Synthesis and characterization of B-site Ni-doped perovskites Sr ₂Fe_1.5-xNi_xMo_0.5O _6-δ (x = 0, 0.05, 0.1, 0.2, 0.4) as cathodes for SOFCs

Sr₂Fe_1.5-xNi_xMo_0.5O _6-δ (x = 0, 0.05, 0.1, 0.2, 0.4) (SFNM) materials have been synthesized by a sol-gel combustion method and studied towards application as cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs). The crystal structure, microstructure, thermal expansion, element valence, conductivity and electrochemical properties have been characterized as a function of Ni content. The symmetrical structure of the cubic lattice in perovskite oxides is confirmed. SFNM powders possess the 3D interconnected network microstructure composed of nanoparticles. An increasing Ni substitution results in the unit cell shrinkage and the increase of thermal expansion coefficient (TEC). Furthermore, X-ray photoelectron spectroscopy (XPS) analysis shows that Ni basically exhibits a low oxidation state (Ni²⁺). Doped Ni²⁺ affects the equilibrium between Fe³⁺/Mo⁵⁺ and Fe²⁺/Mo⁶⁺, which is directly related to the conductivity. The SFNM conductivity was apparently improved, reaching 60 S cm^-1 at 450 °C when x = 0.1, which is more than twice that of the Sr₂Fe_1.5Mo_0.5O_6-δ (SFM) sample. In addition, Sr₂Fe_1.4Ni_0.1Mo _0.5O_6-δ (SFN_0.1M) cathodes showed excellent electrochemical performance and lowest interface polarization resistance (R_p). The R_p of the SFN_0.1M cathode was approximately 50% of that of the SFM cathode. Moreover, the maximum power densities of a single cell based on the SFN_0.1M cathode were 0.92, 1.27 W cm^-2 at 700, 750 °C, respectively. The SFNM material is a type of potential cathode for IT-SOFCs.