A novel one step synthesized Co-free perovskite/brownmillerite nanocomposite for solid oxidefuel cells

Abstract

Cobalt-free perovskite oxides Pr_1−xCa_xFe_0.8Ni_0.2O₃ (PCFN) were investigated as novel cathodes for intermediate temperature solid oxide fuel cells (IT-SOFCs). Ca and Ni substitution in the PrFeO₃ material shows that a wide range of perovskites Pr_1−xCa_xFe_0.8Ni_0.2O₃ (0 < x < 0.9) can be prepared by sintering in air at 600 °C. Perovskites with 0 < x < 0.4 exhibit orthorhombic single phases (Pnma space group), whereas 0.4 < x < 0.9 show a coexistence of the perovskite and the brownmillerite-type structure (Ca₂Fe₂O₅). The structure of the polycrystalline powders was analyzed by X-ray powder diffraction, Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The analysis on the synthesized powders shows the presence of clusters formed by 30–100 nm nanoparticles. High-Resolution TEM (HRTEM) studies were carried out to confirm the existence of Ca₂Fe₂O₅. The dc four-probe measurement exhibits a total electrical conductivity, over 100 S cm⁻¹ at T ≥ 600 °C when 0 < x < 0.6, pointing out that strontium can be substituted for calcium without modifying the electrochemical properties. The Pr_0.4Ca_0.6Fe_0.8Ni_0.2O₃/Ca₂Fe₂O₅ composite cathode presents the better performance in electrochemical measurements, showing an area specific resistance value of 0.09 ohm cm² at 850 °C.