(499a) 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 Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-d (BSCF) has recently been proposed as a promising cathode material for Solid Oxide Fuel Cells (SOFC). The performance of BSCF for this application is dictated by the structure, oxygen stoichiometry, 3-d, and chemical and thermal expansion properties of the material. These properties are traditionally determined by combining separate X-ray diffraction and thermogravimetric measurements. In-situ neutron diffraction is an elegant and accurate technique enabling simultaneous measurement of these properties. In addition, neutron diffraction yields detailed information on the oxygen sublattice and structural transitions within the material.

A novel sample holder was designed to characterize the materials at temperatures and oxygen partial pressures of interest to application; 873 to 1173 K and pO₂ between 5x10^-4 to 1 atm. Diffraction data collected on the related material, SrCo_0.8Fe_0.2O_3-d (SCF), a promising mixed-conducting oxygen separation membrane material, provides an informative comparison point. The neutron diffraction results are complemented by measurements of the mixed oxygen ion-electron transport rates of BSCF and SCF.

It was found that BSCF exhibits significantly lower oxygen stoichiometry than SCF and that 3-d can be as low as ~2.2, and ~2.3 respectively. 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 lead to increased oxygen transport rates in BSCF and help to explain the observed high performance of BSCF cathodes.