Oxygen Chemical Potential Gradient Research Articles

A design of solar-driven thermochemical reactor integrated with heat recovery for continuous production of renewable fuels

To cope with the increasingly serious energy crisis and environmental problems, the transformation from traditional energy to renewable energy is urgent. Solar-driven thermochemical conversion of CO2 and H2O into renewable fuels technology provides a favorable path for alternative energy. However, the temperature/pressure swing required for the reduction and oxidation steps during thermochemical process incurs irreversible energy losses and severe material stresses. Moreover, the two-step redox cycle mode cannot achieve a continuous fuel production in the reactor. To address these issues, this study proposes a novel design of thermochemical reactor integrated with heat recovery for continuous production of renewable fuels. In this design, an oxygen-conducting ceria membrane is employed to conduct oxygen ions, electrons and vacancies induced by the oxygen chemical potential gradient to achieve an isothermal and continuous splitting of CO2 and H2O. At the fuel production side, a counter-flow and tube-in-tube alumina reticulate porous ceramic filling heat exchange design is developed to recover the sensible heat of gas products and excessive reactants. In addition, a comprehensive model is developed by coupling the computational dynamics of flow, radiation, conduction and convection, and is solved by the Monte Carlo method coupled with ANSYS FLUENT software through User-defined Functions. Results indicate that the integrating membrane reactor/heat recovery design can not only realize continuous production of renewable fuels, but also recover sensible heat of gas products, which is beneficial for conversion of solar energy to chemical energy. Optimizing heat recovery design can help reactor achieve a solar-to-fuel efficiency of 10.58%. Moreover, if the theoretical CO2 conversion rate could be achieved, the solar-to-fuel efficiency of reactor can reach as high as 40.43%, which means that such a reactor design has a huge potential in conversion of solar energy into renewable fuels.

Solutocapillary transport of oxygen bubbles in a diffusion-bubbling membrane core.

Bubbles are extensively explored as gas and energy carriers. However, despite notable progress, the bubble transport mechanisms are still poorly understood. At the present time there is not sufficient understanding of whether the body or surface forces play a major role in bubble transport in liquid interfacial systems. This understanding is important to be able to drive oxygen bubble transport. Here, we show the crucial role of solutocapillary forces in oxygen bubble transport in the core of a diffusion-bubbling membrane with a high density of solid/liquid and gas/liquid interfaces that operates under the oxygen chemical potential gradient. In order to describe the transport of oxygen bubbles in the membrane core, we developed a mathematical model. Both the velocity of bubbles and oxygen flux through this membrane predicted by this model agree with experiments. An in-depth understanding of the bubble transport mechanism presented in this study could eventually lead the way to more efficient bubble membrane gas separation, bubble energy generation, and bubble-assisted therapy in the future.

Electrochemical and Degradation Studies on One-Dimensional Tunneled Sodium Zirconogallate (NZGO) + Yttria-Stabilized Zirconia (YSZ) Composite, Mixed Sodium and Oxygen Ion Conductor

In recent years, multi-phase materials capable of multi-ion transport have emerged as attractive candidates for a variety of electrochemical devices. Here, we provide experimental results for fabricating a composite electrolyte made up of a one-dimensional fast sodium-ion conductor, sodium zirconogallate, and an oxygen-ion conductor, yttria-stabilized zirconia. The composite is synthesized through a vapor phase conversion mechanism, and the kinetics of this process are discussed in detail. The samples are characterized using diffraction, electron microscopy, and electrochemical impedance spectroscopy techniques. Samples with a finer grain structure exhibit higher kinetic rates due to larger three-phase boundaries (TPBs) per unit area. The total conductivity is fitted to an Arrhenius type equation with activation energies ranging from at temperatures below to above The electrochemical performance of multi-phase multi-species, mixed and conductor, is tested under both oxygen chemical potential gradient as well as sodium chemical potential gradient are discussed using the Goldman-Hodgkin-Kats (GHK) and the Nernst equation.

Unlocking bulk and surface oxygen transport properties of mixed oxide-ion and electron conducting membranes with combined oxygen permeation cell and oxygen probe method

Surface exchange kinetics and bulk diffusion of oxygen are of paramount importance to the activity of oxygen electrocatalysis and performance of electrochemical devices such as fuel cell, metal-air batteries, and oxygen separation membranes. Conventional approaches to obtaining these transport properties are often limited to single property under a specific non-operation related condition. Here we use a combined oxygen permeation cell and oxygen probe methodology to simultaneously attain rates of oxygen surface exchange and bulk conductivity/chemical diffusivity of three representative mixed oxide-ion and electron conductors, namely SrCo0.9Ta0.1O3-δ (SCT), La0.6Sr0.4CoO3-δ (LSC) and La0.6Sr0.4FeO3-δ (LSF), operated under a steady-state oxygen flux. The results explicitly show that SCT exhibit the highest oxide-ion conductivity/chemical diffusivity, fastest rates of surface oxygen exchange kinetics, thus promising to be the best oxygen electrocatalyst. We have also mapped out the distribution of oxygen chemical potential gradient across the membranes and applied B-transport number concept to illustrate the rate-limiting steps in the overall oxygen permeation process.

Nanoscale segregation mechanism of cation dopant at the matrix/oxide interface in oxide dispersion-strengthened alloys

The current study has demonstrated that cation dopant segregation at matrix/oxide interface opened up a new route to refine and disperse secondary oxide particles in oxide dispersion-strengthened (ODS) alloys. Thus, a unified theory that explains the physical origins of this interfacial segregation phenomenon is needed for designing ODS alloys with excellent oxide dispersity and ensuing high performance. Here, taking W–Y2O3 system for example, we firstly assess the possible driving forces for cation dopant interfacial segregation based on the experimental observation from Sc3+-, La3+-, Ti4+-, Zr4+- and Hf4+-doped W–Y2O3 alloys. It was suggested that elastic energy, oxygen chemical potential gradient and interfacial energy reduction are three main driving forces for the cation dopant segregation at W/Y2O3 interface. Then, an analytical model was developed in this work to quantitatively calculate the contributions of these three factors to the total segregation energy. Finally, the coupled results are further validated with the density functional theory (DFT)-calculated total segregation energy, and the good consistency confirms again the underlying mechanism behind cation dopant segregation phenomenon in W-based ODS alloy. On this basis, it can be predicted that a chemically expanded lattice and a large oxygen affinity will promote dopant interfacial segregation and enable the microstructure of ODS alloys to be tailored desirably. More importantly, the results and analytical model in our work can provide theoretical guidance for choosing proper cation dopant for other ODS alloys and then enhancing their strength and ductility simultaneously. Besides, the high-temperature instability of secondary oxide particles under extreme working environment also can be solved easily using this method.

Spatial distribution of oxygen chemical potential under potential gradients and performance of solid oxide fuel cells with Ce0.9Gd0.1O2−δ electrolyte

In this work, maximum power density as the function of electrolyte thickness of a solid oxide fuel cell (SOFC) with Ce0.9Gd0.1O2-δ (GDC10) electrolyte was calculated by integrating partial conductivities of charge carriers under various DC bias conditions at a fixed oxygen chemical potential gradient at both sides of the electrolyte. Partial conductivities as a function of temperature and oxygen partial pressure (PO2) were calculated using Hebb-Wagner polarization method and spatial distribution of PO2 across the electrolyte was calculated based on Choudhury and Patterson's model [1] by considering reversible electrode conditions. At terminal voltages corresponding to SOFC and electrolysis cell operation modes, the oxygen chemical potential gradient at a electronic-stoichiometric point became maximum and minimum to compensate the contribution from electrochemical potential gradient of electron. The current-voltage characteristics in different fuel cell conditions with temperature and thickness dependence were calculated with cathodic and anodic PO2 of 0.21 and 10−22 atm, respectively. The theoretical maximum power density increased from 1.26 W·cm−2 at 500 °C to 7.39 W·cm−2 at 700 °C. Similarly, at 500 °C, power density increased two fold on reducing electrolyte thickness from 20 μm to 10 μm. The implications of these results on the development of GDC10 based SOFC systems was discussed.

Solar-Driven Thermochemical Splitting of CO2 and In Situ Separation of CO and O2 across a Ceria Redox Membrane Reactor

SummarySplitting CO2 with a thermochemical redox cycle utilizes the entire solar spectrum and provides a favorable path to the synthesis of solar fuels at high rates and efficiencies. However, the temperature/pressure swing commonly applied between reduction and oxidation steps incurs irreversible energy losses and severe material stresses. Here, we experimentally demonstrate for the first time the single-step continuous splitting of CO2 into separate streams of CO and O2 under steady-state isothermal/isobaric conditions. This is accomplished using a solar-driven ceria membrane reactor conducting oxygen ions, electrons, and vacancies induced by the oxygen chemical potential gradient. Guided by the limitations imposed by thermodynamic equilibrium of CO2 thermolysis, we operated the solar reactor at 1,600°C, 3·10−6 bar and 3,500 suns radiation, yielding total selectivity of CO2 to CO + ½O2 with a conversion rate of 0.024 μmol·s−1 per cm2 membrane. The dynamics of the oxygen vacancy exchange, tracked by GC and XPS, further validated stable fuel production.

Spatial Distribution of Oxygen Chemical Potential Profile across Zr0.84Y0.16O1.92 / Ce0.9Gd0.1O1.95 Bilayer Electrolyte under SOFC Operating Conditions

Partial electronic conductivity and total conductivity of Zr0.84Y0.16O1.92 and Ce0.9Gd0.1O1.95 have been measured by Hebb-Wagner polarization method and D.C. 4-probe conductivity measurements, respectively as a function of temperature and oxygen partial pressure. (600 ≤ temp/˚C ≤ 900, 10-18 ≤ PO 2/atm ≤ 0.21) Theoretical current-voltage characteristics and power density were calculated by integrating partial conductivities of charge carriers under various DC bias condition at fixed oxygen chemical potential gradient at both sides of the electrolyte (YSZ side: PO 2=10-22 GDC side: PO 2=0.21). The spatial distribution of oxygen partial pressure across the bilayer electrolyte with various thickness ratio was calculated based on Choudhury and Patterson’s model by considering zero electrode polarization. From the spatial distribution, oxygen partial pressures of interface were calculated with various operating condition. Interfacial oxygen chemical potential with various thickness ratio (YSZ/GDC) shows strong dependence upon cell operating potential.

Spatial Distribution of Oxygen Chemical Potential Profile across Zr0.84Y0.16O1.92 / Ce0.9Gd0.1O1.95 Bilayer Electrolyte under SOFC Operating Conditions

Theoretical current-voltage characteristics and power density were calculated by integrating partial conductivities of charge carriers under various DC bias condition at fixed oxygen chemical potential gradient at both sides of the electrolyte (YSZ side: PO 2=10-22 GDC side: PO 2=0.21). The spatial distribution of oxygen partial pressure across the bilayer electrolyte with various thickness ratios was calculated based on Choudhury and Patterson’s model by considering zero electrode polarization. From the spatial distribution, oxygen partial pressures of an interface were calculated with the various operating condition. Interfacial oxygen chemical potential with different thickness ratios (YSZ/GDC) shows strong dependence upon cell operating potential.

High‐rate hydrogen separation using an MIEC oxygen permeable membrane reactor

In this study, we propose using mixed ionic‐electronic conducting (MIEC) oxygen permeable membrane to separate hydrogen via the water splitting reaction. To do that, steam was fed to one side of the membrane (side I) and a low‐purity hydrogen was fed to the other side (side II). Oxygen from water splitting on side I permeates through the membrane driven by an oxygen chemical potential gradient across the membrane to react with the low‐purity hydrogen on side II. After condensation and drying, high‐purity hydrogen is acquired from side I. Thus, the hydrogen separation process is realized based on the fact that the low‐purity hydrogen is consumed and high‐purity hydrogen is acquired. We achieved a high hydrogen separation rate (13.5 mL cm−2 min−1) at 950°C in a reactor equipped with a 0.5‐mm‐thick Ba0.98Ce0.05Fe0.95O3‐δ membrane. This research proofed that it is feasible to upgrade hydrogen purity using an MIEC oxygen permeable membrane. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1278–1286, 2017

WITHDRAWN: Electrochemical properties of CaZrO 3 -α-based solid electrolyte

Abstract In this study, yttrium-doped calcium zirconate powders were synthesized by the conventional solid-state reaction method. The improved emf apparatus has been applied to measure the electrochemical values in the Y-doped CaZrO 3 system. The electromotive force of CaZr 0.96 Y 0.04 O 3− α has been measured by electrochemical cells of the type. ( − , ref. ) , Pt , gas mixture ( p ′ O 2 , p ″ H 2 ) | CaZr 0.96 Y 0.04 O 3 − α | gas mixture ( p ″ O 2 , p ″ H 2 ) , Pt , ( meas , + . ) The electromotive force (emf) was measured for various oxygen and hydrogen chemical potential gradients. The measurements were performed in a hydrogen-rich atmosphere for the temperature range from 973 to 1473 K. For almost all the conditions in the experiment, the measured emfs were well explained by regarding that the substantial predominant charge carrier is the proton. Under the hydrogen-rich atmosphere investigated, it was found that the emf generated only with the hydrogen activity gradient. The detailed analysis of the obtained emf data showed that the transport number of proton in CaZr 0.96 Y 0.04 O 3− α is almost unity for the hydrogen-rich atmosphere and its proton conduction domain is a little wider than that of the In-doped CaZrO 3 system.

A Multiphysics Study of Chemical Potential Distributions As Drivers for Solid Oxide Cell Mechanical Degradation

Solid oxide cells are promising energy conversion devices that enable the generation of power from diverse fuel sources in fuel cell operation and the production of hydrogen and syngas fuels when operating as electrolyzers. Accelerated degradation has been observed for electrolytic operation in comparison to fuel cell operation. Of particular note, oxygen electrode delamination has been observed for solid oxide electrolyzers. It has been proposed that this mechanical failure mode arises from adverse gradients in the oxygen chemical potential across the electrode-electrolyte interface. A multiphysics model of solid oxide cell operation has been developed for mass and charge transport in the presence of a distributed electrochemical reactions. Both 1D and 2D transport within the cross-section of a planar positive electrode-electrolyte-negative electrode (PEN) unit cell are addressed. Mass transport in the porous composite electrodes is treated using the dusty-gas model. Charge transport is cast in terms of ion and electron electrochemical potentials, a feature that facilitates analysis of mixed ion and electron conduction. The present work focuses on analyzing oxygen chemical potential gradients within the solid electrolyte and the porous composite electrodes, with an emphasis on gradients near interfaces between the PEN components. Two general modeling cases are compared: one neglecting electronic conduction within the solid electrolyte and one accounting for small yet present electronic conduction. Electrochemical potential distributions are assessed across the cell structure over a range of electrolytic and galvanic operating conditions. These distributions are used to assess the likelihood of adverse oxygen chemical potential gradients within the cell. The effects of interconnect geometry for planar cells are also considered. It is found that variations in electrochemical potential arise within the PEN structure, a phenomenon that may drive mechanical failure. These gradients may increase the likelihood of electrolyzer failure for certain combinations of electrode and interconnect geometry. The use of device scale geometric design is discussed as a means of mitigating these gradients and reducing the likelihood of cell mechanical failure from adverse distributions of reactant chemical potentials.

Spatial Distribution of Oxygen Chemical Potential under Potential Gradients and Theoretical Maximum Power Density with 8YSZ Electrolyte.

The maximum power density of SOFC with 8YSZ electrolyte as the function of thickness was calculated by integrating partial conductivities of charge carriers under various DC bias conditions at a fixed oxygen chemical potential gradient at both sides of the electrolyte. The partial conductivities were successfully taken using the Hebb-Wagner polarization method as a function of temperature and oxygen partial pressure, and the spatial distribution of oxygen partial pressure across the electrolyte was calculated based on Choudhury and Patterson’s model by considering zero electrode polarization. At positive voltage conditions corresponding to SOFC and SOEC, the high conductivity region was expanded, but at negative cell voltage condition, the low conductivity region near n-type to p-type transition was expanded. In addition, the maximum power density calculated from the current-voltage characteristic showed approximately 5.76 W/cm2 at 700 oC with 10 μm thick-8YSZ, while the oxygen partial pressure of the cathode and anode sides maintained ≈0.21 and 10−22 atm.

In-Operando Raman Spectroscopy Study on Oxygen Chemical Potential Gradient in Ni-SDC Cermet Anode for SOFCs

Solid oxide fuel cells are promising energy conversion systems for the next generation. High-temperature operation of this electrochemical device offers high fuel flexibility as well as high energy conversion efficiency. Currently in Japan, methane is utilized as the main fuel source for the residential SOFC cogeneration system and the pre-reformed gas is supplied to the cell chamber. In this case, the anode is requested to be active for the reforming reaction of hydrocarbons as well as the electrochemical oxidation of fuels. The tolerance to carbon deposition is also an important function to provide against accidents of the reforming system because the hydrocarbon gas will be introduced directly to the anode chamber. The cermet of Ni‒oxide ion conductor is widely used as an anode in SOFCs. However, the usage of nickel induces various degradation phenomena during discharge operation. In most cases, the agglomeration and/or oxidation of nickel catalyst are the major degrading factors under steady operation. For instance, such degradation phenomena can be observed in the downstream part of the fuel gas flow channel accompanied with the microstructural evolution because the anode is exposed to the severe condition due to lean fuel and high partial pressure of steam. The electrochemical oxidation of nickel also sometimes proceeds during discharge under the fuel shortage condition. These will lead to the microstructural change accompanied with the reduction in the length of triple phase boundary (TPB). As mentioned above, furthermore, the carbon deposition leads to the performance degradation due to the obstruction of gas diffusion path and the metal dusting of nickel. A series of degradation phenomena is triggered by the change in the oxygen chemical potential in the anode. In general, the partial pressure of oxygen in the vicinity of anode/electrolyte interface is higher than that in the anode surface region under anodic polarization because the steam is generated via the electrochemical reaction. Therefore, it is of importance to elucidate the oxygen chemical potential gradient in the anode layer under polarization. At this stage, however, appropriate measurement methods have not been developed. Ceria-based oxides are widely used as oxide components in Ni-based cermet anodes for SOFCs. These oxides show the mixed conduction in reducing atmospheres because the partial reduction of Ce4+ to Ce3+ occurs accompanied with a change in oxygen nonstoichiometry. Mineshige et al. reported that this variation in oxygen vacancy concentration in quenched samples can be detected quantitatively by Raman spectroscopy since the band related to oxygen vacancy changes depending on the partial pressure of oxygen in the atmosphere exposed [1]. In this study, then, Ce0.8Sm0.2O2-δ (SDC) was used as a probe for the detection of oxygen chemical potential. The oxygen chemical potential gradient in Ni−SDC cermet anode at 700ºC under polarization was investigated by applying in-operando Raman spectroscopy. The change in oxygen nonstoichiometry of doped ceria in the cermet anode was successfully quantified. Furthermore, the effective reaction zone was estimated from impedance spectra. Acknowledgements This study was partially supported by Japan Science and Technology Agency (JST), CREST.

Analysis of Interfacial Processes at the SOFC Electrodes by I n-Situ Raman Spectroscopy

Kinetics of redox reactions and the variations of real charge-carrier concentrations across the SOFC electrode / electrolyte interfaces have critical importance for the fuel cell performance. The microscopic mechanisms of electrochemical processes in the vicinity of triple-phase boundary (TPB) can be assessed employing impedance spectroscopy, current-voltage measurements and various pulse techniques. Except for the micro-electrode approaches, however, these methods are cumulative. Moreover, unambiguous interpretation of the electrochemical measurement results requires, as a rule, to introduce complimentary experimental methods and/or to use simplified models. The present work is centered on the developments of a new combined technique for in-situ Raman spectroscopy studies of local chemical and electrochemical reactions, phase transitions, strains and morphological alterations in the SOFC electrodes under working conditions. The Raman spectroscopy was already successfully used to investigate both cathodic [1,2] and anodic [3] processes in SOFC, but the resultant information was mainly related to outer boundaries of the model electrochemical cells, primarily electrode surface, due to low penetration depth of the excitation radiation. The most important zones of the electrode systems, where the electrochemical reactions occur and ionic charge carriers are generated, cannot be achieved viewing the surface and edge areas. In the present work, this problem was solved by employing optically transparent, single-crystal membranes made of 10 mol.% Sc2O3 and 1% mol.% Y2O3 stabilized zirconia (10Sc1YSZ). In order to provide simultaneous Raman and electrochemical measurements of the cell placed under an oxygen chemical potential gradient, such as air/H2, a special controlled-atmosphere chamber was elaborated and tested. An appropriate selection of the electrodes geometry makes it possible to directly collect Raman spectra from the TPB zone, passing the beam through single-crystal solid electrolyte onto the interface, as a function of temperature, atmosphere, current density and/or overpotential. The results of case studies focused on redox kinetics of Ni-containing cermet anodes, are presented. This work was supported by grant 14-29-04031 of Russian Foundation for Basic Research. References S. Loridant, L. Abello, E. Siebert, G. Lucazeau, Solid State Ion. 78 (1995) 249-258K. Blinn, H. Abernathy, M.Liu, Advances in Solid Oxide Fuel Cells V. (2010) 63-73W. Bessler, M.Vogler, H. Stoermer, D. Gerthsen, A. Utz, A. Weber, E. Ivers-Tiffee, Phys. Chem. Chem. Phys., 12 (2010) 13888-13903

Mixed ionic-electronic conducting (MIEC) membranes for hydrogen production from water splitting

Hydrogen as a clean energy carrier for power generation through fuel cells has attracted much attention. Water is a suitable source for hydrogen production by the splitting reaction. Significant amount of hydrogen can be produced at moderate temperature if a mixed ionic-electronic conducting (MIEC) membrane is used to remove the produced oxygen from water splitting reaction, although the equilibrium constant for water splitting is small. In this review, the principle of hydrogen production via water splitting in MIEC membrane reactor is illustrated, and the factors affecting the hydrogen production rate (such as membrane materials, thickness, modification of membrane surface) and the stabilities of membranes under high oxygen chemical potential gradient are discussed. Furthermore, following the concept of process intensification, the researches on water splitting coupling with other catalytic reactions in membrane reactors are summarized.

Stability and oxygen transport property of La0.8Sr0.2Cr0.5Fe0.5O3-δ

Abstract The stability of La0.8Sr0.2Cr0.5Fe0.5O3 -δ (LSCrF) in reducing atmosphere was investigated by examining the extent of its reaction with hydrogen at elevated temperature. LSCrF powder exposed to diluted hydrogen was found to loss a weight of only ~ 0.5%, corresponding to the formation of oxygen vacancies in the lattice. LSCrF powder exposed to flowing concentrated hydrogen for 30 h was found to decompose partially. The decomposition oxygen partial pressure of LSCrF at 950 °C was estimated to be 6.3 × 10− 28 atm from thermodynamic calculations. The stability of LSCrF under an oxygen chemical potential gradient was also examined by exposing a disk-shaped dense sample to air at one side and to reducing atmosphere (CO) at the other side at elevated temperatures. A thin, porous layer was found to form on the CO side surface. An oxygen permeation flux of 2.5 × 10− 7 mol cm− 2 s− 1 was observed at 950 °C under given air/CO gradient. The occurrence of oxygen permeation revealed the presence of mixed oxygen ionic and electronic conductivity. The oxygen ionic conductivity was estimated to be ~ 0.01 S/cm at 950 °C.

Investigation of Transport Properties of Bzcy (BaZrxCe1-xY0.15O3- δ, x=0, 0.2, 0.4, 0.6)

Some perovskite-type oxides have shown high proton conductivity in wet hydrogen atmosphere. In these perovskite oxides the partial substitution of an acceptor dopant for B-site results in formation of oxygen vacancies, which lead to the formation of protonic defects by the absorption of water in wet atmosphere. Among various proton conducting perovskite-type oxides, Y-doped Barium cerate and barium zirconate are nearly isomorphic. Therefore, the basic phases of a binary solid solution where B-sites are randomly occupied by either Zr4+ or Ce4+. This is because the chemical instability of Y-doped BaCeO3- δ in the presence of CO2-, H2O- or SO2-containing atmosphere below 973K was substantially improved by replacing Ce with Zr to form a solid solution, and the mechanical strength was also enhanced when the proton conductivity and thermal stability remained adequate over a wide range of operation conditions for the fuel cell.To elucidate the transport properties and defect structure of a Ce/Zr-coexisting preovskite structure, BZCY(BaZrxCe1-xY0.15O3- δ, x=0, 0.2, 0.4, 0.6) was chosen as the model system. First, we study the electrical properties of BZCY in various oxygen and water vapor atmospheres by four-probe DC measurements as a function of temperature. And then, these properties are analyzed analytically, based on the defect structure of the material. Furthermore, we use the conductivity relaxation measurement technique to monitor the monotonic/non-monotonic(two-fold) relaxation for the BZCY under various themodynamic conditions and extract the chemical diffusivities by solving Fick’s second law for each case on the basis of hydrogen- and oxygen- decoupled diffusion as proposed by Professor Yoo1-3.Fig. 1 shows the typical relaxation profiles of the mean total conductivity upon oxidation and reduction across an identical oxygen activity window. During the redox reaction driven by the oxygen chemical potential gradient at a fixed water vapor activity, oxygen may be incorporated into the BZCY by ambipolar diffusion of V•• o and 2h• in the p-type oxidizing regime. In Fig. 2, conductivity relaxation profiles upon hydration and dehydration at a fixed pO2(≈0.21 atm) are shown as a function of water vapor activities at 1023K. The figures clearly reveal the violation of monotonic relaxation governed by the chemical diffusion of water, according to the ambipolar diffusion of oxygen vacancies and proton. As shown in the fig. 2, non-monotonic twofold relaxation behavior was clearly observed for the BCZY system.1. H.I. Yoo et al., Phys Chem Chem Phys, 10, 974 (2008)2. H.I. Yoo et al., Solid State Ionics, 180, 1443 (2009)3. J.I. Yeon et al., Solid State Ionics, 181, 1323 (2010)4. S. Hamakawa et al., Solid State Ionics, 148, 71 (2002)5. S.-J. Song et al., Solid State Ionics, 167, 99 (2004)6. S.M. Haile et al., Nature, 125, 271 (1999)7. D. Pergolesi et al., Nature Materials, 9, 846 (2010)8. H. Iwahara et al., Solid State Ionics, 125, 271 (1999)9. Y.C. Yang et al., Sensors and Actuators B, 140, 273 (2009)10. C. Zuo et al., Advanced Materials, 18, 3318 (2006).

Correlation between defect structure and electrochemical properties of mixed conducting La0.1Sr0.9Co0.8Fe0.2O3−δ

The high catalytic properties of LSCF1982 arise from its defect structure. In this work, the oxygen nonstoichiometry (δ) of LSCF1982 was analyzed as a function of oxygen partial pressure (PO2) and temperature for the -6⩽log(PO2/atm)⩽0 and 800⩽T/°C⩽1000 ranges. A defect structure model for LSCF1982 was presented, which fitted well with the experimental data for δ. The equilibrium constants of appropriate defect reactions were determined. Analysis of the defect structure of LSCF1982 suggested that the conduction mechanism of LSCF1982 is governed by hopping conduction and band conduction of p-type carriers, which was determined by the analysis of thermoelectric power. The characteristic membrane thickness (Lc), indicating the transition from predominantly bulk-diffusion controlled reaction to surface-exchange controlled reaction, had a value of 3.5±0.9×10−2cm. The oxygen vacancy diffusivity was calculated from the relationship between oxygen flux and oxygen chemical potential gradient. The chemical expansion was measured as a function of PO2 and temperature in the 10-3⩽PO2/atm⩽0.21 and 800⩽T/°C⩽1000 ranges. The chemical expansion model based on the relative change in mean ionic radius was employed to compute the chemical expansion vs. δ, which indicated that the spin states of B-site transition metal ions are a mixture of high-spin and low-spin states, and made the transition from the low-spin to the high-spin state with an increase in δ and temperature.

On the steady-state chemical potential profiles in bilayer solid electrolytes

Abstract