Preparation, Characterization, and Electrochemical Properties of Pure and Composite LaNi_0.6Fe_0.4O₃-Based Cathodes for IT-SOFC

Recently LaNi 1– x Fe x O 3 materials have been indicated as good candidates for cathodes for intermediate temperature solid oxide fuel cells (IT-SOFCs), and LaNi 0.6 Fe 0.4 O 3 (LNF) showed the highest specific conductivity in the LaNi 1– x Fe x O 3 series. Here we report the results of our investigation on the performances of LNF, synthesized by the coprecipitation method. A detailed local structure analysis, performed using X-ray absorption near edge spectroscopy (XANES ) and extended X-ray absorption fine structure (EXAFS) techniques, indicated that both Fe and Ni are in the (III) oxidation state with coordination numbers of ∼6, suggesting the absence of appreciable amount of oxygen vacancies. The fresh LNF material was used as a precursor for LNF–YSZ (yttria-stabilized zirconia) and LNF–SDC (samaria-doped ceria) electrodes. In the case of the LNF–YSZ nanocomposite, even a short (5 h) treatment at 1000 °C induced a strong reaction between the cathode components, leading to the formation of an undesirable La 2 Zr 2 O 7 insulating layer. In the case of LNF–SDC, no evidences of reaction between the two components have been observed even at 1100 °C. Low area specific resistance (ASR) values were obtained by electrochemical impedance spectroscopy (EIS) for the composite LNF–SDC electrode in accordance with the good electronic conductivity of LNF and the high ionic conductivity of SDC. Pure LNF and composite LNF–SDC electrodes showed identical apparent activation energy, suggesting the presence of the same rate-determining step. However, ASR values for LNF–SDC were significantly smaller with respect to pure LNF. This is consistent with the hypothesis that the addition of SDC does not change the reaction mechanism, but it increases the triple phase boundary (TPB) length where the reaction occurs. Notably, the polarization of the electrode further improved the cathodic performance, increasing the number of active sites for the O 2 adsorption and activation.