Search

A series of strontium titanates-vanadates (STVN) with nominal cation composition Sr1-xTi1-y-zVyNizO3-δ (x = 0–0.04, y = 0.20–0.40 and z = 0.02–0.12) were prepared by a solid-state reaction route in 10% H2–N2 atmosphere and characterized under reducing conditions as potential fuel electrode materials for solid oxide fuel cells. Detailed phase evolution studies using XRD and SEM/EDS demonstrated that firing at temperatures as high as 1200 °C is required to eliminate undesirable secondary phases. Under such conditions, nickel tends to segregate as a metallic phase and is unlikely to incorporate into the perovskite lattice. Ceramic samples sintered at 1500 °C exhibited temperature-activated electrical conductivity that showed a weak p(O2) dependence and increased with vanadium content, reaching a maximum of ~17 S/cm at 1000 °C. STVN ceramics showed moderate thermal expansion coefficients (12.5–14.3 ppm/K at 25–1100 °C) compatible with that of yttria-stabilized zirconia (8YSZ). Porous STVN electrodes on 8YSZ solid electrolytes were fabricated at 1100 °C and studied using electrochemical impedance spectroscopy at 700–900 °C in an atmosphere of diluted humidified H2 under zero DC conditions. As-prepared STVN electrodes demonstrated comparatively poor electrochemical performance, which was attributed to insufficient intrinsic electrocatalytic activity and agglomeration of metallic nickel during the high-temperature synthetic procedure. Incorporation of an oxygen-ion-conducting Ce0.9Gd0.1O2-δ phase (20–30 wt.%) and nano-sized Ni as electrocatalyst (≥1 wt.%) into the porous electrode structure via infiltration resulted in a substantial improvement in electrochemical activity and reduction of electrode polarization resistance by 6–8 times at 900 °C and ≥ one order of magnitude at 800 °C.

In the past decade, intensive research on proton-conducting oxide materials has provided a basis for the development of intermediate-temperature protonic ceramic electrochemical cells, which constitute a real alternative to conventional cells based on oxygen-conducting electrolytes. To achieve both high efficiency and excellent performance, not only electrolytes but also electrode materials should be carefully selected considering their functional properties. Compared to the traditional ABO3 perovskite electrode materials, Ln2NiO4+δ with a layered structure has unique advantages (high chemical stability, mechanical compatibility, improved oxygen transport, and hydration ability), and thus is now becoming a hot topic in this field, offering both scientific and practical interests. However, a comprehensive and in-depth review is still lacking in the literature to date. Accordingly, this work presents a comprehensive overview of the prospects of layered nickelates (Ln2NiO4+δ, where Ln = La, Nd, and Pr) as one of the most attractive oxygen (steam) electrode materials for protonic ceramic electrochemical cells. In particular, the crystalline features, defect structure, stability, chemical properties, and mechanical compatibility of this class of materials, contributing to their transport functionality, are discussed with the primary emphasis on revealing the relationship between the composition of the materials and their properties. The presented systematic results reveal the main strategies regarding the utilisation of Ln2NiO4+δ-based electrodes and existing gaps related to fundamental and applied research aspects.

Advances in design of materials for solid oxide fuel cells, oxygen and hydrogen separation membranes, and catalysts for biofuel conversion into syngas and hydrogen are reviewed. Application of new efficient techniques of material synthesis and characterization of their atomic-scale structure, transport properties, and reactivity allowed to develop new types of efficient cathodes and anodes for solid oxide fuel cells, asymmetric supported oxygen, and hydrogen separation membranes with high permeability and structured catalysts with nanocompositeactive components demonstrating high performance and stability to coking in steam/autothermal reforming of biofuels. FCT published

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.

Ruddlesden-Popper Nd0.8Sr1.2Ni0.8M0.2O4-δ (M = Ni, Co, Fe) nickelates have been characterized as prospective oxygen electrode materials for solid electrolyte cells. XRD studies showed that these oxides retain tetragonal K2NiF4-type structure in air until at least 900°C. Average thermal expansion coefficients of Nd0.8Sr1.2Ni0.8M0.2O4-δ calculated from the structural data are in the range 14.5-15.8 ppm/K. TGA studies revealed that these nickelates are oxygen-deficient in air at temperature above 700°C but tends to oxygen stoichiometry or minor excess on cooling. Incorporation of cobalt or iron into nickel sublattice of Nd0.8Sr1.2NiO4-δ reduces oxygen deficiency and electrical conductivity. Electrochemical impedance spectroscopy studies of symmetrical cells showed that porous Nd0.8Sr1.2Ni0.8M0.2O4-δ electrodes applied onto Ce0.9Gd0.1O2-δ electrolyte exhibit quite similar performance, with lowest values of polarization resistance (0.8 Ω⸱cm2 at 800°C) observed for M = Ni. The polarization resistance can be further decreased (down to 0.04 Ω⸱cm2 at 800°C for M = Ni) by surface modification with PrOx. published

Ceramics samples with the nominal composition [(ZrO2)0.95(Y2O3)0.05]1-x[PrOy]x and praseodymia contents of x = 0.05–0.15 were prepared by the direct firing of compacted 5YSZ + PrOy mixtures at 1450–1550 °C for 1–9 h and characterized for prospective applicability in reversible solid oxide cells. XRD and SEM/EDS analysis revealed that the dissolution of praseodymium oxide in 5YSZ occurs via the formation of pyrochlore-type Pr2Zr2O7 intermediate. Increasing PrOy additions results in a larger fraction of low-conducting pyrochlore phase and larger porosity, which limit the total electrical conductivity to 2.0–4.6 S/m at 900 °C and 0.28–0.68 S/m at 700 °C in air. A longer time and higher temperature of firing promotes the phase and microstructural homogenization of the ceramics but with comparatively low effect on density and conductivity. High-temperature processing leads to the prevailing 3+ oxidation state of praseodymium cations in fluorite and pyrochlore structures. The fraction of Pr4+ at 600–1000 °C in air is ≤2% and is nearly independent of temperature. 5YSZ ceramics with praseodymia additions remain predominantly oxygen ionic conductors, with p-type electronic contribution increasing with Pr content but not exceeding 2% for x = 0.15 at 700–900 °C. The average thermal expansion coefficients of prepared ceramics are in the range of 10.4–10.7 ppm/K.

Manganese-substituted 5 mol.% yttria-stabilized zirconia (5YSZ) was explored as a prospective material for protective interlayers between electrolyte and oxygen electrodes in reversible solid oxide fuel/electrolysis cells. [(ZrO2)0.95(Y2O3)0.05]1−x[MnOy]x (x = 0.05, 0.10 and 0.15) ceramics with cubic fluorite structure were sintered in air at 1600 °C. The characterization included X-ray diffraction (XRD), scanning electron microscopy (SEM)/energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), thermogravimetry and dilatometry in controlled atmospheres, electrical conductivity measurements, and determination of oxygen-ion transference numbers by the electromotive force (EMF) technique. Mn-substituted 5YSZ solid solutions exhibit variable oxygen nonstoichiometry with manganese cations in a mixed 2+/3+ oxidation state under oxidizing conditions. Substitution by manganese gradually increases the extent of oxygen content variation on thermal/redox cycling, chemical contribution to thermal expansion and dimensional changes on reduction. It also deteriorates oxygen-ionic conductivity and improves p-type electronic conductivity under oxidizing conditions, leading to a gradual transformation from predominantly ionic to prevailing electronic transport with increasing x. Mn2+/3+→Mn2+ transformation under reducing atmospheres is accompanied by the suppression of electronic transport and an increase in ionic conductivity. All Mn-substituted 5YSZ ceramics are solid electrolytes under reducing conditions. Prolonged treatments in reducing atmospheres, however, promote microstructural changes at the surface of bulk ceramics and Mn exsolution. Mn-substituted 5YSZ with 0.05 ≤ x &lt, 0.10 is considered the most suitable for the interlayer application, due to the best combination of relevant factors, including oxygen content variations, levels of ionic/electronic conductivity and thermochemical expansion.

The present work aims to evaluate relevant effects of the size of substituting rare-earth cation on electrical conductivity and thermal expansion of donor-doped SrTiO3 as ceramic components of electrochemical solid electrolyte cells. A series of strontium titanate-based ceramics with a moderate doping level and different cation stoichiometry, Sr0.90Ln0.10TiO3±δ and Sr0.85Ln0.10TiO3±δ (Ln = La–Yb), are prepared under identical conditions with the final reductive treatment in 10%H2–N2 atmosphere at 1500 °C for 10 h. The solubility of rare-earth cations in the strontium sublattice of perovskite-type SrTiO3 is found to decrease with reducing ionic radius leading to a segregation of secondary pyrochlore-type Ln2Ti2O7 phase in the case of smaller rare-earth cations (Ln = Dy–Yb). The results of electrical, dilatometric and thermogravimetric studies suggest that the size of donor Ln3+ cation and cation stoichiometry have no evident impact on the electrical conductivity or thermal expansion behavior as long as the content of Ln3+ is within the solubility limits. All reduced donor-doped titanate ceramics (except Yb-doped) show a similar level of electrical conductivity, 100–155 S cm−1 at 800 °C, determined by the content of Ln3+ in the strontium sublattice, and moderate thermal expansion coefficients, 11.9–12.1 ppm K−1 at 25–1100 °C, compatible with that of conventional solid electrolytes.

In the present work, the anionic doping of a Ba-containing Nd2NiO4+δ mixed conductor is proposed as an efficient means of tuning its functional properties for application as an oxygen/steam electrode material in protonic ceramic electrolysis cells (PCECs). Single-phase Nd1.9Ba0.1NiO4+δFγ (γ = 0, 0.03, 0.05, 0.07 and 0.1) nickelates having a K2NiF4-type structure were prepared and comprehensively characterised in the range from room temperature to 1000 °C. A combination of complimentary techniques, including 4-probe DC electrical measurements, an electron-blocking method, electrochemical impedance spectroscopy and analysis of equivalent circuit schemes and distribution of relaxation times, was employed to reveal the fundamental correlations between electrical properties, oxygen-ionic transport and electrochemical performance of fluorinated nickelates. The highest ionic conductivity in combination with the lowest electrode polarisation resistance was found for the composition with γ = 0.05. The enhanced transport properties of this material were attributed to mixed anion lattice effect. Electrochemical tests of an electrolysis cell based on a proton-conducting BaCe0.5Zr0.3Y0.1Y0.1O3–δ electrolyte with a Nd1.9Ba0.1NiO4+δF0.05 oxygen electrode demonstrated competitive performance compared to state-of-the-art PCECs, thus supporting the prospective viability of the proposed approach.

In this work, the concept of biomass direct (air) gasification was demonstrated in a pilot-scale bubbling fluidized bed and the influence of in-situ application of low-cost catalytic materials on the produced gas characteristics and gasifier performance was analyzed. Three different low-cost catalysts were tested: bottom bed ashes resulting from combustion of residual forest biomass derived from eucalyptus, char particles resulting from wood pellets direct (air) gasification, and synthetic fayalite (Fe2SiO4). Without using catalysts, the produced gas composition was 7.7–16.9%v CO, 3.2–8.3%v H2, 0.5–3.4%v CH4 and 9.5–14.6%v CO2, with 2.4–4.3 MJ/Nm3 lower heating value, specific dry gas production between 1.0 and 1.8 Nm3 dry gas/kg biomass (dry basis), cold gas efficiency between 13.7 and 30.5% and carbon conversion efficiency between 30.7 and 50.9%. With the use of catalysts, the produced gas composition was 14.2–37.6%v CO, 9.5–14.7%v H2, 2.6–3.5%v CH4 and 3.6–14.8%v CO2, with 3.9–6.3 MJ/Nm3 lower heating value, specific dry gas production between 1.4 and 2.0 Nm3 dry gas/kg biomass (dry basis), cold gas efficiency between 38.1 and 66.3% and carbon conversion efficiency between 56.8 and 86.6%. The highest increase in H2 concentration (352% increase) was observed on experiments using wood pellets char as catalyst while the highest increase in CO (305% increase), lower heating value (123% increase), specific dry gas production (62% increase), cold gas efficiency (262% increase) and carbon conversion efficiency (174% increase), was observed on experiments using synthetic Fe2SiO4 as catalyst.

The applicability of perovskite-type SrVO3-δ in high-temperature electrochemical energy conversion technology is hampered by the limited stability domain of the perovskite phase. The aim of the present work was to find a compromise between the phase stability and electrical performance by designing solid solutions in the SrVO3 -SrTiO3 system. Increasing titanium content in SrV1-y Tiy O3-δ (y=0-0.9) perovskites is demonstrated to result in a gradual shift of the upper-p(O2 ) phase stability boundary toward oxidizing conditions: from ≈10-15 bar at 900 °C for undoped SrVO3-δ to ≈10-11 -10-5 bar for y=0.3-0.5. Although the improvement in the phase stability is accompanied by a decrease in electrical conductivity, the conductivities of SrV0.7 Ti0.3 O3-δ and SrV0.5 Ti0.5 O3-δ at 900 °C remain as high as 80 and 20 S cm-1 , respectively, and is essentially independent of p(O2 ) within the phase-stability domain. Combined XRD, thermogravimetric analysis, and electrical studies revealed very sluggish kinetics of oxidation of SrV0.5 Ti0.5 O3-δ ceramics under inert gas conditions and a nearly reversible behavior after exposure to an inert atmosphere at elevated temperatures. Substitution by titanium in the SrV1-y Tiy O3-δ system results also in a decrease of oxygen deficiency in perovskite lattice and a favorable suppression of thermochemical expansion. Variations of oxygen nonstoichiometry and electrical properties in the SrV1-y Tiy O3-δ series are discussed in combination with the simulated defect chemistry of solid solutions.

Despite an ongoing strenuous effort to understand the compositional and structural drivers controlling the chemical durability of oxide glasses, there is still no complete consensus on the basic mechanism of glass dissolution that applies to a wide composition space. One major reason for this problem is the structural complexity contained within the multicomponent silicate glasses chosen for glass corrosion studies. The nonsilicate network polyhedra present in these glasses interact with one another, often in unpredictable ways, by forming a variety of structural associations, for example, Al[IV]-B[III] and B[III]-B[IV], resulting in significant influence on both the structure of the glass network and related macroscopic properties. Likewise, the formation of a variety of next-neighbor linkages, as well as increasingly complex interactions involving Si and differently coordinated next-nearest neighbor cations, is very difficult to decipher experimentally. Consideration of these factors motivates instead a different strategy: that is, the study of a sequence of SiO2-free ternary or quaternary glass compositions, whose structures can be unambiguously determined and robustly linked to their corrosion properties. With this aim, the present study is focused on understanding the structural drivers governing the kinetics and mechanism of corrosion of ternary Na2O-Al2O3-B2O3 glasses (in water) over a broad composition space comprising compositions with distinct structural features. It has been shown that the addition of Al2O3 to binary sodium borate glasses decreases their corrosion rate in water and converts their dissolution behavior from congruent to incongruent leading to the formation of six-coordinated alumina, and higher concentration of four-coordinated boron (in comparison to pre-dissolution glasses) in post-dissolution glass samples. The drivers controlling the corrosion kinetics and mechanism in these glasses based on their underlying structure have been elucidated. Some open questions have been proposed which require an extensive analysis of surface chemistry of pre- and post-dissolution samples and will be investigated in our future work.

The proton conductivity and solid solubility limits of acceptor strontium doped vanadium stabilised lanthanum niobate (SryLa1−yNb1-xVxO4-δ, x = 0.25, 0.30 and y = 0 to 0.10) were explored as potential proton conducting ceramic electrolytes. All samples were synthesized via a solid-state method. The phase purity, microstructure and thermal expansion behaviour of the materials were studied using powder X-ray diffraction, scanning electron microscopy and dilatometry, respectively. A maximum solid solution limit of 5% Sr in the A-site of SryLa1−yNb1-xVxO4-δ samples is observed for a vanadium content of x = 0.25, while further increases in the Sr or vanadium contents lead to the presence of Sr3(VO4)2 as a secondary phase. This acceptor dopant content of 5%Sr in the current scheelite material exceeds that possible in the parent vanadium-free fergusonite SryLa1−yNbO4-δ material by a factor of 5. All Sr doped scheelite materials show linear thermal expansion behaviour, successfully avoiding the scheelite to fergusonite structural phase change during thermal cycling. The average grain size is shown to be increased by increasing vanadium content. In humid conditions, all phase pure samples show predominantly proton conductivity at lower temperatures, while p-type conductivity is noted at higher temperatures under dry oxidising conditions. In the low temperature range, the Sr0.05La0.95Nb0.75V0.25O4-δ sample, containing the largest acceptor dopant concentration, exhibits slightly higher bulk and specific grain boundary conductivities in comparison to other phase pure compositions.

Undoped LnVO 4 and magnesium-doped Ln 0.95 V 0.95 Mg 0.10 O 4-δ (Ln = Pr, Sm, Gd, Dy and Er) orthovanadates were synthesized by solid state reaction method and characterized by XRD, SEM/EDS, electrical conductivity measurements in controlled atmospheres, and modified e.m.f. technique for determination of oxygen-ion transference numbers. XRD analysis showed the formation of phase-pure materials with tetragonal zircon-type structure and a decrease in lattice parameters with a decrease of ionic radius of rare-earth cations. Trace amounts of MgO and Mg-V-O phases revealed by SEM/EDS suggest that the solid solubility limit of magnesium cations in LnVO 4 lattice is somewhat lower than the nominal doping level, and that magnesium substitutes preferentially into the vanadium sublattice. LnVO 4 and Ln 0.95 V 0.95 Mg 0.10 O 4-δ orthovanadates show semiconducting behavior under oxidizing conditions at 450–950 °C and are predominantly oxygen-ionic conductors, except PrVO 4 that shows mixed conductivity. In the LnVO 4 series, electrical conductivity is the highest for PrVO 4 and SmVO 4 (~4 × 10 −4 S/cm at 800 °C) and decreases with increasing atomic number of rare-earth cation for the other compositions. Additions of magnesium results in a drop of electrical conductivity, by 1.5–2 times for most of compositions. Interstitial oxygen diffusion is discussed as a prevailing mechanism of ionic transport in undoped LnVO 4 , whilst acceptor-type magnesium doping suppresses the formation of interstitial oxygen ions. Humidity has a rather negligible impact on the electrical properties of substituted ceramics, indicating only minor (if any) protonic contribution to the total electrical transport of the studied orthovanadates.

Magnesium-substituted europium vanadate ceramics, Mg x EuVO 4 ± δ ( x = 0–0.5), were prepared by solid state method and characterized by XRD, SEM/EDS, dilatometry, UV–vis spectroscopy, impedance spectroscopy, and measurements of oxygen-ion transference numbers ( t o ¯ ). Magnesium was found to substitute preferentially into vanadium sublattice of zircon-type EuVO 4 with limited solubility of ∼5 at.%. Additions of magnesium increase slightly coefficients of thermal expansion (3.2–6.0 ppm/K at 150–400 °C) and have negligible effect on the optical properties. Undoped EuVO 4 is predominantly an oxygen-ionic conductor with t o ¯ = 0.96–0.99 at 700–900 °C under oxidizing conditions. Acceptor-type substitution suppresses total conductivity and oxygen-ionic transport. The variations of electrical transport properties are discussed in terms of interstitial oxygen diffusion in the parent EuVO 4 and oxygen vacancy diffusion in Mg-substituted vanadate. Humidified atmosphere has negligible impact on the electrical properties of substituted ceramics but results in increased electrical conductivity of EuVO 4 indicating a protonic contribution to the total electrical transport.

Cubic perovskite-type Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) is one of the mixed ionic-electronic conductors with the highest oxygen permeability known to date. It serves as a parent material for the development of functional derivatives for electrochemical applications including oxygen separation membranes, solid electrolyte cell electrodes and electrocatalysts for the oxygen evolution reaction. The present study is focused on the determination of the precise stability boundaries of cubic perovskite BSCF employing a coulometric titration technique in combination with thermogravimetric analysis, X-ray and neutron diffraction, and molecular dynamics simulations. Both the low-p(O2) and high-p(O2) stability boundaries at 700-950 °C were found to correspond to a fixed value of oxygen content in the perovskite lattice of 3 - δ = ∼2.13 and ∼2.515, respectively. The stability limits in this temperature range are expressed by the following equations: high-p(O2) boundary: log p(O2) (atm) (±0.1) = -10 150/T (K) + 8.055; low-p(O2) boundary: log p(O2) (atm) (±0.03) = -20 750/T (K) + 4.681. The p(O2)-T phase diagram of the BSCF system under oxidizing conditions is addressed in a wider temperature range and is shown to include a region of precipitation of a "low-temperature" phase occurring at 400-500 °C. The fraction of the low-temperature precipitate, which co-exists with the cubic perovskite phase and is stable up to 790-820 °C, increases upon increasing p(O2) in the range 0.21-1.0 atm.

In this study, a novel Fe2-xNixTiO5 catalyst for potential applications in biomass gasification gas cleaning/upgrading was investigated. The material was successfully synthesized through combined mechanical activation and microwave firing. Catalytic steam reforming was studied in a fixed bed tubular reactor, using a mixture of toluene and naphthalene as model tar compounds as well as downstream a fluidized bed gasifier. Fe2-xNixTiO5 catalyst showed high activity in converting the model compounds at temperatures higher than 700 °C. The catalyst exhibited a tar conversion of 78% at 800 °C when exposed to biomass-derived gas from a bubbling fluidized bed gasifier. Nevertheless, the catalytic activity declined with increased time on stream due to structural changes in iron active phases, caused by redox conditions of the producer gas. Furthermore, thermodynamic calculations suggest that sulfur chemisorption on the nickel surface, may also contribute to the catalyst deactivation.

Development of the cathode contact materials (CCMs) for intermediate-temperature solid oxide fuel cells (IT-SOFCs) is a relatively new task promoted by the appearance of full-sized cells, which may operate below 700 °C with sufficient performance, on the market. CCM must ensure a low-resistive interface between interconnect and cathode and is supposed to be sintered at temperatures below 800 °C, as imposed by the specifications of IT-SOFCs and corresponding sealants. The present work is focused on the elaboration of CCMs derived from perovskite-like La0.6Sr0.4Co0.2Fe0.8O3-δ employing two approaches: introduction of the A-site cation vacancies and partial substitution by copper in B sublattice. Both approaches were found to result in a higher electrical conductivity below 800 °C if compared to the parent material. All studied materials exhibit acceptable coefficients of thermal expansion, 13.5–14.8 ppm K−1 at 25–700 °C. Area-specific resistance (ASR) of CCM/chromium barrier (Mn1.5Co1.5O4)/interconnect (Crofer 22APU) assemblies prepared by the screen-printing was measured in air at 660–750 °C. The studies revealed that morphology of CCM powder should be considered as a key parameter in the formation of interfaces with a low resistivity. The best ASR values, below 4 mΩ cm2 at 660–700 °C, were obtained for La0.6Sr0.4Co0.15Cu0.10Fe0.75O3-δ, La0.6Sr0.4Co0.15Cu0.05Fe0.80O3-δ and (La0.60Sr0.40)0.995Co0.20Fe0.80O3-δ as CCMs.

Neutron powder diffraction is a powerful technique for probing oxygen in nonstoichiometric oxides, but often gives lower oxygen occupancy compared to traditional characterization methods. In this work, the oxygen nonstoichiometry of a model oxide system, cubic perovskite-type Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF), was studied by thermogravimetric analysis, coulometric titration and room-temperature neutron diffraction, with the aim to identify the possible cause of such a discrepancy. Comparative analysis of the obtained results and available literature data indicates that the Rietveld refinement of neutron diffraction data yields strongly overestimated δ values, by 0.20–0.31 oxygen atoms per formula unit, compared to other methods. The uncertainty of oxygen content in BSCF from the neutron diffraction data is discussed in terms of strong diffuse scattering leading to undervalued oxygen occupancy in a standard Rietveld refinement.

Nano-sized composite La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF)-Ce0.8Sm0.2O1.9 (SDC) powder was synthesised by one-step microwave self-assisted combustion synthesis and characterized by X-ray diffraction and transmission electron microscopy. Electrochemical performance of composite LSCF-SDC cathode in contact with Ce0.8Gd0.2O1.9 (GDC) solid electrolyte under steady state polarisation was studied employing electrochemical impedance spectroscopy in 3-electrode cell configuration. The evolution of elementary steps of oxygen reduction reaction was analysed under cathodic polarisation in potentiostatic mode (up to −0.8 V) in the temperature range 650–750 °C. The composite LSCF-SDC cathode prepared by one-step microwave combustion route demonstrated a lower overall polarisation resistance when compared to literature reports of similar cathodes produced by other synthesis routes.

Perovskite-like LaNiO3-δ was evaluated as potential oxygen electrode material for solid oxide electrolysis cells. Compared to the Ruddlesden-Popper Lan+1NinO3n+1 (n = 1,2,3) counterparts, LaNiO3-δ exhibits higher p-type metallic-like conductivity under oxidizing conditions (450 S×cm-1 at 800°C for highly porous ceramics) together with a moderate thermal expansion coefficient (13.7 ppm×K-1 in air at 25-800°C) compatible with common solid electrolytes. The measured electrode polarization resistance (Rη) in contact with YSZ, CGO and LSGM solid electrolytes was 1.4, 0.77 and 0.22 Ω×cm2 at 800°C, and 208, 123 and 7.1 Ω×cm2 at 600°C, respectively, under zero-current conditions in air. Surface modification of via PrOx infiltration resulted in lower values of Rη (0.024 Ω×cm2 at 800°C and 0.76 Ω×cm2 at 600°C) and low anodic overpotentials (20 mV at 800°C and 500 mA×cm-2) in contact with LSGM. published

The present work explores mechanosynthesis of lanthanum aluminate-based perovskite ceramics and corresponding effects on ionic-electronic transport properties. La1-xCaxAlO3-δ (x = 0.05-0.20) nanopowders were prepared via one-step high-energy mechanochemical processing. Sintering at 1450°C yielded dense ceramics with submicron grains. As-prepared powders and sintered ceramics were characterized by XRPD, XPS and SEM. Electrochemical studies showed that partial oxygen-ionic conductivity in prepared La1-xCaxAlO3-δ increases with calcium content up to 10 at.% in the lanthanum sublattice and then levels off at ~6×10-3 S/cm at 900°C. La1-xCaxAlO3-δ ceramics are mixed conductors under oxidizing conditions and ionic conductors with negligible contribution of electronic transport in reducing atmospheres. Oxygen-ionic contribution to the total conductivity is 20-68% at 900°C in air and increases with Ca content, with temperature and with reducing p(O2). Impedance spectroscopy results showed however that electrical properties of mechanosynthesized La1-xCaxAlO3-δ ceramics below ~800°C are determined by prevailing grain boundary contribution to the total resistivity. This work was supported by the Slovak Research and Development Agency APVV (contracts SK-PT-18-0039 and 15-0438) and the Slovak Grand Agency (contract No. 2/0055/19). BIAS and AAY would like to acknowledge financial support by the FCT, Portugal (bilateral project Portugal-Slovakia 2019-2020, project CARBOSTEAM (POCI01-0145-FEDER-032295) and project CICECO-Aveiro Institute of Materials (FCT ref. UID/CTM/50011/2019), financed by national funds through the FCT/MCTES and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement). HK thanks to SAIA, n.o. for financial support within National Scholarship Programme of the Slovak republic (NSP). in publication

Perovskite-related (La1-xSrx)(2)NiO4-delta (x= 0.5-0.8) phases were explored for possible use as oxygen electrodes in solid electrolyte cells with a main focus on the effect of oxygen deficiency on ...

The influence of elevated oxygen pressure (from 1 to 50 bar) on phase separation in Ba0.5Sr0.5Co0.8Fe0.2O3-delta(BSCF) cubic perovskite was investigated in the range of 300to 1300 K. X-ray diffraction and thermogravimetric analysis revealed two separation processes occurring in the high temperature (HT), 1043 K, and low temperature (LT), 713 K, ranges. Increasing oxygen pressure shifts slightly the LT phase separation to lower temperature, but has a rather minor effect on the volume fraction growth of the secondary phase. Phase transformation in the HT range is a slow process and the degree of decomposition is strongly influenced by the cooling rate from higher temperatures. On the contrary, the LT separation is relatively fast and is essentially independent of the cooling rate. The BSCF ceramics thermally treated at p(O-2)=50 bar was analyzed by different electron microscopy techniques. The structures of separation products were identified as trigonal Ba3Co10O17-like lamellae and 2H-hex-agonal Ba-0.5 (+/-) Sr-x(0.5) (+/-) xCoO3-delta perovskite, respectively. (C) 2016 Elsevier B.V. All rights reserved.

The present study focuses on investigating the solubility of RuO2 in a borosilicate-based model high-level waste glass and understanding its impact on the crystallization behavior and electrical conductivity of the resulting vitrified waste forms. The solubility limit of RuO2 in the investigated glass composition has been determined to be 460 ppm by weight. The higher concentration of RuO2 results in sub-micron sized Ru-rich inclusions in the glassy matrix, which eventually agglomerate to form needle-like and polyhedral RuO2 crystals. It is observed that RuO2 selectively promotes the crystallization of the rare-earth apatite phase over the powellite phase. The as-synthesized RuO2-containing glasses exhibit semiconducting behavior with a similar level of electrical conductivity below the glass transition. The percolation of non-uniformly distributed RuO2 inclusions may result in a formation of short-range low-resistive conduction pathways in the host glass matrix leading to an apparent metallic-like behavior of selected thin samples with the highest ruthenium content.

Long-term degradation remains the main issue for the viability of solid oxide electrolysis cell (SOEC) technology as a practical hydrogen production system. The major specific degradation mechanism in SOECs relates to delamination phenomena at or near electrolyte/anode interface. The principle of so-called fuel-assisted electrolysis is to supply the carbon-containing species which can react with oxygen at the anode side thus bringing down the oxygen chemical potential at the electrolyte/anode interface and improving its stability. The present work is aimed at the characterization of PrMnO3-based perovskites for potential application as anodes in solid oxide fuel-assisted electrolysis cells. Pr0.6-x A0.4MnO3 ±δ (A = Sr, Ca; x = 0 and 0.05) powders were synthesized by glycine-nitrate combustion technique. The characterization included XRD, SEM/EDS, XPS, dilatometry and thermogravimetry, measurements of electrical properties, and determination of oxygen nonstoichiometry by coulometric titration. XRD analysis confirmed the formation of solid solutions with orthorhombic perovskite-like structure. Pr0.6-x A0.4MnO3 ±δ exhibit negligible variations of oxygen content with temperature and oxygen partial pressure under oxidizing conditions, while reducing p(O2) below 10-4 atm results in a progressive oxygen losses from the lattice and reduction of variable-valence cations. The low-p(O2) stability boundary of the perovskite phase at 800°C corresponds to ~10-17-3×10-16 atm; the stability domain is wider for Ca-substituted compositions and narrows with introduction of A-site vacancies. Dilatometric studies confirmed a good thermomechanical compatibility with common solid electrolytes. The electrical conductivity of Pr0.6-x A0.4MnO3 ±δ ceramics is p-type electronic and decreases with reducing p(O2), but still exceeds 40-50 S/cm under oxygen chemical potentials anticipated for the oxygen electrode operation conditions at 800°C. The electrochemical performance of Pr0.6-x A0.4MnO3 ±δ porous electrode layers was evaluated in contact with yttria-stabilized zirconia solid electrolyte using symmetrical cell configuration as function of relevant parameters (electrode fabrication conditions, with and without ceria-based interface layers, modifications via impregnations by praseodymia or gadolinia-doped ceria).

The applicability of perovskite-type SrVO

Cubic perovskite-type Ba

Starting from a previously published stoichiometric model for the commensurate Type III phase in the (1–x)Bi2O3∙xNb2O5 system, Bi94Nb32O221 (x = 0.254), we have developed a crystal-chemical model of this phase across its solid-solution range 0.20 ≤ x ≤ 0.26. After using annular dark-field scanning transmission electron microscopy to identify the metal sites that support non-stoichiometry, we show that the maximum possible range of that non-stoichiometry is 0.198 ≤ x ≤ 0.262, perfectly consistent with the experimental result. Inter-site cation defects on these sites provide some local coordinative flexibility with respect to the surrounding oxygen sublattice, but not enough to create continuous fluorite-like channels like those found in the high-temperature incommensurate Type II phase. This explains the reduced oxide-ionic conductivity of Type III compared to Type II at all temperatures and compositions, regardless of which phase is thermody-namically stable under those conditions. The solid-solution model shows that oxygen disorder and vacancies are both re-duced as x increases, which also explains why Type III becomes relatively more stable, and why oxide ionic conductivity decreases, as x increases.

Inspired by recent research results that have demonstrated appealing thermoelectric performance of A-site cation-deficient titanates, this work focuses on detailed analysis of the changes in performance promoted by altering the defect chemistry mechanisms. The series of cation-stoichiometric SrTi1-xTaxO3 +/-delta and A-site deficient Sr1-x/2Ti1-xTaxO3-delta compositions (0.05

In this study, nanostructured ceria (CeO2) films are deposited on Si(100) and ITO coated glass substrates by pulsed DC magnetron sputtering using a CeO2 target. The influence on the films of using various gas ambients, such as a high purity Ar and a gas mixture of high purity Ar and O-2, in the sputtering chamber during deposition are studied. The film compositions are studied using XPS and SIMS. These spectra show a phase transition from cubic CeO2 to hexagonal Ce2O3 due to the sputtering process. This is related to the transformation of Ce4+ to Ce3+ and indicates a chemically reduced state of CeO2 due to the formation of oxygen vacancies. TGA and electrochemical cyclic voltammetry (CV) studies show that films deposited in an Ar atmosphere have a higher oxygen storage capacity (OSC) compared to films deposited in the presence of O-2. CV results specifically show a linear variation with scan rate of the anodic peak currents for both films and the double layer capacitance values for films deposited in Ar/O-2 mixed and Ar atmosphere are (1.6 +/- 0.2) x 10(-4) F and (4.3 +/- 0.5) x 10(-4) F, respectively. Also, TGA data shows that Ar sputtered samples have a tendency to greater oxygen losses upon reduction compared to the films sputtered in an Ar/O-2 mixed atmosphere. (C) 2015 Elsevier B.V. All rights reserved.

Previously unexplored oxygen-deficient Ruddlesden–Popper Nd2−xSrxNiO4−δ (x = 1.0–1.6) nickelates were evaluated for potential use as oxygen electrode materials for solid oxide fuel and electrolysis cells, with emphasis on structural stability, oxygen nonstoichiometry, dimensional changes, and electrical properties. Nd2−xSrxNiO4−δ ceramics possess the K2NiF4-type tetragonal structure under oxidizing conditions at 25–1000 °C. Acceptor-type substitution by strontium is compensated by the generation of electron–holes and oxygen vacancies. Oxygen deficiency increases with temperature and strontium doping reaching ∼1/8 of oxygen sites for x = 1.6 at 1000 °C in air. Strongly anisotropic expansion of the tetragonal lattice on heating correlated with oxygen nonstoichiometry changes results in an anomalous dilatometric behavior of Nd2−xSrxNiO4−δ ceramics under oxidizing conditions. Moderate thermal expansion coefficients, (11–14) × 10−6 K−1, ensure however thermomechanical compatibility with common solid electrolytes. Reduction in inert atmosphere induces oxygen vacancy ordering accompanied by a contraction of the lattice and a decrease of its symmetry to orthorhombic. Nd2−xSrxNiO4−δ ceramics exhibit a p-type metallic-like electrical conductivity at 500–1000 °C under oxidizing conditions, with the highest conductivity (290 S cm−1 at 900 °C in air) observed for x = 1.2. The high level of oxygen deficiency in Sr-rich Nd2−xSrxNiO4−δ implies enhanced mixed ionic–electronic transport favorable for electrode applications.

The development of electrochemical devices towards clean energy generation has been intensified in the last years. In this context, fuel cells, and especially intermediate temperature solid oxide fuel cells (IT-SOFC) have received much attention. Perovskite type materials of composition Ba1-xSrxCo1-yFeyO3-δ, doped or not with rare earth ions, have been quite promising because they have good ionic conductivity and operate at relatively low temperatures (500 750 °C). In this study, powders of compositions (BaSr)0.5Sm0.5Co0.8Fe0.2O3-δ (BSSCF) and (BaSr)0.5Nd0.5Co0.8Fe0.2O3-δ (BSNCF) were prepared using commercial gelatin powder as a polymerizing agent for its use as cathode. The as-prepared powders were calcined at 1000 oC and characterized by X-ray diffraction and scanning electron microscopy. Screen-printed symmetrical cells were sintered 1150 °C and studied by electrochemical impedance spectroscopy in order to assess the cathode kinetics for the oxygen reduction reaction. The best area specific resistance was found for the BSSCF cathode sintered at 1150 °C for 4 h (0.157 Ω.cm2 at 750 °C), demonstrating that this novel composition is suitable for application as SOFC cathode material. key words: Cathode, microstructure, impedance spectroscopy, IT-SOFC.

Phase formation under high pressure in the chalcopyrite–delafossite system has been studied at 5 GPa and 500–1000 °C. The parent chalcopyrite composition used in this study was slightly Cu-deficient, corresponding to the chemical formula (Cu,Ga)0.905GaS2. It was revealed that under high-pressure and high-temperature conditions, a mixture of the parent phases partly decomposes to form Cu2S and Ga2O3. The defect chalcopyrite (Cu,Ga)0.905GaS2 splits into a near-stoichiometric CuGaS2 and a Cu-deficient cubic (sphalerite-like) phase of the (Cu,Ga)0.8S composition. Such a phenomenon has been considered in terms of characteristic features of chalcopyrite and sphalerite crystal structures. Delafossite CuGaO2 at 5 GPa and 1000 °C was found to recrystallize entirely, which results in the disappearance of the stacking faults in its layered structure.

Spinel-type magnetite-based oxides, possessing relatively high electrical conductivity, are considered as promising consumable anode materials for high temperature pyroelectrolysis, a breakthrough low-CO2 steel technology to overcome the environmental impact of classical extractive metallurgy. The present work was focused on the analysis of phase stability, thermal expansion and high-temperature electrical conductivity in (Fe,Mg,Al)(3)O-4 system under oxidizing and mildly reducing conditions. Metastable, nearly single-phase at room temperature (Fe,Mg,Al)(3)O-4 ceramics was obtained by sintering at 1753-1773 K for 10 h in argon atmosphere. Thermal expansion and redox induced dimensional changes were studied on heating, using TG, XRD and dilatometry. The results revealed that magnesium improves the tolerance against oxidative decomposition and minimizes unfavorable dimensional changes in ceramic samples upon thermal cycling. Co-substitution of iron with aluminium and magnesium was proved to be a promising strategy for improvement of refractoriness and phase stability of Fe3O4-based spinels at elevated temperatures, without significant reduction in the electrical conductivity. (C) 2013 Elsevier Ltd. All rights reserved.

Electrical conductivity, thermal expansion and redox behavior of Y- and Al-substituted SrVO3 were evaluated for potential use in solid oxide fuel cell anodes. Sr1-xYxV1-yAlyO3-delta (x = 0-0.4, y = 0-0.2) ceramics were prepared by solid state reaction route and sintered in hydrogen-containing atmosphere. Solid solubility limits of yttrium and aluminum cations in SrVO3 perovskite lattice were found close to similar to 25% of strontium sublattice and 8-10% of vanadium sublattice, respectively. Under reducing conditions, all ceramics exhibit nearly p(O-2)-independent metallic-like electrical conductivity slightly decreasing with substitution level. Undoped SrVO3-delta is characterized with unfavorably high thermal expansion coefficient (TEC = 21.6 x 10(-6) K-1 at 823-1273 K) impeding thermomechanical compatibility with common solid electrolytes. Substitutions with yttrium and aluminum suppress to some extent oxygen nonstoichiometry variations and thermal expansion decreasing average TEC values down to (14.5-15.1) x 10(-6) K-1 at 300-1273 K. Substitutions extend also phase stability domain of perovskite phase, enabling processing of Sr(Y)V(Al)O3-delta electrodes in inert atmosphere with p(O-2) similar to 5 x 10(-5) atm. Slow decomposition into Y5+-based phases is observed however at slightly higher oxygen pressure of similar to 10(-4) atm. Oxidative decomposition into Sr2V2O7 and YVO4 phases on heating in air results in significant bulk dimensional changes, contributed by progressive sintering of Sr2V2O7 phase fraction. (C) 2013 Elsevier B.V. All rights reserved.

Ln-substituted SrTiO3 is a promising material for energy conversion technologies such as. thermoelectric generators and solid oxide fuel/electrolysis cells. In this study, formation of local structures enabling accommodation of excess oxygen in perovskite matrix of SrTiO3 and related redox behavior were assessed employing static lattice simulations in combination with experimental methods (XRD, SEM/EDS, XPS, TGA, and electrical measurements) using Sr(0.90-x)Ln(0.10)TiO(3 +/-delta) (Ln = Ce, Pr; x = 0-0.10) as model systems. Although strontium-vacancy formation is found to be a preferable mechanism for donor compensation in oxidized Sr(Ln)TiO3, oxygen excess still can be accommodated by extended defects quenched from high temperatures. Linear Ln(Sr)(3+)center dot center dot center dot O-i(2-) defect clusters and SrO shear planes characteristic of Ruddlesden Popper phases are found to be the most probable extended defects enabling the accommodation of excess oxygen in oxidized titanates with Sr(1-x)Ln(x)TiO(3+delta) cation stoichiometry. The presence of oxygen-rich local structures is shown to be strongly correlated with the faster redox kinetics and higher electrical conductivity critical for practical applications. Easy oxidation of reduced Sr(1-x)Ln(x)TiO(3+delta) (with electronic donor compensation) provide further evidence in favor of Ln(Sr)(3+)center dot center dot center dot O-i(2-) defect clusters as mechanism of excess oxygen accommodation.

The synthesis of Y1-x/2MgxV1-x/2O4-delta (MgxYV) (x = 0.0-0.5) by solid state reaction technique is reported. This preparation method is compared with chemical synthesis method such as nitrate-citrate auto-combustion route. The crystal structure and the surface morphology of the samples were determined by X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) techniques, respectively. Substantial decrease in phase formation temperature was observed for combustion method. The solubility limit of Mg in YVO4 lattice was found to be low i.e., x

Phase relationships, transport properties and thermomechanical behavior of acceptor- and donor-co-substituted Gd2Ti2O7 were studied for possible application in solid oxide fuel cell anodes. The range of (Gd1-xCax)(2)(Ti1-yMoy)(2)O7-delta solid solutions with a cubic pyrochlore-type structure was found to be limited to 0.10 < x < 0.15 and 0.05 < y < 0.10 under oxidizing conditions. No evidence of phase instability of substituted materials was detected in the course of the electrical and thermogravimetric studies down to oxygen partial pressures as low as 10(-19) atm at 950 degrees C. (Gd1-xCax)(2)(Ti1-yMoy)(2)O7-delta ceramics possess moderate thermal expansion coefficients compatible with solid electrolytes, (10.5-10.7) x 10(-6) K-1 at 25-1100 degrees C in air, and demonstrate remarkable dimensional stability with nearly zero chemical expansion down to p(O-2) x 10(-1)2 atm at 950 degrees C. Though co-substitution by Mo suppresses oxygen-ionic conduction under oxidizing conditions, reducing oxygen partial pressure increases both ionic and n-type electronic transport. (Gd1-xCax)(2)(Ti0.95Mo0.05)(2)O7-delta (x = 0.07-0.10) pyrochlores are mixed conductors under SOFC anode operation conditions, but comparatively low total conductivity limits its applicability as electrode materials.

Strontium vanadate-based perovskites are considered as promising anode materials for hydrocarbon-fueled solid oxide fuel cells due to high electronic conductivity, sulfur tolerance and resistance to coking, but possess a narrow phase stability domain precluding their use in practice. This study aimed at expanding the perovskite phase stability domain by donor-type substitutions focusing on Sr(0.8)Ln(0.2)V(1-y)Nb(y)O(3-delta) (Ln = La or Y, y = 0-0.10) solid solutions. The upper-p(O-2) stability boundary at 900 degrees C was found to shift from similar to 10(-15) atm for the parent strontium vanadate to similar to 6 x 10(-13) atm for Sr0.8Y0.2VO3-delta, whereas oxidative decomposition of Sr0.8La0.2VO3-delta occurs in the p(O-2) range between 10(-10) and 10(-5) atm. Co-substitution by niobium in the vanadium sublattice has rather minor (Y-containing series) or even negative (La-containing series) effects on the perovskite phase stability boundary, but results in a slower kinetics of oxidative decomposition in an inert atmosphere. Sluggish oxidation kinetics in inert gas environments, demonstrated by electrical, thermogravimetric, dilatometric and structural studies, results in a nearly reversible behavior of Sr(0.8)Ln(0.2)V(1-y)Nb(y)O(3-delta) after exposure to inert atmosphere, thus enabling the fabrication of solid-electrolyte cells with SrVO3-delta-based anodes under these conditions. Donor-type substitution is demonstrated also to decrease the electronic conductivity, which still remains sufficiently high for electrode application (>100 S cm(-1) at temperatures

Iron-based oxides are considered as promising consumable anode materials for high temperature pyroelectrolysis. Phase relationships, redox stability and electrical conductivity of Fe 3− x Al x O 4 spinels were studied at 300–1773 K and p (O 2 ) from 10 −5 to 0.21 atm. Thermogravimetry/XRD analysis revealed metastability of the sintered ceramics at 300–1300 K. Low tolerance against oxidation leads to dimensional changes of ceramics upon thermal cycling. Activation energies of the total conductivity corresponded to the range of 16–26 kJ/mol at 1450–1773 K in Ar atmosphere. At 1573–1773 K and p (O 2 ) ranging from 10 −5 to 0.03 atm, the total conductivity of Fe 3− x Al x O 4 is nearly independent of the oxygen partial pressure. The conductivity values of Fe 3− x Al x O 4 (0.1 ≤ x ≤ 0.4) at 1773 K and p (O 2 ) ∼10 −5 to 10 −4 atm were found to be only 1.1–1.5 times lower than for Fe 3 O 4 , showing high potential of moderate aluminium additions as a strategy for improvement of refractoriness for magnetite without significant deterioration of electronic transport.

The effects of K 2 O content on sintering and crystallization of glass powder compacts in the Li 2 O–K 2 O–Al 2 O 3 –SiO 2 system were investigated. Glasses featuring SiO 2 /Li 2 O molar ratios of 2.69–3.13, far beyond the lithium disilicate (LD-Li 2 Si 2 O 5 ) stoichiometry, were produced by conventional melt-quenching technique. The sintering and crystallization behaviour of glass powders was explored using hot stage microscopy (HSM), scanning electron microscopy (SEM), differential thermal (DTA) and X-ray diffraction (XRD) analyses. Increasing K 2 O content at the expense of SiO 2 was shown to lower the temperature of maximum shrinkage, eventually resulting in early densification of the glass-powder compacts. Lithium metasilicate was the main crystalline phase formed upon heat treating the glass powders with higher amounts of K 2 O. In contrast, lithium disilicate predominantly crystallized from the compositions with lower K 2 O contents resulting in strong glass–ceramics with high chemical and electrical resistance. The total content of K 2 O should be kept below 4.63 mol% for obtaining LD-based glass–ceramics.

Conversion-electron Moessbauer spectroscopy analysis of iron surface states in the dense ceramic membranes made of {sup 57}Fe-enriched SrFe{sub 0.7}Al{sub 0.3}O{sub 3-{delta}} perovskite, shows no traces of reductive decomposition or carbide formation in the interfacial layers after operation under air/CH{sub 4} gradient at 1173 K, within the limits of experimental uncertainty. The predominant trivalent state of iron cations at the membrane permeate-side surface exposed to flowing dry methane provides evidence of the kinetic stabilization mechanism, which is only possible due to slow oxygen-exchange kinetics and enables long-term operation of the ferrite-based ceramic reactors for natural gas conversion. At the membrane feed-side surface exposed to air, the fractions of Fe{sup 4+} and Fe{sup 3+} are close to those in the powder equilibrated at atmospheric oxygen pressure, suggesting that the exchange limitations to oxygen transport are essentially localized at the partially reduced surface. - Graphical Abstract: Conversion-electron Moessbauer spectroscopy analysis of dense ceramic membranes made of {sup 57}Fe-enriched SrFe{sub 0.7}Al{sub 0.3}O{sub 3-{delta}} perovskite, shows no reductive decomposition in thin interfacial layers after testing under air/CH{sub 4} gradient, enabling stable operation of the ferrite-based ceramic reactors for partial oxidation of methane. Highlights: > Conversion-electron Moessbauer spectroscopy is used for mixed-conducting membranes. > No decompositionmore » is detected in the membrane surface layers under air/CH{sub 4} gradient. > Due to kinetic stabilization, Fe{sup 3+} states prevail at the surface exposed to methane. > Transmission Moessbauer spectra show perovskite decomposition on equlibration in CH{sub 4}. > Ferrite-based ceramic reactors can stably operate under air/CH{sub 4} gradient.« less

Phase relationships, thermal expansion and electrical properties of Mg1 − xFexO (x = 0.1–0.45) cubic solid solutions and Fe3 − x − yMgxCryO4 ± δ (x = 0.7–0.95; y = 0 or 0.5) spinels were studied at 300–1770 K in the oxygen partial pressure range from 10 Pa to 21 kPa. Increasing iron content enlarges the spinel phase stability domain at reduced oxygen pressures and elevated temperatures. The total conductivity of the spinel ceramics is predominantly n-type electronic and is essentially p(O2)-independent within the stability domain. The computer simulations using molecular dynamics technique confirmed that overall level of ion diffusion remains low even at high temperatures close to the melting point. Temperature dependencies of the total conductivity in air exhibit a complex behavior associated with changing the dominant defect-chemistry mechanism from prevailing formation of the interstitial cations above 1370–1470 K to the generation of cation vacancies at lower temperatures, and with kinetically frozen cation redistribution in spinel lattice below 700–800 K. The average thermal expansion coefficients of the spinel ceramics calculated from dilatometric data in air vary in the range (9.6–10.0) × 10− 6 K− 1 at 300–500 K and (13.2–16.1) × 10− 6 K− 1 at 1050–1370 K. Mg1 − xFexO solid solutions undergo partial decomposition on heating under oxidizing and mildly reducing conditions, resulting in the segregation of spinel phase and conductivity decrease.

Cation-stoichiometric and strontium-deficient Sr1−xCexTiO3 ± δ (0

Layered LnBaCo2O5+δ (Ln = Nd, Sm) with the cation-ordered double perovskite structure were synthesized by the solid-state reaction route and characterized by X-ray diffraction, thermogravimetric analysis and dilatometry. For NdBaCo2O5.73 and SmBaCo2O5.61 equilibrated with atmospheric oxygen at low temperatures, tetragonal and orthorhombic polymorphs were found to form, respectively. The oxygen content at 300–1300 K decreases with decreasing rare-earth cation size, whilst δ variations and chemical contribution to the apparent thermal expansion in air are substantially lower compared to the disordered (Ln, A)CoO3−δ (A = Ca, Sr) analogues. The average thermal expansion coefficients are 23.1 × 10−6 K−1 for NdBaCo2O5+δ and 20.8 × 10−6 K−1 for SmBaCo2O5+δ at 300–1370 K and atmospheric oxygen pressure. These values are comparable to those of Bi2O3-based ionic conductors, but are incompatible with common electrolytes such as stabilized zirconia or doped ceria. The oxygen partial pressure dependencies of the total conductivity and Seebeck coefficient, studied in the P(O2) range from 10−10 to 1 atm, confirm predominant p-type electronic conductivity.

A series of alkaline-earth aluminosilicate glass-ceramics (GCs) were appraised with respect to their suitability as sealants for solid oxide fuel cells (SOFCs). The parent composition with general formula Ca 0.9 MgAl 0.1 La 0.1 Si 1.9 O 6 was modified with Cr 2 O 3 and BaO. The addition of BaO led to a substantial decrease in the total electrical conductivity of the GCs, thus improving their insulating properties. BaO-containing GCs exhibited higher coefficient of thermal expansion (CTE) in comparison to BaO-free GCs. An extensive segregation of oxides of Ti and Mn, components of the Crofer22 APU interconnect alloy, along with negligible formation of BaCrO 4 was observed at the interface between GC/interconnects diffusion couples. Thermal shock resistance and gas-tightness of GC sealants in contact with yttria-stabilized zirconia electrolyte (8YSZ) was evaluated in air and water. Good matching of CTE and strong, but not reactive, adhesion to the solid electrolyte and interconnect, in conjunction with a high level of electrical resistivity, are all advantageous for potential SOFC applications.