This work presents the results from a study of the structure and transport properties of Ca-doped La2NiO4+δ. La2−xCaxNiO4+δ (x = 0–0.4) materials that were synthesized via combustion of organic-nitrate precursors and characterized by X-ray diffraction (XRD), in situ XRD using synchrotron radiation, thermogravimetric analysis (TGA) and isotope exchange of oxygen with C18O2. The structure was defined as orthorhombic (Fmmm) for x = 0 and tetragonal (I4/mmm) for x = 0.1–0.4. Changes that occurred in the unit cell parameters and volume as the temperature changed during heating were shown to be caused by the excess oxygen loss. Typical for Ruddlesden–Popper phases, oxygen mobility and surface reactivity decreased as the Ca content was increased due to a reduction in the over-stoichiometric oxygen content with the exception of x = 0.1. This composition demonstrated its superior oxygen transport properties compared to La2NiO4+δ due to the enhanced oxygen mobility caused by structural features. Electrochemical data obtained showed relatively low polarization resistance for the electrodes with a low Ca content, which correlates well with oxygen transport properties.
The main aspects of the cathode materials
morphology for Intermediate Temperature Solid Oxide Fuel
Cells (IT SOFC) are considered in this paper. The approaches
for estimation of their basic properties, e.g. oxygen mobility
and surface reactivity, are described and the results
of different techniques (e.g. weight and conductivity
relaxation, oxygen isotope exchange) application for studies
of powders and dense ceramic materials are compared.
The Ruddlesden-Popper type phases (e.g. Pr2NiO4) provide
enhanced oxygen mobility due to cooperative mechanism
of oxygen interstitial migration. For perovskites, the oxygen
mobility is increased by doping, which generates oxygen
vacancies or decreases metal-oxygen bond strength. Nonadditive
increasing of the oxygen diffusion coefficients
found that for perovskite-fluorite nanocomposites, it can be
explained by the fast oxygen migration along perovskitefluorite
interfaces. Functionally graded nanocomposite
cathodes provide the highest power density, the lowest
area specific polarization resistance, and the best stability
to degradation caused by the surface layer carbonization/
hydroxylation, thus being the most promising for thin film
IT SOFC design.
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