Defect and electrical transport properties of Nb-doped SrTiO 3 Peter Blennow a,b, ⁎, Anke Hagen a , Kent K. Hansen a , L. Reine Wallenberg b , Mogens Mogensen a a Fuel Cells and Solid State Chemistry Department, Risø National Laboratory, Technical, University of Denmark, DK-4000 Roskilde, Denmark b nCHREM, Polymer and Materials Chemistry, Kemicentrum, Lund University, P.O. Box, 124, SE-221 00 Lund, Sweden ABSTRACT ARTICLE INFO Article history: Received 5 September 2007 Received in revised form 7 May 2008 Accepted 30 June 2008 Keywords: Nb-doped SrTiO3 Defect chemistry Conductivity XANES XRD This study reports the defect and electrical transport properties of Nb-doped SrTiO 3 . Samples with various A/B- ratios were synthesized by a modified glycine-nitrate combustion process and evaluated as a constituent in a SOFC anode. The phase purity and defect structure of the materials have been analyzed with SEM, XRD, TGA, and XANES. The electrical conductivity of Nb-doped strontium titanate (Sr 0.94 Ti 0.9 Nb 0.1 O 3 — sintered in 9% H 2 /N 2 at 1400 °C for 12 h) decreased with increasing temperature and showed a phonon scattering conduction mechanism with σ N 120 S/cm at 1000 °C (in 9% H 2 /N 2 ). The results were in agreement with the defect chemistry model of donor-doped SrTiO 3 where the charge compensation changes from Sr vacancy compensation to the electronic type when samples are sintered in reducing atmosphere. XANES in combination with TGA indicated that Ti is the only species that is reduced to a lower oxidation state (from Ti 4+ to Ti 3+ ). The pre-edge fine structure (PEFS) from the XANES results indicated that Nb improved the overlap of the Ti atomic orbitals and thereby provided one more explanation for the positive effect of Nb on the electronic conductivity of Nb-doped SrTiO 3 . © 2008 Elsevier B.V. All rights reserved. 1. Introduction Solid oxide fuel cells (SOFC) are high temperature electrochemical devices, which convert the energy of a chemical reaction directly into electrical energy. They are normally operated between 600–1000 °C. In this study, the main objective has been to develop a material with high electronic conductivity and investigate its potential as a constituent in a SOFC anode. Generally, the target for electronic conductivity for anode materials in a reducing atmosphere is often set to be 100 S/cm. However, depending on cell design this value might be lowered to 1 S/cm [1]. A recently developed synthesis route [2] based on the glycine nitrate combustion process (GNP) [3], was used for fabricating submicron sized particles of Nb-doped strontium titanate. Several authors have manifested that anodes based on a perovskite structure are promising candidates for future fuel cell anodes [1,4,5]. Some n-doped titanates have been found to be dimensionally phase stable during redox cyclings [6] and highly tolerant to extremely high sulfur-containing (up to 1% H 2 S) fuel atmospheres [7]. In this paper, the structural characterization of Nb-doped strontium titanate, with special focus on the defect chemistry, and its effect on electrical conductivity is presented and discussed. 1.1. Defect and conductivity model for Nb-doped SrTiO 3 Nb-doped SrTiO 3 represents a donor-doped perovskite. A donor dopant has a higher cationic charge than the host cation that it replaces. In the case of Nb in SrTiO 3 , the Nb 5+ is substituting Ti 4+ and is thus trying to bring either more oxide ions or more electrons into the system than the host oxide (Nb 2 O 5 for 2 TiO 2 ). The properties of the Nb-doped SrTiO 3 depend significantly on where the extra oxygen incorporates in the structure and to which extent the extra charge of Nb 5+ is compensated. There exist essentially two different alternatives for the extra oxygen; it must be either a structural excess in the form of interstitial oxygen in perovskites, or it must be incorporated into the oxygen sublattice with the consequent formation of cation vacancies in order to satisfy the electroneutrality condition (ENC). There are no known cases of compensation of interstitial oxygen in perovskites, but there have been reports that suggest that strontium vacancies are the predominant defect in donor-doped SrTiO 3 (see e.g. Ref [8] and references therein). The effect of this for an ideal perovskite composition, where the number of A-site and B-site cations are equal (i.e. Sr/(Ti + Nb) = 1), would be a separation of a phase which is rich in the oxide of the species whose vacancy is the compensating defect. If Sr-vacancy (written as V ″ Sr , according to the Kröger–Vink formalism used throughout this paper) is the preferred species, the incorporation reaction will be similar to: SrO þð1−xÞTiO 2 þ x=2Nb 2 O 5 →ð1−x=2ÞSr x Sr þ x=2V ″ Sr þð1−xÞTi x Ti þ xNb • Ti þ 3O x O þ x=2SrO: ð1Þ Solid State Ionics 179 (2008) 2047-2058 ⁎ Corresponding author. Fuel Cells and Solid State Chemistry Department, Risø National Laboratory, Technical, University of Denmark, DK-4000 Roskilde, Denmark. Tel.: +45 4677 5868; fax: +45 4677 5858. E-mail address: peter.blennow@risoe.dk (P. Blennow). 0167-2738/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2008.06.023 Contents lists available at ScienceDirect Solid State Ionics journal homepage: www.elsevier.com/locate/ssi