Bias polarization study of steam electrolysis by composite oxygen electrode Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ/BaCe_0.4Zr_0.4Y_0.2O_3-δ

The polarization behavior of Ba0.5Sr0.5Co0.8Fe0.2O3-δ-BaCe0.4Zr0.4Y0.2O3-δ (BSCF-BCZY) electrode under steam electrolysis conditions was studied in detail. The composite oxygen electrode supported by BCZY electrolyzer has been assessed as a function of temperature (T), water vapor partial pressures (pH2O), and bias polarization voltage for electrodes of comparable microstructure. The Electrochemical impedance spectra show two depressed arcs in general without bias polarization. And the electrode resistance became smaller with the increase of the bias polarization under the same water vapor partial pressures. The total resistance of the electrode was shown to be significantly affected by temperature, with the same level of pH2O and bias polarization voltage. This result highlights BSCF-BCZY as an effective oxygen electrode under moderate polarization and pH2O conditions.</p

Oxygen transport in La2NiO4 + [delta]: Assessment of surface limitations and multilayer membrane architectures

http://www.sciencedirect.com/science/article/B6TY4-4VMBNT9-3/2/2c6a74f1bb2f51b7b9503333d1045f0

Oxygen Nonstoichiometry, Mixed Conductivity, and Mössbauer Spectra of Ln0.5A0.5FeO3−δ (Ln = La−Sm, A = Sr, Ba): Effects of Cation Size

Increasing the difference of the Ln3+ and A2+ cation radii in perovskite-type Ln0.5A0.5FeO3−δ (Ln = La, Pr, Nd, Sm; A = Sr, Ba) results in higher oxygen deficiency and lower oxygen-ionic and p-type electronic conductivities, determined using the oxygen permeation and total conductivity measurements at 973−1223 K. The relationships between the anion transport and A-site cation size mismatch remain essentially similar in air and under reducing conditions when most iron cations become trivalent, thus confirming critical influence of oxygen-vacancy trapping processes induced by the lattice strain. At low temperatures, analogous correlation is also observed for quadrupole splittings derived from the Mssbauer spectra of oxygen-stoichiometric Ln0.5A0.5FeO3. Contrary to the ionic conductivity variations, the role of surface exchange kinetics as a permeation-limiting factor, evaluated from the membrane thickness dependence of oxygen fluxes, tends to decrease on Ba2+ doping and on decreasing Ln3+ size in Ln0.5Sr0.5FeO3−δ series. The n-type electronic conduction and low-p(O2) stability at 1223 K are substantially unaffected by the cation radius mismatch

Oxygen permeability of mixed-conducting Ce0.8Tb0.2O2-delta membranes: Effects of ceramic microstructure and sintering temperature

Nanopowder of Ce0.8Tb0.2,O2-delta, a fluorite-type mixed conductor stable in wide p(O-2) range, was successfully synthesised by the hydrothermal method. Dense ceramics sintered at relatively low temperatures (900 and 1200 degrees C), by employing minor additions of cobalt oxide sintering additive (2 mol%), were compared to those sintered at 1500 degrees C without additives. The effect of the sintering temperature on oxygen transport was analysed in O-2 and N-2 atmospheres. Oxide-ion transference numbers were determined by the modified electromotive force ([ME) method under oxygen/air and nitrogen/air gradients, showing positive temperature dependencies for all sintering temperatures. The partial ionic and electronic conductivities, calculated from the transference numbers and total conductivity, both increase with cobalt additions. A detailed transmission electron microscopy (TEM) study shows a grain boundary location of the cobalt sintering additive at the lowest sintering temperature, 900 degrees C. On increasing sintering temperature the grain-boundary concentration of cobalt is depleted, leading to the presence of segregated grains of cobalt oxide. This factor is shown to be highly relevant with respect to measured oxygen permeation fluxes. Ce0.8Tb0.2O2-delta ceramics sintered at 900 degrees C show significantly higher oxygen permeation, related to improved surface exchange due to the grainboundary enrichment of cobalt and larger grain-boundary area. (C) 2014 Elsevier B.V. All rights reserved

Oxygen permeability of transition metal-containing La(Sr,Pr)Ga(Mg)O_3-δ ceramic membranes

Acceptor-type doping of perovskite-type La1-xSrxGa0.80-yMgyM0.20O3-δ (x = 0-0.20, y = 0.15-0.20, M = Fe, Co, Ni) leads to significant enhancement of ionic conductivity and oxygen permeability due to increasing oxygen vacancy concentration. The increase in strontium and magnesium content is accompanied, however, with increasing role of surface exchange kinetics as permeation-limiting factor. At temperatures below 1223 K, the oxygen permeation fluxes through La(Sr)Ga(Mg,M)O3-δ membranes with thickness less than 1.5 mm are predominantly limited by the exchange rates at membrane surface. The oxygen transport in transition metal-containing La(Sr)Ga(Mg)O3-δ ceramics increase in the sequence Co < Fe < Ni. The ionic conduction in these phases is, however, lower than that in the parent compounds, La1-xSrxGa1-yMgyO3-δ. The highest level of oxygen permeation, comparable to that of La(Sr)Fe(Co)O3- and La2NiO4-based phases, is observed for La0.90Sr0.10Ga0.65Mg0.15Ni0.20O3-δ membranes. The average thermal expansion coefficients of La(Sr)Ga(Mg,M)O3-δ ceramics in air are in range (11.6–18.4) × 10-6 K-1 at 373-1273 K. Doping of LaGa0.65Mg0.15Ni0.20O3-δ with praseodymium results in moderate increase of the permeation fluxes, lower thermal expansion and an improved phase stability in reducing environments.<br><br>El dopado aceptor de cerámicas tipo perovskita La1-xSrxGa0.80-yMgyM0.20O3-δ (x = 0-0.20, y = 0.15-0.20, M = Fe, Co, Ni) da lugar a una mejora significativa de la conductividad iónica y de la permeabilidad al oxígeno debido al aumento de la concentración de vacantes de oxígeno. Sin embargo, el aumento de la cantidad de estroncio y magnesio viene acompañado de un aumento de la participación de las cinéticas de intercambio superficial como factor limitante de la permeabilidad. A temperaturas por debajo de 1223 K la permeabilidad al flujo de oxígeno a través de las membranas de La(Sr)Ga(Mg,M)O3-δ con espesor menor de 1.5 mm está limitado principalmente por las velocidades de intercambio en la superficie de la membrana. El transporte de oxígeno en las cerámicas La(Sr)Ga(Mg)O3-δ que contienen M aumenta en la secuencia Co < Fe < Ni. La conductividad iónica en estas fases es, sin embargo, menor que en la de los compuestos La1-xSrxGa1-yMgyO3-δ. El mayor nivel de permeabilidad de oxígeno, comparable a la de las fases basadas en La(Sr)Fe(Co)O3 y La2NiO4, se observa para las membranas de La0.90Sr0.10Ga0.65Mg0.15Ni0.20O3-δ. Los coeficientes de dilatación térmica medios de las cerámicas La(Sr)Ga(Mg,M)O3-δ en aire son del orden de (11.6–18.4) × 10-6 K-1 a 373-1273 K. El dopado de LaGa0.65Mg0.15Ni0.20O3-δ con praseodimio da lugar a un incremento moderado de los flujos de permeabilidad, menor dilatación térmica y una mejor estabilidad de fase en entornos reductores

Oxygen Ionic Transport in Brownmillerite-Type Ca2Fe2O5-delta and Calcium Ferrite-Based Composite Membranes

Oxygen ionic transport in mixed-conducting Ca2Fe2O5-delta brownmillerite was analyzed in light of potential applications in the composite materials for oxygen separation membranes and solid oxide fuel cell cathodes. The lattice defect formation and oxygen diffusion mechanisms were assessed by the computer simulations employing molecular dynamics and static lattice modeling. The most energetically favorable oxygen-vacancy location is in the octahedral layers of the brownmillerite structure, which provide a maximum contribution to the ionic migration in comparison with the structural blocks comprising iron-oxygen tetrahedra. The activation energies for the vacancy and interstitial diffusion in the tetrahedral layers, and also between the octahedral and tetrahedral sheets, are several times higher. The calculated values were found comparable to the experimental activation energy for ionic conduction in Ca2Fe2O5-delta, 147 kJ/mol, determined by the steady-state oxygen permeation measurements. The dense membranes of model composite Ca2Fe2O5-delta - Ce0.9Gd0.1O2-delta with equal weight fractions of the components (CGCF5) were sintered and characterized. No critical interdiffusion of the composite constituents, leading to their decomposition, was found by X-ray diffraction and electron microscopic analyses. The electrical conductivity of this composite, with an activation energy of 37 kJ/mol, is intermediate between two parent compounds and is dominantly p-type electronic as for Ca2Fe2O5-delta. Since the ion- and electron-conducting phases are well percolated in the composite ceramics, the oxygen permeation fluxes through CGCF5 are considerably higher than those of both constituents

Over 25 Years of Partnering to Conserve Chiricahua Leopard Frogs (<i>Rana chiricahuensis</i>) in Arizona, Combining Ex Situ and In Situ Strategies

The Phoenix Zoo has partnered with US Fish and Wildlife Service, Arizona Game and Fish Department, US Forest Service, and other organizations for more than 25 years to help recover Chiricahua leopard frogs (Rana [=Lithobates] chiricahuensis) in Arizona, USA. This federally threatened species faces declines due to habitat loss and degradation, long-term drought, disease, and invasive species. Over 26,000 larvae, froglets, and adults, as well as 26 egg masses produced by adults held at the Phoenix Zoo have been released to the wild, augmenting and/or re-establishing wild populations. Chiricahua leopard frog-occupied sites in Arizona have increased from 38 in 2007, when the species’ recovery plan was published, to a high of 155 in the last five years, as a result of ex situ and in situ conservation efforts. As one of the longest-running programs of its kind in the United States, communication among partners has been key to sustaining it. Recovery strategies and complex decisions are made as a team and we have worked through numerous management challenges together. Though Chiricahua leopard frogs still face significant threats and a long road to recovery, this program serves as a strong example of the positive effects of conservation partnerships for native wildlife

Mixed conducting materials for partial oxidation of hydrocarbons

Thermodynamic calculations with additional conditions for the conservation of carbon and hydrogen were used to predict the gas composition obtained by partial oxidation of methane as a function of oxygen partial pressure and temperature; this was used to assess the stability and oxygen permeability requirements of mixed conducting membrane materials proposed for this purpose. A re-examination of known mixed conductors shows that most materials with highest permeability still fail to fulfil the requirements of stability under reducing conditions. Other materials possess sufficient stability but their oxygen permeability is insufficient. Different approaches were thus used to attempt to overcome those limitations, including changes in composition in the A and B site positions of ABO3 perovskites, and tests of materials with different structure types. Promising results were obtained mainly for some materials with perovskite or related K2NiF4-type structures. Limited stability of the most promising materials shows that one should rely mainly on kinetic limitations in the permeate side to protect the mixed conductor from severe reducing conditions.&lt;br&gt;&lt;br&gt;Se han usado cálculos termodinámicos con condiciones adicionales para la conservación del carbono e hidrógeno para predecir la composición del gas obtenido mediante la oxidación parcial del metano en función de la presión parcial de oxígeno y de la temperatura; esto se ha usado para asegurar los requerimientos de estabilidad y permeabilidad al oxígeno de los materiales conductores mixtos empleados como membrana para este propósito. Un nuevo exámen de los conductores mixtos conocidos muestra que la mayoría de los materiales con la mayor permeabilidad todavía fallan en el cumplimiento de los requerimientos de estabilidad bajo condiciones reductoras. Otros materiales poseen suficiente estabilidad, pero su permeabilidad al oxígeno es insuficiente. Por ello se han empleado diferentes aproximaciones para intentar superar esas limitaciones, incluyendo cambios en la composición en las posiciones A y B de de las perovsquitas ABO3, y pruebas con materiales con estructuras diferentes. Se han obtenido resultados prometedores principalmente en algunos materiales con estructura perovsquita o estructuras relacionadas con K2NiF4. La estabilidad limitada de los materiales más prometedores muestra que los estudios deben hacerse principalmente sobre las limitaciones cinéticas del lado permeable para proteger el conductor mixto en condiciones reductoras severas