Defect formation, ordering, and transport in SrFe1–x Si x O3–δ (x = 0.05–0.20)

Novel Anion-Doped Cathode Material SrFe1-x Si x O3-δF y for Intermediate-Temperature Solid Oxide Fuel Cells

SrFe1-x Si x O3-δF y cathode materials (x = 0.05, 0.1, 0.15; y = 0, 0.1, 0.5) were prepared via a solid-state method. X-ray diffraction results show that the synthesized F doping samples were perovskite structure. X-ray photoelectron spectroscopy findings show that F- anions were doped into SrFe1-x Si x O3-δ. Transmission electron microscopy and energy-dispersive spectroscopy were performed to analyze the microstructure and element distribution in the materials, respectively. Double-layer composite cathode symmetric cells were prepared through a screen printing method. Scanning electron microscopy images revealed that the double-layer composite cathode adhered well to the electrolyte. The doping with F- can increase the coefficient of thermal expansion of SrFe1-x Si x O3-δ. The electrochemical impedance spectroscopy results indicate that the oxygen transport capacity of the SrFe0.95Si0.05O3-δ material can be improved by doping with F-, but such a method can decrease the oxygen transport capacity of SrFe0.9Si0.1O3-δ. At 800 °C, the peak power density of the single cell supported by an anode and SrFe0.9Si0.1O3-δF0.1 as the cathode reached 388.91 mW/cm2. Thus, the incorporation of F- into SrFe1-x Si x O3-δ cathode materials can improve their electrochemical performance and enable their application as cathode materials for solid-oxide fuel cells.

The effect of temperature and oxygen partial pressure on the concentration of iron and manganese ions in La1/3Sr2/3Fe1-xMnxO3-δ

The oxygen content was measured in cubic perovskite-type La1/3Sr2/3Fe1-xMnxO3-δ (x = 0.1, 0.17, 0.25, and 1/3) in the range of oxygen partial pressure from 10-22 to 0.5 atm at 750-950 °C with a step of 50 °C by coulometric titration. Gradual removal of oxygen from the oxides during the measurements was carried out until the stability limit was achieved and the reductive decomposition began. An increase in manganese content was shown to lead to a decrease in the stability of La1/3Sr2/3Fe1-xMnxO3-δ under reducing conditions. The obtained data on oxygen content were used for defect chemistry modeling in the oxides. The enthalpy of the Fe3+ to Fe4+ and Mn3+ to Mn4+ oxidation reactions (ΔHox0) was determined to be -103.2 ± 0.3 and -250 ± 2 kJ mol-1, respectively, for the x = 0.1 composition, and increased slightly with increasing manganese content. The large difference in ΔHox0 determines a strong distinction between the behavior of iron and manganese in perovskite-type oxides. An increase in manganese content in La1/3Sr2/3Fe1-xMnxO3-δ was found to lead to a decrease in the concentration of Fe4+ ions, but did not affect the concentration of Fe2+ ions. The impact of La/Sr ratio was evaluated by comparison of the obtained data with that for La0.5Sr0.5Fe1-xMnxO3-δ, and found to be different for iron and manganese. An increase in lanthanum fraction causes a decrease in the concentration of Fe2+ ions and an increase in the concentration of Mn2+ under reducing conditions.

Impact of Silica Additions on the Phase Composition and Electrical Transport Properties of Ruddlesden-Popper La2NiO4+δ Mixed Conducting Ceramics

The present work explores the possibility of incorporation of silicon into the crystal structure of Ruddlesden-Popper La2NiO4+δ mixed conducting ceramics with the aim to improve the chemical compatibility with lanthanum silicate-based solid electrolytes. Ceramics with the nominal composition La2Ni1−ySiyO4+δ (y = 0, 0.02 and 0.05) were prepared by the glycine nitrate combustion technique and sintered at 1450 °C. While minor changes in the lattice parameters of the tetragonal K2NiF4-type lattice may suggest incorporation of a small fraction of Si into the Ni sublattice, combined XRD and SEM/EDS studies indicate that this fraction is very limited (≪2 at.%, if any). Instead, additions of silica result in segregation of apatite-type La10−xSi6O26+δ and La2O3 secondary phases as confirmed experimentally and supported by the static lattice simulations. Both total electrical conductivity and oxygen-ionic transport in La2NiO4+δ ceramics are suppressed by silica additions. The preferential reactivity of silica with lanthanum oxide opens a possibility to improve the compatibility between lanthanum silicate-based solid electrolytes and La2NiO4+δ-based electrodes by appropriate surface modifications. The promising potential of this approach is supported by preliminary tests of electrodes infiltrated with lanthanum oxide.

Impact of oxygen content on preferred localization of p- and n-type carriers in La0.5Sr0.5Fe1-xMnxO3-δ

The oxygen content in La_0.5Sr_0.5Fe_1-xMn_xO_3-δ, measured by coulometric titration in a wide range of oxygen partial pressure at various temperatures, was used for defect chemistry analysis. The obtained data were well approximated by a model assuming defect formation in La_0.5Sr_0.5Fe_1-xMn_xO_3-δvia Fe³⁺ and Mn³⁺ oxidation reactions and charge disproportionation on Fe³⁺ and Mn³⁺ ions. The partial molar enthalpy and entropy of oxygen in La_0.5Sr_0.5Fe_1-xMn_xO_3-δ obtained by statistical thermodynamic calculations were found to be in satisfactory agreement with those obtained using the Gibbs-Helmholtz equations, thus further confirming the adequacy of the model. The impact of manganese substitution on defect equilibrium in La_0.5Sr_0.5Fe_1-xMn_xO_3-δ was shown to be attributed to a lower enthalpy of Mn³⁺ oxidation reaction (vs. for the oxidation of Fe³⁺) and the charge disproportionation reaction on Mn³⁺ (vs. for that on Fe³⁺). The former makes Mn⁴⁺ ions more resistant to reduction than Fe⁴⁺. The latter favors the presence of Mn²⁺, Mn³⁺, and Mn⁴⁺ ions in oxides in comparable concentrations. The distribution of charge carriers over iron and manganese ions was determined as a function of oxygen content in La_0.5Sr_0.5Fe_1-xMn_xO_3-δ.

High-temperature charge transport in Nd0.25Sr0.75FeO3-δ: the influence of various factors

The oxygen content and electrical conductivity in Nd_0.25Sr_0.75FeO_3-δ were measured in the range of oxygen partial pressure from 10^-19 to 0.5 atm at 750-950 °C by coulometric titration and four-probe dc techniques. The thermodynamic analysis of defect equilibrium in the oxide allowed successful simulation of the oxygen content data and calculation of charge carrier concentrations that were used for the analysis of electrical conductivity. The electrical conductivity data were accurately described in the models, which implied that the hole mobility increased upon an increase in the oxygen content in the oxide. The results suggest that only some of the Fe³⁺ sites are available for hole transport, and their fraction increases with an increase in the oxygen content. The migration energy for oxygen ions, electrons and holes was found to be 0.89 ± 0.02, 0.62 ± 0.01 and 0.230 ± 0.006 eV, respectively. Nd_0.25Sr_0.75FeO_3-δ was shown to have a considerable oxygen conductivity (0.12 S cm^-1 at 950 °C) and fairly good stability under reducing conditions, which is a good recommendation for using this oxide as a functional material in high-temperature electrochemical applications.

Carbon dioxide and water incorporation mechanisms in SrFeO3-δ phases: a computational study

With a higher propensity for low temperature synthesis routes along with a move toward lower solid oxide fuel cell operating temperatures, water and carbon dioxide incorporation in strontium ferrite is of importance. Despite this, the mechanisms are not well understood. In this work, classical-potential-based computational techniques are used to determine the favourability of water and CO₂ incorporation mechanisms in both SrFeO_3-δ and SrFeO_2.5. Our studies suggest that intrinsic Frenkel and Schottky type defects are unlikely to form, but that water and carbon dioxide incorporation are favourable in both phases. Water incorporation is likely for both the cubic and brownmillerite phases, with hydroxyl ions preferring to sit on octahedral oxygen sites in both structures, causing slight tilting of the shared octahedra. Interstitial hydroxyl ions are only likely for the brownmillerite phase, where the hydroxyl ions are most stable between adjacent FeO₄ tetrahedral chains. Carbon dioxide incorporation via carbonate defects is most favourable when a carbonate molecule exists on an iron site, preferring the iron site with lower oxygen coordination. This involves formation of multiple oxygen vacancies surrounding the iron site, and thus we conclude that carbonate can trap oxygen vacancies.

Low-temperature NO oxidation using lattice oxygen in Fe-site substituted SrFeO3-δ

Improvement of the low-temperature activity for NO oxidation catalysts is a crucial issue to improve the NO_x storage performance in automotive catalysts. We have recently reported that the lattice oxygen species in SrFeO_3-δ (SFO) are reactive in the oxidation of NO to NO₂ at low temperatures. The oxidation of NO using lattice oxygen species is a powerful means to oxidize NO in such kinetically restricted temperature regions. This paper shows that Fe-site substitution of SFO with Mn or Co improves the properties of lattice oxygen such as the temperature and amount of oxygen release/storage, resulting in the enhancement of the activity for NO oxidation in a low-temperature range. In particular, NO oxidation on SrFe_0.8Mn_0.2O_3-δ is found to proceed even at extremely low temperatures <423 K. From oxygen release/storage profiles obtained by temperature-programmed reactions, Co doping into SFO increases the amount of released oxygen owing to the reducibility of the Co species and promotes the phase transformation to the brownmillerite phase. On the other hand, Mn doping does not increase the oxygen release amount and suppresses the phase transformation. However, it significantly decreases the oxygen migration barrier of SFO. Substitution with Mn renders the structure of SFO more robust and maintains the perovskite structure after the release of oxygen. Thus, the oxygen release properties are strongly dependent on the crystal structure change before and after oxygen release from the perovskite structure, which has a significant effect on NO oxidation and the NO_x storage performance.