂的钴酸镧类(La1-xSrxCoyFe1-yO3,LSCF)钙钛矿结构材料具有高氧离子自扩散系数,良好的电子和氧离子混合导电能力,是一种优良的氧还原催化剂,非常适合作为固体氧化物燃料电池的阴极材料。除了LSCF元素组成对性能有着重要影响之外,初始粉体微观形貌以及电极的微观结构对电化学性能也有着非常重要的影响。而粉体合成方法直接决定了粉体的微观形貌,并对电极微观结构也有着重要影响。因此,本文从研究LSCF粉体的合成方法出发,将微波引入到LSCF的合成和电极烧结过程中,提出了一种理想的快速高效合成方法——微波溶胶凝胶法;在此基础上,用极化曲线、电化学阻抗谱和单电池等实验手段,比较研究了LSCF电极、LSCF-GDC复合电极和LSCF-GDC梯度电极三种电极结构的电化学性能及其影响因素,初步分析它们的氧还原反应动力学过程。主要研究结果如下:1)提出了微波溶胶凝胶法(微波法)合成LSCF粉体,尝试使用了微波烧结法制备LSCF电极。采用微波法合成了LSCF纳米粉体,借助X-射线荧光探针、X-射线衍射、傅立叶红外、BET、扫描电镜和透射电镜等分析手段对材料性能和反应过程进行了分析。初步将LSCF形成过程分为下面几个阶段:(1)在反应初期,LSCF相主要由硝酸盐反应生成;(2)除硝酸锶之外的其它硝酸盐同时分解成为相应的氧化物,这些刚分解形成的氧化物呈无定形状态;(3)硝酸锶与有机物分解产生的二氧化碳生成比较稳定的碳酸锶;(4)在反应中后期,LSCF相主要由碳酸锶与其它金属氧化物之间相互发生固相反应生成。采用正交实验分析了混合液初始浓度,PVA加入量,微波处理时间和微波处理功率对合成粉体比表面积的影响。结果发现,微波处理时间对比表面积影响最大,表面活性剂影响次之,微波功率影响较小,而混合液初始浓度基本没有影响。在700W微波功率下,辐射35min可制备出比表面积为38.9(m2/g),粒度为23nm,粒径分布在16nm范围内的纳米粉体。2)采用极化曲线、电化学阻抗等方法比较研究了LSCF电极、LSCF-GDC复合电极和LSCF-GDC梯度电极的电化学性能。实验发现,复合电极(800℃,Rp=0.03～0.06??cm2)具有最小的极化电阻,梯度电极(800℃,Rp=0.061～0.085??cm2)次之,LSCF电极(800℃,Rp=0.13??cm2)极化电阻最大。与相同条件下的La1-xSrxMnO3(LSM)电极比较后发现,LSCF阴极的极化电阻比LSM电极要小10倍左右。GDC掺混在一定程度上降低了LSCF电极的极化电阻,但降低幅度只有25%～50%,这与LSM电极极化电阻是掺杂YSZ的LSM-YXZ复合电极的10倍相比,优化效果不是很明显。计算比较了LSCF和LSM的三相线长度,900℃时,LSM电极的三相线长度为0.12μm,而LSCF电极为8.6μm。也就是说,对LSM电极,电化学反应基本就发生在电极/电解质界面的几个分子层内,而LSCF电极反应可以深入到电极内部近10个微米的范围内。当添加GDC制成LSCF-GDC复合电极后,三相线长度基本不变。而YSZ掺混的LSM-YSZ复合电极三相反应线长度由原来的0.12μm提高到了6.5μm。由三相线长度及其变化可以初步解释LSM和LSCF电极在分别使用YSZ和GDC掺混后性能的变化趋势。根据极化电阻与温度的阿累尼乌斯关系,可计算相应电极上氧还原反应的表观活化能分别为LSCF电极的1.53eV(P.Murray,1.63eV),LSCF-GDC20复合电极的1.54eV(P. Murray ,1.62eV),LSCF-GDC40复合电极的1.47eV(P.Murray,1.69eV),LSCF-GDC60复合电极的1.53eV(P.Murray,1.30eV),LSCF-GDC80复合电极的1.49eV,与P.Murray研究结果接近。研究发现,随着温度升高,交换电流密度增大。温度每升高10K,交换电流密度就增大1～3倍,与范霍夫(Vant Hoff)近似规律有类似规律。随着GDC掺混量的增加,交换电流密度呈先升后降趋势。800℃下,LSCF阴极的交换电流密度69.14 mA/cm2,相应复合电极的交换电流密度分别为147.66mA/cm2,77.90mA/cm2,57.19mA/cm2,28.79mA/cm2。3)研究了GDC含量对复合电极性能的影响。随着GDC的增加,极化电阻先降后升。当GDC掺杂量在20～40%时,复合电极具有最低的极化电阻。交换电流密度随GDC含量的变化与极化电阻有类似的规律。不同的是,交换电流的最大值出现在20%的GDC掺杂。显然,掺混一定的GDC可提高电极性能,其最佳GDC掺杂量在20～40wt%之间。4)从理论和实验两个方面对偏置电压影响进行了分析。研究发现,电解质电阻和导电活化能基本不随偏置电压的变化而变化;在大偏置电压下,偏置电压与电极极化电阻的对数(lnRP)之间具有线性关系,而在小偏置电压下,Rp基本不受偏置电压影响,同时,电极反应的导电活化能随偏置电压的增大而逐渐减小。显然,偏置电压对电化学反应的影响主要是通过影响界面双电层电场,进而影响反应活化能来实现的。也就是说,偏置电压变化只对容抗有影响,而对纯电阻没有作用。5)对于LSCF电极和LSCF-GDC复合电极,电极反应对称系数均接近0.5,反应电子数均约等于2。初步判断氧还原过程速控步骤为氧的吸附脱附步骤。更多还原

【Abstract】 Strontium and iron-doped lanthanum cobaltite materials (LSCF)are well known to have high catalytic activity towards the reduction of oxygen, high oxygen self-diffusion coefficients, and high electrical conductivity with mixed conduction that make them suitable as cathode materials. The properties of LSCF depend very much on the powder and cathode morphology besides their composition. In the sense, the synthesis method of the electrode powders plays a very important role in obtaining homogeneous and porous electrodes,which can produce significant improvements on the oxygen conducting properties and oxygen reduction activity of the cathode. In this dissertation, A novel synthesis method, microwave assisted sol-gel method, was used to synthesis the LSCF nanopowders. The properties of three different electrode like pure electrode, composite electrode and functionally graded electrode were studied and compared one another by polarization curves, electrochemical impedance spectroscopy and single cell tests. The kinetic mechanism of oxygen reduction reaction (ORR)was also studied. The major research results are as follows:(1)A novel route, microwave assisted sol-gel method (MWSG), was brought forward to synthesis LSCF naopowders; the cathode were also tried to fabricated by microwave.Microwave assisted sol-gel route was used for preparation the novel materials. By use of EDAX, X-ray diffraction and FT-IR at different temperatures, SEM and TEM, the properties and synthesized process were studied. The reaction process could be divided into some steps: at the beginning, (a)LSCF phase formation from nitrates, (b)decomposition of reactants into amorphous powders except for strontium nitrate, (c)formation of strontium carbonate from strontium nitrate and carbon bioxides from the decomposition of PVA, and at the medium and terminal stage, (d)LSCF phase formation mainly from strontium carbonate with other oxides.The effect of experiment conditions like starting concentration of mixed solution,PVA, microwave treated time and power was systematically studied by orthogonal experiment. LSCF nanopowder could be obtained at 700W for 35 min with a 38.9 m2/g for the BET surface areas and ~23 nm for the grain size. The synthesis period of 20 h usually observed for conventional heating mode is reduced to a few minutes. Thus, the MWSG method is proved to be a novel, extremely facile, time-saving and energy-efficient route to synthesize LSCF powders.(2)The properties of pure electrode, composite electrode and functionally graded electrode were studied and compared one another by polarization curves, electrochemical impedance spectroscopy.Results shows: the LSCF-GDC composite cathode is highest with a polarization resistance (Rp)of 0.03～0.06??cm2 at 800℃, followed the FGC with a Rp of 0.061～0.085??cm2, the lowest is pure cathode with a Rp of 0.13??cm2.The Rp for the LSM and LSCF cathodes at various temperatures is compared. The Rp for LSCF cathodes is as 10% lower as that of the LSM cathode. Mixed with GDC, the Rp for the LSCF composite cathode decreased to 20%~50% compared to pure LSCF cathode; for the LSM composite cathode, the Rp change to the 10% compared to pure LSCF cathode. The difference in cathode performance may be explained by the change of the varies of the TPB(three phase boundary). According to Adler et al. studies, the reactions can occur at internal surfaces away from the electrolyte for mixed conductor cathode and electrode reaction zone can extend up to 10μm from the electrolyte.The apparent activation energies for the corresponding cathode are 1.53eV(pure LSCF cathode ) ,1.54eV ( LSCF-GDC20 ) ,1.47eV ( LSCF-GDC40 ) , 1.53eV(LSCF-GDC60), 1.49eV(LSCF-GDC80)respectively. That is close to the results reported by P.Murray.The exchange current density for the corresponding cathode at 800℃are 69.14 mA/cm2 (pure LSCF cathode), 147.66 mA/cm2 (LSCF-GDC20), 77.90 mA/cm2 (LSCF-GDC40), 57.19mA/cm2 (LSCF-GDC60), 28.79mA/cm2 (LSCF-GDC80)respectively. Reslts also show that the exchange current density increase with the temperature increase. The exchange current density increase to 1~3 times When the temperature increase 10 K. The rule is similar to the Vant Hoff`s approximately law.(3)The effects of the GDC on the properties are studied. With the increase of the GDC content, the Rp firstly increases and then decreases. When added 20～40% GDC in the LSCF cathode, the Rp has a lowest value. The exchange current densities also has a highest value when added 20% GDC in LSCF composite cathode.(4)The effects of the DC bias on the properties are also studied from experiments and theory. The DC bias are in linear with the ln(Rp)under a large DC bias value, while at small value, the DC bias has no influence on the Rp. At the same time, the apparent activation energies decrease with the increase of the DC bias. So, the DC bias affects electric field of the double layer at electrode-electrolyte interfaces, and then affects the activation energy. That is, the DC bias can only affects the capacitance resistances, but have no effects on the ohm resistances.(5)The symmetry factors are near to 0.5, and the charge transfer numbers of the rate-determining step (RDS)are 2 to all cathodes. These phenomena can only be explained by the existence of intermediate species and multi-step reactions, and it is narrow down to the possible mechanisms of the oxygen reduction reaction to three mechanisms: (1)the direct exchange of oxygen vacancies at electrode-electrolyte interface;(2)reduction of intermediately adsorbed oxygen;(3)the adsorption and desorption step of oxygen. The mathematics models are used to explain the mechanisms. Compared to the experiment results, it is found that: the polarization curves simulated by the adsorption and desorption step of oxygen are in agreement with the experimental results at low overpotentials and a little lower at high overpotentials. so we can draw an conclusion that the ORR RDS may be the adsorption and desorption step of oxygen at the LSCF and LSCF-GDC cathode.更多还原