Evaluation of the R 2RuMnO 7 pyrochlores as cathodes in solid-oxide fuel cells

Abstract

The use of pyrochlore oxides as solid-oxide fuel cells (SOFC) cathodes is a promising and rather unexplored alternative. The presence of some cationic disorder in pyrochlores promotes the Frenkel-defect formation responsible of a high-ionic conductivity in these compounds. Moreover, the use of the suitable transition metals at the B position gives rise to an adequate electronic conductivity constituting disordered pyrochlores as mixed, ionic–electronic conductors. In this work, we have synthesized and characterized a new family of pyrochlore oxides with the formula R 2MnRuO 7 (R = Tb, Dy, Ho, Er, Tm, Yb, Lu and Y) using the smallest rare-earth cations in order to promote disorder in the crystal lattice. Polycrystalline samples were prepared by a soft chemistry procedure involving citrates of the different metal ions, followed by thermal treatments in air or O 2 pressure. They are all cubic, space group F d 3 ¯ m . The Rietveld-refinement from neutron powder diffraction data at room-temperature evidences that the cation disorder (distribution of Mn between A and B positions) increases when the size of R decreases. This disorder is accompanied by an increment of the oxygen-vacancy concentration due to the reduction of Mn 4+ at the B position to Mn 2+ at the A position. The obtained compounds display a semiconductor-like behavior with a maximum conductivity of 8.9 S cm −1 for Er 2MnRuO 7 at 900 °C. Moreover, the measured thermal expansion coefficients are in the range of 9.8–10.7 × 10 −6 K −1 between 100 and 900 °C that perfectly match those of the usual electrolytes, YSZ, LSGM or CGO. The obtained results present the R 2MnRuO 7 pyrochlores as alternative cathodes for IT-SOFCs.