Thermodynamic Guidelines for the Mechanosynthesis or Solid-State Synthesis of MnFe2O4 at Relatively Low Temperatures.

Herein, thermodynamic assessment is proposed to screen suitable precursors for the solid-state synthesis of manganese ferrite, by mechanosynthesis at room temperature or by subsequent calcination at relatively low temperatures, and the main findings are validated by experimental results for the representative precursor mixtures MnO + FeO3, MnO2 + Fe2O3, and MnO2 +2FeCO3. Thermodynamic guidelines are provided for the synthesis of manganese ferrite from (i) oxide and/or metallic precursors; (ii) carbonate + carbonate or carbonate + oxide powder mixtures; (iii) other precursors. It is also shown that synthesis from metallic precursors (Mn + 2Fe) requires a controlled oxygen supply in limited redox conditions, which is hardly achieved by reducing gases H2/H2O or CO/CO2. Oxide mixtures with an overall oxygen balance, such as MnO + Fe2O3, act as self-redox buffers and offer prospects for mechanosynthesis for a sufficient time (>9 h) at room temperature. On the contrary, the fully oxidised oxide mixture MnO2 + Fe2O3 requires partial reduction, which prevents synthesis at room temperature and requires subsequent calcination at temperatures above 1100 °C in air or in nominally inert atmospheres above 750 °C. Oxide + carbonate mixtures, such as MnO2 +2FeCO3, also yield suitable oxygen balance by the decomposition of the carbonate precursor and offer prospects for mechanosynthesis at room temperature, and residual fractions of reactants could be converted by firing at relatively low temperatures (≥650 °C).

SrTiO3-SrVO3 Ceramics for Solid Oxide Fuel Cell Anodes: A Route from Oxidized Precursors.

Perovskite-type Sr(Ti,V)O3-δ ceramics are promising anode materials for natural gas- and biogas-fueled solid oxide fuel cells, but the instability of these phases under oxidizing conditions complicates their practical application. The present work explores approaches to the fabrication of strontium titanate-vanadate electrodes from oxidized precursors. Porous ceramics with the nominal composition SrTi1-yVyOz (y = 0.1-0.3) were prepared in air via a solid state reaction route. Thermal processing at temperatures not exceeding 1100 °C yielded composite ceramics comprising perovskite-type SrTiO3, pyrovanadate Sr2V2O7 and orthovanadate Sr3(VO4)2 phases, while increasing firing temperatures to 1250-1440 °C enabled the formation of SrTi1-yVyO3 perovskites. Vanadium was found to substitute into the titanium sublattice predominantly as V4+, even under oxidizing conditions at elevated temperatures. Both perovskite and composite oxidized ceramics exhibit moderate thermal expansion coefficients in air, 11.1-12.1 ppm/K at 30-1000 °C, and insignificant dimensional changes induced by reduction in a 10%H2-N2 atmosphere. The electrical conductivity of reduced perovskite samples remains comparatively low, ~10-1 S/cm at 900 °C, whereas the transformation of oxidized vanadate phases into high-conducting SrVO3-δ perovskites upon reduction results in enhancement in conductivity, which reaches ~3 S/cm at 900 °C in porous composite ceramics with nominal composition SrTi0.7V0.3Oz. The electrical performance of the composite is expected to be further improved by optimization of the processing route and microstructure to facilitate the reduction of the oxidized precursor and attain better percolation of the SrVO3 phase.

Glass-Zirconia Composites as Seals for Solid Oxide Cells: Preparation, Properties, and Stability over Repeated Thermal Cycles.

This study focuses on the preparation and characterization of composite gaskets designed for the sealing of the solid oxide cell stacks operating below 700 °C. The seals were fabricated with the addition of various amounts (10-90 wt.%) of 3 mol.% yttria partially stabilized zirconia to a BaO-Al2O3-CaO-SiO2 glass matrix. The sample gaskets in the form of thin frames were shaped by tape casting. The quality of the junctions between the composites and Crofer 22APU steel commonly used as an SOC interconnect was evaluated after thermal treatment of heating to 710 °C, then cooling to the working temperature of around 620 °C and then leaving them for 10h in an air atmosphere, before cooling to room temperature. The samples were also studied after 3, 5, and 10 thermal cycles to determine the changes in microstructure and to evaluate the porosity and possible crystallization of the glass phase. The compression of the seals was calculated on the basis of differences in thickness before and after thermal treatment. The influence of zirconia additions on the mechanical properties of the seals was studied. The experimental results confirmed that glass-ceramic composites are promising materials for gaskets in SOC stacks. The most beneficial properties were obtained for a composite containing 40 wt.% of YSZ.

Characterization of Ruddlesden-Popper La2-xBaxNiO4±Î´ Nickelates as Potential Electrocatalysts for Solid Oxide Cells.

Ruddlesden-Popper La2-xBaxNiO4±Î´ (x = 0-1.1) nickelates were prepared by a glycine-nitrate combustion route combined with high-temperature processing and evaluated for potential application as electrocatalysts for solid oxide cells and electrochemical NOx elimination. The characterization included structural, microstructural and dilatometric studies, determination of oxygen nonstoichiometry, measurements of electrical conductivity and oxygen permeability, and assessment of chemical compatibility with other materials. The formation range of phase-pure solid solutions was found to be limited to x = 0.5. Exceeding this limit leads to the co-existence of the main nickelate phase with low-melting Ba- and Ni-based secondary phases responsible for a strong reactivity with Pt components in experimental cells. Acceptor-type substitution of lanthanum by barium in La2-xBaxNiO4+δ is charge-compensated by decreasing oxygen excess, from δ ≈ 0.1 for x = 0 to nearly oxygen-stoichiometric state for x = 0.5 at 800 °C in air, and generation of electron-holes (formation of Ni3+). This leads to an increase in p-type electronic conductivity (up to ~80 S/cm for highly porous La1.5Ba0.5NiO4+δ ceramics at 450-900 °C) and a decline of oxygen-ionic transport. La2-xBaxNiO4+δ (x = 0-0.5) ceramics exhibit moderate thermal expansion coefficients, 13.8-14.3 ppm/K at 25-1000 °C in air. These ceramic materials react with yttria-stabilized zirconia at 700 °C with the formation of an insulating La2Zr2O7 phase but show good chemical compatibility with BaZr0.85Y0.15O3-δ solid electrolyte.

Development of La1.7Ca0.3Ni1-yCuyO4+δ Materials for Oxygen Permeation Membranes and Cathodes for Intermediate-Temperature Solid Oxide Fuel Cells.

The La1.7Ca0.3Ni1-yCuyO4+δ (y = 0.0-0.4) nickelates, synthesized via a solid-state reaction method, are investigated as prospective materials for oxygen permeation membranes and IT-SOFC cathodes. The obtained oxides are single-phase and possess a tetragonal structure (I4/mmm sp. gr.). The unit cell parameter c and the cell volume increase with Cu-substitution. The interstitial oxygen content and total conductivity decrease with Cu-substitution. The low concentration of mobile interstitial oxygen ions results in a limited oxygen permeability of Cu-substituted La1.7Ca0.3NiO4+δ ceramic membranes. However, increasing the Cu content over y = 0.2 induces two beneficial effects: enhancement of the electrochemical activity of the La1.7Ca0.3Ni1-yCuyO4+δ (y = 0.0; 0.2; 0.4) electrodes and decreasing the sintering temperature from 1200 °C to 900 °C. Enhanced electrode activity is due to better sintering properties of the developed materials ensuring excellent adhesion and facilitating the charge transfer at the electrode/electrolyte interface and, probably, faster oxygen exchange in Cu-rich materials. The polarization resistance of the La1.7Ca0.3Ni1.6Cu0.4O4+δ electrode on the Ce0.8Sm0.2O1.9 electrolyte is as low as 0.15 Ω cm2 and 1.95 Ω cm2 at 850 °C and 700 °C in air, respectively. The results of the present work demonstrate that the developed La1.7Ca0.3Ni0.6Cu0.4O4+δ-based electrode can be considered as a potential cathode for intermediate-temperature solid oxide fuel cells.

Prospects of Using Pseudobrookite as an Iron-Bearing Mineral for the Alkaline Electrolytic Production of Iron.

The alkaline electrolytic production of iron is gaining interest due to the absence of CO2 emissions and significantly lower electrical energy consumption when compared with traditional steelmaking. The possibility of using an iron-bearing pseudobrookite mineral, Fe2TiO5, is explored for the first time as an alternative feedstock for the electrochemical reduction process. To assess relevant impacts of the presence of titanium, similar electroreduction processes were also performed for Fe2TiO5·Fe2O3 and Fe2O3. The electroreduction was attempted using dense and porous ceramic cathodes. Potentiostatic studies at the cathodic potentials of -1.15--1.30 V vs. an Hg|HgO|NaOH reference electrode and a galvanostatic approach at 1 A/cm2 were used together with electroreduction from ceramic suspensions, obtained by grinding the porous ceramics. The complete electroreduction to Fe0 was only possible at high cathodic polarizations (-1.30 V), compromising the current efficiencies of the electrochemical process due to the hydrogen evolution reaction impact. Microstructural evolution and phase composition studies are discussed, providing trends on the role of titanium and corresponding electrochemical mechanisms. Although the obtained results suggest that pseudobrookite is not a feasible material to be used alone as feedstock for the electrolytic iron production, it can be considered with other iron oxide materials and/or ores to promote electroreduction.

Mixed Ionic-Electronic Conductivity, Redox Behavior and Thermochemical Expansion of Mn-Substituted 5YSZ as an Interlayer Material for Reversible Solid Oxide Cells.

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 < 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.

Development of Ni-Sr(V,Ti)O3-δ Fuel Electrodes for Solid Oxide Fuel Cells.

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.

Compromising Between Phase Stability and Electrical Performance: SrVO3 -SrTiO3 Solid Solutions as Solid Oxide Fuel Cell Anode Components.

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.

Structural and Chemical Approach toward Understanding the Aqueous Corrosion of Sodium Aluminoborate Glasses.

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 p(O2)-T stability domain of cubic perovskite Ba0.5Sr0.5Co0.8Fe0.2O3-δ.

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.

Impact of Oxygen Deficiency on the Electrochemical Performance of K2 NiF4 -Type (La1-x Srx )2 NiO4-δ Oxygen Electrodes.

Perovskite-related (La1-x Srx )2 NiO4-δ (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 electrocatalytic activity. (La1-x Srx )2 NiO4-δ solid solutions were demonstrated to preserve the K2 NiF4 -type tetragonal structure under oxidizing conditions. Acceptor-type substitution by Sr is compensated by the formation of oxygen vacancies and electron holes and progressively increases high-temperature oxygen nonstoichiometry, which reaches as high as δ=0.40 for x=0.8 at 950 °C in air. The electrical conductivity of (La1-x Srx )2 NiO4-δ ceramics at 500-1000 °C and p(O2 )≥10-3  atm is p-type metallic-like. The highest conductivity, 300 S cm-1 at 800 °C in air, is observed for x=0.6. The average thermal expansion coefficients, (14.0-15.4)×10-6  K-1 at 25-900 °C in air, are sufficiently low to ensure the thermomechanical compatibility with common solid electrolytes. The polarization resistance of porous (La1-x Srx )2 NiO4-δ electrodes applied on a Ce0.9 Gd0.1 O2-δ solid electrolyte decreases with increasing Sr concentration in correlation with the concentration of oxygen vacancies in the nickelate lattice and the anticipated level of mixed ionic-electronic conduction. However, this is accompanied by increasing reactivity between the cell components and necessitates the microstructural optimization of the electrode materials to reduce the electrode fabrication temperature.

A (3 + 3)-dimensional "hypercubic" oxide-ionic conductor: type II Bi2O3-Nb2O5.

The high-temperature cubic form of bismuth oxide, δ-Bi2O3, is the best intermediate-temperature oxide-ionic conductor known. The most elegant way of stabilizing δ-Bi2O3 to room temperature, while preserving a large part of its conductivity, is by doping with higher valent transition metals to create wide solid-solutions fields with exceedingly rare and complex (3 + 3)-dimensional incommensurately modulated "hypercubic" structures. These materials remain poorly understood because no such structure has ever been quantitatively solved and refined, due to both the complexity of the problem and a lack of adequate experimental data. We have addressed this by growing a large (centimeter scale) crystal using a novel refluxing floating-zone method, collecting high-quality single-crystal neutron diffraction data, and treating its structure together with X-ray diffraction data within the superspace symmetry formalism. The structure can be understood as an "inflated" pyrochlore, in which corner-connected NbO6 octahedral chains move smoothly apart to accommodate the solid solution. While some oxide vacancies are ordered into these chains, the rest are distributed throughout a continuous three-dimensional network of wide δ-Bi2O3-like channels, explaining the high oxide-ionic conductivity compared to commensurately modulated phases in the same pseudobinary system.