The Optimization And Electrochemical Performance Of LaBiMn₂O₆ Cathode Materials With Double Perovskite Structure

Solid oxide fuel cells（SOFCs）are attracting attention due to their high current density,wide fuel adaptability,and no pollution.They can be used in power generation,cogeneration,transportation,space aerospace and many other fields,and be known as the 21st century green energy.As an important part of SOFC,cathode has a great influence on the overall performance of SOFC.Therefore,to develop cathodes with high catalytic activity is one of the important factors to accelerate the application of solid oxide fuel cells.In recent years,double perovskite-type oxides have received widespread attention due to their excellent physical properties.In this thesis,the double perovskite LaBiMn₂O_6-δ（LBM）,LaBiMn₂O₆-xSm_0.2Ce_0.8O_1.9,LaBiMn_2-xNi_xO_6-δ,LaBiMn_1.8Ni_0.2O_6-δ-xSm_0.2Ce_0.8O_1.9.9 and LaBiMn_2-xCo_xO_6-δwere synthesized,and their high temperature electro-chemical properties were studied.Firstly,LaBiMn₂O₆ was synthesized by glycine-nitrate combustion method.Studies proved that the oxide is an orthorhombic double perovskite with a space group of P n m a.The material has excellent high temperature chemical compatibility and thermal compatibility with the electrolyte SDC.The results of impedance spectroscopy indicated that the polarization resistance of LaBiMn₂O₆ cathode was 0.71Ω·cm² at 700^oC in air.The output power test of the electrolyte supported cell showed that the power density of LaBiMn₂O₆ was only 122 mW·cm^-22 at 700 ^oC.In order to improve the electrochemical properties of the material,LaBiMn₂O₆-xSm_0.2Ce_0.8O_1.9.9 composite cathode was prepared.The test showed that the polarization resistance of the LaBiMn₂O₆-30%Sm_0.2Ce_0.8O_1.9.9 composite cathode has a minimum value of 0.186Ω·cm² at 700 ^o C and a power density of 234 mW·cm^-2.The improvement of electrochemical performance is mainly due to the fact that the recombination of SDC increases the three-phase reaction interface of the electrode and promotes the oxygen reduction reaction process.After 90 hours of electrochemical stability test,the output power of LaBiMn₂O₆-30Sm_0.2Ce_0.8O_1.9.9 composite cathode was stable at 230 mW·cm^-2,and the performance attenuation was about 4%.LaBiMn_2-xNi_xO_6-δ（LBMN_x,x=0.1-0.3）were synthesized by glycine-nitrate method.The results show that the Ni-doped LBM material has an orthorhombic structure and the space group is P n m a.With the increase of nickel doping amount,the unit cell volume of the material becomes larger,and the thermal expansion coefficient and electrical conductivity also increase.In the LBMN_x material,LaBiMn_1.7Ni_0.3O_6-δhas a total conductivity of 25.67 S·cm^-11 in air at 800 ^oC and has good high temperature chemical compatibility and thermal compatibility with electrolyte SDC.The results of impedance spectroscopy indicated that the control step of oxygen reduction reaction was the adsorption-dissociation process of oxygen molecules.The single cell output power test showed that the power density of LaBiMn_1.8Ni_0.2O_6-δwas 157 mW·cm^-22 at700 ^oC.The optimization of the performance of the electrode material is mainly due to the introduction of excessive metal elements in the low-cost state,which causes the material to generate more oxygen vacancies,promotes the transport of oxygen ions,dissociative adsorption of oxygen,and provides more reaction sites for the diffusion of oxygen at the cathode interface.LaBiMn_1.8Ni_0.2O_6-δhas good high temperature chemical compatibility and thermal matching with electrolyte SDC.The two were mixed to form a LaBiMn_1.8Ni_0.2O_6-δ-30Sm_0.2Ce_0.8O_1.9.9 composite cathode,and the polarization resistance was only 0.11Ω·cm² under air conditions of 700°C.Tests have shown that the bulk charge transfer process is the rate-control step of the oxygen reduction reaction.The single cell output power test showed that the power density of the LaBiMn_1.8Ni_0.2O_6-δ-30Sm_0.2Ce_0.8O_1.9.9 composite cathode reached 236 mW·cm^-2,which was double that of LBM.LaBiMn_2-xCo_xO_6-δ（x=0.1-0.3）series cathode materials were synthesized by glycine-nitrate method.The synthesized material has an orthorhombic double perovskite crystal structure with a space group of P n m a.The thermal expansion coefficient of LaBiMn_2-x-x Co_xO_6-δincreases slightly with the increase of Co incorporation,but it has good high temperature chemical compatibility and thermal matching with electrolyte SDC.The results of impedance spectroscopy indicated that the polarization resistance of LaBiMn_1.7Co_0.3O_6-δcathode in air atmosphere was 0.29Ω·cm² at 700°C.The single cell output power test results show that the power density of LaBiMn_1.7Co_0.3O_6-δcathode at 700°C reaches 201 mW·cm^-2,which indicates that LaBiMn_1.7Co_0.3O_6-δcathode has certain potential research value.