| Cathode material is one of the most important coments in intermediate-tmeperature solid oxide fuel cells?IT-SOFCs?.Cathode materials with a highly electrochemical performance have a significant influence on improved power in SOFCs at low temperatures.Layered perovskite LnBaCo2O6-?oxides have been widely investigated as potential cathode materials because of their high oxygen ionic diffusion coefficient?D*?and surface exchange coefficient?k*?.Additionally,there are a certain number of oxygen vacancy in LnBaCo2O6-?materials,good compatibility between cathode and electrode,and the electrochemical conductivity of LnBa Co2O6-?is higher than other perovskite materials,in view of this reason,these materials have been caused great attention for intermediate-temperature solid oxide fuel cell.Among all the LnBa Co2O6-?materials,EuBaCo2O6-?cathodes have a superior electrochemical performance and lower thermal expansion coefficient,thus being considered as a potential cathode material.In order to improve the properities of EuBaCo2O6-?,A-site Ba deficiency,A-site calcium doping and composite electrolyte Sm0.2Ce1.8O1.9?SDC?are studied.The EuBa1-xCo2O6-??x=0-0.06?oxides were synthesized by EDTA-citrate method.The synthesized powders hold orthorhombic double-layered perovskite oxide at room temperature,the space group is Pmmm.The results show that these oxides have good chemical compatibility with CGO at high temperature.The Ba-deficiency leads to an increase in the electrical conductivity,reduction of thermal expansion coefficient and the increase of concentration of oxygen vacancies,high oxygen ion transport capacity is beneficial to the improvement of the electrochemical performance of cathode materials.The results of electrochemical tests showed that EuBa0.98Co2O6-?have lowest cathodic polarization resistance,the polarization resistance of the EuBa0.98Co2O6-?materials is0.107?cm2 at 700 ?.Ni-YSZ anode-supported cell(Ni-YSZ/YSZ/CGO/EB0.98CO)delivers a maximum power density of 505 mW cm-2.The rate-limiting steps of the surface reaction of these cathode material determined by the impedance spectroscopy under different oxygen partial pressure conditions are the charge transfer and the dissociation of adsorbed oxygen molecule.In order to improve the electrochemical performance,the material with the lowest polarization resistance EB0.98CO and the electrolyte Sm0.2Ce1.8O1.9?SDC?were composited and their effects on the electrochemical performance were investigated.Result show that the thermal expansion coefficient decreases with the increase in SDC content.When the amount of SDC was 20%,thermal expansion coefficient decreased to16.3×10-66 K-11 at the temperature range of 50-800 o C.EuBa0.98Co2O6-?-20SDC have the lowest polarization resistance at 700 ?,and the polarization resistance was only 0.027?cm2,the resistance has been significantly reduced comperaed to EuBa0.98Co2O6-?.A maximum power density of 969 m W cm-22 is achieved on Ni-YSZ anode supported single cell(Ni-YSZ/YSZ/CGO/EB0.98CO-20SDC)at 700 ?.The results of impedance spectroscopy measured under different oxygen partial pressures show that the rate-limiting steps are oxygen ions transfer to the electrode and the charge transfer process.Eu1-xCaxBaCo2O6-??x=0-0.25?cathode materials were synthesized by solid-state reaction,according to the result,the thermal expansion coefficient reduced with Ca doping in the material and the oxygen vacancy are improved.Eu0.8Ca0.2BaCO2O6-?posseses the lowest cathode polarization resistance of 0.03?cm2 at 700 ?,and the polarization resistance of the cathode decreases 72%compared to EuBaCO2O6-?.The results of the pressure test show that the rate-limiting steps of the cathode material of Eu0.8Ca0.2BaCo2O6-?are charge transfer process and oxygen molecule adsorption process.The Ni-YSZ anode-supported single cell delivers a maximum power density of941 mW cm-22 at 700 ?. |
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