Design of Sr 0.7 R 0.3 CoO 3-δ (R = Tb and Er) Perovskites Performing as Cathode Materials in Solid Oxide Fuel Cells

Abstract

Sr 0.7 R 0.3 CoO 3-δ (R = Tb and Er) tetragonal perovskites have been prepared and evaluated as mixed ionic-electronic cathodes for SOFC. Neutron powder diffraction (NPD) measurements evidenced that both compounds are oxygen hypo-stoichiometric with long-range order of oxygen vacancies that leads to a tetragonal perovskite-type superstructure (s.g. I4/mmm) stable within the whole temperature range under study. The oxygen vacancies located mainly in the equatorial oxygen positions exhibit large displacement factors. The high oxygen mobility in Sr 0.7 Tb 0.3 CoO 3-δ was confirmed by 18 O oxygen labeling followed by Secondary Ion Mass Spectrometry (SIMS) with values of oxygen self-diffusion of 1.29 × 10 −10 cm 2 /s at 525 • C. Polarization resistances with LSGM as electrolyte gave values as low as 0.011 ·cm 2 and maximum output powers of 570 mW/cm 2 at 850 • C were obtained in test cells set in electrolyte-supported configuration. Electrical conductivity, thermal and chemical expansion and stability measurements confirm the potential of these materials as cathodes for SOFC. Nowadays it is frequent to hear about the so-called "hydrogen economy", which is based on replacing fossil fuels with this gas. The combustion of hydrogen is clean and releases more energy than any other fuel per unit weight. The utilization of hydrogen as an energy vector relies on the replacement of the current couple constituted by "hydrocarbon fuel-combustion engine" by the "hydrogen-fuel cell" binomial. Among the fuel cells, solid oxide fuel cells (SOFC) are intended to be used for stationary applications and exhibit a superior performance compared to other fuel-cell types. 1–3 For practical reasons it would be ideal the reduction of the SOFC operating temperature to lower temperatures (650–850 • C) without detriment of the cell efficiency. Therefore, it is essential to innovate in electrode materials in order to enhance their catalytic properties and their mixed ionic–electronic conductivity, achieving much faster kinetics for the two half-reactions. 4 In this work we have designed, prepared and characterized new mixed ionic-electronic conductors (MIECs) based on the family of perosvkites Sr 0.7 R 0.3 CoO 3-δ (R = Tb and Er) as cathode materials for SOFC. Cobalt-based perovskite compounds have attracted huge atten-tion as IT-SOFC (intermediate-temperature SOFC) cathodes due to their mixed-conducting characteristics and high ion-conductivity in the intermediate temperature range. 5–7 Due to their properties, high-temperature SrCoO 3–δ phases with cubic 3C-like crystal structures (where the CoO x coordination polyhedra share corners) are presented as mixed conductors with very high oxygen permeability. 8,9 Unfor-tunately, this cubic polymorph is not stable below 900 • C where a 3C-cubic to 2H-hexagonal phase transition takes place when the ma-terial undergoes a slow cooling process. In the 2H-hexagonal phase, characterized by a tolerance factor higher than 1, the crystal structure contains columns of CoO 6 octahedra sharing faces, which is detri-mental for its electronic transport properties. The stabilization of SrCoO 3-δ in a tetragonal perovskite phase has been achieved in previous works by different doping strategies. 10–16