Electrochemical Performance Of La_0.33Ba_0.67Co_0.7Fe_0.3O_3-δ Perovskite Cathode Material

Intermediate-temperature solid oxide fuel cells（IT-SOFCs）have been considered to be one of the most promising devices that directly convert chemical energy into the electricity,and the electrochemical performance of the cathodes dominates their output performance.Nowadays,mixed ionic and electronic conductors（MIECs）have been the effective cathode materials.As a kind of MIECs,the perovskite oxides have attracted much attention because of flexible element composition,stable crystal structure,high electrical conductivity,and excellent electrocatalytic oxygen reduction reaction（ORR）activity.In this thesis,La_0.33Ba_0.67Co_1-xFe_xO_3-δ（x=0-0.4）perovskite oxides have been synthesized by glycine-nitrate firing method.The as-synthesized La_0.33Ba_0.67Co_1-xFe_xO_3-δhas cubic perovskite structure with space group Pm-3m.X-ray photoelectron spectroscopy（XPS）demonstrates that La_0.33Ba_0.67Co_0.7Fe_0.3O_3-δ（LBCF0.3）has abundant surface oxygen vacancies and adsorbed oxygen,facilitating the oxygen surface exchange and accelerating the ORR kinetics.As expected,the LBCF0.3cathode possesses the great ORR activity,as evidenced by the polarization resistance（R_p）of 0.068Ωcm² at 700 ^oC.The maximum power density of 278 m W cm^-2 is achieved in an electrolyte-supported fuel cell with a configuration of Ni-GDC|GDC|LBCF0.3.Rare earth ion-doped La_0.33Ba_0.62-xLn_0.05Co_0.7Fe_0.3O_3-δ（Ln=Pr、Nd、Sm、Gd）perovskite oxides have been successfully synthesized.We systematically study the impacts of A-site rare earth doping on their electrochemical properties.The results indicate that La_0.33Ba_0.62Gd_0.05Co_0.7Fe_0.3O_3-δ（LBGCF）has the largest concentration of oxygen vacancies among all compositions.The electrical conductivity and oxygen surface exchange coefficient(k_chem)of LBGCF reach 199 S cm^-1 and 5.47×10^-3 cm s^-1at 700 ^oC,respectively.Distribution of relaxation time（DRT）analysis reveals that the Gd doping promotes the change transfer and oxygen surface exchange rate.The LBGCF cathode exhibits the highest electrocatalytic activity,as evidenced by the R_p value of0.035Ωcm^-2 at 700 ^oC.The LBGCF cathode-based electrolyte-supported fuel cell delivers a maximum power density of 371 m W cm^-2.Moreover,LBGCF shows superior CO₂ durability to LBCF0.3.To improve electrochemical performance of the LBCF0.3 cathode,mechanically mixing the LBCF0.3 and Ce_0.9Gd_0.1O_1.95（GDC）powder to obtain homogeneous La_0.33Ba_0.67Co_0.7Fe_0.3O_3-δ-x GDC（x=10-50 wt%）composite cathode materials.Among this series of materials,the La_0.33Ba_0.67Co_0.7Fe_0.3O_3-δ-40GDC（LBCF-40GDC）cathode has the highest ORR activity.The R_p value of LBCF-40GDC reaches as low as 0.025Ωcm² at 700 ^oC.The maximum power density of 375 m W cm^-2is achieved in an electrolyte-supported fuel cell with a configuration of Ni-GDC|GDC|LBCF-40.The rate-limiting step for ORR is determined to be the mass transport process,as confirmed by impedance spectra under different oxygen partial pressures.Furthermore,this composite cathode has a good CO₂ durability.