Phase equilibriums, oxygen exchange kinetics and diffusion in oxides CaZr1 ? x Sc x O3 ? x/2 ? δ

Oxides CaZr_{1 ? x} Sc _x O_{3 ? x/2 - ??} (x = 0.00?C0.20) were synthesized according to the ceramic technology. The solubility boundary of scandium with formation of solid solutions on the basis of calcium zirconate CaZrO_{3 ? ??} corresponds to x = 0.07?C0.08. The second phase of CaSc₂O₄ is present in the samples with scandium content of x = 0.10, 0.15, 0.20. Its fraction grows at an increase in x. The method of full-profile Rietveld analysis was used to calculate the structure parameters for oxides CaZr_0.99Sc_0.01O_{2.995 ? ??} and CaZr_0.95Sc_0.05O_{2.975 ? ??}. The method of isotopic exchange with gas phase analysis was used to study the kinetics of gas-phase oxygen interaction with the CaZr_0.95Sc_0.05O_{2.975 ? ??} oxide in the temperature range of 700?C850°C and at oxygen pressures of 0.13?C6.67 kPa. The values of effective activation energies of the oxygen exchange and diffusion processes were 1.36 ± 0.32 and 1.92 ± 0.21 eV, accordingly. The dependence of the interphase exchange rate on the pressure of oxygen corresponds to the power law with the exponent of 0.31 ± 0.04 at the temperature of 750°C.     

Preparation and characterization of ceria-Based electrolytes for intermediate temperature solid oxide fuel cells (IT-SOFC)

Solid-oxide fuel cells (SOFCs) can be used for clean, efficient and environment-friendly energy conversion with a variety of fuels at high temperature (1273 K). The high temperature operation accelerates unwanted reactions and creates materials challenges; so, intermediate-temperature SOFCs (IT-SOFCs) have been developed. Reduction of the operating temperature (between 873–1073 K) requires solid electrolyte materials with higher conductivities. In this study, partially substituted ceria as solid electrolyte is experimented systematically for use in solid oxide fuel cells operating below 1073 K (intermediate temperature range). Nine compositions namely, CeO₂, Ce_0.95Gd_0.05O_2-δ (CGO9505), Ce_0.90Gd_0.10O_2-δ (CGO9010), Ce_0.85Gd_0.15O_2-δ (CGO8515), Ce_0.80Gd_0.20O_2-δ (CGO8020), Ce_0.95Sm_0.05O_2-δ (SDC9505), Ce_0.90Sm_0.10O_2-δ (SDC9010), Ce_0.85Sm_0.15O_2-δ (SDC8515) and Ce_0.80Sm_0.20O_2-δ (SDC8020) were synthesized by Glycine Nitrate (GN) combustion technique and investigated. The physical properties and the other relevant features of the data obtained are analyzed with a view to use these alternate electrolyte materials in IT-SOFC.     

Exchange kinetics and diffusion of oxygen in systems based on lanthanum gallate

The oxygen exchange and diffusion kinetics are studied by means of isotopic exchange in complex oxides La_0.80Sr_0.20Ga_{0.85 ? x} Mg_0.15Co _x O_{3 ? δ} (x = 0.00, 0.05, 0.15, 0.20, 0.25) and in electrolyte La_0.88Sr_0.12Ga_0.82Mg_0.18O_{3 ? δ} with gold-coated surface. The oxygen exchange rates and diffusion coefficients are determined in the bulk material and near the surface within the range of temperatures from 600 to 900°C and oxygen pressures from 0.3 to 1.0 kPa. The activation energies of the oxygen exchange and diffusion processes are calculated. By cobalt doping, gallium sublattice is shown to increase the interphase exchange rate; however, the oxygen transport properties in the bulk phase are not significantly affected by cobalt, while cobalt additive decreases the activation barrier of oxygen diffusion in the near-surface area. Gold activates the electrolyte surface, affecting the exchange rate and increasing the first-type exchange share. Cobalt substitution increases the third-type exchange share as compared to that of La_0.88Sr_0.12Ga_0.82Mg_0.18O_{3 ? δ} (x = 0.00, 0.05, 0.15, 0.20, 0.25) and in elec. The oxygen exchange with gas phase is assumed to be limited by molecular oxygen adsorption-desorption process on the electrolyte surface.     

Mechanism of oxygen transfer in layered lanthanide nickelates Ln2 − x NiO4 + δ (Ln = La, Pr) and their nanocomposites with Ce0.9Gd0.1O2 − δ and Y2(Ti0.8Zr0.2)1.6Mn0.4O7 − δ solid electrolytes

The mechanism of oxygen transfer in layered nickelates having a Ruddlesden-Popper structure and their nanocomposites with Ce_0.9Gd_0.1O_{2 ? δ} (GDC) and Y₂(Ti_0.8Zr_0.2)_1.6Mn_0.4O_{7 ? δ} (YTZM) solid electrolytes having fluorite and pyrochlore structures were studied by the oxygen isotope heteroexchange method in a flow and static reactor, thermoprogrammed desorption, and semiempirical interacting bonds method. The experimental heteroexchange data were adequately described by assuming that all atoms were equivalent in exchange in the bulk of layered nickelates, which was consistent with the cooperative oxygen migration model with fast exchange between the interstitial and regular positions. Strong interaction between the domains of the nickelate phases and solid electrolytes in nanocomposites, accompanied by a redistribution of cations between the phases, hindered the cooperative oxygen migration and led to a decrease in the diffusion coefficient as the exchange rate increased.     

The effect of phase composition on the transport properties of composites La0.8Sr0.2Fe0.7Ni0.3O3 ? δ-Ce0.9Gd0.1O1.95

Full conductivity, diffusion and oxygen exchange processes in composites (100 − x)La_0.8Sr_0.2Fe_0.7Ni_0.3O_{3 − δ}−xCe_0.9Gd_0.1O_1.95 (x is the volume fraction, 0 ≤ x ≤ 71.1%) at 700°C over the oxygen partial pressure range from 0.2 to 3 × 10⁻³ atm are studied by the electrical conductivity relaxation method. The composites’ conductivity was shown to decrease monotonically with the increasing of Ce_0.9Gd_0.1O_1.95 fraction, while the oxygen chemical diffusion coefficient increased. The oxygen exchange constant is higher for the composites than for the individual phases of La_0.8Sr_0.2Fe_0.7Ni_0.3O_{3 − δ} and Ce_0.9Gd_0.1O_1.95. Possible reason of the dependence of the parameters D _chem and k _chem on the temperature, oxygen pressure, and the composite composition is the effect of the interface on the oxygen transfer processes. Most effective oxygen transfer occurs in the composites whose composition approaches La_0.8Sr_0.2Fe_0.7Ni_0.3O_{3 − δ}-Ce_0.9Gd_0.1O_1.95 (x = 71%).     

Kinetics of gas-phase oxygen exchange with La0.6Sr0.4MeO3 ? δ (Me = Mn, Co)

The method of isotopic exchange was used to study the kinetics of interaction between the gasphase oxygen and oxides, La_0.6Sr_0.4MnO_{3 ? ??} (LSM) and La_0.6Sr_0.4CoO_{3 ? ??} (LSC), in the temperature range of 600?C850°C at the oxygen pressures of 0.13?C8.53 kPa. The values of the interphase exchange rate and oxygen diffusion coefficient were determined. Effective activation energies of the oxygen exchange and diffusion processes were 0.71 ± 0.16 and 1.42 ± 0.32 eV for LSM and 0.11 ± 0.03 and 1.08 ± 0.19 eV for LSC, accordingly. The contributions of the three oxygen exchange types were calculated. It was found that the exponent in the dependence of the interphase exchange rate on $P_{O_2 }$ (n) and the exponent in the dependence of the concentration of oxygen vacancies in the oxide on $P_{O_2 }$ (?) are related as: n = 1 + ?. Fulfillment of this relationship implies participation of the molecular form of oxygen (O₂)_a on the surface of the studied oxides as the rate-determining stage of exchange.     

Thermal characterization of Er-doped and Er–Gd co-doped ceria-based electrolyte materials for SOFC

Ce_1?xEr_xO₂ and Ce_1?2xEr_xGd_xO₂ co-doped ceria electrolyte nanopowder materials were successfully prepared by sol–gel method. Depending on the temperature, the crystal structure changes were analyzed by X-ray diffraction. It was observed that the crystal size of the electrolytes decreased depending on the temperature and the time. X-ray diffraction results confirmed cubic fluorite structure in the samples. The microstructural properties of the samples were analyzed by scanning electron microscopy, and thermal stability measurement was performed by thermogravimetric and differential thermal analyses. The total electrical conductivity of the nanopowder electrolytes was determined by the dc four-point probe technique in air at temperatures ranging from room temperature to 1373 K. The four-probe conductivity results revealed that Ce_0.8Er_0.1Gd_0.1O₂ has a higher ionic conductivity compared to Ce_0.83Er_0.17O₂ at 1123 K. The four-probe conductivity results show that both Ce_1?xEr_xO₂ and Ce_1?2xEr_xGd_xO₂ solid electrolytes have potential application to oxide ionic conductor for solid oxide fuel cells.     

Conductivity,oxygen interfacial exchange and diffusion in oxides based on lanthanum gallate

The method of isotopic exchange was used to study the oxygen exchange kinetics in the oxides of La_0.88Sr_0.12Ga_0.82Mg_0.18O_{3 − δ} and La_0.80Sr_0.20Ga_0.85−x Mg_0.15Co _x O_{3 − δ} (x = 0.05, 0.15, 0.20, 0.25). The rates of oxygen exchange and its diffusion coefficients were determined in the temperature range of 600–900°C at the oxygen pressure of 5 torr. The fractions of the three exchange types for the oxides studied were determined at the temperature of 817°C and oxygen pressure of 5 torr. The total conductivity of the oxides of La_0.80Sr_0.20Ga_0.85−x Mg_0.15Co _x O_{3 − δ} (x = 0.05, 0.15, 0.20, 0.25) was measured in the temperature range of 550–850°C in air and at the temperature of 800°C in the range of oxygen pressures of 1–760 torr. It was shown that an increase in the electronic conductivity component due to an increase in the cobalt fraction in the gallium sublattice results in growing interfacial exchange rate, total conductivity of the studied systems, and a decrease in the effective conductivity activation energy.     

The Effect of Chromium Oxide on the Conductivity of Ce<Subscript>0.9</Subscript>Gd<Subscript>0.1</Subscript>O<Subscript>2</Subscript>, a Solid-Oxide Fuel Cell Electrolyte

To study the effect of chromium oxide on the electric properties of Ce_0.9Gd_0.1O₂, a solid-oxide fuel cell electrolyte, two approaches were used: (a) the studying of electrochemical properties of the Ce_0.9Gd_0.1O₂- electrolyte after the spontaneous adsorption of chromium-containing molecules from a gas phase and (b) the analyzing of transport properties of the Ce_0.9Gd_0.1O₂-based chromium-containing compositions obtained by the mixing of solid-oxide electrolyte with chromium(III) oxide. It was found that the chromium reduction at the electrolyte surface dominates when chromium is adsorbed from gas phase. Both approaches allow concluding that the chromium presence in Ce_0.9Gd_0.1O₂ deteriorates the electrolyte transport properties at temperatures above 735°С. This is caused by the chromium incorporation into the electrolyte’s fluorite structure, as well as surface microheterogeneity induced by the chromium presence at the Ce_0.9Gd_0.1O₂ surface and the cerium and gadolinium cation redistribution between the grains’ bulk and surface. At intermediate temperatures (below 735°С) the electric conductivity of the Ce_0.9Gd_0.1O₂-based chromium-containing composition exceeds that of the initial solid-oxide electrolyte, which can be due to changes in transport properties of the chromium-containing phases formed at the Ce_0.9Gd_0.1O₂ surface and grain boundaries.     

On the sol–gel synthesis and catalytic activity of Ce1−xAxO2−δ (A = Pr, Sm, Gd) SOFCs anode materials for reforming of methane

A sol–gel route to synthesize nanocrystalline praseodymium-, samarium- and gadolinium-doped ceria powders for solid oxides fuel Cells SOFCs is presented. The method involves metal nitrates with propionic acid (both as chelating ligand and solvent), gel formation, liquid nitrogen quenching, drying at 150 °C/24 h, and finally decomposition at 450 °C in nitrogen followed by calcination at 650 °C in air. TG–DTA, BET, XRD, FTIR, UV–vis and catalytic tests were used to characterize the samples. Ce_0.8Pr_0.2O_2?δ sample exhibited the best catalytic performance in methane steam reforming under water deficient conditions, closely followed by Ce_0.9Gd_0.1O_2?δ, Ce_0.8Sm_0.2O_2?δ and Ce_0.8Gd_0.2O_2?δ catalysts. The superior catalytic performance of Ce_0.8Pr_0.2O_2?δ sample was attributed to the existence of praseodymium species (Pr⁴⁺/Pr³⁺) strongly interacting with ceria. The two systems act synergistically in the catalytic steam reforming of methane.     

Studies of oxygen transport mechanism in electrolytes based on doped lanthanum silicate with apatite structure using techniques of oxygen isotopic heteroexchange and impedance spectroscopy

The work presents the results of studying the mechanism of oxygen transport for a new promising class of oxygen-containing electrolytes based on lanthanum silicate with an apatite structure using impedance spectroscopy and isotopic oxygen heteroexchange. At 1000 K, in the case of samples with an optimum composition including codoped Fe and Al, σ ～ 3 × 10^?3 to 10^?2 S/cm and D* reaches ～10^?8 cm²/s, which is close to the values of YSZ and Ce_0.9Gd_0.1O_{2 ? δ} (GDC). Lower energies of conductivity activation and oxygen diffusion for doped apatites (～0.5–0.8 eV instead of ～1 eV for GDC) and also equivalence as regards exchange of all oxygen atoms within apatite agree with the model, in which oxygen mobility is determined by a nonlinear cooperative migration process of oxygen atoms with fast exchange between interstitial and regular sites.     

Preparation of aqueous sols of Ce1 − x Gd x O2-δ, Y0.9Eu0.1VO4 and nanocomposites Ce1 − x Gd x O2-δ/Y0.9Eu0.1VO4 stabilized by polyacrylic acid

A method for the synthesis of stable aqueous sols of nanocrystalline solid solutions Ce_1?x Gd _x O_2-δ (x = 0.10, 0.15, 0.20) and Y_0.9Eu_0.1VO₄ and nanocomposites Ce_{1 ? x} Gd _x O_2-δ/Y_0.9Eu_0.1VO₄ stabilized by biocompatible low-molecular-weight polyacrylic acid was proposed. Polyacrylic acid was shown to be a promising matrix for the preparation of polyfunctional composite materials.     

A Mixed Ionic and Electronic Conducting Dual‐Phase Membrane with High Oxygen Permeability

To combine good chemical stability and high oxygen permeability, a mixed ionic‐electronic conducting (MIEC) 75 wt % Ce_0.85Gd_0.1Cu_0.05O_2?δ‐25 wt % La_0.6Ca_0.4FeO_3?δ (CGCO‐LCF) dual‐phase membrane based on a MIEC–MIEC composite has been developed. Copper doping into Ce_0.9Gd_0.1O_2?δ (CGO) oxide enhances both ionic and electronic conductivity, which then leads to a change from ionic conduction to mixed conduction at elevated temperatures. For the first time we demonstrate that an intergranular film with 2–10 nm thickness containing Ce, Ca, Gd, La, and Fe has been formed between the CGCO grains in the CGCO‐LCF one‐pot dual‐phase membrane. A high oxygen permeation flux of 0.70 mL min^?1 cm^?2 is obtained by the CGCO‐LCF one‐pot dual‐phase membrane with 0.5 mm thickness at 950 °C using pure CO₂ as the sweep gas, and the membrane shows excellent stability in the presence of CO₂ even at lower temperatures (800 °C) during long‐term operation.     

Effect of graphite pore former on oxygen electrodes prepared with La0.6Sr0.4CoO3−δ nanoparticles

This study aimed at fabricating porous crack-free and delamination-free La_0.6Sr_0.4CoO_3?δ electrodes using nanopowders and investigating oxygen reduction (occurring at solid oxide fuel cell cathodes) and oxygen evolution (occurring at solid oxide electrolysis cell anodes) at 600 °C in air. The electrodes were deposited by screen-printing on Ce_0.8Gd_0.2O_1.9 substrates. The pastes were prepared with nanoparticles synthesised by flame spray synthesis and graphite pore former. Without graphite, the electrodes sintered at 1000 °C exhibit relatively low porosity and significant densification which led to partial delamination and large overpotentials. The addition of graphite, which was removed by combustion at ca. 650 °C during sintering, markedly improves electrode performance by increasing porosity and reducing densification. A minimal overpotential for both the oxygen reduction and oxygen evolution was reached for a layer porosity of ca. 50–60 vol.%.     

The electrochemical behavior of the LaSrCuO<Subscript>4 − δ</Subscript>/Ce<Subscript>0.9</Subscript>Gd<Subscript>0.1</Subscript>O<Subscript>2 − δ</Subscript> interface

The electrochemical behavior of the LaSrCuO_{4 − δ}/Ce_0.9Gd_0.1O_{2 − δ} interface is studied by impedance spectroscopy and cyclic voltammetry methods. By analyzing the dependence of the impedance frequency spectra on the oxygen partial pressure, the rate-determining stages of oxygen exchange are determined in the temperature interval of 500–900°C. For temperatures above 700°C, the adsorption of oxygen molecules and their dissociation to oxygen atoms are shown to make a substantial contribution to the polarization resistance of the overall electrode process, besides the charge-transfer resistance.     

Study on the oxygen exchange capacities of Ce<Subscript>2</Subscript>Sn<Subscript>2</Subscript>O<Subscript>7</Subscript> pyrochlore

Ce₂Sn₂O₇ pyrochlore was synthesized by a hydrothermal method. X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to characterize the composition and valence state of the sample. The oxygen exchange property of the Ce₂Sn₂O₇ phase was measured by an oxidation reaction in sealed air atmosphere and a followed reduction reaction in 5% H₂-95% N₂ atmosphere. Gas chromatography (GC) was used to analyze the oxygen change in the reaction. The results show that Ce₂Sn₂O₇ sample has excellent oxygen absorption capacity at 250°C as Ce³⁺ ions are oxidized to Ce⁴⁺ ions. The oxidized sample can be reduced by 5% H₂-95% N₂. The refreshed sample remains the capacity of oxygen absorption, while the oxygen exchange capacity degrades with the reduction times.     

Oxygen exchange properties in the new pyrochlore solid solution Ce2Sn2O7-Ce2Sn2O8

The phase-pure cerium stannate pyrochlore (Ce₂Sn₂O₇) has been prepared for the first time. The structure and oxidation states of both cations were carefully reviewed, and the compound was unambiguously replaced within the rare-earth stannate series. As a consequence of the low stability of trivalent cerium in oxide phases, one oxygen per formula unit could be intercalated by calcination under O₂ at 400 °C, leading to the new Ce₂Sn₂O₈ pyrochlore. This latter phase is subject to oxygen under-stoichiometry from 400 to 700 °C. However, oxygen deintercalation seems to be in competition with cerium oxide segregation at high temperature, leading to the formation of cerium deficient pyrochlore phases.     

Microwave-hydrothermal synthesis of gadolinium-doped nanocrystalline ceria in the presence of hexamethylenetetramine

A new method is proposed for producing nanodisperse ceria-based solid solutions, involving microwave-hydrothermal (MW-HT) treatment of aqueous solutions containing salts of cerium and dopant elements and hexamethylenetetramine (HMTA). The specifics of HMTA hydrolysis under hydrothermal conditions are studied. X-ray and electron diffraction, Raman spectroscopy, and transmission electron microscopy are used to analyze the physicochemical characteristics of Ce_{1 ? x} Gd _x O_{2 ? x/2} (x = 0.10, 0.15, 0.20) nanocrystalline solid solutions produced using the new method.     

Enhanced Surface Interactions Enable Fast Li+ Conduction in Oxide/Polymer Composite Electrolyte

Li⁺‐conducting oxides are considered better ceramic fillers than Li⁺‐insulating oxides for improving Li⁺ conductivity in composite polymer electrolytes owing to their ability to conduct Li⁺ through the ceramic oxide as well as across the oxide/polymer interface. Here we use two Li⁺‐insulating oxides (fluorite Gd_0.1Ce_0.9O_1.95 and perovskite La_0.8Sr_0.2Ga_0.8Mg_0.2O_2.55) with a high concentration of oxygen vacancies to demonstrate two oxide/poly(ethylene oxide) (PEO)‐based polymer composite electrolytes, each with a Li⁺ conductivity above 10^?4 S cm^?1 at 30 °C. Li solid‐state NMR results show an increase in Li⁺ ions (>10 %) occupying the more mobile A2 environment in the composite electrolytes. This increase in A2‐site occupancy originates from the strong interaction between the O^2? of Li‐salt anion and the surface oxygen vacancies of each oxide and contributes to the more facile Li⁺ transport. All‐solid‐state Li‐metal cells with these composite electrolytes demonstrate a small interfacial resistance with good cycling performance at 35 °C.     

Solid Solubility,Raman Spectra and Electrical Property of the Solid Solutions Ce1‐xNdxO2‐δ by Sol‐gel Route

Ce_1‐xNd_xO_2‐δ (x = 0.05–0.55) solid solutions prepared by sol‐gel route were crystallized in a cubic fluorite structure. The solid limit was determined to be as high as x = 0.45. Raman spectra of the solid solutions with lower composition exhibited only one band, which was assigned to F_2g mode. Increasing composition produced broad and asymmetric F_2g mode with an appearance of low frequency tail. The new broad peak observed at higher frequency side of the F_2g mode associated with the oxygen vacancy in the lattice. The impedance spectra of the solid solutions showed definitely ionic conduction, and Ce_0.80Nd_0.20O_2‐δ solid solution possessed a maximum conductivity. At 500 °C, the conductivity and activation energy were 2.65 × 10^?3S/cm and 0.82 eV, respectively.