Co-free, iron perovskites as cathode materials for intermediate-temperature solid oxide fuel cells

Abstract

We have developed a Co-free solid oxide fuel cell (SOFC) based upon Fe mixed oxides that gives an extraordinary performance in test-cells with H2 as fuel. As cathode material, the perovskite Sr0.9K0.1FeO3−δ (SKFO) has been selected since it has an excellent ionic and electronic conductivity and long-term stability under oxidizing conditions; the characterization of this material included X-ray diffraction (XRD), thermal analysis, scanning microscopy and conductivity measurements. The electrodes were supported on a 300-μm thick pellet of the electrolyte La0.8Sr0.2Ga0.83Mg0.17O3−δ (LSGM) with Sr2MgMoO6 as the anode and SKFO as the cathode. The test cells gave a maximum power density of 680 mW cm−2 at 800°C and 850 mW cm−2 at 850 °C, with pure H2 as fuel. The electronic conductivity shows a change of regime at T ≈ 350 °C that could correspond to the phase transition from tetragonal to cubic symmetry. The high-temperature regime is characterized by a metallic-like behavior. At 800 °C the crystal structure contains 0.20(1) oxygen vacancies per formula unit randomly distributed over the oxygen sites (if a cubic symmetry is assumed). The presence of disordered vacancies could account, by itself, for the oxide-ion conductivity that is required for the mass transport across the cathode. The result is a competitive cathode material containing no cobalt that meets the target for the intermediate-temperature SOFC.