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).     

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.     

Phase and valence transitions in Ba2LnSnxSb1−xO6−δ (Ln=Pr and Tb)

Compounds in the double perovskites series Ba₂LnSn_xSb_1−xO_6−δ (Ln=Pr and Tb) have been synthesised and structurally characterised using synchrotron X-ray and neutron powder diffraction. It was found that the two end-members of the Ba₂PrSn_xSb_1−xO_6−δ series both adopt rhombohedral symmetry but the antimonate is a fully ordered double perovskite while the stannate has no B-site cation ordering. X-ray absorption near-edge structure (XANES) and near-infrared spectroscopy indicate that the Pr cations gradually change oxidation state from Pr³⁺ to Pr⁴⁺ with increasing x and that this is likely to be the cause of the loss of B-site ordering. Similarly, both Ba₂TbSbO₆ and Ba₂TbSnO_6−δ are cubic with B-site ordering present in the former but absent in the latter due to the oxidation state change of the Tb from Tb³⁺ to Tb⁴⁺. Multiple linear regression analysis of the Pr and Tb L_III-edge XANES indicates that the rate of Ln³⁺ transforming to Ln⁴⁺ is such that there are no oxygen vacancies in Ba₂PrSn_xSb_1−xO_6−δ but in Ba₂TbSn_xSb_1−xO_6−δ there is a small amount of oxygen vacancies, with a maximum of δ≈0.05 present.     

Magnetic and electrical properties of quadruple perovskites with 12 layer structures Ba4LnM3O12 (Ln=rare earths; M=Ru, Ir): The role of metal-metal bonding in perovskite-related oxides

Structures and magnetic and electrical properties of quadruple perovskites containing rare earths Ba₄LnM₃O₁₂ (Ln=rare earths; M=Ru, Ir) were investigated. They crystallize in the 12L-perovskite-type structure. Three MO₆ octahedra are connected to each other by face-sharing and form a M₃O₁₂ trimer. The M₃O₁₂ trimers and LnO₆ octahedra are alternately linked by corner-sharing, forming the perovskite-type structure with 12 layers. For Ln=Ce, Pr, and Tb, both the Ln and M ions are in the tetravalent state (Ba₄Ln⁴⁺M⁴⁺₃O₁₂), and for other Ln ions, Ln ions are in the trivalent state and the mean oxidation state of M ions is +4.33 (Ba₄Ln³⁺M^4.33+₃O₁₂). All the Ba₄Ln³⁺Ru^4.33+₃O₁₂ compounds show magnetic ordering at low temperatures, while any of the corresponding iridium-containing compounds Ba₄Ln³⁺Ir^4.33+₃O₁₂ is paramagnetic down to 1.8 K. Ba₄Ce⁴⁺Ir⁴⁺₃O₁₂ orders antiferromagnetically at 10.5 K, while the corresponding ruthenium-containing compound Ba₄Ce⁴⁺Ru⁴⁺₃O₁₂ is paramagnetic. These magnetic results were well understood by the magnetic behavior of M₃O₁₂. The effective magnetic moments and the entropy change for the magnetic ordering show that the trimers Ru^4.33+₃O₁₂ and Ir⁴⁺₃O₁₂ have the S= ground state, and in other cases there is no magnetic contribution from the trimers Ru⁴⁺₃O₁₂ or Ir^4.33+₃O₁₂.Measurements of the electrical resistivity of Ba₄LnM₃O₁₂ and its analysis show that these compounds demonstrate two-dimensional Mott-variable range hopping behavior.     

Composite electrodes for proton conducting electrolyte of CaZr<Subscript>0.95</Subscript>Sc<Subscript>0.05</Subscript>O<Subscript>3 – δ</Subscript>

A new method of obtaining gastight ceramic based on CaZr_0.95Sc_0.05O_{3 – δ} is presented. The microstructure and electric properties of the obtained samples, same as the behavior of composite electrodes in contact with this electrolyte, are studied for application of the obtained results in the technology of formation of electrochemical devices. The design of bilayer electrodes is suggested, in which the materials tested as the functional layer were layered lanthanum nickelate La2NiO_{4 + δ} and substituted lanthanum nickelate La_1.7Ca(Sr,Ba)_0.3NiO_{4 + δ} in combination with the electrolyte components of Ce_0.8Sm_0.2O_{2 – δ} and BaCe_0.89Gd_0.1Cu_0.01O_{3 – δ}. The collector layer used was lanthanum nickelate–ferrite LaNi_0.6Fe_0.4O_{3 – δ} and manganite La_0.6Sr_0.4MnO_{3 – δ} that are characterized by high electron conductivity, low layer resistance and are close by their values of coefficient of linear thermal expansion to the materials of functional layers. Electrochemical activity of the obtained electrodes are compared with the characteristics of composite electrodes based on lanthanum ferrite–cobaltite La_0.6Sr_0.4Fe_0.8Co_0.2O_{3 – δ} and deficient lanthanum manganite La_0.75Sr_0.2MnO_{3 – δ}.     

Homogeneity regions of neodymium,samarium, and europium manganites in air

XRD phase analysis of homogeneous phases and heterogeneous compositions of general formula Ln_2?x Mn_xO_3±δ (Ln = Nd, Sm, Eu; 0.90 ≤ x ≤ 1.20; Δx = 0.22) prepared by ceramic synthesis from oxides in air at 900–1400°C was used to determine the solubility boundaries for Ln₂O₃ oxides and maganese oxides in LnMnO_3±δ. The results were represented as fragments of the phase diagrams for the Ln-Mn-O systems in air. It was assumed that the solubility of Ln₂O₃ oxides in LnMnO_3±δ is determined by lattice defects, while that of manganese oxides, in addition to above mechanism, by the disproportionation reaction 2Mn³⁺ = Mn²⁺ + Mn⁴⁺ followed by the partial substitution of divalent magnesium for Ln³⁺ at cuboctahedral positions of the perovskitelike crystal lattice.     

Microwave synthesis of LiMg0.05Mn1.95O4 and electrochemical performance at elevated temperature for lithium-ion batteries

The microwave sintering method is used to synthesize the spinel LiMg_0.05Mn_1.95O₄ materials, and the structures and electrochemical performances of as-prepared powders are investigated. The powders resulting from the microwave synthesis are single crystalline phases with cubic spinel structure and exhibit outstanding structural stability. The discharge capacity and cycling stability of LiMg_0.05Mn_1.95O₄ are found to be superior with lower capacity fading over the investigated 100 cycles at elevated temperature (55 °C). The XRF and EIS measurements reveal that the doped LiMn₂O₄ synthesized by this simple method has lower dissolution of manganese into the electrolyte and higher electronic conductivity at high temperature for lithium ion batteries.     

Phase equilibria and crystal structure of the complex oxides in the Ln-Ba-Co-O (Ln=Nd, Sm) systems

The phase equilibria in the Ln-Ba-Co-O (Ln=Nd, Sm) systems were systematically studied at 1100 °C in air. The homogeneity ranges and crystal structure of the solid solutions: Ln_2−xBa_xO_3−δ (0<x≤0.1 for Ln=Nd and 0<x≤0.3 for Ln=Sm), Nd_3−yBa_yCo₂O₇ (0.70≤y≤0.80), BaCo_1−zSm_zO_3−δ (0.1≤z≤0.2) were determined by X-ray diffraction of quenched samples. The values of oxygen content (5+δ) for slowly cooled LnBaCo₂O_5+δ (Ln=Nd, Sm) samples were estimated as 5.73 for Ln=Nd, and 5.60 for Ln=Sm. The unit cell parameters were refined using Rietveld full-profile analysis. It was shown that NdBaCo₂O_5.73 possesses tetragonal structure and SmBaCo₂O_5.60 - orthorhombic structure. The projections of isothermal-isobaric phase diagrams for the Ln-Ba-Co-O (Ln=Nd, Sm) systems to the compositional triangle of metallic components were presented.     

Potentiometric cells with oxygen-conducting solid electrolyte based on ceria

The present work presents studies of potentiometric cells with electrolyte of 0.98Ce_0.8(Sm_0.75Sr_0.2Ba_0.05)_0.2O_1.875 + 0.02TiO₂ and electrodes of silver or various materials with a perovskite structure. The oxygen activity limit is determined, above which electrolyte features preferred oxygen conductivity. Electrode materials are suggested that feature an equilibrium potential in the gas mixtures with the oxygen content from 1 to 21% and their lower temperature applicability limit is determined. The rate of potential response of the electrodes to a fast change in the oxygen partial pressure over the electrodes is determined. The results of studies are of interest for development of electrochemical devices with electrolyte based on ceria operating at 500–750°C.     

Phase and valence transitions in Ba2LnSnxNb1−xO6−δ

The structures of compounds in the perovskite series Ba₂LnSn_xNb_1−xO_6−δ (Ln=Pr and Tb and x=0, 0.1, 0.2, …, 1.0) have been examined using synchrotron X-ray and neutron diffraction. It was found that niobate members of both series feature full B-site cation ordering but that this order is lost with increasing x. X-ray absorption near-edge structure (XANES) and near-infrared spectroscopies indicate that the oxidation state of the lanthanide cations gradually changes from Ln³⁺ to Ln⁴⁺ with increased Sn⁴⁺ doping. This is believed to be the cause of the loss of B-site ordering. Least squares analysis of the XANES spectra suggests that the rate of the transformation of Ln³⁺ cations to the tetravalent state is such that the Pr series contains no oxygen vacancies while the Tb series may contain a very small amount of vacancies, with δ≈0.02.     

Magnetic properties of quadruple perovskites Ba4LnRu3O12 (Ln=La, Nd, Sm-Gd, Dy-Lu)

Quadruple perovskites Ba₄LnRu₃O₁₂ (Ln=La, Nd, Sm-Gd, Dy-Lu) were prepared and their magnetic properties were investigated. They adopt the 12L-perovskite-type structure consisting of Ru₃O₁₂ trimers and LnO₆ octahedra. All of these compounds show an antiferromagnetic transition at 2.5-30 K. For Ba₄NdRu₃O₁₂, ferrimagnetic ordering has been observed at 11.5 K. The observed magnetic transition is due to the magnetic behavior of the Ru^4.33+₃O₁₂ trimer with S=. Magnetic properties of Ba₄LnRu₃O₁₂ were compared with those of triple perovskites Ba₃LnRu₂O₉ and double perovskites Ba₂LnRuO₆.     

Crystal growth of Ln2NiO4 (Ln = La,Pr, Nd) by skull melting

Growth of large single crystals of quasi-two-dimensional Ln₂NiO₄ (Ln = La, Pr, Nd) has been accomplished using the radio frequency technique of skull melting under controlled oxygen partial pressures. No evidence of intergrowth of other phases such as Ln₃Ni₂O₇ or higher homologs was observed. Determination of the Ni³⁺ content indicates that the crystals are cation deficient.     

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.     

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.     

Preparation of Y2Ba4Cu7O15-δ Superconductor without High Oxygen Pressure

The synthesis of bulk Y₂Ba₄Cu₇O_15-δ superconductor at atmospheric oxygen pressure via solid state sintering is reported. Temperature ranging from 860 to 890 °C as well as time interval over 2 to 15 days were used to investigate the formation of the Y₂Ba₄Cu₇O_15-δ phase. A time-temperature profile characterizing the conditions for the preparation of Y₂Ba₄Cu₇O_15-δ phase suggests the optimal condition to be sintering at 890 °C for over 10 days. Detailed results of X-ray diffraction, electrical resistivity, iodometric titration and magnetization measurements are described.     

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.     

Zur Kristallchemie der Oxoplatinate

On the Crystal Chemistry of Oxoplatinates Referring to the coordination of Pt²⁺ and Pt⁴⁺ by oxygen the crystal structures of oxoplatinates will be systemized. Pt²⁺ exclusively shows in solids a square and planar coordination well known and typical for Cu²⁺, Pd²⁺ and Ni²⁺ too. Often mixed valences (Pt²⁺/Pt⁴⁺) are observed in square planar oxygen surrounding. Some exceptionally rare coordination is the dumbbell like O—Pt²⁺—O. The crystal chemistry of Pt⁴⁺ resembles the countless metal ions showing octahedral coordination by oxygen. Despite different compositions the compounds BaLn₂PtO₅, Ba₃Cu₂Ln₂PtO₁₀, Ba₅Ln₈Zn₄O₂₁, Ba₁₃Ln₈Zn₄Pt₄O₃₇ and Ba₁₇Ln₁₆Zn₈Pt₄O₅₇ show amazing relationships of polyhedra connections. Additionally will be shown that Ba₄Sm₄Zn₃PtO₁₅ is isotypic to Ba₆Nd₂Al₄O₁₅ and Ba₄NaCu_{0, 5}Pt_{1, 5}O₈ to Ba₁₅Pt₆O₂₇. Oxoplatinates containing lone pair active elements show one side open polyhedra. The positions of the free (s²) electrons are calculated using the Coulomb terms of lattice energy.     

Luminescence enhancement of Eu, Ce co-doped Ba3Si5O13−δNδ phosphors

Host lattice Ba₃Si₅O_13−δN_δ oxonitridosilicates have been synthesized by the traditional solid state reaction method. The lattice structure is based on layers of vertex-linked SiO₄ tetrahedrons and Ba²⁺ ions, where each Ba²⁺ ion is coordinated by eight oxygen atoms forming distorted square antiprisms. Under an excitation wavelength of 365 nm, Ba₃Si₅O_13−δN_δ:Eu²⁺ and Ba₃Si₅O_13−δN_δ:Eu²⁺,Ce³⁺ show broad emission bands from about 400-620 nm, with maxima at about 480 nm and half-peak width of around 130 nm. The emission intensity is strongly enhanced by co-doping Ce³⁺ ions into the Ba₃Si₅O_13−δN_δ:Eu²⁺ phosphor, which could be explained by energy transfer. The excitation band from the near UV to the blue light region confirms the possibility that Ba₃Si₅O_13−δN_δ:Eu²⁺, Ce³⁺ could be used as a phosphor for white LEDs.     

Investigations on chemical interactions between alternate cathodes and lanthanum gallate electrolyte for Intermediate Temperature Solid Oxide Fuel Cell (ITSOFC)

The doped perovskite oxides such as La_0.65Sr_0.30MnO_3-δ (LSM), La_0.70Sr_0.30CoO_3-δ (LSC), La_0.65Sr_0.30FeO_3-δ (LSF), La_0.65Sr_0.30NiO_3-δ (LSN) and La_0.60Sr_0.40Co_0.20Fe_0.80O_3-δ (LSCF) are proposed as alternate cathode materials for solid oxide fuel cells working at reduced temperature (< 1073 K). The critical requirement for their applicability is their chemical compatibility in conjunction with an alternate solid electrolyte, La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-δ (LSGM) without any new phase formation. To understand the chemical reactivity between these two components, thoroughly mixed different cathode and LSGM electrolyte (1:1 by wt.) powders were pressed as circular components and subjected to annealing at 1573 K for 3 h in air. XRD and SEM were used for the characterization of the annealed samples. XRD measurements revealed that no new secondary phases were formed in LSM, LSC, and LSF with LSGM mixtures whereas LSN and LSCF with LSGM resulted in the formation of new secondary phases after high temperature treatment. The sintering shrinkage for all the components (cathode + electrolyte mixture) was also estimated. For comparison of data, the individual powders (cathode/electrolyte) were also compacted and studied in the same manner. The obtained results are discussed keeping in view the requirements that the candidate cathode material must meet out with respect to its chemical compatibility to qualify for the LSGM based ITSOFC systems at 1073 K.