New Material Design of Electron-Proton Mixed Conductor Ru-Doped BaCe0.90Y0.10O3-
d Thin Film As Fuel Cell Anode Electrode
Authors: Ochi Masanori , Tsuchiya Takashi , Higuchi Tohru 
Published: February 2020

Perovskite oxide Y3+-doped BaCeO3 (BaCe1-x
Y
x
O3-
d: BCY) is a promising material as an electrolyte of solid oxide fuel cells (SOFCs) that operate in the intermediate- temperature (IMT) region.  The electrical conductivity of the bulk crystal exhibits a high proton conductivity of ~7.0×10-2 S·cm-1 at 800 ºC and ~1.0×10-2 S·cm-1at 400 ºC in a wet atmosphere.  The proton adsorption and conduction mechanisms have been extensively studied by theoretical and experimental approaches.

 In order to realize hydrogen separation membranes for hydrogen production in the high-temperature region, Matsumoto and co-workers have developed a Ru5+-doped BCY (BCRY) ceramic that exhibits the electron-proton mixed conduction in moist reducing atmosphere at approximately 800 ºC.  When the Ru5+ concentration is in the range of  5~10 mol%, BCRY ceramics exhibit the mixed conduction of 3.0~6.0×10-3 S·cm-1.  Although such a mixed conductor can be used as an anode electrode of SOFCs, BCRY thin films have not been realized thus far.  Therefore, understanding of the detailed electrical and structural properties of BCRY thin films is most important for the material design of SOFC electrodes.

 In this study, we present the structural and electrical properties at the surface and bulk regions of BCRY thin films and discuss the role of Ru5+substitution in terms of electron structure.

 The BCRY thin film was deposited on Al2O3 and Nb-doped SrTiO3 (Nb-STO) substrates by RF magnetron sputtering.  The flow rate of Ar gas, the deposition pressure, and substrate temperature were set at 20 sccm, 5.0×10-3 Torr and 600 ºC, respectively.  The film thicknesses were changed between ~100 and ~477 nm.  The structural property of the BCRY thin films were characterized as a function of film thickness 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 200 to 600 ºC.  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 (P
O2).

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 h
n= 700 eV.

 The a- and c-axes-oriented BaCe0.85Ru0.05Y0.10O3-
d thin films have been deposited by RF magnetron sputtering.  The thin films have mixed valence states of Ce4+ and Ce3+.  The activation energy for the conductivity of film thicker than 200 nm at the surface region and bulk region are 0.26 eV and 0.60 eV, respectively.  The thin films exhibit ion conduction at the bulk region and electron-ion mixed conduction at the surface region.  Proton conductions are also observed in the surface state and bulk state.  The Fermi levels (E
F) locate at the middle position in the band gap region, although E
F of the BaCe0.90Y0.10O3-
d thin films locates on the valence band side.  These results indicate that the Ru5+ions and protons act as donor ions in BCRY thin films.