中温固体氧化物燃料电池La_(1-x)Ba_(x)FeO_(3-δ)阴极材料的制备及性能表征

Preparation and Characterization of La_(1-x)Ba_(x)FeO_(3-δ) Cathode Materials for Intermediate-Temperature Solid Oxide Fuel Cell

采用柠檬酸-硝酸盐自蔓延燃烧法合成了中温固体氧化物燃料电池阴极材料La_(1-x)Ba_(x)FeO_(3-δ)(LBF)(x=0,0.1,0.2,0.3,0.4,0.5)。通过X射线衍射(XRD)、直流四端引线法和热膨胀仪分别对LBF的晶体结构、电导率和热膨胀系数进行了研究,分析了Ba掺杂量对材料性能的影响。同时制备了电解质Gd_(0.1)Ce_(0.9)O_(2-δ)(GDC),构建对称电池LBF/GDC/LBF并进行了电化学性能测试,用扫描电子显微镜观察(SEM)对称电池的断面微结构,交流阻抗谱测试极化阻抗。结果表明,合成的阴极材料LBF均为钙钛矿结构,A位Ba掺杂提高了材料的电导率,同时增大了材料的热膨胀系数,改善了阴极材料与电解质GDC的热膨胀匹配性。SEM结果显示对称电池具有理想的微观结构,阴极与电解质之间的界面展现出良好的烧结结合;电化学交流阻抗分析结果表明,A位Ba掺杂可以降低阴极的极化电阻,当Ba^(2+)掺杂量为x=0.4时,LBF-0.4具有最小的极化电阻,在800℃空气气氛下极化电阻为0.127Ω·cm^(2),电荷转移阻抗是电池极化阻抗的主要来源。实验结果表明LBF材料是一种电化学性能较为优良的中温固体氧化物燃料电池(IT-SOFC)阴极材料。

As a promising energy conversion device,solid oxide fuel cell(SOFC)has become one of the most promising green energy sources in the future due to its high efficiency,low pollution,high energy conversion rate,strong fuel adaptability and environmental friendliness.The traditional SOFC operated at 800~1000℃,which lead to the problems of unsuitable thermal matching between mate‐rials and instability of electrode.At the same time,the cost of battery operating at high temperature is high and the material selection was difficult.The exploration of cathode materials with good electrocatalytic activity and low-cost at intermediate temperature(600~800℃)is crucial for improving the performance of solid oxide fuel cells(SOFC).LaFeO_(3)with perovskite structure as cathodes have at‐tracted researchers’interest due to the advantages of high ionic conductivity,good chemical and thermal stability and low cost.In this paper,LaFeO_(3)(LF)was used as the research object,which was modified by single doping of A-sites.Cathode materials La_(1-x)Ba_(x )FeO_(3-δ)(LBF)(x=0,0.1,0.2,0.3,0.4,0.5)of medium-temperature solid oxide fuel cell were synthesized by a citric acid-nitrates self-prop‐agating combustion method.The crystal structure,electrical conductivity and thermal expansion coefficient of the LBF cathode materi‐al were studied by X-ray diffraction(XRD),direct current four-terminal method and thermal dilatometer,respectively.And the ef‐fects of Ba-doping on performance and properties of materials were analyzed.The solid electrolyte Gd_(0.1)Ce_(0.9)O_(2-δ)(GDC)was prepared using the same method simultaneously,and a symmetrical cell of LBF/GDC/LBF was fabricated and the electrochemical performance of the cell was analyzed.The cross-section microstructure and polarization of the symmetrical cell were detected using a scanning elec‐tron microscope(SEM)and AC impedance spectroscopy.XRD results showed that the undoped LaFeO_(3)was judged to have formed a single orthorhombic perovskite structure.Two phases of LaFeO_(3)and BaFeO_(3)were observed after calcination of Ba^(2+)doped LBF(0.2≤x≤0.4).Moreover,it showed that diffraction peaks shifted to lower angles with increasing content of Ba^(2+),this can be explained as the larger ionic radius of Ba^(2+)compared with that of La^(3+)caused distortion of the crystal lattice of the material.When the doping amount x=0.5,BaO impurity was generated,which would adversely affect the electrochemical performance of the material.The thermal expan‐sion coefficient(TEC),electrical conductivity gradually increased with A-site Ba doping.On the one hand,the increase of electrical conductivity was due to the fact that in the LBF material,A-site was doped with low-valent alkaline earth ions Ba^(2+).Based on the elec‐trical neutrality requirement of the system,part of the Fe^(3+)at the B-site was converted to Fe^(4+),generating electron holes.On the other hand,according to the above XRD results,it was known that the formation of BaFeO_(3)phase played a certain role in promoting conduc‐tivity.When the doping content was less than 0.2,the decrease of conductivity wasn’t observed in the range of medium temperature be‐cause the oxygen loss temperature was higher than 800℃.When the doping amount x=0.4,the maximum value about 140 S·cm^(-1)at550℃was reached,which met the requirements of the conductivity of cathode materials for fuel cells.It could be seen in the ln(σT)vs.1000/T results that the curve of the high temperature section deviated from the linear relationship,which is mainly due to the in‐creased of lattice oxygen loss at high temperature,which increased the compensation effect of ions and weakened the contribution of small polaron transition to conductivity.It was revealed that the doping of Ba^(2+)reduced the activation energy of carrier migration in the low temperature section.It was shown that when the Ba^(2+)doping amount were 0.1 and 0.4,the average thermal expansion coefficients were 11.5×10^(-6)and 13.5×10^(-6)K^(-1),respectively.The thermal expansion matching of the cathode material to the electrolyte GDC was im‐proved and ensured the integrity of the battery in the process of preparation and operation.SEM image revealed that the symmetrical cell has an ideal microstructure and a good adhesion among cathode and electrolyte.It showed that they had good thermal compatibili‐ty,which was conducive to the transport of carriers and the diffusion of gas,and reduced the interface polarization,so as to improve the battery performance.A good porous structure was observed,which provided the necessary channel for oxygen and increased the tri‐ple-phase boundary.The analysis of electrochemical AC impedance spectra indicated that Ba-doping at A site reduced the polarization resistance of the cathode.When the Ba^(2+)doping amount was x=0.4,LBF-0.4 had the lowest polarization resistance of 0.127Ω·cm^(2)at800°C in air.The reason might be that the doping of Ba^(2+)increased the oxygen vacancy concentration and improved the electrocatalytic activity of the electrode material.According to the fitting results,the R_(LF)was higher than the R_(HF)for all of the investigated samples,which indicated that the charge transfer,gas adsorption and desorption processes were the step-limiting process of the electrochemical reaction on the electrode.The encouraging results indicated that the LBF was a promising cathode candidate for IT-SOFCs.