Search

The La1.5Sr0.5Ni1–yCoyO4+δ (y = 0, 0.1, 0.2, 0.3, 0.4) complex oxides were synthesized by the citrate-nitrate route. The phase purity of the samples was confirmed by the powder X-ray diffraction (PXRD). All studied samples possess the K2NiF4-type structure with space group I4/mmm. The cobalt doping in La1.5Sr0.5Ni1–yCoyO4+δ leads to the expansion of oxygen octahedra in the ab plane and their shrinkage in the c direction, thus, decreasing the structural microstrain. The increase in cobalt concentration results in a gradual increase in oxygen over-stoichiometry, while the oxygen content in La1.5Sr0.5Ni1–yCoyO4+δ shows weak temperature dependence. It is shown that the majority of cobalt cations in these oxides are in the Co3+ state in the whole temperature range studied. The fitting results for the temperature dependencies of the Seebeck coefficient show that the Ni3+ cations are mostly in the low-spin state in La1.5Sr0.5Ni1–yCoyO4+δ in the range of 25–1000°C, although the increase in cobalt content leads to the non-monotonous increase in the fraction of high-spin Ni3+ cations at T > 600°C. The cobalt doping of La1.5Sr0.5Ni1–yCoyO4+δ decreases total conductivity due to the increase in localization of electron holes on the 3d-metal cations.

The La1.5Sr0.5Ni1-yCoyO4+δ (y = 0, 0.2, 0.4) complex oxides were synthesized by the citrate-nitrate method. The disk-shaped dense ceramic membranes with thickness of 0.8 and 1.2 mm for each composition were prepared by uniaxial pressing of the powders and sintering at 1350 °C in air. The phase purity of the samples was confirmed by the X-ray diffraction (XRD). The oxygen excess in the samples gradually increased with cobalt substitution and was almost independent of temperature. Oxygen permeation studies through the membranes with different thickness revealed that the cobalt doping eliminated the surface exchange limitations leading to a significant increase in the oxygen surface exchange coefficients (kex). On the contrary, the bulk oxygen diffusion was deteriorated by the cobalt doping, although the oxygen-ion conductivity remained almost unchanged. The electron backscatter diffraction (EBSD) analysis of the surface for the studied ceramic membranes indicated that the observed increase in the kex values could be explained by the effect of cobalt on the crystallographic orientation of crystallites. It is suggested that the cobalt doping increases the surface area of the ( 1 ‾ 10 ) , ( 1 ‾ 11 ) and ( 011 ) faces leading to a higher number of 3d-metals in the surface layer of the membranes.

The NdBaMnFeO6-δ layered perovskite was synthesized by a topotactic reaction in the N2/H2 atmosphere for the first time while controlling the sample mass during the experiment. The X-ray powder diffraction (XRPD) patterns of the samples with different oxygen content refined by the Rietveld method revealed the YBaCuFeO5-type tetragonal structure of NdBaMnFeO6-δ (sp. gr. P4/mmm). The results obtained by selected area electron diffraction (SAED) and by high-resolution transmission electron microscopy (HRTEM) clearly evidenced the doubling of the c parameter without any additional superstructure reflections. The NdBaMnFeO6-δ phase exhibited narrower variation in oxygen content compared with that for NdBaMn2O6-δ and NdBaFe2O6-δ. This was explained by a higher distortion of square-pyramidal/octahedral surrounding of 3d-metals in NdBaMnFeO6-δ, which significantly increased with oxygen content, reducing the upper limit of oxygen content down to ≈5.45.

The effect of A-site substitution by alkaline earth metals (A Ba, Sr, Ca) on the crystal structure and properties of the Nd1−xAxMn0.5Fe0.5O3−δ (x = 0, 0.25) perovskite-like oxides were studied in the temperature range of 25–1000 °C in air. The Nd1−xAxMn0.5Fe0.5O3−δ samples were synthesized by the citrate-nitrate combustion technique. X-ray powder diffraction (XRPD) patterns were refined by the Rietveld method using orthorhombic unit cell for NdMn0.5Fe0.5O3−δ and Nd0.75Ba0.25Mn0.5Fe0.5O3−δ (sp. gr. Pnma); and monoclinic unit cell for Nd0.75Ca0.25Mn0.5Fe0.5O3−δ and Nd0.75Sr0.25Mn0.5Fe0.5O3−δ (sp. gr. P21/n). The refinement results for the single-phase samples revealed that the unit cell volume decreased while decreasing the size of A-dopant in Nd1−xAxMn0.5Fe0.5O3−δ. Oxygen content and oxidation state of Mn in Nd1−xAxMn0.5Fe0.5O3−δ increased in the row: Nd0.75Ca0.25Mn0.5Fe0.5O2.81, Nd0.75Sr0.25Mn0.5Fe0.5O2.85, Nd0.75Ba0.25Mn0.5Fe0.5O2.93 at all temperature studied. Oxygen non-stoichiometry of the Nd1−xAxMn0.5Fe0.5O3−δ samples was almost independent of temperature even at high temperatures except for NdMn0.5Fe0.5O3−δ. The thermal expansion coefficient (TEC) for Nd0.75Ba0.25Mn0.5Fe0.5O3−δ was found to be 10.1 × 10−6 K−1 in the range of 25–800 °C. Temperature dependencies of total conductivity possessed semiconductor-type behavior and were interpreted within the small polaron hopping mechanism. The maximum value of conductivity (25 S/cm) was obtained for Nd0.75Ba0.25Mn0.5Fe0.5O3−δ at 1000 °C in air. The Seebeck coefficient values decreased with temperature from positive to negative values indicating that both electrons and electron holes may contribute to the electric conductivity of the oxides.

The La2-xCaxNi1-y(Fe/Cu)yO4+δ solid solutions with х = 0.2, 0.4, 0.5 and y = 0, 0.1, 0.2, 0.3 were synthesized by decomposition of citrate–nitrate precursors. The single-phase La2-xCaxNi1-y(Fe/Cu)yO4+δ materials crystallized in the K2NiF4-type structure, sp. gr. I4/mmm. The crystal structure parameters of La2-xCaxNi1-y(Fe/Cu)yO4+δ were refined by the Rietveld method. The unit cell volume of La2-xCaxNi1-y(Fe/Cu)yO4+δ decreased with x and increased with y. All studied samples exhibited oxygen excess compared to the stoichiometric composition (δ > 0) in the temperature range 30–1100 °C except for La1.6Ca0.4Ni1-yCuyO4+δ (y = 0.2 and 0.3). Calcium and copper doping decreased the total oxygen content while iron doping increased it in La2-xCaxNi1-y(Fe/Cu)yO4+δ. Thermal expansion coefficients (TECs) of La2-xCaxNi1-y(Fe/Cu)yO4+δ slightly increased with dopant concentration and varied in the range (14.5–15.1) × 10−6 K−1. The contribution of chemical expansion was shown to be negligible. The total conductivity of La2-xCaxNi1-y(Fe/Cu)yO4+δ was thermally activated within the whole studied temperature range 30–1000 °C and could be described in terms of small polaron conduction mechanism. The values of total conductivity increased with x and decreased with y in La2-xCaxNi1-y(Fe/Cu)yO4+δ. The analysis of the Seebeck coefficient vs temperature for La2-xCaxNi1-y(Fe/Cu)yO4+δ showed that the main charge carriers are electron holes localized on Ni2+ cations represented by low-spin Ni3+ cations which had a tendency to change the spin configuration gradually to high-spin when the temperature and dopant content increased.

The phase equilibria in the NdFeO3 − δ – SrFeO3 − δ – SrCoO3 − δ – NdCoO3 − δ system were studied at 1373 K in air. The homogeneity ranges and crystal structure of Nd1 − xSrxFe1 − yCoyO3 − δ solid solutions were determined by the X-ray powder diffraction, the unit cell parameters were refined by the Rietveld analysis. It was found that Nd1 − xSrxFe1 − yCoyO3 − δ with 0.0 ≤ x ≤ 0.6 and 0.1 ≤ y ≤ 0.9 possesses orthorhombic structure (sp. gr. Pbnm) that reveals O-type → O′-type → O-type structural transition. The orthorhombic distortion decreases with the increasing of strontium content, as a result Nd1 − xSrxFe1 − yCoyO3 − δ solid solutions with 0.7 ≤ x ≤ 0.9 and 0.1 ≤ y ≤ 0.9 possess the cubic structure (sp. gr. Pm3m). Isothermal-isobaric phase diagram for the NdFeO3 − δ – SrFeO3 − δ – SrCoO3 − δ – NdCoO3 − δ system at 1373 K in air has been presented. A series of samples with the overall composition Nd0.2Sr0.8Fe1 − yCoyO3 − δ (0.0 ≤ y ≤ 1.0) were characterized by the thermogravimetric analysis, iodometric titration, dilatometry and electrical conductivity measurements. The oxygen nonstoichiometry in Nd0.2Sr0.8Fe1 − yCoyO3 − δ (0.0 ≤ y ≤ 1.0) increases with increasing temperature and cobalt content. Gradual substitution of iron by cobalt in the Nd0.2Sr0.8Fe1 − yCoyO3 − δ oxides leads to the increase of the average thermal expansion coefficients. The electrical conductivity exhibits maximum within the range of 600–750 K and significantly increased with cobalt content at T

Nd0.5Ba0.5Mn0.5Fe0.5O3−δ (NBMF55) adopts a cubic perovskite structure in the temperature range of 25–1000 °C in air. The oxygen deficit in NBMF55, equal to 0.14 at room temperature (RT), noticeably increased with temperature due to oxygen release at T > 330 °C. The sample exhibits a moderate value of linear thermal expansion coefficient (LTEC), 14.4 × 10−6 K−1, within 25–800 °C. Total conductivity in NBMF55 increased with temperature yielding 25 S/cm at 1000 °C in air. The conduction was thermally activated with the activation energy of 0.33 eV suggesting a hopping mechanism. The impedance measurements for a symmetrical cell based on the Ce0.8Sm0.2O2−δ (SDC) electrolyte material revealed the area-specific resistance (ASR) of 2.2 Ω cm2 at 700 °C in air.

Oxygen-ion transport in La2−xSrxNi1−yFeyO4+δ complex oxides (x = 0.5, y = 0.1–0.4 and x = 0.8, y = 0.1, 0.2, and 0.4) was studied by steady-state oxygen permeation and modified Hebb–Wagner polarization methods. The samples were obtained by a decomposition of citrate–nitrate precursors followed by three times subsequent annealing at 1100 °C in air, pelletizing and sintering at 1300–1350 °C for 20 h in air. The phase purity of the samples was confirmed by an X-ray diffraction (XRD) method. Energy dispersive X-ray (EDX) analysis of the pellets after the steady-state oxygen permeation measurements revealed negligible diffusion of dopant cations and an almost constant oxygen gradient along the samples' thickness. Strontium was shown to decrease the oxygen permeation through the overstoichiometric La2−xSrxNi1−yFeyO4+δ (δ > 0) membranes and to increase it for the oxygen deficient ones (δ < 0). Iron promoted bulk diffusion in all studied samples. The oxygen surface exchange in the series La1.5Sr0.5Ni1−yFeyO4+δ was slightly deteriorated by iron doping up to y = 0.3 but significantly enhanced at y = 0.4. The values of oxygen-ion conductivity calculated from the oxygen permeation data were in good agreement with those measured by the modified Hebb–Wagner polarization method. The highest oxygen permeation flux was obtained for the La1.5Sr0.5Ni0.6Fe0.4O4+δ membrane. La1.5Sr0.5Ni0.6Fe0.4O4+δ showed lower oxygen-ion conductivity compared with undoped La2NiO4+δ, but substantially improved oxygen surface exchange. The latter factor plays a crucial role in the cathode performance of the studied La2NiO4+δ-based oxides and makes La1.5Sr0.5Ni0.6Fe0.4O4+δ a promising candidate for IT-SOFC application.

The effect of air humidity on the transport properties of the La2-xAxNi1-yFeyO4+δ (A=Ca, Sr; x = 0.5, 0.6 and y = 0.4, 0.5) oxides was studied in the temperature range of 25–950 °C. Total conductivity of the samples decreased with increasing relative humidity (RH), most significantly in the range of 250–550 °C. Thermogravimetric analysis in dry and wet air indicated that the observed changes in conductivity could be attributed to the hydration of the samples. The high-temperature X-ray powder diffraction (HT-XRPD) results for La1.4Sr0.6Ni0.5Fe0.5O4+δ (LSNF0605) suggested that the hydration led to the insertion of hydroxide ions into the interstitial sites resulting in the expansion of the rock-salt type layers and shrinkage of the perovskite layers. The Electrochemical Impedance Spectroscopy (EIS) measurements for the LSNF0605/Ce0.8Sm0.2O2-δ (SDC) and LSNF0605/La28−zW4+zO54+1.5z (LWO) symmetrical cells showed that the hydration affected the surface exchange process in LSNF0605. The activation energy of surface exchange decreased with the increase in RH for the LSNF0605/LWO cell indicating that water could participate in the surface exchange. The activation energy of the charge transfer – ionic diffusion process in the LSNF0605 electrode for the LSNF0605/LWO cell possessed smaller values compared to that for the LSNF0605/SDC cell in wet air, suggesting the presence of proton conduction.

The effect of A-site doping by alkaline earth metals (A = Ca, Sr and Ba) on optical, magnetic, and electrical properties of Nd1-xAxMn0.5Fe0·5O3-I´ (x = 0, 0.25) has been investigated. The UVâ��vis absorption spectra show that the A-site doping decreases the absorption. Two values of optical band-gap energy (Eg) can be estimated for each studied sample: the higher Eg value is associated with the charge transfer involving iron cations and the lower Eg value â�� with the charge transfer via manganese cations. Partial substitution of neodymium by alkaline earth metals decreases the NA©el temperature (TN) and induces significant irreversibility between the zero-field cooled (ZFC) and field-cooled (FC) data below TN. The field-dependent magnetization at 3 K indicates antiferromagnetic ordering with a spin canting. Temperature dependencies of resistivity demonstrate the change from metallic to semiconductor-type conduction (with increasing temperature) at T = Tp TN. Within the range of Tp < T < TN, the resistivity data are interpreted by the variable range hopping (VRH) mechanism. A significant value of magnetoresistance (â�¼44) is observed only for Nd0.75Sr0·25Mn0·5Fe0·5O3-I´ at 120 K. © 2020 Elsevier B.V.

The effect of A-site substitution by alkaline earth metals on properties of NdMnO3-δ have been investigated. NdxA1-xMnO3 (A=Ba, Sr, Ca; x=0, 0.25) have been synthesized by a citrate-nitrate combustion technique. The samples were characterized by X-ray powder diffraction (XRPD), thermo-gravimetric analysis and the standard 4-probe DC technique in the temperature range of 25-1000 °C in air. The XRPD patterns revealed that all samples were single-phase and possessed an orthorhombic structure (Pnma space group). The Rietveld refinement of the patterns showed that the unit cell volume reduced with a decrease in the dopant size. The temperature dependencies of mass loss indicated that oxygen release from the samples started only at temperatures above 600°C. The total conductivity of the samples indicated a semiconducting behaviour in the whole temperature range studied. The highest value of total conductivity (205 S/cm) was observed for Nd0.75Ba0.25MnO3 at 1000 °C in air.

A series of La2−xSrxNi1−yFeyO4+δ complex oxides adopting the K2NiF4-type structure (sp.gr. I4/mmm) was prepared via the decomposition of citrate–nitrate precursors, followed by multiple annealing treatments at 1100 °C in air. Strontium for lanthanum substitution in La2−xSrxNi1−yFeyO4+δ leads to a progressive increase of iron solubility (y) which reaches maximum values at x = 1.0–1.2. The crystal structure of single-phase samples was refined by the Reitveld method. The unit cell volume increases with y and decreases with x in La2−xSrxNi1−yFeyO4+δ. The solid solutions of La2−xSrxNi1−yFeyO4+δ were shown by TGA to be over-stoichiometric (δ > 0) at x = 0.5 and 0.6 and oxygen deficient (δ < 0) at x = 0.8 within the temperature range of 25–1050 °C in air. The thermal expansion coefficient of La2−xSrxNi1−yFeyO4+δ increases with x and y, varying within the range of (12–16) × 10−6 K−1 up to 700 °C in air. In the higher temperature range, the value of the TEC increases up to 20 × 10−6 K−1 due to a chemical expansion contribution. The total conductivity and Seebeck coefficient were measured in air from RT to 1100 °C. The maximum conductivity value equal to 289 S cm−1 was obtained for La1.2Sr0.8Ni0.9Fe0.1O4+δ at 460 °C in air. The conduction is temperature activated for all samples within the composition range under study. The temperature dependencies of the Seebeck coefficient were explained in the approximation of the small-polaron hopping mechanism. The charge carriers were electron holes localized on nickel forming Ni3+ cations in low- and high-spin states.

The series of complex oxides, La2 -xSrxNi1 -yMnyO4 -δ (х = 0.7–1, y = 0.1, 0.2, 0.3, 0.5, 0.7), were synthesized via glycine–nitrate and citrate–nitrate combustion techniques followed by sintering at 1350–1400°C in air. All single-phase compounds have the K2NiF4-type structure (sp.gr. I4/mmm). The unit cell volume increases with y and decreases with x in La2 -xSrxNi1 -yMnyO4 ± δ. At room temperature, the solid solutions LaSrNi1 -yMnyO4 ± δ (y = 0.1, 0.3, 0.5, and 0.7) show a small excess of oxygen (δ > 0) which increases with the manganese doping level y. Total conductivity and Seebeck coefficient were determined in air from room temperature to 1100°C, with the maximum value ~ 100 S/cm for La1.2Sr0.8Ni0.9Mn0.1O4 ± δ and La1.3Sr0.7Ni0.9Mn0.1O4 ± δ. The conduction is temperature activated for all phases under study. Seebeck coefficient has tendency to decrease from low temperature to high temperature and predominantly adopts slightly negative values varying from − 15 to − 5 μV/K.

This study focuses on the application of the La 2 NiO 4 -based cathodes in the electrolyte-supported fuel cells with La 0.88 Sr 0.12 Ga 0.82 Mg 0.18 O 3-δ (LSGM1218) as an electrolyte, Sr 2 Ni 0.75 Mg 0.25 MoO 6-δ (SNMM) as an anode and Ce 0.8 Sm 0.2 O 2-δ (SDC) as an interlayer between the cathode and electrolyte. La 2 NiO 4+δ and La 1.5 Ca 0.5 Ni 1- y Fe y O 4+δ ( y = 0.3, 0.33, 0.35, 0.37, 0.4) have been synthesized by a citrate-nitrate combustion technique. La 1.5 Ca 0.5 Ni 0.7 Fe 0.3 O 4+δ (LCNF73), La 1.5 Ca 0.5 Ni 0.67 Fe 0.33 O 4+δ (LCNF6733) and La 2 NiO 4+δ have been tested as cathodes for SOFCs. LCNF6733 cathode material has been prepared with different microstructure in fuel cells of similar configuration (designated as I-­LCNF6733 and II-­LCNF6733). Higher power densities of the II-LCNF6733/SDC/LSGM1218/SNMM fuel cell than those of LCNF73/SDC/LSGM1218/SNMM indicate that iron doping might improve the performance of these oxides as cathode materials. The cells with the II-­LCNF6733 and La 2 NiO 4+δ cathodes show similar power densities reaching 277 and 270 mW cm − 2 at 800 °C, respectively.

The authors would like to acknowledge that the equipment of the Ural Center for Shared Use ‘Modern nanotechnology’ SNSM UrFU was used in this research. This work was supported in parts by the Ministry of Education and Science of the Russian Federation (State Task 4.2288.2017) and by Act 211 Government of the Russian Federation, agreement 02.A03.21.0006.

The oxygen permeation flux through dense La 1.2 Sr 0.8 Ni 0.9 Fe 0.1 O 4+δ membranes with different thicknesses (0.8 and 1.2 mm) was measured versus oxygen partial pressure gradient in the temperature range of 800–1000 ∘ C. The calculated values of critical membrane thickness clearly indicated the change of rate-limiting step from bulk oxygen diffusion (T < 900 ∘ ) to surface exchange (T > 900 ∘ ). In contrast to the promising cathode material, such as oxygen-excessive La 1.5 Sr 0.5 Ni 0.6 Fe 0.4 O 4+δ , the oxygen-deficient La 1.2 Sr 0.8 Ni 0.9 Fe 0.1 O 4+δ showed similar values of surface exchange coefficients, while oxygen-ion conductivity possessed higher activation energy and was significantly lower in the intermediate temperature range. The research was supported by the Ministry of Education and Science of the Russian Federation Agreement No. 02.A03.21.0006.

The MIEC La1.5Sr0.5Ni1-yFeyO4 (y=0.1-0.4) oxides have been studied as cathode materials with La0.88Sr0.12Ga0.82Mg0.18O3-δ (LSGM) electrolyte. Total conductivity, thermal expansion, oxygen nonstoichiometry, and chemical compatibility with LSGM and Ce0.8Sm0.2O1.9 (SDC) were determined. The following fuel cells were tested: La1.5Sr0.5Ni1-yFeyO4 (y=0.1, 0.2, 0.3, 0.4)/SDC/LSGM/Sr2N0.75Mg0.25MoO6 (SNMM) and La1.5Sr0.5Ni0.6Fe0.4O4/SDC/LSGM/SDC/NiO-SDC. For the former, the maximum power densities were 218, 274, 222, and 390 mW/cm2 at 850 °C in case of y equal to 0.1, 0.2, 0.3, and 0.4, respectively. The latter cell showed maximum power density of 341 mW/cm2 at 850°C.