Geometric parameterization of the YBaCo4O7 structure type: implications for stability of the hexagonal form and oxygen uptake.

We explore the stability of the hexagonal form of MBaCo4O7 cobaltites in terms of geometric characteristics of the crystal structure and Global Instability Index (GII) based on the bond-valence considerations. Mismatch between an M3+/2+ and the three-dimensional network of CoO4 tetrahedra, whether expressed using an M ionic radii or GII, is shown to essentially determine both the temperature of structural transition to an orthorhombic modification and oxygen storage properties. A number of M cations not reported in the literature are identified to be suitable for the octahedral sites in an MBaCo4O7 structure. © 2010, Elsevier Ltd

Mossbauer spectroscopy analysis of Fe-57-doped YBaCo4O7+delta: effects of oxygen intercalation.

Mossbauer spectroscopy of layered YBaCo3.96Fe0.04O7+delta (delta=0.02 and 0.80), where 1% cobalt is substituted With 57 Fe isotope, revealed no evidence of charge ordering at 4-293 K. The predominant state of iron cations was found trivalent, irrespective of their coordination and oxygen stoichiometry variations determined by thermogravimetric analysis. The extremely slow kinetics of isothermal oxidation at 598 K in air, and the changes of Fe3+ fractions in the alternating triangular and Kagome layers in oxidized YBaCo3.96Fe0.04O7.80, may suggest that oxygen intercalation is accompanied with a substantial structural reconstruction stagnated due to sluggish cation diffusion. Decreasing temperature below 75-80 K leads to gradual freezing of the iron magnetic moments in inverse correlation with the content of extra oxygen. The formation of metal-oxygen octahedra and resultant structural distortions extend the temperature range where the paramagnetic and frozen states co-exist, down to 45-50 K. © 2008, Elsevier Ltd

Mixed conductivity and stability of CaFe2O4−δ.

The total conductivity of CaFe2O4-delta, studied in the oxygen partial pressure range from 10(-17) to 0.5 atm at 1023-1223 K, is predominantly p-type electronic under oxidizing conditions. The oxygen ion transference numbers determined by the steady-state oxygen permeation and faradaic efficiency measurements vary in the range of 0.2 to 7.2 x 10(-4) at 1123-1273 K, increasing with temperature. No evidence of any significant cationic contribution to the conductivity was found. The Mossbauer spectroscopy, thermogravimetry, and X-ray diffraction (XRD) showed that the orthorhombic lattice of calcium ferrite is essentially intolerant to the oxygen vacancy formation and to doping with lower-valence cations, such as Co and Ni. The oxygen nonstoichiometry (delta) is almost negligible, 0.0046-0.0059 at 973-1223 K and p(O-2) = 10(-5)-0.21 atm, providing a substantial dimensional stability of CaFe2O4-delta ceramics. The average linear thermal expansion coefficients, calculated from the controlled-atmosphere dilatometry and high-temperature XRD data, are (9.6-13.9) x 10(-6) K-1 in the oxygen pressure range from 10(-8) to 0.21 atm at 873-1373 K. Decreasing P(02) results in a modest lattice contraction and in the p-n transition indicated by the conductivity and Seebeck coefficient variations. The phase decomposition of CaFe2O4-delta occurs at oxygen chemical potentials between the low-p(O-2) stability limit of Ca2Fe2O5-delta brownmillerite and the hematite/magnetite boundary in binary Fe-O system. © 2008, Electrochemical Society Inc

Mixed conductivity and electrochemical behavior of (La_0.75Sr_0.25)_0.95Cr_0.5Mn_0.5O_{3 −} _δ

The electronic and oxygen-ionic transport in (La0.75Sr0.25)(0.95)Cr0.5Mn0.5O3-delta, a member of promising family of solid oxide fuel cell (SOFC) anode materials, was studied at 1023-1273 K in the oxygen partial pressure range from 10(-2)0 to 0.5 atm. In oxidizing and moderately reducing atmospheres, this perovskite exhibits a predominant p-type electronic conductivity, which ties in the range 20-35 S/cm and is essentially p(O-2)-independent. Reducing p(O-2) below 10(-16)-10(-12) atm leads to a drastic increase in the oxygen vacancy concentration, ionic conductivity and oxygen permeability, whilst the total conductivity decreases down to 1-3 S/cm. The ion transference numbers, calculated from the oxygen permeation data and measured by the faradaic efficiency technique controlling oxygen pressures at both sides of dense ceramic membranes, vary in the range 9 x 10(-7) to 8 x 10(-5) at 1223-1273 K, increasing with temperature. The average thermal expansion coefficients in air increases from 10.8 x 10(-6) K-1 at 373-923 K up to 14.1 x 10(-6) K-1 at 1223-1523 K. Under both oxidizing and reducing conditions, the electrochemical behavior of porous (La-0.75 Sr-0.25)(0.95)Cr0.5Mn0.5O3-based electrodes applied onto (La0.9Sr0.1)(0.98)Ga0.8Mg0.2O3-delta solid electrolyte suggests a key role of electronic transport-related processes. As a result, the electrode performance can be significantly enhanced by optimizing current collector and/or by introducing an additional electronically-conductive component, such as metallic Ni or Ag. Further decrease of overpotentials may be achieved via incorporation of electrocatalytically active additions, including praseodymium oxide in oxidizing atmospheres and ceria at low P(O-2). (c) 2006 Elsevier B.V. All rights reserved.</p

Behavior of (La,Sr)CoO3- and La2NiO4-based ceramic anodes in alkaline media: compositional and microstructural factors.

The behavior of dense ceramic anodes made of perovskite-type La1-x-ySrxCo1-zAlzO3-δ (x=0.30-0.70; y=0-0.05; z=0-0.20) and K2NiF4- type La2Ni1-xMexO4+δ (Me=Co, Cu; x=0-0.20) indicates significant influence of metal hydroxide formation at the electrode surface on the oxygen evolution reaction (OER) kinetics in alkaline solutions. The overpotential of cobaltite electrodes was found to decrease with time, while cyclic voltammetry shows the appearance of redox peaks characteristic of Co(OH)(2)/CoOOH. This is accompanied with increasing effective capacitance estimated from the impedance spectroscopy data, because of roughening of the ceramic surface. The steady-state polarization curves of (La,Sr)CoO3-δ in the OER range, including the Tafel slope, are very similar to those of model Co(OH)(2)-La(OH)(3) composite films where the introduction of lanthanum hydroxide leads to decreasing electrochemical activity. La2NiO4-based anodes exhibit a low electrochemical performance and poor stability. The effects of oxygen nonstoichiometry of the perovskite-related phases are rather negligible at high overpotentials but become significant when the polarization decreases, a result of increasing role of oxygen intercalation processes. The maximum electrocatalytic activity to OER was observed for A-site-deficient (La0.3Sr0.7)(0.97)CoO3-δ, where the lanthanum content is relatively low and the Co4+ concentration determined by thermogravimetric analysis is highest compared to other cobaltites. Applying microporous layers made of template-synthesized nanocrystalline (La0.3Sr0.7)(0.97)CoO3-δ leads to an improved anode performance, although the effects of microstructure and thickness are modest, suggesting a narrow electrochemical reaction zone. Further enhancement of the OER kinetics can be achieved by electrodeposition of cobalt hydroxide- and nickel hydroxide- based films. © 2008, Springer. The original publication is available at www.springerlink.co

Oxygen permeability, stability and electrochemical behavior of Pr2NiO4+δ-based materials.

The high-temperature electronic and ionic transport properties, thermal expansion and stability of dense Pr2NiO4+δ Pr2Ni0.9Fe0.1O4+δ ceramics have been appraised in comparison with K2NiF4-type lanthanum nickelate. Under oxidizing conditions, the extensive oxygen uptake at temperatures below 1073-1223 K leads to reversible decomposition of Pr2NiO4-based solid solutions into Ruddlesden-Popper type Pr4Ni3O10 and praseodymium oxide phases. The substitution of nickel with copper decreases the oxygen content and phase transition temperature, whilst the incorporation of iron cations has opposite effects. Both types of doping tend to decrease stability in reducing atmospheres as estimated from the oxygen partial pressure dependencies of total conductivity and Seebeck coefficient. The steady-state oxygen permeability of Pr2NiO4+δ ceramics at 1173-1223 K, limited by both surface-exchange kinetics and bulk ionic conduction, is similar to that of La2NiO4+δ. The phase transformation on cooling results in considerably higher electronic conductivity and oxygen permeation, but is associated also with significant volume changes revealed by dilatometry. At 973-1073 K, porous Pr2Ni0.8Cu0.2O4+δ electrodes deposited onto lanthanum gallate-based solid electrolyte exhibit lower anodic overpotentials compared to Pr2Ni0.8Cu0.2O4+δ, whilst cathodic reduction decreases their performance. © 2007, Springer. The original publication is available at www.springerlink.co