Synthesis and characterization of Ba(Fe,Zr,Ni)O3−δ perovskites for potential electrochemical applications

Abstract

Phase relationships in the BaFe1-x-yZryNixO3-δ system were evaluated in the search for Ba-rich electrode materials with potential application in solid electrolyte cells for electrochemical NOx reduction. Co-substitutions by zirconium and nickel into the iron sublattice of barium ferrite stabilize the cubic perovskite structure with a solid solution formation range limited to (x + y) ⪅ 0.3. Characterization of BaFe0.7Zr0.3-xNixO3-δ (x = 0.10–0.20) perovskites included the determination of oxygen nonstoichiometry variations by TGA, measurements of electrical conductivity as a function of temperature and oxygen partial pressure, studies of thermal expansion, and assessment of chemical compatibility with selected solid electrolytes. BaFe0.7Zr0.3-xNixO3-δ perovskites exhibit substantial oxygen losses from the lattice above 350 °C, reaching Δδ ∼ 0.2 on heating to 1000 °C in air, with effects on electrical properties and thermochemical expansion. Electrical conductivity is p-type under oxidizing conditions, increases with Ni content, and reaches 3.5 S/cm at 400 °C. The perovskite phase is stable down to p(O2) ∼ 10‐17 at 900 °C. The average thermal expansion coefficients in air increase from (15−18)× 10‐6 K‐1 at lower temperatures to ∼ 30 × 10‐6 K‐1 above 400 °C. The optimum co-doping level was found to be x = y = 0.15 as this composition exhibits the best chemical compatibility with yttria-stabilized zirconia and BaZr0.85Y0.15O3-δ solid electrolytes.