A new scandium and niobium co-doped cobalt-free perovskite cathode for intermediate-temperature solid oxide fuel cells

In exploring new cobalt-free perovskite oxide cathodes for intermediate temperature SOFCs (solid oxide fuel cells), Sc and Nb co-doped SrFeO3-based perovskite, SSNF (SrSc0.175Nb0.025Fe0.8O3-delta) was synthesized. Powder X-ray diffraction, high resolution transmission electron microscopy and SAED (selected area electron diffraction) showed that SSNF crystallized in the tetragonal space group P4/mmm (#123) with lattice parameters of a = 3.912 angstrom and c = 7.794 angstrom. Upon heating from 400 degrees C to 1000 degrees C, the oxygen non-stoichiometry (delta) of SSNF increased from 0.19 to 0.47, concurrent with a decrease in the average valence state of iron from 3.73 to 3.03. The electrical conductivity of SSNF followed semi-conducting thermal dependence behavior, with values of 3.7-9.4 S cm(-1) at 350-750 degrees C. O-2-TPD (oxygen temperature programmed desorption) indicated the onset of Fe4+ reduction at 500 degrees C. SSNF exhibited a low thermal expansion coefficient value of 16.2 x 10(-6) K-1 (70-900 degrees C), as anticipated by excluding cobalt from the perovskite lattice. SSNF was relatively inert to the SDC (Sm0.2Ce0.8O1.9) electrolyte up to 1000 degrees C. These features, and its low ASR (area specific resistance) of 0.236 n cm(2) at 600 degrees C, denote the potential of SSNF and the opportunity to create new structures with improved performance through a co-doping strategy. (C) 2015 Elsevier Ltd. All rights reserved.