abstract
Strontium additions in (La1-x Sr (x) )(1-y) Mn0.5Ti0.5O3-delta (x = 0.15-0.75, y = 0-0.05) having a rhombohedrally distorted perovskite structure under oxidizing conditions lead to the unit cell volume contraction, whilst the total conductivity, thermal and chemical expansion, and steady-state oxygen permeation limited by surface exchange increase with increasing x. The oxygen partial pressure dependencies of the conductivity and Seebeck coefficient studied at 973-1223 K in the p(O-2) range from 10(-19) to 0.5 atm suggest a dominant role of electron hole hopping and relatively stable Mn3+ and Ti4+ states. Due to low oxygen nonstoichiometry essentially constant in oxidizing and moderately reducing environments and to strong coulombic interaction between Ti4+ cations and oxygen anions, the tracer diffusion coefficients measured by the O-18/O-16 isotopic exchange depth profile method with time-of-flight secondary-ion mass spectrometric analysis are lower compared to lanthanum-strontium manganites. The average thermal expansion coefficients determined by controlled-atmosphere dilatometry vary in the range 9.8-15.0 x 10(-6) K-1 at 300-1370 K and oxygen pressures from 10(-21) to 0.21 atm. The anodic overpotentials of porous La0.5Sr0.5Mn0.5Ti0.5O3-delta electrodes with Ce0.8Gd0.2O2-delta interlayers, applied onto LaGaO3-based solid electrolyte, are lower compared to (La0.75Sr0.25)(0.95)Cr0.5Mn0.5O3-delta when no metallic current-collecting layers are introduced. However, the polarization resistance is still high, similar to 2 Omega x cm(2) in humidified 10 % H-2-90 % N-2 atmosphere at 1073 K, in correlation with relatively low electronic conduction and isotopic exchange rates. The presence of H2S traces in H-2-containing gas mixtures did not result in detectable decomposition of the perovskite phases.
keywords
OXIDE FUEL-CELLS; OXYGEN TRACER DIFFUSION; ELECTRODE MATERIALS; REACTION-MECHANISMS; ANODE MATERIALS; CERAMIC MICROSTRUCTURE; SURFACE EXCHANGE; SOFC CATHODES; TRANSPORT; CONDUCTIVITY
subject category
Electrochemistry
authors
Kolotygin, VA; Tsipis, EV; Ivanov, AI; Fedotov, YA; Burmistrov, IN; Agarkov, DA; Sinitsyn, VV; Bredikhin, SI; Kharton, VV
our authors
Vladislav Kharton
Invited Principal ResearcherGroups
acknowledgements
This work was supported by the Ministry of Education and Science of the Russian Federation (state contract 02.740.11.5214) and by FCT-Portugal (projects SFRH/BPD/28629/2006 and SFRH/BD/45227/2008).