Synthesis and characterization of B-site Ni-doped perovskites Sr2Fe1.5−xNixMo0.5O6−δ (x = 0, 0.05, 0.1, 0.2, 0.4) as cathodes for SOFCs

Sr2Fe1.5−xNixMo0.5O6−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 Sr2Fe1.5Mo0.5O6−δ (SFM) sample. In addition, Sr2Fe1.4Ni0.1Mo0.5O6−δ (SFN0.1M) cathodes showed excellent electrochemical performance and lowest interface polarization resistance (Rp). The Rp of the SFN0.1M cathode was approximately 50% of that of the SFM cathode. Moreover, the maximum power densities of a single cell based on the SFN0.1M 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.