Synthesis and Evaluation of Nb-Doped Lanthanum Strontium Ferrite Oxide Cathodes with Different Compositions for Solid Oxide Fuel Cells

The structural, electrical, and electrochemical properties of Nb-doped lanthanum strontium ferrite perovskite oxide (LSFNb) strongly depend on their chemical composition. With this in mind, in the present study, the effect of the extensive compositional changes of LSFNb perovskite oxides on the structural, electrical and electrochemical characteristics of the resulting cathodes was investigated. Moreover, LSFNb-samarium doped cerium oxide (SDC) composite was used as the cathode material and the electrochemical behavior of the cell with the aforesaid cathode was evaluated. LSFNb perovskites were synthesized through solid-state reactions and evaluated with X-ray diffraction (XRD), conductivity measurement, and electrochemical impedance spectroscopy (EIS). The XRD results corroborated that in LSFNb_x cathodes, when x > 0.1, impurity phases were formed along with perovskite structure, and the change in the ratios of other elements did not significantly affect the perovskite structure. Electrical conductivity measurement verified that in compositions with variable Nb ratios, with increasing Nb ratio to higher than 0.1, the conductivity decreased by several orders of magnitude, and in compositions with a constant Nb ratio, the conductivity increased proportionally to the Sr ratio. EIS implied that the polarization resistance (R_p) decreased with increasing Sr ratio. The R_p of La_0.5Sr_0.5Fe_0.9Nb_0.1O_2.85-30%SDC composite was as small as 0.88 Ω cm² at 800 °C, confirming the suitability of this composite as the electrode of solid oxide fuel cells.