The Oxygen Non-Stoichiometry and Structure of Solid Oxide Fuel Cell Cathode Materials Measured by in-Situ Neutron Diffraction

The mixed ion-electron conducting perovskite oxide Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) has recently been proposed as a promising cathode material for solid oxide fuel cells. The performance of BSCF for this application is dictated by the structure, oxygen stoichiometry, 3-δ, and chemical and thermal expansion properties of the material. In-situ neutron diffraction provides a novel approach to simultaneously measure of all of these properties at temperatures and oxygen partial pressures of interest to application; 873 to 1173 K and pO2 between 5x10 to 1 atm. The related material, SrCo0.8Fe0.2O3-δ (SCF), a promising mixed-conducting membrane material, provides an informative comparison. It was found that BSCF exhibits significantly lower oxygen stoichiometry than SCF and that 3-δ can be as low as ~2.2, and ~2.3 respectively. This was surprising as the increased basicity of Ba relative to Sr may be expected to stabilize higher B-site oxidation states. In addition, where oxygen vacancies in SCF order to form the brownmillerite structure at reduced pO2 and temperature, BSCF maintains the vacancy disordered cubic perovskite structure under all conditions investigated. Both of these factors will lead to increased oxygen transport rates in BSCF and may explain the observed high performance of BSCF cathodes.