Preparation And Electrochemical Stability Of Ba Doped La_0.6Sr_0.4Co_0.2Fe_0.8O_3-? Cathode Materials

With the energy crisis and environmental problems brought about by the decrease of fossil energy and the burning of non-renewable energy sources,the search for efficient and clean energy utilization methods has become the focus of worldwide attention.Solid oxide fuel cell?SOFC?has a wide range of application prospects because of its high power generation efficiency,clean emissions and no pollution.La_0.6Sr_0.4Co_0.2Fe_0.8O_3-??LSCF?cathodes is the most suitable as cathode materials for IT-SOFC due to its favorable mixed conductivity of oxygen ion/electron in the range of 600-800?.However,there are still some problems in the electrochemical performance of the LSCF cathode materials when operating at intermediate temperature,and its polarization resistance needs to be further improved.Moreover,the LSCF cathode has poor electrochemical stability over time at intermediate temperature?600-800??,these problems limit the wide application of LSCF cathode material in SOFC.In this paper,the electrochemical modification of LSCF cathode material La³⁺at the A site by Ba²⁺was replaced and the feasibility of this material as an IT-SOFC cathode material was verified.The long term operation tests under Pt and Fe-Cr alloy connectors were performed and the effects of Ba²⁺doping on the electrochemical stability of LSCF cathodes were compared in these two cases,the results show that:La_0.6-xBa_xSr_0.4Co_0.2Fe_0.8O_3-??x=0,0.05,0.10,0.15,0.20?series cathode materials were synthesized by sol-gel method,and their phase structure and electrochemical performance were tested.The LBSCF?x=0-0.20?cathode powders calcined at 1000?has formed a single oblique hexagonal perovskite structure,and no other heterogeneous phase formed.The doping of Ba²⁺can effectively improves the conductivity of the LSCF cathode.The cathode conductivity first increases and then decreases with the increase of the doping amount of Ba²⁺.The conductivity of LBSCF cathode is the highest when the doping amount x=0.10.The conductivity up to 392 Scm^-1,nearly double the performance compared to the undoped cathode conductivity(204 Scm^-1)at 800?.Doping the Ba²⁺can significantly improves the electro-chemical performance of LSCF.In particular,the doping of Ba²⁺greatly improves the transport capacity of O^2-in the cathode and the adsorption,dissociation,and diffusion transport capacity of oxygen on the electrode surface.The polarization resistance of the LBSCF?x=0.20?cathode is 0.04?cm²,which is nearly 82%lower than that of an undoped LSCF cathode?0.22?cm²?at 800?.Adopting Ni-SDC as the anode,SDC as the electrolyte,and LBSCF as the cathode to prepare the anode-supported single cell.The maximum power density of Ni-SDC?SDC?LBSCF?x=0.20?single cells is 0.7 W/cm² at 750?,improved by nearly 30%compared to Ni-SDC?SDC?LSCF single cells?0.54 W/cm²?performance at the same temperature.The stability of the cathode materials under different connectors was measured by electro-chemical AC impedance.The results show that:the polarized resistance of LBSCF?x=0.20?decreased from 0.05?cm² to 0.05?cm² under the Pt connector after continuous testing at800?for 7200 minutes,its almost no attenuation of polarization resistance compared with LSCF?0.22?cm²?0.38?cm²?;The polarization resistance of LBSCF?x=0.20?cathode increased from 0.04?cm² to 0.05?cm² after continuous testing under Fe-Cr alloy connector at800?for 7200 minutes.The electrochemical stability performance is significantly improved,compared with LSCF cathode?0.26?cm²?2.18?cm²?.XRD,SEM,and Raman spectroscopy analysis results show that the LSCF cathode grains grow and the cathode porosity decreases by SEM result in an increase in the LSCF cathode polarization resistance under a Pt connector after testing at 800?for 7200 minutes.However,the cathode materials doped with Ba²⁺has no significant difference in grains before and after the test,indicating that Ba²⁺can inhibit the growth of LSCF cathode grains in a certain extent;The spinellike surfaces structure SrCrO₄,Cr₂O₃ insulation phase were formed on the surface of the LSCF cathode under Fe-Cr alloy connectors after 7200 minutes testing,and LBSCF?x=0.10,x=0.20?cathode not only found the formation of SrCrO₄,Cr₂O₃,but also formed the BaCrO₄ phase.Because the electronic conductivity of BaCrO₄ is higher than that of SrCrO₄,the increases in the cathodic polarization resistance is relatively reduced,regardless of whether the Pt connector or the Fe-Cr alloy connector testing at 700?and the amount of the insulating phase in the cathode is also relatively reduced.It shows that reducing the operating temperature of the cells can reduce the attenuation of the cathode materials.Based on the above analysis,it is concluded that the LBSCF cathode material has excellent electrochemical performance,both in Pt connector or Fe-Cr alloy connection,and has the potential to become the key to the future commercial application of SOFC cathode materials.