Ion Conduction of BaPrO3-δ Thin Film with Mixed Valence State for SOFC Anode Electrode

Abstract

The BaPrO3 (BPO) is expected as a candidate for anode materials in solid oxide fuel cell (SOFC) using oxide protonics material as a solid electrolyte, due to a possible presence of mixed conductivity or holes (electrons)and protons. BPO ceramics has the possibility of electron-ion mixed conduction because of auto-ionization of Pr+4/Pr+3. Therefore, the electrode of fuel cell should be used as thin film form. However, details of high mixed conduction of BPO thin film have not been clarified thus far. In this study, we have deposited the BPO thin films on Al2O3 (0001) single crystal and Nb-doped SrTiO3 substrates by radio frequency (RF) magnetron sputtering. The deposition pressure and substrate temperature were set at 4.0×10-3 Torr and 500 °C, respectively. In order to obtain the high mixed conduction of the thin film, the influence of film thickness was probed between 60 and 170 nm. The BPO thin film was characterized by XRD, AC impedance method, X-ray photoemission spectroscopy (XPS), and X-ray absorption spectroscopy (XAS). The AC impedance was measured by means of a frequency response analyzer in the frequency range of 0.1 Hz – 32 MHz. Impedance spectra were collected at temperatures between 200 °C and 600 °C with a step of 25 °C. The measurement atmosphere used dry Air (the mix gas of O2 and Ar) and wet Air (the mix gas of O2 and Ar and H2O). The electrode made a Pt comb type electrode by RF magnetron sputtering. In order to probe conducting carriers, the conductivity was measured by changing by gas partial pressure. The prepared thin films exhibit highly a-axis orientation. The lattice constant of a-axis decreases with increasing film thickness. We conformed the electron structure by XPS and RPES in order to prove the mixed valent states of Pr4+ and Pr3+. This originates the changes of lattice constant with film thickness. The electrical conductivity increases and the activation energy decreases with increasing film thickness. The activation energy does not match with the energy separation between the Fermi level and the top of valence band. However, the conductivity at 300 °C depends on the oxygen partial pressure, indicating the existence of electron-oxygen ion mixed conduction. The thin films below 100 nm have proton conductivity in wet atmosphere. The above results indicates that the thin film has proton conduction in addition to electron-oxygen ion mixed conduction.