Preparation And Performance Research Of Electrolyte And Cathode Materials For Intermediate-Temperature Solid Oxide Fuel Cell

Solid oxide fuel cells are a type of fuel cell that features the use of solid oxides as electrolytes.Compared to other types of fuel cells,they operate at higher temperatures（600°C to 800°C）and therefore do not require expensive platinum metal,which is susceptible to carbon monoxide poisoning,as the catalyst.However,the higher operating temperature makes it a cost and safety barrier to commercialization.To further reduce the operating temperature of solid oxide fuel cells,it is necessary to develop a new type of electrolyte material with high oxygen ion conductivity at intermediate temperatures.The development of novel cathode material is also critical for decreasing the high polarization resistance due to lower operating temperature.Recently,NBT(Na_0.5Bi_0.5Ti O₃),originally a well-known lead-free piezoelectric material,has been regarded as a potential electrolyte material candidate due to its fast oxygen ion transport within the twisted perovskite lattice.Meanwhile,there are also some mixed ionic-electronic conductors in perovskite oxides,such as LCF(La_0.65Ca_0.35Fe O_3-δ),which has good chemical and thermal compatibility because it does not contain elements such as Sr and Co,could be a promising cathode material to replace traditional LSCF.In this paper,the electrochemical properties of NBT-based electrolyte materials and LCF-based cathode materials were investigated,respectively.（1）NBT oxygen ion conductor material is taken as the research object and the solid-state reaction method is used to prepare the samples with Sr²⁺substituting Bi³⁺at the A site,Cu²⁺or Zn²⁺replacing Ti⁴⁺at the B site,and Bi deficiency in the components.The results show that:A-site and B-site doping,and Bi deficiency all increase the lattice volume and decrease the sintering temperature,and significantly improve the ionic conductivity of the material.The doping at the A site has a greater effect on the bulk conductivity,while the doping of the B site has a greater effect on the conductivity of the grain boundary.The solid solution limit of Cu²⁺and Zn²⁺in NBT-based materials is about 2mol%.NB49STC2 and NB49STZ2 exhibit the highest bulk conductivities at600°C,6.31×10^-3S/cm and 6.24×10^-3 S/cm,which are higher than 2×10^-3S/cm of commercial electrolyte YSZ,with activation energies of 0.45 e V and 0.47 e V,respectively.The doping of metal ions Cu²⁺or Zn²⁺does not lead to the electronic conductance in the materials.All the samples have good chemical compatibility with the anode material Ni O,NB49STC2 has a slight reaction phenomenon with the cathode material LSCF,while NB49STZ2 has poor chemical compatibility with it,and within both two samples,Bi³⁺and Ti⁴⁺are reduced in the reducing atmosphere.The above studies indicate that NB49STC2 can be regarded as a promising electrolyte material for intermediate-temperature solid oxide fuel cells.The doping at the B site is taken as a research object,B-site Zr⁴⁺single-doped samples are first prepared by the solid-state reaction method,and then the B-site Zr⁴⁺and Mg²⁺double-doped samples are prepared according to the specific conditions,then a series of studies are carried out on their properties.The results show that:the doping of Zr⁴⁺makes the lattice volume of the NBT larger,and the doping amount of Mg²⁺is successfully increased from 3mol%to 5mol%in the NBTZr10 system.Among the sample groups with relatively stable chemical properties,NBTZr10M5 exhibits the highest bulk conductivity of 4.23×10^-3 S/cm at 600°C,which is higher than 2×10^-3S/cm of commercial electrolyte YSZ,and activation energy of 0.46 e V.In the sample group with less stable chemical properties,NBTZr20M20 and NBTZr40M40 exhibit the highest total conductivities of 1.98×10^-2 S/cm and 4.4×10^-2 S/cm at 650°C and the activation energies at high temperature（650°C and above）are 0.44 e V and 0.36 e V,respectively,where the latter exceeds 3.15×10^-2 S/cm of commercial electrolyte GDC.The doping of Zr⁴⁺and Mg²⁺does not lead to the electronic conductance in the materials.Each sample material has good chemical compatibility with the anode material Ni O,and NBTZr10M5 and NBTZr20M20 have a slight reaction phenomenon with the cathode material LSCF.In addition,Bi³⁺and Ti⁴⁺are reduced in NBTZr10M5 in reducing atmosphere,which indicates that the doping of Zr⁴⁺does not improve its chemical properties of being easily reduced,while NBTZr20M20 was completely reduced to decomposite,and its resistance to reducibility is even worse than that of pristine NBT.The above studies show that NBTZr10M5 can be regarded as a promising electrolyte material for intermediate-temperature solid oxide fuel cells,and the higher-doped materials which have higher conductivities than that of commercial electrolytes,are extremely poor in reduction resistibility and needs further improvement or could be considered for other purposes.The novel cathode materials are regarded as a research object,solid-state reaction method with ball milling is utilized to prepare LCF6535(La_0.65Ca_0.35Fe O_3-δ),LCF73(La_0.7Ca_0.3Fe O_3-δ),BCF73(Bi_0.7Ca_0.3Fe O_3-δ),BCF82(Bi_0.8Ca_0.2Fe O_3-δ)pristine samples and LBCF(La_0.65-xBi_xCa_0.35Fe O_3-δ)series samples based on LCF6535,the electrochemical properties are studied.The results show that:The effects of the preparation methods on the crystalline size and properties of the materials are compared using manual grinding,10mm and 1mm ball milling,showing that the smaller the grain size,the better the electrochemical properties.Porous cathodes are successfully fabricated by screen printing and lowering sintering temperature,no peeling occurred between the electrolyte layer,cathode layer and gold electrode layer in the symmetrical cell after the high-temperature test,maintaining a good delamination relationship and microstructure.The number of cathodes printed layers,sintering temperature,sintering time,and sintering temperature of gold electrodes all have great effects on the performance of the symmetrical cells of cathode materials.The LBCF series materials combine the electrochemical properties of both LCF and BCF materials.When the doping amount of Bi is 0.3,the ASR of the symmetrical cell reaches 0.04Ωcm² at750°C,which is 1/3 of 0.12Ωcm² of commercial LSCF cathode materials.The LBCF3sample exhibits the lowest thermal expansion coefficient of 12.97 ppm/K,showing good thermal matching with commonly used electrolytes.The conductivity of the LCF6535 sample reaches 157 S/cm at 600°C,while LBCF3 only reaches 21 S/cm at600°C.The doping of Bi³⁺at the A site increases the ionic conductivity of the material but suppresses its electronic conductivity to some extent.LBCF3 has good chemical compatibility with commercial electrolyte GDC,but its stability in the air is not good enough as its performance drops by about 17.5%after dwelling at 600°C for 100 hours.In summary,LBCF3 is still expected to become the novel cathode material for next-generation intermediate-temperature solid oxide fuel cells.