Jean Claude Grenier

Hydration and transport properties of the Pr2−xSrxNiO4+δ compounds as H+-SOFC cathodes

The result of the substitution of Pr3+ by Sr2+ in the 214 Ruddlesden-Popper Pr2NiO4+δ material was studied with regard to its electrochemical properties as a H+-SOFC cathode. Structural characterizations as well as physical properties of the Pr2−xSrxNiO4+δ compounds (x ≤ 0.50), in particular hydration as a function of water partial pressure, have shown that oxygen over-stoichiometry and oxygen exchange with atmosphere decrease with increasing x, which has been correlated with the stabilization of the 214 structure by Sr2+ substitution. Electrochemical studies on the oxygen reduction versus hydration have allowed determination of the rate determining steps of the formation of water and evidence the role of protons in Pr2NiO4+δ in contrast to Pr2−xSrxNiO4+δ oxides. It has been concluded that triple mixed conductivity (i.e. protonic, ionic as well as electronic conductivities) exists in this nickelate Pr2NiO4+δ. In addition, there was evidence for strong correlation between the insertion of protonic defects and additional oxygen in the interstitial position of Pr2NiO4+δ.

New preparation method of Lan+1NinO3n+1–δ (n=2, 3)

Samples Lan+1NinO3n+1–δ(n=2,3) have been prepared by two different methods, which lead to different oxygen stoichiometry values. Materials obtained by the citrate route always show higher content of Ni3+ , which can be rationalized by the high reactivity and the original morphology of the precursors obtained by this method.

Double electron exchange in Fe1−xO : A Mössbauer study

Abstract Mossbauer spectroscopy investigations have been performed at room temperature upon Fe 1−x O samples with 0.05 ⩽ x ⩽ 0.094 which have been previously characterized by X-ray diffraction and thermomagnetic analysis and the composition of which have been determined by thermogravimetric oxidation to Fe 2 O 3 . A model allowing to calculate the amount of Fe 3+ ions has been proposed; it assumes a fast electron exchange over iron atoms in octahedral sites between Fe 2+ and Fe 3+ . The amount of Fe 3+ ions calculated from the Mossbauer data agrees very well with those obtained directly from the thermogravimetric analysis.

A2MO4+δ Oxides: Flexible Electrode Materials for Solid Oxide Cells

Until now, most cathode materials used in high temperature ceramic solid electrolyte oxide cells (SEOC) are perovskite-type MEIC compounds, AMO3-δ, showing oxygen sub-stoichiometry. Recently, a new family of overstoichiometric oxides, formulated A2MO4+δ with A = La, Nd, Pr, Sr and M = Ni, Cu, has been investigated. Due to their basic properties (large D* and k coefficients, high electrical conductivity), the nickelate compounds have been especially used in various applications. Nd2NiO4 and Pr2NiO4 exhibit very promising electrochemical properties down to temperature as low as 600{degree sign}C, as ITSOFC cathodes as well as PCFC cathodes (they are stable under moist air (3 -10 % H2O). In addition, encouraging cell tests (SOFC and PCFC) have been performed with both nickelates. Furthermore, these materials have been also used as HTSE anodes and first results show excellent performances. Thus, it appears that these materials show an extremely high flexibility for being used as air electrode in very different conditions in terms of oxygen partial pressure or/and water content. This is explained on the basis of structural features of these materials and the great ability of these compounds for accommodating the oxygen non-stoichiometry.

A Density Functional Study of Oxygen Mobility in Ceria-Based Materials

In CeO2-based solid electrolytes, it has been shown that point defects are directly responsible for oxygen ionic conduction. The ionic conductivity is strongly affected by the anion vacancy concentration which is enhanced by doping with aliovalent cations. When rare earth sesquioxides such as La2O3, Gd2O3, Sm2O3, Y2O3 are added to CeO2, the dopant cation substitutes for the cerium ion, and oxygen vacancies are created for charge compensation. Incorporation of trivalent dopants into CeO2 at the Ce4+ sites can be depicted by the following defect reaction (expressed in Kröger-Vink notation):