Methods of Isotopic Relaxations for Estimation of Oxygen Diffusion Coefficients in Solid Electrolytes and Mixed Ionic-Electronic Conductors
Solid oxide fuel cells are environment friendly power generators with efficiency determined by the oxide-ion conductivity of solid electrolytes and mixed ionic–electronic conducting cathodes. Their tailor-made design requires reliable characterization of the oxygen self-diffusion coefficient DO of these materials. A novel technique of Do estimation based upon analysis of the oxygen heteroexchange dynamics in the temperature-programmed mode (TPIE) between the oxide powder and C18O2 in the gas phase was developed and verified by comparison with results of traditional methods -SIMS isotope profiling and conductivity measurements. For solid electrolytes –Sc+Ce-doped zirconia, Fe/Al –doped lanthanum silicates with apatite structure, etc. calcined at moderate temperatures a strong negative effect of composition inhomogeneity on the oxygen mobility was demonstrated. For oxides with asymmetric structures where oxygen migration occurs via cooperative mechanisms, doping (i.e., La2Mo2O9 by W, La2NiO4 by Sr, etc.) disrupts such a cooperation movement, so fast and slow diffusion channels were revealed by TPIE C18O2 as two separate peaks of exchange. For cathode nanocomposites comprised of ionic conductors (Y-doped ceria) and perovskites (PrNi1-xCoxO3, etc) TPIE also revealed such two channels, with DO values and their shares depending upon the cations redistribution between domains of phases and their interface.