Structural and Electrochemical Characterization of NdBa1-xCo2-yFeyO5+δ as Cathode for Intermediate Temperature Solid Oxide Fuel Cells

NdBa1-xCo2-yFeyO5+δ (NBCFd) layered perovskites are investigated as cathodes in Intermediate Temperature Solid Oxide Fuel Cells. The effects of Fe doping (up to 20%) and Ba deficiency (up to 10%) are explored. The compounds are prepared via molten citrate route and characterized by XRPD, SEM, TGA and cerimetric titration. The electrochemical properties are tested via 4-probe conductivity measurement and impedance experiments on symmetric cells at varying temperature (550 °C-700 °C) and O2 pressure (5%-100% v/v). The compositional tailoring leads to activity variation and distinct oxygen reduction behavior. An ordered layered structure with tetragonal lattice (P4/mmm) is found, whose cell volume increases with Fe doping and decreases with Ba deficiency. High total conductivity (150-450 S cm-1 at 700 °C) is measured despite the hindering effect of Fe. In stoichiometric compounds, Fe doping reduces the polarization resistance, while an increase is observed in Ba-deficient samples. The impedance results are rationalized by distribution of relaxation times analysis and equivalent circuit modeling, which reveal that the steps of surface electronation and ion transfer across the electrode/electrolyte interface are determining. Acting on electronation, Fe doping influences the electrode's activity. The target performance for application is achieved at 10% Ba deficiency and 5% Fe doping (0.14 Ωcm2 at 700 °C).