Phase stability and structural distorsions of nanostructured iron-cobaltite mixed conductors

LAURA BAQUÉ; FEDERICO NAPOLITANO; ANALÍA SOLDATI; ALBERTO CANEIRO; ADRIANA SERQUIS

Campinas

Encuentro; 22ª Reunión Anual de Usuarios del Laboratorio Nacional de Luz Sincrotrón (22 RAU); 2012

Laboratorio Nacional de Luz Sincrotrón

Mixed conductors oxides such as La1-xSrxCo1-yFeyO3-d are good candidates for intermediate-temperature solid oxide fuel cells cathodes. Our previous studies showed that the electrochemical performance is largely improved when using cathodes prepared with nanosized La0.4Sr0.6Co0.8Fe0.2O3-d (LSCF) powders (i.e. crystallite sizes 20-30 nm). However, an irreversibility increase in the polarization resistance occurs after measurements under reductive atmospheres in the 500-700 C range, which seemed to be related to the presence of some unidentified lattice distortions or an additional phase that could not be identified by conventional laboratory X-ray powder diffraction (XPD). In this work, nanocrystalline LSCFO powders  were prepared by two different chemical methods. In-situ experiments were carried out using the D10BXPD beamline at LNLS under different atmospheres (pure He or synthetic air) between 20 and 600 C. In addition, some samples were prepared ex-situ under Ar and O2 and studied by transmission electron microscope and laboratory XRD. The formation of phases with oxygen vacancy ordering was observed for first time in the studied composition. The XRD data indicate that only a portion of the sample is transformed while the rest keeps the original phase. These results are in agreement with TEM observations that show the coexistence of two phases in the same grain. All these changes can be reverted with heat treatments under oxygen or air atmospheres.