Fast Oxygen Diffusion Research Articles

Superoxide Stabilization and a Universal KO2 Growth Mechanism in Potassium-Oxygen Batteries.

Rechargeable potassium-oxygen (K-O2 ) batteries promise to provide higher round-trip efficiency and cycle life than other alkali-oxygen batteries with satisfactory gravimetric energy density (935 Wh kg-1 ). Exploiting a strong electron-donating solvent, for example, dimethyl sulfoxide (DMSO) strongly stabilizes the discharge product (KO2 ), resulting in significant improvement in electrode kinetics and chemical/electrochemical reversibility. The first DMSO-based K-O2 battery demonstrates a much higher energy efficiency and stability than the glyme-based electrolyte. A universal KO2 growth model is developed and it is demonstrated that the ideal solvent for K-O2 batteries should strongly stabilize superoxide (strong donor ability) to obtain high electrode kinetics and reversibility while providing fast oxygen diffusion to achieve high discharge capacity. This work elucidates key electrolyte properties that control the efficiency and reversibility of K-O2 batteries.

Superoxide Stabilization and a Universal KO2 Growth Mechanism in Potassium–Oxygen Batteries

AbstractRechargeable potassium–oxygen (K‐O2) batteries promise to provide higher round‐trip efficiency and cycle life than other alkali–oxygen batteries with satisfactory gravimetric energy density (935 Wh kg−1). Exploiting a strong electron‐donating solvent, for example, dimethyl sulfoxide (DMSO) strongly stabilizes the discharge product (KO2), resulting in significant improvement in electrode kinetics and chemical/electrochemical reversibility. The first DMSO‐based K‐O2 battery demonstrates a much higher energy efficiency and stability than the glyme‐based electrolyte. A universal KO2 growth model is developed and it is demonstrated that the ideal solvent for K‐O2 batteries should strongly stabilize superoxide (strong donor ability) to obtain high electrode kinetics and reversibility while providing fast oxygen diffusion to achieve high discharge capacity. This work elucidates key electrolyte properties that control the efficiency and reversibility of K‐O2 batteries.

Rapid oxygen diffusion during high temperature alteration of zircon

The mineral zircon through its isotopic and elemental signatures comprises the greatest archive recording the evolution of Earth’s continental crust. Recognising primary from secondary zircon compositional signatures is thus important for the accurate interpretation of this archive. We report two examples of metasedimentary rocks from high-grade shear zones within the Southern Granulite Belt of India, where anomalously high and homogeneous oxygen isotope signatures indicate disturbance of this isotopic system. Utilising the combined U-Pb-Hf-O and trace element signatures from these zircon grains, we postulate that fluid-assisted alteration has led to complete resetting of the oxygen isotope signatures. This case study presents a rarely observed natural example of potentially fast diffusion of oxygen under hydrous conditions. Given the pervasive nature of fluid interaction within high-grade and highly deformed rocks, we expect that such isotopic disturbance might be more common to nature than is currently reported. A lack of correlation between isotopic disturbance with cathodoluminescence or Th/U values, suggests that these altered zircon grains would not clearly be classified as metamorphic, in which case they would be expected to yield primary compositions. Caution is therefore advised when using detrital δ18O zircon compilations without a high level of scrutiny for primary versus secondary compositions.

Oxygen transport properties of Ca-doped Pr2NiO4

Praseodymium nickelate Pr2NiO4+δ with layered Ruddlesden–Popper (R–P) structure is a promising material for the ceramic membranes for oxygen separation and for SOFC cathodes due to its mixed ionic-electronic conducting nature and high oxygen mobility and surface reactivity. However, a low thermal stability and the inadequate electronic conductivity of Pr2NiO4+δ require its doping for a more efficient performance. This work presents results of study of the effect of Ca doping on the structural and transport properties of Pr2NiO4. Pr2−xCaxNiO4 oxides (x = 0–0.6) were synthesized by a co-precipitation method and characterized by XRD, XPS and TEM methods. The oxygen content in these materials and its variation with increasing temperature was evaluated using TGA. The oxygen transport properties of powdered samples were studied by the temperature-programmed oxygen isotope heteroexchange with C18O2 (TPIE). Electrical conductivity of compact samples was measured by a dc four-probe technique. Electrochemical measurements were performed using an impedance spectroscopy method with symmetrically arranged electrodes on the Ce0.8Sm0.2O1.9 electrolyte. It was shown that doping resulted in enhanced electrical conductivity and at a low Ca content the electrochemical activity of electrodes increased while the interstitial oxygen content and oxygen diffusivity gradually decreased. For x > 0.3 increasing the lattice anisotropy resulted in the co-existence of 2–3 channels of oxygen diffusion revealed as separate peaks in TPIE curves. The fast diffusion channel (D⁎ ~ 10−8 cm2/s at 700 °C), with a share in the total diffusion drastically decreasing at big doping levels, corresponds to the fast interstitial oxygen diffusion via the cooperative mechanism while the slow channels (D⁎ < 10−10 cm2/s) are probably related to the oxygen transport in perovskite layers and the complicated transport involving interlayer positions near the dopant cation sites.

Strain effects on oxygen vacancy formation energy in perovskites

Oxygen vacancy formation energy is an important quantity for enabling fast oxygen diffusion and oxygen catalysis in technologies like solid oxide fuel cells. Both previous literature in various systems and our calculations in LaMnO3, La0.75Sr0.25MnO3, LaFeO3, and La0.75Sr0.25FeO3, show mixed results for the direction and magnitude of the change in vacancy formation energy with strain. This paper develops a model to make sense of the different trend shapes in vacancy formation energy versus strain. We model strain effects using a set of consistent ab initio calculations, and demonstrate that our calculated results may be simply explained in terms of vacancy formation volume and changes in elastic constants between the bulk and defected states. A positive vacancy formation volume contributes to decreased vacancy formation energy under tensile strain, and an increase in elastic constants contributes to increases in vacancy formation energy with compressive and tensile strains, and vice versa. The vacancy formation volume dominates the linear portion of the vacancy formation energy strain response, while its curvature is governed by the vacancy-induced change in elastic constants. We show results sensitive to B-site cation, A-site doping, tilt system, and vacancy placement, which contributions may be averaged under thermally averaged conditions. In general, vacancy formation energies for most systems calculated here decreased with tensile strain, with about a 30–100meV/% strain decrease with biaxial strain for those systems which showed a decrease in vacancy formation energy. Experimental verification is necessary to confirm the model outside of calculation.

Enhanced Stability of BaCoO3-δ Using Doping Process as a Cathode Material for IT-SOFCs

Solid oxide fuel cells (SOFCs) are energy conversion devices that transform the chemical energy stored in fuels to electricity without combustion process involved. The environmentally friendly operation, high energy conversion efficiency, as well as the fuel flexibility are currently attracting a great deal of attention. However, the high operating temperature restricts the commercial application of SOFCs with traditional cathode materials. Then researchers had expended considerable efforts in reducing operating temperature by lowering the polarization resistance of cathodes. In addition, the power density of traditional cathode ceramic oxides shows a severe degradation as the operating temperature drops. The most common ceramic materials used as cathode materials for intermediate temperature solid oxide fuel cell (IT-SOFC) are mixed ionic-electronic conducting (MIECs) perovskites. Therefore, more advanced cathode materials with adequate conductivity and desirable electrochemical performance under intermediate temperature (600-800oC) need to be researched and developed. Many investigations suggest that cobalt-based perovskite cathode is one of the most competitive candidates with the most excellent electrocatalytic activity for oxygen reduction and sufficient conductivity around intermediate temperature comparing with other ABO3-type perovskite cathode based on chromium and manganese, etc. The electrocatalytic activity, thermal expansion coefficient (TEC), chemical compatibility with different types of electrolytes as well as structural stability of Cobalt-containing perovskites as cathodes for SOFCs had been widely studied, for example, La(Sr)CoO3-δ and BaCoO3-δ. In A-site ordered perovskites fast oxygen diffusion can be achieved. Base on the study of oxygen permeation rate, A-site sublattice fully occupied by Ba2+ provides the highest oxygen permeability in contrast to Sr2+and Ca2+ in La0.6A0.4Co0.6Fe0.2O3-δ (A=Ca, Sr, Ba). Unfortunately, the sensitivity to CO2 / H2O as well as the structural transition of cubic-phase to hexagonal-phase at RT limited its further application. Cheng, et.al. pointed out that the structural-stabilization can be efficaciously improved by low concentration Nb5+ doping in the B-site. Therefore, researchers made a lot of attempts on improving the structural stability and maintaining the high oxygen permeation rate by B-site doping high oxidation state cation, such as Ta, Ni, Nb, etc. In this study, partial substitution of Fe3+ for Co4+ is aimed at reducing the TEC of BaCoO3-δ parent oxide. Ta5+ having similar ionic size with Co4+ as another dopant in B-site will help to improve the oxygen nonstoichiometry corresponding to structural stability by introducing high oxidation states cations into B-stie. BaCo 0.7 Fe 0.2 Ta 0.1 O3-δ (BCFT) oxide as cathode material for proton conducting IT-SOFC had been reported. It shows low cathode polarization resistance in a symmetric cell and a fair cell performance around 255 mWcm-2 fueled with humidified H2. However, the chemical compatibility and the electrocatalytic activity of BFCT as a cathode material for IT-SOFC supported by an oxygen-ion conducting electrolyte are not reported. BFCT powder was synthesized by a solid-state reaction. La 0.9 Sr 0.1 Ga 0.8 Mg 0.2O3-δ (LSGM) was used as supporting electrolyte. LSGM-supported symmetrical cells were fabricated using screen-painting process. X-Ray diffraction and SEM were performed to show the crystallization (see figure. 1). TEC and symmetric cell resistance were investigated to analyze the structural stability of BCFT to be a potential candidate of cathode for IT-SOFCs. A detailed reference list can be found in the manuscript. Figure 1

Fast oxygen diffusion and iodide defects mediate oxygen-induced degradation of perovskite solar cells

Methylammonium lead halide perovskites are attracting intense interest as promising materials for next-generation solar cells, but serious issues related to long-term stability need to be addressed. Perovskite films based on CH3NH3PbI3 undergo rapid degradation when exposed to oxygen and light. Here, we report mechanistic insights into this oxygen-induced photodegradation from a range of experimental and computational techniques. We find fast oxygen diffusion into CH3NH3PbI3 films is accompanied by photo-induced formation of highly reactive superoxide species. Perovskite films composed of small crystallites show higher yields of superoxide and lower stability. Ab initio simulations indicate that iodide vacancies are the preferred sites in mediating the photo-induced formation of superoxide species from oxygen. Thin-film passivation with iodide salts is shown to enhance film and device stability. The understanding of degradation phenomena gained from this study is important for the future design and optimization of stable perovskite solar cells.

Macroporous Carbon Nanotube (CNT) Foams As Lithium Air Battery Cathodes

In recent years Lithium air (Li-air) battery has attracted considerable attention mainly due to its extreme high specific energy1,2. The theoretical specific energy is estimated up to 3,000 Wh kg-1, which is approximately one order of magnitude higher than that of conventional Li-ion battery3. The cathode of Li-air battery is chosen as porous carbon material in most studies, which plays a significant role of Li2O2 storage and electron transfer. Good electronic conductivity, fast oxygen diffusion, stable mechanical integrity and sufficient surface area and porosity are required for Li-air battery cathode4. In this study, a simple and scalable method for manufacturing three-dimensional carbon nanotube (CNT) foams as the Li-air battery cathode was being reported. CNT foams were synthesized using poly(methyl methacrylate) (PMMA) microspheres as a template and Polyacrylonitrile (PAN) as a precursor to create crosslinks among CNTs5,6. The tunable porosity and major pore size afford the opportunity to estimate the influence of cathode pore structure with controlled configuration and properties. A series of CNT foams with high porosity and desired major pore size were tested as cathodes in Li-air cells for single galvanostatic discharge performance. As shown in Fig. 1, the cathode with major pore size of 6 μm achieved a specific capacity over 10,000 mAh g-1 at a current density of 0.13 mA cm-2. Meanwhile, a direct inverse correlation between cathode major pore size and cell discharge specific capacity was observed. The electrochemical performance was combined with microscopy, porosimetry and modeling for further discussion. The results revealed that discharge product of Li2O2 were mainly deposited in micro-size pores and the reduced pore size favored the discharge reaction due to the increased volume specific surface area. 1. N. Akhtar and W. Akhtar, Int. J. Energy Res., 39, 303–316 (2015). 2. N. Imanishi and O. Yamamoto, Mater. Today, 17, 24–30 (2014). 3. J. P. Zheng, R. Y. Liang, M. Hendrickson, and E. J. Plichta, J. Electrochem. Soc., 155, A432-A437. 4. J. Lu, L. Li, J. Park, Y. Sun, F. Wu and K. Amine, Chem. Rev., 114, 5611–5640 (2014). 5. Y. Cui and M. Zhang, ACS Appl. Mater. Interfaces, 5, 8173–8178 (2013). 6. Y. Cui and M. Zhang, J. Mater. Chem. A, 1, 13984-13988 (2013). Figure 1

Synergistic Effect of A-Site Cation Ordered-Disordered Perovskite as a Cathode Material for Intermediate Temperature Solid Oxide Fuel Cells

The effects of Ca on the properties of PrBa1-xCaxCo2O5+δ (x = 0.0, 0.3, and 0.5) as a cathode material for intermediate temperature solid oxide fuel cells were studied. Higher Ca content in PrBa1-xCaxCo2O5+δ led to a gradual change in phase symmetry from A-site cation ordered double perovskite (x = 0.0) to mixed A-site cation ordered-disordered perovskite (x = 0.3) to A-site cation disordered simple perovskite (x = 0.5). It also led to a decreased oxygen content. Comparing the electrochemical performance of the samples with x = 0.0 and x = 0.5, PrBa0.7Ca0.3Co2O5+δ exhibited the best catalytic activity for the oxygen reduction reaction and maximum power density. This was ascribed to the synergistic effect of the A-site cation ordered-disordered structure, which combined the advantages of a fast oxygen diffusion rate and a relatively short oxygen diffusion distance from the oxygen reduction site to the cathode/electrolyte interface. The maximum power density, which was as high as 2.16 W cm−2, was achieved for the PrBa0.7Ca0.3Co2O5+δ-based single cell with a commercial anode-supported half cell at 800°C. PrBa0.7Ca0.3Co2O5+δ was shown to be a potential cathode material for IT-SOFCs.

Novel Nanocomposite Materials Based On Praseodymium Nickelate-Cobaltite for Oxygen Separation Membranes

The key problems in developing catalytic membranes for producing syngas from biofuels by selective oxidation with oxygen separated from air are design of materials for functional layers and optimization of techniques of their deposition. Nanocomposite materials based upon praseodymium nickelate-cobaltite are promising for these purposes due to their high oxygen mobility provided by the fast oxygen diffusion channel. The oxygen permeability of membrane obtained meets criteria of the practical application. Syngas yield and methane conversion increase with temperature and contact time. Stable performance of the membrane was demonstrated for at least 200h time-on-stream.

Fast oxygen diffusion in bismuth oxide probed by quasielastic neutron scattering

We present the first, to our knowledge, study of solid state oxygen translational diffusion by quasielastic neutron scattering. Such studies in the past might have been precluded by relatively low diffusivities of oxygen anions in the temperature range amenable to neutron scattering experiments. To explore the potential of the quasielastic scattering technique, which can deduce atomic diffusion jump length of oxygen anions through the momentum transfer dependence of the scattering signal, we have selected the fastest known oxygen conductor, bismuth oxide. We have found the oxygen anion jump length in excellent agreement with the nearest oxygen-vacancy distance in the anion sublattice of the fluorite-related structure of bismuth oxide.

Sm-doped praseodymium nickelates-cobaltites and their nanocomposites with Y-doped ceria as promising cathode materials

ABSTRACTThis work aims at elucidating effects of composition and microstructure of materials based upon Pr1-ySmyNi1-xCoxO3-δ perovskites on their oxygen mobility. Pr1-ySmyNi1-xCoxO3-δ and their composites with Y-doped ceria are perspective materials for intermediate temperature solid oxide fuel cells and oxygen separation membranes. Doping PrNi1-xCoxO3-δ by Sm results in suppression of the fast channel of oxygen diffusion along extended defects due to Sm segregation in their vicinity. For nanocomposites moderate doping by Sm enhances fast oxygen diffusion in fluorite phase domains and along perovskite-fluorite interface due to a higher disordering caused by incorporation of both Pr and Sm cations into Y-doped ceria.

Competing descriptions of diffusion profiles with two features: Surface space-charge layer versus fast grain-boundary diffusion

Two different physical processes, (i) fast grain-boundary diffusion (FGBD) of oxygen and (ii) hindered oxygen diffusion in a surface space-charge layer, yield oxygen isotope diffusion profiles in a similar form. Two features are observed, with the short, sharp profile close to the surface being followed by a longer, shallower profile. In this study, we develop a procedure for deciding which of the two descriptions applies to experimentally measured profiles. Specifically, we solve Fick's second law, using finite-element simulations, to obtain oxygen isotope diffusion profiles for the two cases. Each set of profiles is then analysed in terms of the competing description. In this manner, we derive falsifiable conditions that allow physical processes to be assigned unambiguously to the two features of such isotope profiles. Applying these conditions to experimental profiles for SrTiO3 single crystals published in the literature, we find that FGBD is an invalid model for describing the diffusion processes.

Investigation of a Layered Perovskite for IT-SOFC Cathodes: B-Site Fe-Doped YBa0.5Sr0.5Co2-xFexO5+δ

Cobalt-based layered perovskite oxides have been recognized as pioneering cathode materials for intermediate-temperature solid oxide fuel cells (IT-SOFCs) due to their fast oxygen diffusion, surface exchange kinetics, and high electrical conductivity. Herein, we study novel Fe doped layered perovskites, YBa0.5Sr0.5Co2-xFexO5+δ (x = 0, 0.25, 0.5, and 0.75) in terms of their structural properties, thermal expansion behavior, electrical properties, and electrochemical performances for potential usage as a cathode. Among them, YBa0.5Sr0.5Co1.75Fe0.25O5+δ (YBSCF25, x = 0.25) not only shows enhanced structural stability but also meets the essential required properties, including an appropriate thermal expansion coefficient (TEC), chemical compatibility with the electrolyte, and suitable electrical conductivity. Moreover, power density as high as 1.5 W cm−2 at 600°C is achieved for the YBSCF25-GDC composite cathode with a full cell with a dense thin gadolinium doped ceria (GDC) electrolyte and a Ni-GDC composite anode in humidified H2. These favorable features highlight the potential of YBSCF25 as a promising cathode material for IT-SOFCs.

Application of SR Methods for the Study of Nanocomposite Materials for Hydrogen Energy

This work summarizes results of synchrotron radiation (SR) studies of the real/defect structure of nanocrystalline/nanocomposite oxide materials, which determines their functional properties in hydrogen energy field as catalysts and mixed ionic electronic conductors (cathodes and anodes of solid oxide fuel cells, oxygen separation membranes). For nanocrystalline ceria-zirconia mixed oxide prepared via modified Pechini route using ethanol solution of reagents, a high spatial uniformity of cations distribution between domains along with the oxygen sublattice deficiency revealed by full-profile Rietveld refinement of SR diffraction data provide structure disordering enhancing oxygen mobility. For PrNi0.5Co0.5O3–δ – Ce0.9Y0.1O2–δ nanocomposite extensive transfer of Pr cations into fluorite domains generates a new path of fast oxygen diffusion along chains of Pr3+ – Pr4+ cations as directly proved by analysis of the unit cell relaxation after changing pO2 in perfect agreement with data obtained by oxygen isotope heteroexchange.

First principles study of oxygen diffusion in a α-alumina twin grain boundary

We have investigated atomistic scale behaviour of oxygen diffusion along the twin grain boundary in α-Al2O3 (alumina) using molecular dynamics simulation and first principles total energy calculations. Based on the GB structure model which is verified by atomic-scale STEM observations on the bicrystal sample, quantitative evaluation of migration energies for dominant migration paths were performed by atomistic calculations. The preset calculation results confirmed fast oxygen diffusion behaviour along the GB. Our analysis shows that the dominant migration path or difference in the migration energies can be well correlated with the geometry of local atomic coordination around the migrating oxygen; lower migration energies are generally expected for paths with less change in coordination environment on migration. This trend holds both among gain boundary paths and bulk paths in α-alumina examined in the present study.

Microstructure design for fast oxygen conduction

Abstract

Oxygen Transfer in a Fluctuating Capillary Fringe: Impact of Microbial Respiratory Activity

Laboratory‐scale flow‐through experiments in a sand‐filled chamber were performed to investigate the impact of aerobically growing Escherichia coli HB101‐K12 on oxygen transfer across the capillary fringe (CF) to oxygen‐depleted groundwater. In the experiments, both the effects of different nutrient concentrations and transient flow conditions were tested. The results of biotic experiments under oligotrophic and eutrophic conditions (6.8 g organic C L−1) were compared with those of an abiotic experiment. Moreover, in each experiment steady‐state and transient conditions were considered, the latter induced by changing the water‐table height. Growth of E. coli was quantified by cell counting in effluent samples and could be monitored due to intracellular production of the green fluorescent protein (GFP). Under eutrophic conditions, highest cell densities and strongest cell attachment were observed in the transition region of the CF. In this region, intensive bacterial respiration decreased oxygen concentrations relatively quickly, causing a steep oxygen gradient that resulted in a higher oxygen flux across the unsaturated–saturated interface. Under oligotrophic conditions, this effect was considerably reduced, but was still detectable. Due to oxygen supply from entrapped air, bacterial growth was slightly enhanced within the upper, newly formed CF region, directly after raising the water table. After lowering the water table to the initial height only a minor microbial impact on oxygen transfer was noticed, even under eutrophic conditions, because of the fast diffusion of oxygen in the (partially) air‐filled pore spaces. Bacteria that remained in the transition region of the CF grew to higher densities due to better oxygen supply.

Oxygen storage capacity and thermal stability of the CuMnO2–CeO2 composite system

Fast oxygen diffusion and improved oxygen storage capacity of crednerite CuMnO2 have been achieved at reduced temperatures by surface modification with CeO2.

Fast oxygen exchange and diffusion kinetics of grain boundaries in Sr-doped LaMnO3 thin films.

In this study, the contribution of grain boundaries to the oxygen reduction and diffusion kinetics of La0.8Sr0.2MnO3 (LSM) thin films is investigated. Polycrystalline LSM thin films with columnar grains of different grain sizes as well as epitaxial thin films were prepared by pulsed laser deposition. (18)O tracer exchange experiments were performed at temperatures from 570 °C to 810 °C and subsequently analyzed by secondary ion mass spectrometry (SIMS). The isotope concentration depth profiles of polycrystalline films clearly indicate contributions from diffusion and surface exchange in grains as well as in grain boundaries. Measured depth profiles were analyzed by finite element modeling and revealed the diffusion coefficients D and oxygen exchange coefficients k of both the grain bulk and grain boundaries. Values obtained for grain boundaries (Dgb and kgb) are almost three orders of magnitude higher than those of the grains (Dg and kg). Hence, grain boundaries may not only facilitate fast oxygen diffusion but also fast oxygen exchange kinetics. Variation of the A-site stoichiometry ((La0.8Sr0.2)0.95MnO3) did not lead to large changes of the kinetic parameters. Properties found for epitaxial layers without grain boundaries (Db and kb) are close to those of the grains in polycrystalline layers.