Electron-Ion Mixed Conduction of Nd0.6Sr0.4FeO3 Cathode Electrode Thin Film for Solid Oxide Fuel Cell

Abstract

Ln1-x Sr x FeO3 (Ln=La, Nd) bulk crystal exhibits various electrical and magnetic properties by Ln substitution. In particular, Ln1-xSrxFeO3 has been extensively studied in the bulk and thin film forms due to electrochemical application of cathode electrode for Solid Oxide Fuel Cell (SOFC). However, La1-x Sr x FeO3 thin film as electrode is not practical material since the mixed conduction is low. Therefore, new cathode material with high mixed conduction and durability is necessary for realization a practical SOFC device. In this study, we have prepared the Nd0.6Sr0.4FeO3 thin film on Al2O3 substrate by RF magnetron sputtering and characterized its electrical conductivity. The Nd0.6Sr0.4FeO3 (NSFO) thin film was deposited on Al2O3 (0001) substrates by RF magnetron sputtering using a ceramics target. The NSFO ceramics target for sputtering was prepared by the solid-state reaction method. The flow rate of Ar gas, the deposition pressure , and substrate temperature were set at 0.67 sccm , 8.0×10-3 Torr and 700 ˚C, respectively. The film thickness was changed between ~42 and ~160 nm. The structural properties of the NSFO thin films were characterized by powder X-ray diffraction (XRD) analysis. The electrical conductivity was measured by the AC impedance method in dry atmosphere in a temperature region from 300 to 700 ˚C. The measurement frequency region was from 0.1 Hz to 32 MHz. In order to probe conducting carriers, the conductivity was measured by changing oxygen gas partial pressure (PO2). The electronic structure was characterized by photoemission spectroscopy (PES) and X-ray absorption spectroscopy (XAS). The energy resolutions of PES and XAS were 100 and 60 meV, respectively, at hν=1010 eV. The lattice constant of the (112)-oriented thin film increases with increasing film thickness due to relaxation of lattice distortion. The chemical composition of the thin film was confirmed by PES. The electrical conductivity depends on film thickness. The conductivity increases with film thickness in 40~112 nm and decreases in 156 nm. This result is closely related to the lattice constant. The 112 nm thick film exhibits high conductivity and low activation energy (EA) of 0.45 eV. The EA is in a good agreement with the band gap obtained by PES and XAS, indicating the electron conduction. Furthermore, the conductivity above 500 ˚C does not depend on oxygen gas partial pressure, which corresponds to ionic conduction. The above results indicate that the Nd0.6Sr0.4FeO3 thin film has the oxygen ion-electron mixed conduction.