First Principles Study On The La_1-xSr_xMnO_3–?Coated Ba_1–xSr_xCo_1–yFe_yO_3–?Cathode For Solid Oxide Fuel Cells

Solid oxide fuel cell?SOFC?is considered to be one of the most promising new energy technologies due to its high energy conversion efficiency,pollution-free and fuel fexibility.Reducing operating temperature has become a hot topic in current research to fulfill the requirement of the long-term stable operation of solid oxide fuel cell systems.The low catalytic activity of traditional SOFC cathode materials at intermediate temperature restricts the development of SOFC technology.Perovskite-type cathode,such as La₁–x SrxMnO₃–??LSM?,La_1-xSr_xCo_1-yFe_yO_3-??LSCF?and Ba₁–x SrxCo₁–y FeyO₃–??BSCF?,has been widely used as intermediate temperature SOFC cathode.BSCF has attracted much attention due to its high electrocatalytic activity,but its stability is poor.LSM is a commonly used high temperature SOFC cathode material with good stability,whereas the ionic conductivity of LSM at intermediate temperature becomes very low.A core-shell structured composite cathode LSM coated BSCF?LSM@BSCF?can achieve dual optimization of high power output and long-term stability.By the density functional theory calculation of LSM@BSCF,the binding energy between the La/Sr-O plane of LSM?100?and the Co/Fe-O plane of BSCF?100?is-11.72eV,lower than that of other binding,suggesting that such a combination is the most possible interface structure.The LSM@BSCF surface phase has a good adsorption for O₂.The adsorbed oxygen and the matrix are chemically bonded and there is charge transfer.The oxygen vacancy formation energy of LSM is 1.73 eV,and 0.06 eV for BSCF,thus the oxygen vacancy is more likely to be formed in BSCF,which also accounts for better oxygen ion conductivity of BSCF than LSM.BSCF incorporation can enhance the oxygen-vacancy concentration of LSM.CO₂ is hard to be adsorbed onto the LSM@BSCF surface.Thus,the structure modification can protect the BSCF cathode from CO₂ poisoning.The oxygen diffusion barrier from LSM to BSCF at the interface is determined to be 0.66 eV,indicating an easy oxygen ionic transport between the interface.The calculation results theoretically reveal the reason why the core-shell cathode has high catalytic activity and high stability.LSCF has better stability than BSCF at intermediate temperature.The ORR performance of LSCF can be improved by doping appropriate proportion of F at the anion position of LSCF?LSCFF?.The anion doping model of SOFC cathode has been constructed to calculate and analyse the mechanism of F doping in LSCFF.The process of oxygen adsorption,oxygen vacancy and oxygen diffusion have been investigated before and after F doping.The result shows both LSCF and LSCFF have good adsorption of oxygen,and F doping has an improvement on the oxygen adsorption capacity of LSCF.Doping of an appropriate proportion of F can increase the ORR activity of LSCF by reducing the oxygen ion migration barrier,but the oxygen vacancy concentration in LSCF can be reduced by doping excessive F,which will reduce the performance of LSCF cathode.