Electrochemical evaluation of La2NiO4+δ -based composite electrodes screen-printed on Ce0.8Sm0.2O1.9 electrolyte

La₂NiO_4+δ , 60 wt.% La₂NiO_4+δ –40 wt.% La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ , and 60 wt.% La₂NiO_4+δ –40 wt.% Ce_0.8Sm_0.2O_1.9 electrodes were prepared from fine powders on dense Ce_0.8Sm_0.2O_1.9 electrolyte substrates by screen-printing technique. Electrochemical impedance spectroscopy and chronopotentiometry techniques were employed to evaluate the electrochemical properties of the composite electrodes in comparison with the La₂NiO_4+δ electrode. For the three electrodes, main electrode processes were resolved to be charge-transfer at the electrode/electrolyte interface and oxygen exchange on the electrode surface. The contribution of the surface oxygen exchange process was detected to be dominant for the overall electrode polarization. The addition of Ce_0.8Sm_0.2O_1.9 into La₂NiO_4+δ was favorable for the charge transfer process whereas it was undesired for the surface oxygen exchange process. On comparison, adding La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ into La₂NiO_4+δ was found to benefit both the two electrode processes. The La₂NiO_4+δ -La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ composite electrode showed optimum electrochemical properties among the three electrodes. At 800 °C, the composite electrode achieved a polarization resistance of 0.20 Ω cm², an overpotential of 45 mV at a current density of 200 mA cm^?2, together with an exchange current density of ～200 mA cm^?2.     

Anomalies in magnetic susceptibility of nonstoichiometric Nd2NiO4+δ (δ=0.049, 0.065, 0.077, 0.234)

We have investigated the influence of oxygen excess on structural and physical properties of the Nd₂NiO_4+δ compounds. Using the citrate method and subsequent annealing in air and in a reducing atmosphere a various oxygen-doped compounds were prepared. X-ray diffraction at room temperature shows that structure is strongly oxygen excess dependent. Thus, by increasing δ by up to 0.077, the compounds adopt a tetragonal structure gradually with a biphasic domain between orthorhombic and tetragonal structures. And at higher δ values, the structure becomes orthorhombic. Moreover, Rietveld analysis shows that for δ<0.077 the presence of two crystalline phases with different oxygen excess: it should be the signature of interstitial oxygen, which is distributed in heterogeneous way. The biphasic products are composed of a stoichiometric Nd₂NiO₄ phase (orthorhombic structure) and a tetragonal Nd₂NiO_4.077 phase. Magnetic susceptibility shows a deviation from Curie-Weiss law for lower oxygen excess (δ?0.077). Moreover, some anomalies in dc magnetic susceptibility curves was observed at 45, 95 and 130 K for δ<0.077. These transitions are connected to the tetragonal phase, and were attributed, respectively, to an antiferromagnetic transition, possible charge ordering and structural transition.     

Oxygen nonstoichiometry and chemical stability of Nd2−xSrxNiO4+δ

Nonstoichiometric variation of oxygen content in Nd_2−xSr_xNiO_4+δ (x=0, 0.2, 0.4) and decomposition P(O₂) were determined by means of high temperature gravimetry and coulometric titration. The measurements were carried out in the temperature range from 873 to 1173 K and the P(O₂) range from 10⁻²⁰ to 1 bar. Nd_2−xSr_xNiO_4+δ shows the oxygen excess and the oxygen deficient composition depending on P(O₂), temperature, and the Sr content. To evaluate the characteristics of oxygen nonstoichiometric behavior, partial molar enthalpy of oxygen was calculated. The value of partial molar enthalpy of oxygen slightly approaches zero as δ increases in the oxygen excess region while that is independent of δ in the oxygen deficient region. Discussion was made by comparing data of this study with nonstoichiometric and thermodynamic data of La_2−xSr_xNiO_4+δ: Nd_2−xSr_xNiO_4+δ show more oxygen excess than La_2−xSr_xNiO_4+δ in the higher P(O₂) region, while the nonstoichiometric behavior in the oxygen deficient composition is almost the same. The variation of partial molar enthalpy of oxygen with δ for Nd_2−xSr_xNiO_4+δ in the oxygen excess region is much smaller than that of La_2−xSr_xNiO_4+δ. The oxygen nonstoichiometric behavior of Nd_2−xSr_xNiO_4+δ is more ideal-solution-like than that of La_2−xSr_xNiO_4+δ.     

Thermodynamic quantities and defect equilibrium in La2−xSrxNiO4+δ

In order to elucidate the relation between thermodynamic quantities, the defect structure, and the defect equilibrium in La_2−xSr_xNiO_4+δ, statistical thermodynamic calculation is carried out and calculated results are compared to those obtained from experimental data. Partial molar enthalpy of oxygen and partial molar entropy of oxygen are obtained from δ-P(O₂)-T relation by using Gibbs-Helmholtz equation. Statistical thermodynamic model is derived from defect equilibrium models proposed before by authors, localized electron model and delocalized electron model which could well explain the variation of oxygen content of La_2−xSr_xNiO_4+δ. Although assumed defect species and their equilibrium are different, the results of thermodynamic calculation by localized electron model and delocalized electron model show minor difference. Calculated results by the both models agree with the thermodynamic quantities obtained from oxygen nonstoichiometry of La_2−xSr_xNiO_4+δ.     

Effect of doping, microstructure, and CO2 on La2NiO4+δ-based oxygen-transporting materials

Alkaline earth-free La₂NiO_4+δ based materials were synthesized by a sol-gel method and studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques as well as oxygen permeation experiments. Effects of doping the nickel position with a variety of cations (Al, Co, Cu, Fe, Mg, Ta, and Zr) were investigated with regards to oxygen flux and microstructure. Doping was always found to diminish the oxygen flux as compared to the reference composition. However, larger grains, which were achieved by longer annealing times at 1723 K have a minor negative impact on oxygen permeation flux in case of La₂NiO_4+δ and La₂Ni_0.9Fe_0.1O_4+δ system. Mössbauer spectroscopy shows that the iron-doped system exhibits a secondary phase, which was identified by high-resolution transmission electron microscopy (HRTEM) as a higher Ruddlesden-Popper phase. In-situ XRD in an atmosphere containing 50 vol% CO₂ and long-term oxygen permeation experiments using pure CO₂ as the sweep gas revealed a high tolerance of the materials towards CO₂.     

Compatibility evaluation between La2Mo2O9 fast oxide-ion conductor and Ni-based materials

The chemical reactivity of La₂NiO_4+δ and nickel metal or nickel oxide with fast oxide-ion conductor La₂Mo₂O₉ is investigated in the annealing temperature range between 600 and 1000 °C, using room temperature X-ray powder diffraction. Within the La₂NiO_4+δ/La₂Mo₂O₉ system, subsequent reaction is evidenced at relatively low annealing temperature (600 °C), with formation of La₂MoO₆ and NiO. The reaction is complete at 1000 °C. At reverse, no reaction occurs between Ni or NiO and La₂Mo₂O₉ up to 1000 °C. Together with a previous work [G. Corbel, S. Mestiri, P. Lacorre, Solid State Sci. 7 (2005) 1216], the current study shows that Ni-CGO cermets might be chemically and mechanically compatible anode materials to work with LAMOX electrolytes in solid oxide fuel cells.     

Thin films of Ln1−xSrxCoO3 (Ln=La, Nd and Gd) and SrRuO3 by nebulized spray pyrolysis

Thin films of La_1−xSr_xCoO₃, Nd_0.5Sr_0.5CoO₃, Gd_0.5Sr_0.5CoO₃ and SrRuO₃ have been deposited on Si(100), LaAlO₃(100) and SrTiO₃(100) single crystal substrates by nebulized spray pyrolysis. The films deposited on Si are generally polycrystalline, but they are highly oriented on the oxide substrates. The cobaltate films are generally not metallic, but exhibit low resistivity specially when x=0.3 and 0.5, the latter also exhibiting ferromagnetic characteristics. Films of La_0.7Sr_0.3CoO₃ show negative magnetoresistance of 35% around 180 K. Films of SrRuO₃ are metallic on Si and LaAlO₃ substrates but show an insulator–metal transition on SrTiO₃ around 130 K, around which temperature negative magnetoresistance is observed.     

Epitaxial growth of high purity cubic InN films on MgO substrates using HfN buffer layers by pulsed laser deposition

Cubic InN films have been grown on MgO substrates with HfN buffer layers by pulsed laser deposition (PLD). It has been found that the use of HfN (100) buffer layers allows us to grow cubic InN (100) films with an in-plane epitaxial relationship of [001]_InN//[001]_HfN//[001]_MgO. X-ray diffraction and electron back-scattered diffraction measurements have revealed that the phase purity of the cubic InN films was as high as 99%, which can be attributed to the use of HfN buffer layers and the enhanced surface migration of the film precursors by the use of PLD.     

High-temperature studies of La1−x SrxFeO3−δ solid solutions using synchrotron radiation

A series of La_1?x Sr_xFeO_3?δ samples with a perovskite structure were investigated by high-temperature X-ray analysis using synchrotron radiation. In this series, one can observe a morphotropic phase transition (0.3 ? x ? 0.4) from the orthorhombic (0 ? x < 0.3) to cubic (0.4 < x ? 0.75) modification. The samples from the morphotropic transition region (MTR) at room temperature have a highly disordered microblock structure related to their phase heterogeneity; according to high-temperature X-ray analysis data, this is the result of particle stratification of the high-temperature homogeneous solid solution formed at the temperature of the synthesis (1200°C) in the course of cooling.     

Investigations of the magnetic properties and structures of the pillared perovskites, La5Re3MO16 (M=Co, Ni)

La₅Re₃CoO₁₆ and La₅Re₃NiO₁₆ were synthesized by solid-state reaction and studied by SQUID magnetometry, heat capacity and powder neutron diffraction measurements. These two compounds belong to a series of isostructural Re-based pillared perovskites [Chi et al. J. Solid State Chem. 170 (2003) 165]. Magnetic susceptibility measurements indicate apparent short-range ferri or ferromagnetic correlations and possible long-range antiferromagnetic order for La₅Re₃CoO₁₆ at 35 K, and at 38 and 14 K for La₅Re₃NiO₁₆. Heat capacity measurements of the Co compound show a lambda anomaly, typical of long-range magnetic order, at 32 K. In contrast, the Ni compound displays a broader, more symmetric feature at 12 K in the heat capacity data, indicative of short-range magnetic order. Low-temperature powder neutron diffraction revealed contrasting magnetic structures. While both show an ordering wave vector, k=(0,0,1/2), in La₅Re₃CoO₁₆, the Co²⁺ and Re⁵⁺ moments are ordered ferrimagnetically within the corner-shared octahedral layers, while the layers themselves are coupled antiferromagnetically along the c-axis, as also found in La₅Re₃MnO₁₆ and La₅Re₃FeO₁₆. In the case of the Ni material, the Re⁵⁺ and Ni²⁺ moments in the perovskite layers couple ferromagnetically and are canted 30° away from the c-axis, angled 45° in the ab-plane. The layers then couple antiferromagnetically at low temperature, a unique magnetic structure for this series. The properties of the La₅Re₃MO₁₆ series, with M=Mn, Fe, Co, Ni and Mg are also reviewed.     

Substitutional Effects of 3d Transition Metals on the Magnetic and Structural Properties of Quasi-Two-Dimensional La5Mo4O16

Substituted phases with the composition La₅Mo_4−xT_xO_16−δ (T=Co, Fe, Mn, and Mg and x∼0.7) were prepared by fused-salt electrolysis and/or conventional solid-state methods. The crystal structure of the parent compound, La₅Mo₄O₁₆, contains perovskite-like corner-sharing MoO₆ octahedral units in the ab plane separated by Mo₂O₁₀ bioctahedral units along the c direction. Detailed single-crystal X-ray diffraction studies on the Co-substituted phase, La₅Mo_3.31Co_0.69O_16−δ, indicated that the unit cell is triclinic (space group C-1) with Co exclusively replacing Mo atoms in the perovskite layers. X-ray absorption measurements revealed that the transition metal ions are divalent, consistent with the crystal structure analysis. The anomalous magnetic transition observed at 180 K in the parent compound shifts to lower temperatures upon substitution with transition metal ions. No long-range magnetic order was evident in the Mg²⁺-substituted compositions. The electrical resistivity of all the substituted phases was at least 3 orders of magnitude higher than that of the parent compound. Variations in the magnetic and electrical properties have been ascribed to the disruption of exchange correlations caused by substitutional disorder at the Mo sites.     

Oxygen isotopic effect on the IR reflectivity spectra of a La2NiO4.14 single crystal

Infrared reflectivity spectra measurements have been realised at T=300 K on the oxygen–isotope exchanged single crystals (¹⁶O and ¹⁸O) of La₂NiO_4+δ with δ=0.14. One of the main results lies on transfer of the spectral weight from the mid-infrared spectral range to the far-infrared range upon ¹⁸O substitution. This striking effect is studied in the framework of the harmonic oscillator concept by simulating the experimental data with a dielectric function model based on the generalisation of the Lyddane–Sachs–Teller relationship. It suggests that the lattice strongly influences the behaviour of the charge carriers in such 3d transition metal oxides.     

Ni系钙钛矿氧化物导电性及其CO氧化活性

本文考查了Ni系钙钛矿类希土复合氧化物在不同气氛条件下电导率随温度的变化情况，结合活性评价结果，初步探索了结构、活性及导电率之间的关系。研究结果表明：在测定温度范围内，LaNiO₃具有金属导电性，而La₂NiO₄和LaSrNiO₄为p型半导体；电导率与CO氧化活性之间存在相互对应的关系，其大小顺序均为LaNiO₃>LaSrNiO₄>La₂NiO₄。     

Electrochemical performance of strontium-doped neodymium nickelate mixed ionic–electronic conductor for intermediate temperature solid oxide fuel cells

The Nd_2???x Sr _x NiO_4?+?δ (x?=?0.1–0.5) solid solutions prepared by combustion synthesis are of submicron/superfine crystallite size. The crystal structure estimated by Rietveld analysis reveals increase in space in the rock salt layer on partial replacement of Nd³⁺ by Sr²⁺. The transition from negative temperature coefficient to positive temperature coefficient of conductivity is observed at 913 K. The maximum dc conductivity (σ?=?1.3?±?0.02 S?cm^?1 at 973 K) is obtained for x?=?0.2 in Nd_2???x Sr _x NiO_4?+?δ . The low dc conductivity compared with reported (≈100 S?cm^?1) is due to high porosity (low relative density) resulting from agglomeration of submicron crystallites. The variation in the conductivity with Sr content in Nd_2???x Sr _x NiO_4?+?δ is understood on the basis of defect chemistry. The electrochemical properties of the cathode materials are studied using electrochemical impedance spectroscopy at various temperatures and oxygen partial pressures. Nd_1.8Sr_0.2NiO_4?+?δ cathode exhibits lowest-area-specific resistance?=?0.52?±?0.015 Ohm?cm² at 973 K. At low $ {P_{{{{{\bf O}}_2}}}} $ (<1,000 Pa), oxygen ion transfer from Nd_1.8Sr_0.2NiO_4?+?δ cathode to gadolinium-doped ceria electrolyte is the rate-limiting step, whereas, charge-transfer reaction on the cathode becomes more important at high oxygen partial pressures and temperature (973 K).     

Mixed conductivity, oxygen permeability and redox behavior of K2NiF4-type La2Ni0.9Fe0.1O4+δ

The total conductivity and Seebeck coefficient of La₂Ni_0.9Fe_0.1O_4+δ with K₂NiF₄-type structure, studied in the oxygen partial pressure range from 10⁻⁵ to 0.5 atm at 973-1223 K, were analyzed in combination with the steady-state oxygen permeability, oxygen non-stoichiometry and Mössbauer spectroscopy data in order to examine the electronic and ionic transport mechanisms. Doping of La₂NiO_4+δ with iron was found to promote hole localization on nickel cations due to the formation of stable Fe³⁺ states, although the electrical properties dominated by p-type electronic conduction under oxidizing conditions exhibit trends typical for both itinerant and localized behavior of the electronic sublattice. The segregation of metallic Ni on reduction, which occurs at oxygen chemical potentials close to the low-p(O₂) stability boundary of undoped lanthanum nickelate, is responsible for the high catalytic activity towards partial oxidation of methane by the lattice oxygen of La₂Ni_0.9Fe_0.1O_4+δ as revealed by thermogravimetry and temperature-programmed reduction in dry CH₄-He flow at 573-1173 K. A model for the oxygen permeation fluxes through dense La₂Ni_0.9Fe_0.1O_4+δ ceramics, limited by both bulk ionic conduction and surface exchange kinetics, was proposed and validated.     

Kinetics of Formation of Ruddlesden-Popper Phases: I. Mechanism of La2SrAl2O7 Formation

Phase formation in the La₂O₃-SrO-Al₂O₃ system was studied. Formation of the perovskite LaAlO₃ structure and the perovskite-like LaSrAlO₄ and La₂SrAl₂O₇ layered structures in solid-phase chemical reactions is activated only after reaching certain critical temperatures which correlate with the temperatures of of the transition of intergranular formations to the liquid-like state (melting points of two-dimensional nonautonomous phases).     

Oxygen non-stoichiometry of Ln4Ni2.7Fe0.3O10−δ (Ln=La, Pr)

The oxygen deficiency of iron-substituted nickelates Ln₄Ni_2.7Fe_0.3O_10−δ (Ln=La, Pr) with the orthorhombic Ruddlesden-Popper structure was studied by thermogravimetric analysis and coulometric titration in the oxygen partial pressure range 6×10⁻⁵ to 0.7 atm at 973-1223 K. In air, the non-stoichiometry values vary in the relatively narrow ranges (2.4−4.2)×10⁻² for La- and (0.01−2.0)×10⁻² for Pr-containing compositions, increasing with temperature. Due to the smaller size of praseodymium cations, Pr₄Ni_2.7Fe_0.3O_10−δ exhibits a substantially lower thermodynamic stability in comparison with La₄Ni_2.7Fe_0.3O_10−δ and La₄Ni₃O_10−δ, although the oxygen content in Pr₄Ni_2.7Fe_0.3O_10−δ lattice is higher. The partial substitution of iron for nickel has no essential effect on the low-p(O₂) stability limit corresponding to the transition of Pr₄Ni₃O_10−δ into K₂NiF₄-type Pr₂NiO_4+δ. On the contrary, doping of La₄Ni₃O_10−δ with iron decreases the oxygen vacancy concentration and shifts the phase stability boundary towards lower oxygen chemical potentials, suggesting a stabilization of the transition metal-oxygen octahedra in lanthanum nickelate lattice. The Mössbauer spectroscopy showed that the predominant state of iron cations, statistically distributed between the nickel sites, is trivalent.     

The stripping voltammetric determination of metals in non-aqueous media on the mercury film electrode prepared in situ determination of lead after its extraction with dithizone using a substitution reaction with Hg-Salt

The catalytic activities for oxygen reduction of La_1?xLn_xNiO₃ (Ln: Nd, Sm) and of La?NiO₃ prepared by various methods were studied. La_0.9Nd_0.1NiO₃ was only a single phase perovskite type oxide in La_1?xLn_xNiO₃, and its catalytic activity was the same as that of LaNiO₃, as expected. All the other oxides of La_1?xLn_xNiO₃ investigated were mixtures consisting of a deformed perovskite structure oxide and a monoclinic lanthanoid oxide, and the catalytic activity decreased with promotion of the substitution, i.e., with increase of x. The deformation of the perovskite structure brings a decrease of the catalytic activity, and this was confirmed by comparison of the activities of three kinds of oxides of the single phase LaNiO₃ which had different degrees of crystallization. A fairly high current density of several tens of mA cm^?2 was obtained on an electrode with a form for practical use, when the oxide was prepared by decomposition of nitrates.     

Transport properties and stability of Ni-containing mixed conductors with perovskite- and K2NiF4-type structure

The total conductivity and Seebeck coefficient of a series of Ni-containing phases, including La₂Ni_1−xM_xO_4+δ (M=Co, Cu; x=0.1-0.2) with K₂NiF₄-type structure and perovskite-like La_0.90Sr_0.10Ga_0.65Mg_0.15Ni_0.20O_3−δ and La_0.50Pr_0.50Ga_0.65Mg_0.15Ni_0.20O_3−δ, were studied in the oxygen partial pressure range from 10⁻¹⁸ Pa to 50 kPa at 973-1223 K. Within the phase stability domain, the conductivity of layered nickelates is predominantly p-type electronic and occurs via small-polaron mechanism, indicated by temperature-activated hole mobility and p(O₂) dependencies of electrical properties. In oxidizing conditions similar behavior is characteristic of Ni-containing perovskites, which exhibit, however, significant ionic contribution to the transport processes. The role of ionic conduction increases with decreasing p(O₂) and becomes dominant in reducing atmospheres. All nickelate-based phases decompose at oxygen pressures considerably lower with respect to Ni/NiO boundary. The partial substitution of nickel in La₂Ni(M)O_4+δ has minor effect on the stability limits, which are similar to that of La_0.90Sr_0.10Ga_0.65Mg_0.15Ni_0.20O_3−δ. On the contrary, praseodymium doping enhances the stability of La_0.50Pr_0.50Ga_0.65Mg_0.15Ni_0.20O_3−δ down to p(O₂) values as low as 10⁻¹⁷-10⁻¹⁰ Pa at 1023-1223 K.