Oxygen Permeation Experiments Research Articles

Ba-doped Pr2NiO4+δ electrodes for proton-conducting electrochemical cells. Part 3: Electrochemical applications

Layered nickelates, Ln2NiO4+δ, are promising electrode materials for many electrochemical applications, including solid oxide fuel cells and electrolysis cells. Although Ln2NiO4+δ has been extensively modified by various doping strategies to tune its functional properties, the partial substitution of Ln3+ with Ba2+ remains among the least studied routes. At the same time, such substitution is found to be favorable when Ln2NiO4+δ materials are used for protonic ceramic electrochemical cells based on Ba-containing proton-conducting electrolytes (i.e., BaCeO3, BaZrO3, Ba(Ce,Zr)O3). In this work, which is the third part of a systematic study, Pr2–xBaxNiO4+δ materials are used as electrodes for a proton ceramic fuel cell and as oxygen permeable membranes. The oxygen permeation experiments confirm that the compositions with x = 0.2 and 0.3 prevail over x = 0 and 0.1 in terms of their oxygen-ionic conductivity, while the electrochemical cell characterizations confirm the high electrochemical activity of the Pr1.8Ba0.2NiO4+δ electrode in both fuel-cell- and electrolysis-cell modes. Our research thus confirms that a Ba-doping strategy is highly promising for designing new Ln2NiO4+δ-based phases, simultaneously offering good chemical and thermal compatibility with state-of-the-art proton-conducting electrolytes and high electrochemical performance.

Experimental Study on the Law Prevention of Coal Spontaneous Combustion by Shielding O2 With Hysteresis Oxygen Depletion Inhibitor

ABSTRACT To improve the efficiency of flame retardants in preventing spontaneous combustion of coal, a new type of hysteresis oxygen depletion inhibitor was developed based on the principle of bionics using sodium alginate, NaHCO3, MgCl2, and reduced Fe powder as raw materials. Oxygen permeation rates of different concentrations of sodium alginate were investigated by oxygen permeation experiments, the oxygen consumption characteristics of the inhibitor were analyzed according to the oxygen consumption experiments, and the inhibitory effect of the inhibitor on the oxidative process of spontaneous combustion of coal was analyzed according to the coal oxidative procedure warming experiments. The results showed that oxygen penetration into the inhibitor was slowest when the concentration of sodium alginate was 2%; 1.11 mg/L Fe reduction powder was low exothermic and could continuously consume oxygen; After the addition of this inhibitor, the temperature point where coal samples consumed a lot of oxygen was delayed from 130°C to 170°C, the oxygen consumption rate was reduced from 16.56% to 5.36%, and the inhibition rate of coal oxidation was as high as 67.63%-98.20%. This study used deoxygenation to reduce the coal oxygen contact and provides new ideas and methods for effectively curbing the spontaneous combustion of coal.

Thickness-Dependent Permeation Properties of Quenched and Standard Laser-Sintered Polyamide 12 Sheets.

The laser sintering of polymers is an additive manufacturing technology that is becoming increasingly established in the industrial environment. This study investigated the thickness-dependent permeation properties of laser-sintered (LS) polymers as required to design and produce components with a special barrier performance to gaseous substances. Helium and oxygen permeation experiments were carried out on quenched and standard LS polyamide 12 (PA12) sheets generated with two, four, six, and eight layers at a constant powder layer thickness of 100 µm. The structural properties of the sheets were examined by differential scanning calorimetry, light microscopy, and X-ray micro-computed tomography. A reduction in thickness resulted in higher diffusion coefficients for both types of LS sheets. An explanation could be the large volume fraction of poorly sintered powder particles adhering to the surfaces and incomplete melting and low consolidation of the polymer at small thicknesses. The thickness-dependency of the solubility coefficients was the opposite, especially for the standard LS sheets, which might be related to the larger pore volume in thicker sheets. As both effects compensated for each other, nearly constant permeation coefficients for all thicknesses were observed. The results provide further insights into different material characteristics of thin LS PA12 structures and offer new information on factors relevant to their solution and diffusion behavior.

Oxygen permeation and oxidative coupling of methane with NiFe2O4-Gd0.1Ce0.9O2-δ composite membrane

Oxygen permeability and oxidative coupling of methane (OCM) characteristics of a 30 vol.% NiFe2O4-70 vol.% Gd0.1Ce0.9O2-δ (NFO-GDC) composite membrane were systematically investigated by varying operational conditions for oxygen permeation and OCM experiments. The NFO-GDC membrane showed the maximum oxygen permeation flux of 2.24 mL∙cm−2∙min−1 under an Air/He gradient at 900 °C, and flux increased with increasing oxygen partial pressure gradient in the order He < CH4 < H2. The maximum OCM yield and C2H4/C2H6 ratio were observed at 900 °C and for CH4 flowing at 12.5 mL∙min-1, respectively, and the yield and ratio both increased with increasing temperature in the range 700–900 °C. The overall coupling yield was notably enhanced by applying a Mn/Na2WO4/SiO2 catalyst to the permeate side of the composite membrane. The NFO-GDC membrane with the Mn/Na2WO4/SiO2 catalyst exhibited the maximum yield and a C2H4/C2H6 ratio of 8.6% and 3.21, respectively.

Mass transfer in polycrystalline ytterbium monosilicate under oxygen potential gradients at high temperatures

Mass transfer in polycrystalline Yb2SiO5 wafers with precise composition control was evaluated and analyzed by oxygen permeation experiments at high temperatures using an oxygen tracer. Oxygen permeation proceeded due to mutual grain boundary diffusion of oxide ions and Yb ions without synergistic effects such as acceleration or suppression. The oxygen shielding properties of Yb2SiO5 were compared with those of the other line compounds such as Yb2Si2O7 and Al2O3 based on the determined mass transfer parameters. It was found that the more preferentially an oxide ion diffuses in the grain boundary compared to the interior of the grain, the greater the effect of suppressing the movement of the oxide ion by applying an oxygen potential gradient becomes.

Influence of Mo-doping on structure and oxygen permeation properties of SrCo0.8−xFe0.2MoxO3-δ perovskite membranes for oxygen separation

The effect of cobalt substitution by highly charged Mo6+ cations in SrCo0.8Fe0.2O3-δ (SCF) on the structure, microstructure, oxygen permeation performance and stability in the atmosphere containing carbon dioxide was systematically investigated by XRD, HT-XRD, Mossbauer spectroscopy, oxygen release technique and oxygen permeation experiments. The decrease in oxygen stoichiometry of SrCo0.8-xFe0.2MoxO3-δ (SCFM) materials was accompanied by nanostructuring with observed formation of nanosized domains with ordered oxygen vacancies. Equilibrium "3−δ−lg pO2−T" diagrams showed that SCF doping by molybdenum leads to broadening of the P1 (cubic perovskite phase) stability region. The obtained SCFM membrane materials possess improved phase stability in the intermediate temperature range (T < 700°C) and low oxygen partial pressure (pO2 < 0.06atm) compared to the undoped SCF oxide. The observed non-Arrhenius dependence of oxygen fluxes across SCFM disc membranes is related to combined control by bulk diffusion and surface exchange reactions.

A model for time-dependent grain boundary diffusion of ions and electrons through a film or scale, with an application to alumina

A model for ionic and electronic grain boundary transport through thin films, scales or membranes with columnar grain structure is introduced. The grain structure is idealized as a lattice of identical hexagonal cells – a honeycomb pattern. Reactions with the environment constitute the boundary conditions and drive the transport between the surfaces. Time-dependent simulations solving the Poisson equation self-consistently with the Nernst-Planck flux equations for the mobile species are performed. In the resulting Poisson-Nernst-Planck system of equations, the electrostatic potential is obtained from the Poisson equation in its integral form by summation. The model is used to interpret alumina membrane oxygen permeation experiments, in which different oxygen gas pressures are applied at opposite membrane surfaces and the resulting flux of oxygen molecules through the membrane is measured. Simulation results involving four mobile species, charged aluminum and oxygen vacancies, electrons, and holes, provide a complete description of the measurements and insight into the microscopic processes underpinning the oxygen permeation of the membrane. Most notably, the hypothesized transition between p-type and n-type ionic conductivity of the alumina grain boundaries as a function of the applied oxygen gas pressure is observed in the simulations. The range of validity of a simple analytic model for the oxygen permeation rate, similar to the Wagner theory of metal oxidation, is quantified by comparison to the numeric simulations. The three-dimensional model we develop here is readily adaptable to problems such as transport in a solid state electrode, or corrosion scale growth.

Investigation of Co–doped Ce0.8Sm0.2O2–δ–Ba0.95La0.05Zr0.1Fe0.9–xCoxO3–δDual–phase Oxygen Transport Membranes

In this study, 60 wt% Ce 0.8 Sm 0.2 O 2– δ –Ba 0.95 La 0.05 Zr 0.1 Fe 0.9– x Co x O 3– δ (SDC–BLZFC, x = 0–0.4) dual–phase oxygen transport membranes were prepared by a combined EDTA–citrate complexing sol–gol method. The effects of partial substitution of iron by cobalt on the crystal structure, phase structural stability, oxygen permeation flux and CO 2 –tolerance were systematically investigated by XRD, TG and oxygen permeation experiment. The sample with x = 0.4 had the highest oxygen permeation flux of 0.42 ml min -1 cm -2 in He, while decreased most sharply after switching into pure CO 2 . The result of sample with x = 0 was opposite. From all results, it could indicate that Co–doping contributes to oxygen permeation flux in inert gases and phase structural stability, but would reduce the CO 2 –tolerance of SDC–BLZFC dual–phase membranes.

Cobalt-free gadolinium-doped perovskite GdxBa1−xFeO3−δ as high-performance materials for oxygen separation

Cobalt-free oxides GdxBa1−xFeO3−δ (0.01≤x≤0.1) were achieved by a solid state reaction method. It is found that GdxBa1−xFeO3−δ (0.025≤x≤0.1) exhibits the cubic perovskite structure. Among GdxBa1−xFeO3−δ (0.025≤x≤0.1), the Gd0.025Ba0.975FeO3−δ (GBF2.5) membrane shows the outstanding phase structure stability and the highest oxygen permeation, which can reach 1.44ml·cm−2·min−1 at 950°C under air/He oxygen partial pressure gradient. The GBF2.5 membrane was successfully operated for more than 100h at 800°C and the oxygen permeation flux through the membrane is 0.62ml·cm−2·min−1. After 100h oxygen permeation experiment at 800°C, X-ray diffraction (XRD) and energy dispersive X-ray spectrometer (EDXS) demonstrate that the GBF2.5 exhibits phase structure stability even at intermediate temperature.

Investigation of carbonates in oxygen-transporting membrane ceramics

The performance of perovskite-type oxygen-transporting membranes during CO2 sweeping has been investigated. The series of La1−xSr xCo0.8Fe0.2O3−δ (LSCF) materials with x=1, 0.8, 0.6 and 0.4 (SCF, LSCF2882, LSCF4682, LSCF6482) were synthesized by the sol–gel method and characterized by X-ray diffraction (XRD). To investigate the poisoning effect of CO2 on the phase composition and microstructure of the membranes after long-term oxygen permeation experiments (more than 200h) at 1173K under a CO2 atmosphere, transmission electron microscopy (TEM) investigations were carried out. Electron-energy loss spectroscopy (EELS) and energy-dispersive X-ray spectroscopy (EDXS) revealed the formation of carbonate at the outer surface of the LSCF materials with low lanthanum content, which correlates with a decrease in oxygen permeation flux. Nevertheless, the LSCF 6482 material is tolerant to CO2 gas. No carbonate layer was found after 200h of operation, and the oxygen flux remained stable.

Oxygen permeability and CO2-tolerance of Ce0.8Gd0.2O2−δ-LnBaCo2O5+δ dual-phase membranes

A series of oxygen permeable dual-phase composite oxides 60 wt% Ce0.8Gd0.2O2–δ-40 wt% LnBaCo2O5+δ (CGO-LBCO, Ln = La, Pr, Nd, Sm, Gd and Y) were synthesized through a sol-gel route and effects of the Ln3+ cations on their phase structure, oxygen permeability and chemical stability against CO2 were investigated systemically by XRD, SEM, TG-DSC and oxygen permeation experiments. XRD patterns reveal that the larger Ln3+ cations (La3+, Pr3+ and Nd3+) successfully stabilized the double-layered perovskite structure of sintered LBCO, while the smaller ones (Sm3+, Gd3+, and Y3+) resulted in the partial decomposition of LBCO with some impurities formed. CGO-PBCO yields the highest oxygen permeation flux, reaching 2.8×10−7 mol·s−1·cm−2 at 925 °C with 1 mm thickness under air/He gradient. The TG-DSC profiles in 20 mol% CO2/N2 and oxygen permeability experiments with CO2 as sweep gas show that CGO-YBCO demonstrates the best chemical stability against CO2, possibly due to its minimum basicity. The stable oxygen permeation flux of CGO-YBCO under CO2 atmosphere reveals its potential application in the oxy-fuel combustion route for CO2 capture.

Physical aging in amorphous poly(ethylene furanoate): Enthalpic recovery, density, and oxygen transport considerations

The current work utilizes three separate techniques to study the physical aging process in amorphous poly(ethylene furanoate) (PEF), which is a recently introduced engineering thermoplastic with enhanced properties compared to petroleum-sourced poly(ethylene terephthalate). Differential scanning calorimetry aging experiments were conducted at multiple aging temperatures and times, and the resultant enthalpic recovery values compared to the theoretical maximum enthalpy loss evaluated from calculations involving extrapolation of the equilibrium liquid line. Density measurements reveal densification of the matrix for the aged versus unaged samples, and provide an estimate for the reduction in free volume for the aged samples. Complementary oxygen permeation and pressure-decay sorption experiments provide independent verification of the free volume reduction mechanism for physical aging in glassy polymers. The current work provides the first detailed aging study for PEF. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 389–399

Oxygen Permeation and Stability of Ce0.8Gd0.2O2−δ–PrBaCo2−xFexO5+δ Dual–phase Composite Membranes

Dual-phase membranes of 60 wt% Ce0.8Gd0.2O2−δ–40 wt% PrBaCo2–xFexO3−δ (0 ≤ x ≤ 2) were prepared by a combined citrate and ethylene diamine tetraacetic acid (EDTA) complexing method. X-ray diffraction (XRD) results revealed the good chemical compatibility between ion-conducting phase CGO and electron-conducting phases PBC2−xFxO after sintering in air. The Fe ionic dopant had a significant effect on the phase structure stability and oxygen permeability under CO2 atmosphere, which was confirmed by XRD, thermogravimetry–differential scanning calorimetry (TG–DSC), scanning electron microscopy (SEM) and oxygen permeation experiments. CGO–PBC0.5F1.5O dual-phase membrane demonstrated a stable oxygen permeation flux of 2.71 × 10−7 mol cm−2 s−1 with 50 mol% He/CO2 as the sweep gas at 925 °C, and this value was much higher than that of perovskite-type membranes. The rate-limiting step in the oxygen permeation process changed from the bulk diffusion to the surface oxygen exchange when the CGO–PBC0.5F1.5O membrane thickness decreased to 0.8 mm or less. Due to the high oxygen permeation fluxes and the excellent structural stability under CO2 atmosphere, the CGO–PBC0.5F1.5O membrane is a great potential candidate material for separating oxygen from air in the oxy-fuel combustion techniques.

Oxygen-selective membranes integrated with oxy-fuel combustion

The perovskite-type oxide SrCo0.8Fe0.2O3−δ (SCF), a highly oxygen-permeable material, is restricted for application in the membrane-integrated oxy-fuel combustion process by its low tolerance to CO2. In the present work, we found that the CO2 tolerance of SCF is improved by increasing the oxygen partial pressure in the CO2-containing gas. Long term oxygen permeation experiments, at 950°C, show that mixing 5% of oxygen into the CO2 sweep gas effectively prevents degradation of the SCF membrane. X-ray photoelectron spectroscopy indicates that the increase in CO2 tolerance of SCF is caused by a decrease in basicity of the material with increasing oxygen partial pressure. Based on these experimental results, a modified oxy-fuel combustion process is proposed. Calculation of the required membrane area for operating a 50MW coal-fired power plant showed that the modified process comprises a viable option.

A density functional study of vacancy formation in grain boundaries of undoped [formula omitted]-alumina

Recent oxygen permeation experiments with α-alumina have shown that the two major diffusing species at the grain boundaries, i.e. oxygen and aluminum vacancies, switch over in terms of their dominance depending on the oxygen partial pressure at high temperatures (the so-called p–n transition). In order to investigate this transition behavior the formation energies of charged oxygen and aluminum vacancies at Al2O3 grain boundaries were evaluated using density functional theory. The electronic structures affecting the carrier concentrations are assessed for three types of grain boundaries. The ratios of the grain boundary band gaps relative to the bulk value lie in the range of 0.6–0.85 depending on the degree of grain boundary misorientation. From the formation energies we find that, at band gaps around 60% compared to the bulk value, the dominant vacancy alters depending on the pressure, consistent with the oxygen permeation experiments. Our results suggest that the band bap narrowing occurring at grain boundaries enhances the creation of oxygen vacancies and conduction electrons, facilitating the observed transition.

Oxygen permeation and phase structure properties of partially A-site substituted BaCo0.7Fe0.225Ta0.075O3-δ perovskites

Ba0.9R0.1Co0.7Fe0.225Ta0.075O3-δ (BRCFT, R = Ca, La or Sr) membranes were synthesized by a solid-state reaction. Metal cation Ca2+, La3+ or Sr2+ doping on A-site partially substituted Ba2+ in BaCo0.7Fe0.225Ta0.075O3-δ oxides, and its subsequent effects on phase structure stability, oxygen permeability and oxygen desorption were systematically investigated by XRD, TG-DSC, H2-TPR, O2-TPD techniques and oxygen permeation experiments. The partial substitution with Ca2+, La3+ or Sr2+, whose ionic radii are smaller than that of Ba2+, succeeded in stabilizing the cubic perovskite structure without formation of impurity phases, as revealed by XRD analysis. Oxygen-involving experiments showed that BRCFT with A-site fully occupied by Ba2+ exhibited good oxygen permeation flux under He flow, reaching about 2.3 mL·min−1 ·cm−2 at 900 °C with 1 mm thickness. Of all the membranes, BLCFT membrane showed better chemical stability in CO2, owing to the reduction in alkalinity of the mixed conductor oxide by La doping. In addition, we also found the stability of the perovskite structure under reducing atmospheres was strengthened by increasing the size of A-site cation (Ba2+>La3+>Sr2+>Ca2+).

The effects of Fe-substitution on the crystal structure and oxygen permeability of PrBaCo2O5+δ

A series of double layered perovskites, PrBaCo2−xFexO5+δ (x=0, 0.2, 0.5, 1, 1.5, and 2), are successfully prepared employing a solid state reaction. The effects of Fe substitution on the crystal structure and oxygen transport properties of PrBaCo2O5+δ were investigated by X-ray diffraction and oxygen permeation experiments. X-ray diffraction patterns confirm that PrBaCo2O5+δ, PrBaCo1.8Fe0.2O5+δ, PrBaCo1.5Fe0.5O5+δ, and PrBaFe2O5+δ exhibit an orthorhombic perovskite structure, while PrBaCoFeO5+δ, and PrBaCo0.5Fe1.5O5+δ crystallize in a cubic perovskite. The oxygen permeation flux of PrBaCo2−xFexO5+δ is found to decrease with increasing Fe content. Among the compositions, PrBaCo2O5+δ shows the highest oxygen permeation flux of 1.89×10−7molcm−2s−1 at 900°C under an air/helium atmosphere.

Effect of A-site lanthanum doping on the CO2 tolerance of SrCo0.8Fe0.2O3−-δ oxygen-transporting membranes

The SrCo0.8Fe0.2O3−δ (SCF) perovskite was systematically doped with increasing lanthanum content up to 60wt% on the A-site to investigate the effect on CO2 tolerance. Different powders were prepared by a sol-gel method and the materials were characterized by in-situ X-ray diffraction (XRD) and long-term oxygen permeation measurements in CO2-containing atmospheres. The microstructure was investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). All powders exhibit cubic perovskite structure except the rhombohedral La0.6Sr0.4Co0.8Fe0.2O3−δ (LSCF 6482), which however, shows a phase transition into cubic perovskite structure at higher temperature. By doping 20wt% lanthanum, the tolerance against CO2 is considerably increased and doping with 60wt% lanthanum resulted in a stable oxygen permeation performance in CO2 atmosphere for at least 200h. Oxygen permeation experiments in an air/helium gradient showed that with increasing lanthanum content the oxygen permeation flux decreases. Microstructure analysis of the membranes after CO2 operation showed that the carbonate preferentially forms a dense layer at the carbon dioxide exposed sweep side of the membranes.

Investigation of Zr-doped BSCF perovskite membrane for oxygen separation in the intermediate temperature range

The series of (Ba0.5Sr0.5)(Co0.8Fe0.2)1−zZrzO3−δ (z=0, 0.01, 0.03, 0.05, 0.07, and 0.09) was synthesized by a sol–gel method. The materials with a zirconium content up to 3 mol% were found to be single phase. Further increase results in formation of a mixed (Ba,Sr)ZrO3 by-phase, which was found along the grain boundaries and in the grains. With increasing zirconium content the oxygen permeation flux decreases considerably. The effect of the zirconium substitution on the long-term phase stability was investigated by long-term oxygen permeation experiments and X-ray diffraction. A slight stabilization of the oxygen flux of (Ba0.5Sr0.5)(Co0.8Fe0.2)0.97Zr0.03O3−δ was found after 180 h at 1023 K. However, all compositions show a decrease in permeation flux with time, but the pure BSCF membrane exhibited the strongest drop after 180 h of operation. The decomposition products of the cubic perovskite phase were found to be a hexagonal Ba0.5±xSr0.5±xCoO3 and a rhombohedral Ba1−xSrxCo2−yFeyO5−δ.

Design and experimental investigation of oxide ceramic dual-phase membranes

The design of oxide ceramic dual-phase membranes is discussed in detail from the point of view of solid state chemistry. Dual-phase membranes with high performance have been designed by considering the permeability, stability and compatibility of the dual-phase system. It is deduced that dual-phase membranes made of Fe-based perovskite oxides (such as Sm0.6Sr0.4FeO3, mixed conductors) and Ce-based fluorite oxides (such as Ce0.85Sm0.15O1.925, ionic conductors) have both good permeability and stability. A dual-phase membrane made of an ionic conductor and pure electronic conductor was studied for comparison purposes, with a nominal composition of 75wt.%Ce0.85Sm0.15O1.925–25wt.%Sm0.6Sr0.4CrO3. Experimental investigation of selected membranes is reported with attention to processing, microstructures, conductivity and oxygen permeation properties. Microstructure effects of dual-phase membranes on oxygen exchange reactions and bulk oxide ionic transport were investigated by AC impedance spectroscopy, high resolution transmission electron microscopy and oxygen permeation. Oxygen permeation experiments revealed when the Fe-based mixed conducting perovskite is used in dual-phase membranes, higher oxygen permeation fluxes were achieved than that of the Cr-based pure electronic conducting perovskite was used in the dual-phase membrane. The phenomenon can be well explained by the mutual blocking of electronic and ionic transport by pure electronic conductors and ionic conductors, and the enrichments of Cr-based impurities between the ionic conducting grains. Membranes with various weight ratios of the two phases were synthesized to find the optimal composition. 75wt.%Ce0.85Sm0.15O1.925–25wt.%Sm0.6Sr0.4FeO3 was found to be the best membrane from the point of view of both oxygen permeability and stability. The idea of designing dual-phase membranes was verified by the comparison.