Jean-Marc Bassat

Oxygen transport properties of La2Ni1−xCuxO4+δ mixed conducting oxides

Abstract Materials formulated La2Ni1−xCuxO4+δ (0⩽x⩽1) have been prepared and characterized using various techniques: a particular attention has been paid to their mixed conduction properties in view of application as SOFC cathode. These oxides show a good thermal stability in terms of oxygen over-stoichiometry and thermal expansion coefficient (TEC ≈13×10−6xa0K−1). In addition, they exhibit high electrical conductivity (≈100xa0S·cm−1) as well as high values of oxygen diffusion and surface exchange coefficients ( D ∗ andxa0k) which have been determined using secondary ion mass spectroscopy (SIMS) of the 18O isotope. Highly dense ceramics have been prepared for these measurements, which has required a specific study of the densification of these materials. They exhibit D ∗ and k coefficients one order of magnitude higher than those of the best perovskite materials. The pure nickelate seems to show the most promising properties for the oxygen reduction electrocatalysis.

Oxygen electrode reaction on Nd2NiO4+δ cathode materials: impedance spectroscopy study

Abstract The oxygen reduction at the surface of Nd2NiO4+δ mixed conducting cathodes has been studied in the temperature range 400–1100 K using two kinds of cells: (i) porous cathodes deposited on a standard electrolyte pellet of yttria-stabilized zirconia (YSZ) and (ii) YSZ films sputtered on dense pellets of Nd2NiO4+δ. The studies on dense cathodes allowed to determine their electrical conductivity characteristics independently of their microstructure. The performances of these cathode materials were measured using the complex impedance spectroscopy at various temperatures and oxygen partial pressures. Compared to manganite perovskites, porous electrodes of these oxides exhibit promising electrocatalytic properties (i.e. lower polarization resistances) in agreement with the values of the surface exchange coefficient, k, and of the oxygen tracer diffusion coefficient, D*, previously measured by SIMS. Their behavior, especially the dependence of the polarization resistance on temperature and oxygen partial pressure was tentatively modelled.

Electrode properties of Ln2NiO4 + δ ( Ln = La , Nd , Pr ) AC Impedance and DC Polarization Studies

The perovskite-type materials, Ln 2 NiO 4+δ , were prepared by a modified Pechini method and their electrochemical properties investigated using ac impedance and dc polarization methods from 300 to 800°C under different oxygen partial pressures. Schouler methodology based on the frequency relaxation has been used to assign different electrochemical processes. A transfer process related to oxygen ions moving between the electrolyte and electrode interface was found in the mixed ionic and electronic conducting electrode/electrolyte system. The study shows that the Nd 1.95 NiO 4+δ compound exhibits promising electrode properties due to its high chemical stability and low polarization resistance. The oxygen diffusion process is the rate-limiting step at low oxygen partial pressure, whereas the charge-transfer process becomes more important at higher oxygen partial pressure.

Chemical oxygen diffusion coefficient measurement by conductivity relaxation—correlation between tracer diffusion coefficient and chemical diffusion coefficient

Abstract Chemical oxygen diffusion coefficient ( D ) was measured by conductivity relaxation experiments on La2Cu0.5Ni0.5O4+δ ceramics. This method consists of abruptly changing the oxygen partial pressure in the surrounding atmosphere of the sample. The consequent evolution of the electrical conductivity is recorded as a function of time, at fixed temperature. The transient behaviour during the re-equilibration process is analysed on the basis of the second Ficks law. In parallel, the oxygen tracer self-diffusion coefficient (D*) was derived from isotopic exchange experiments monitored by SIMS analysis. Finally, the oxygen over-stoichiometry (δ) was computed from thermogravimetric experiments, under different pO2, in order to determined the thermodynamic factor (γ). A quantitative agreement between D and γ×D*, observed over a wide temperature range (500–900xa0°C), confirms the validity of the experimental method and of the Wagners theory, applied to the oxygen diffusion in this compound.

Measurement of chemical and tracer diffusion coefficients of oxygen in La2Cu0.5Ni0.5O4+δ

Abstract In mixed electronic and ionic conductors O 2− /e − (MIEC), where the electronic conductivity is dominant, the determination of the oxygen diffusion coefficient is an essential feature. In the present paper, the oxygen chemical diffusion coefficient D O of a MIEC oxide has been evaluated using the conductivity relaxation method, in the temperature range 773–1173 K. Samples of La 2 Cu 0.5 Ni 0.5 O 4+ δ were selected because of the high value of their surface exchange coefficient. In addition, the oxygen diffusion coefficient D * was determined from the 18 O tracer diffusion profile. Finally, the oxygen partial pressure dependence of this compound was determined by thermogravimetric analysis (TGA). On the basis of these three independent experiments, the model of ambipolar diffusion, which describes the diffusion in mixed oxide, has been experimentally verified. For such a determination of the intrinsic diffusion characteristics, dense samples are required.

Synthesis and crystallographic characterization of the mixed-valence reduced nickelate La1.6Sr0.4(Ni+I,Ni+II)O3.47

The new mixed-valence nickelate La1.6Sr0.4(Ni+I,Ni+II)O3.47 was prepared by low-temperature reduction of the oxide La1.6Sr0.4NiO4.10 at 670 K under hydrogen. This strongly reduced nickelate—65% Ni+I—is stable in air, at room temperature. The crystallographic characterization points to an orthorhombic unit cell: a = 3.8728(6)A, b = 3.7242(6)A, c = 12.767(2)A. Structure calculations evidence an oxygen-deficient K2NiF4-type structure. The oxygen vacancies are found only in the “NiO2” equatorial planes, leading to a lowering of the nickel coordination. The main feature is the occurrence of Niue5f8O squares systematically orientated along the c-axis.

A re-investigation of the crystal structure of La2NiO4:Non stoichiometry and “LaO” layers

Abstract The crystal structure of La2NiO4 (a = 3.869(1) A ) , (c = 12.664(3) A ) , Z = 2,I4/mmm) has been re-investigated on single crystal by using a four circle diffractometer. The structure was solved from 143 independent reflections to R = 0.044, Rw = 0.043. The lanthanum deficiency of the compound was taken into account during the refinements. Calculations provide direct evidence that, at the scale experimented by the X-ray determination, the structure can tolerate significant non-stoichiometry. Both holes and oxygen vacancies compensate the lanthanum vacancies: the oxygen vacancies are probably localized on axial oxygens (O1). The possible non-stoichiometry of “LaO” layers and its relation with anisotropy of thermal parameters is discussed for a better understanding of the high Tc superconductivity occuring in the iso-structural Ba-La-Cu-O.

SrVO3 and Sr2VO4, electrical properties below and above room T

Abstract SrVO 3 and Sr 2 VO 4 have been studied with respect to their electrical properties. The resistivity of SrVO 3 has been measured from 4 to 840 K. It is metallic with a T 2 law below 200 K, characteristic of electronic correlation. Above 200 K and up to 600 K the resistivity is linear with T reminding that of high Tc cuprates. Conversly, Sr2VO4 is insulating below room temperature, while the V ion is expected to be in the same electronic configuration.

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.

Anisotropic transport properties of La2NiO4 single crystals

Abstract The temperature dependence of electrical conductivity and thermoelectric power for La2NiO4 single crystals between 77 and 300 K is analysed. The electrical properties are highly anisotropic. The conductivity and the carrier mobility are thermally activated, but the activation energies depend on the orientation (parallel or perpendicular to the basal (a,b) plane). The positive Seebeck coefficient found along the two orientations indicates that holes are the dominant charge carriers. Careful analysis of present data based on a phenomenological model indicates that electrons are also present in the “LaO” layers between the conducting layers.