High Oxide Ion Conductivity Research Articles

Structure and electrical conductivity of Ta doped La2Mo2O9 oxide ion conductors

In this work, we have investigated the crystal structure, thermal, vibrational, microstructural, and electrical properties of Ta doped La2Mo2O9 oxide ion conductors. The Rietveld refinements on the X-ray diffraction patterns confirm the α phase similar to that of undoped La2Mo2O9 at room temperature. α → β phase transition is confirmed by the differential scanning calorimetry analysis. The microcrystalline samples have grain sizes ∼10 μm along with well-defined grain boundaries. The infrared and Raman spectra mainly consist of MoO4 bands. The electrical conductivity of the highly dense pellets was analyzed by the impedance spectroscopy genetic programming method coupled with distribution function of relaxation times (DFRTs). Each DFRT consists of three main peaks conferring bulk, ionic, and electronic contributions of the grain boundary. The higher oxide ion conductivity in the β phase is identified due to the electronic contribution of the grain boundary for the samples. The capacitance for bulk and ionic parts of the grain boundary are almost temperature independent, whereas the electronic contribution varies turbulently. The incorporation reaction suggests that the sample with 5 wt. % Ta has the maximum oxide ion conductivity, which is further verified by the resistances obtained from the DFRT analysis.

Enhanced Ionic Conductivity in Ce<sub>0.8</sub>Gd<sub>0.2</sub>O<sub>2-δ</sub> Nanofiber: Effect of the Crystallite Size

The relationship between the microstructure and the conductivity of nanocrystallized oxygen ionic electrolytes has been received great interest since it provides guidelines for designing electrolytes with high performances which might find applications in fuel cells and oxygen sensors. Here, we present a strategy for controlling the calcination temperature to tune the crystallite size and ionic transport properties of solid electrolyte. Different crystallite sizes of Ce0.8Gd0.2O2-δ (CGO) nanofiber electrolytes were prepared. As the average crystallite size decreased from 27 nm to 8 nm, the conductivity of the nanofibers increased by more than five times. An exceptionally high oxide ion conductivity of 0.023 S∙cm-1 for the nanofibers was observed at 550°C. These insights into the effect of the crystallite size on the structure and the conductivity allow a better control of the electrical properties of solid electrolytes, which might foster their applications in electrochemical devices operable at lower temperatures.

High-Temperature Neutron Diffraction and Oxide Ion Conduction Path for Pb1-XLa2x/3WO4

Introduction Partially substituting lanthanum ions for lead site of the Scheelite-type structured PbWO4, oxide ion interstitials are formed as Pb1-xLaxWO4+x/2 and high oxide ion conduction appears at high temperature [1]. While most of the oxide ion conductors including zirconias, cerias, or LSGM cause high oxide ion conduction by oxide ion vacancies, the present system utilizes oxide ion interstitials. For the vacancy diffusion, conduction paths can be predicted by connecting regular oxide ions by straight or curved lines. On the other hand, for interstitial diffusion as the present system, conduction path would be complicated. We have conducted high-temperature neutron diffraction on the oxide ion conductor Pb1-xLaxWO4+x/2 (x = 0.2) with oxide ion interstitials to estimate the conduction path by means of the Rietveld analysis and Maximum Entropy Method (MEM) [2]. As a result, two types of oxide ion interstitial sites Oi(1) and Oi(2) were detected. The former can be detected even at room temperature, while the latter appears only at high-temperatures. We have concluded that the oxide ion conduction is performed through the regular oxide ion sites and these two interstitial sites. However, it is still uncertain whether the interstitial site is simply a localized position or surely contributes to the oxide ion conduction. In this study, high-temperature neutron diffraction has been carried out on Pb1-xLa2x/3WO4 (x = 0.1), which has the isostructure with the Pb1-xLaxWO4+x/2 but without apparent interstitials. Comparing the occupancy of the interstitials in these systems, we evaluate the contribution of interstitials to the oxide ion conduction. Experimental Pb1-xLa2x/3WO4 (x = 0.1) sample has been prepared by conventional solid-state reaction method starting from PbO, H2WO4 and La2O3. Before the weighing, La2O3 has been calcined at 900 °C for 6 hours. Stoichiometric mixtures of the starting materials were fired at first at 800 °C in air for 10 hours, finely ground, pressed into pellets under a hydrostatic pressure of 100 MPa for 3 minutes, and finally sintered at 900 °C in air for 10 hours. Crystalline phase of sintered samples was identified by X-ray diffraction measurement. About 10 g of the sample pellets were set in vanadium holder through a thin quartz tube and the neutron powder diffraction experiments were performed in the temperature range between 200 °C and 800 °C by using SHRPD diffractometer in J-PARC. The Rietveld refinements were carried out by using Z-Rietveld code, and the oxide ion conduction path at high temperatures was estimated with Maximum Entropy Method using Z-MEM. Results and Discussions Fig. 1 shows the temperature dependence of the lattice constant obtained by the Rietveld analysis. Both a and c-lengths increase linearly with increasing temperature. The a-length of Pb1-xLa2x/3WO4 is close to that of PbWO4, while Pb1-xLaxWO4+x/2 shows a smaller a-length. On the other hand, c-lengths were not largely varied within all the systems. Fig. 2 shows the temperature dependence of the W-O bond length. In Pb1-xLaxWO4+x/2, it decreases with temperature, whereas it increased in Pb1-xLa2x/3WO4 as PbWO4. In the Pb1-xLaxWO4+x/2 system, it is supposed that WO4 tetrahedra shrink with temperature for the activation of the movement of the oxide ions localized at the interstitial sites. For Pb1-xLa2x/3WO4 system, due to the absence of excess oxide ions, WO4 tetrahedra do not shrink with temperature. Considering the possibility of the oxide ion conduction through the oxide ion interstitials, the Rietveld refinements were carried out on Pb1-xLa2x/3WO4 assuming Oi(1) and Oi(2) of the two types of interstitials as represented in Fig. 3. The refined occupation factor of Oi(1) and Oi(2) for Pb1-xLa2x/3WO4 system are 0.0343 and 0.0129, respectively, while those for Pb1-xLaxWO4+x/2 are 0.0316 and 0.0092, respectively. Since the similar occupation factors are obtained between these two systems even for no apparent interstitial, the deduced oxide ion interstitial position would be attributed to the oxide ion conduction.

High oxide-ion conductivity in Si-deficient La9.565(Si5.826□0.174)O26apatite without interstitial oxygens due to the overbonded channel oxygens

Overbonding of the channel oxygens in the apatite-type lanthanum silicates was found to be a key for the high oxide-ion conductivities by the present single-crystal neutron and X-ray diffraction studies.

Ionic conductivity in multiply substituted ceria-based electrolytes

Cerias, appropriately doped with trivalent rare earth ions, have high oxide ion conductivity and are attractive SOFC (solid oxide fuel cell) electrolytes. Here, seven compositions of Ce0.8SmxGdyNdzO1.9 (where x, y and z=0.2, 0.1, 0.0667 or 0 and x+y+z=0.2) are synthesised using a low temperature method in order to determine the effect of multiple doping on microstructure and conductivity. Analysis using scanning and transmission electron microscopy, inductively coupled plasma mass spectrometry, X-ray diffraction and impedance spectroscopy is carried out. Crystallite sizes are determined in the powders and relative densities and grain size distributions were obtained in sintered pellets. Total, bulk and grain boundary conductivities are obtained using impedance spectroscopy and corresponding activation energies and enthalpies of ion migration and defect association are calculated. The highest total conductivity observed at 600°C is 1.80 Sm−1 for Ce0.8Sm0.1Gd0.1O1.9 and an enhancement effect on conductivity for this combination of co-dopants between 300°C and 700°C relative to the singly doped compounds - Ce0.8Sm0.2O1.9 and Ce0.8Gd0.2O1.9 - is seen. This has interesting implications for their application as SOFC electrolytes, especially at intermediate temperatures.

Structural and electrical properties of Bi3Y0.9W0.1O6.15-La0.8Sr0.2MnO3 (BiYWO-LSM) composites

Electrical conductivity and structural behavior in Bi3Y0.9W0.1O6.15-La0.8Sr0.2MnO3 (BiYWO-LSM) composite cathodes for intermediate temperature solid-oxide fuel cells (IT-SOFCs) have been investigated. The ionically conducting component (BiYWO) was selected as it not only exhibits high oxide ion conductivity at intermediate temperatures, but also shows good long-term stability. The LSM component is a well-known electronic conductor, commonly used for SOFC cathodes and also shows high interfacial polarization effects. Both composite components exhibit similar thermal expansion coefficients. Composites of different molar ratios of components were prepared by sintering the components together at 850°C. Electrical behavior was studied by a.c. impedance spectroscopy and transference number measurements were used to determine the ionic and electronic contributions to total conductivity. Percolation type behavior was observed for total conductivity and the percolation threshold value was determined to be in the bismuth rich region of the compositional range, due to the significant difference in the grain sizes of the two components. Ionic and electronic components of the total conductivity at, above and below the percolation threshold are discussed. Stability measurements on a composition close to the percolation threshold show some degree of conductivity decay on prolonged annealing at 650°C, which is partly recoverable on heating to higher temperatures.

“One-Pot” Solar Fuels

Chemists have always taken great pride in simplifying a complex multistep chemical synthesis into a single step process, in just one reactor. The motivation for doing so, apart from demonstrating synthetic competency and elegance, is the pursuit of green and sustainable solutions to chemical problems, as single step strategies have the inherent advantage of being atomically, energetically and economically efficient, as well as environmentally friendly and responsible.

High-Temperature Neutron Diffraction Study on PbWO4 and CaWO4-Based Oxide Ion Conductors with Different Defect Structure

Introduction Oxide ion conductors have been attracted great interests for the application of electrolytes of solid oxide fuel cells (SOFC). Most of the oxide ion conductors employ oxide ion vacancies to perform ionic conduction, while very few possess oxide ion interstitials for the oxide ion migration. When lanthanum ions are partly substituted into lead site in scheelite-type structured PbWO4, oxide ion conduction appears at elevated temperatures [1,2]. In that case, oxide ion interstitials are formed as Pb1-xLaxWO4+x/2 for the oxide ion conduction. On the other hand, CaWO4 with the isostructure with PbWO4 does not show such an oxide ion conduction enhancement due to the lanthanum substation. Recently, we have reported that alkaline-ion substituted CaWO4 also exhibit high oxide ion conduction [3]. Although the defect structure of the latter system is oxide ion vacancy as Ca1-xKxWO4-x/2, activation energies of the oxide ion conduction for both systems are relatively close at high-temperature. In this study, oxide ion conduction paths of these two systems, Pb1-xLaxWO4+x/2 and Ca1-xKxWO4-x/2, are evaluated by means of high-temperature neutron diffraction experiments coupled with maximum entropy method and compared based on the defect structure. Experimental Pb1-xLaxWO4+x/2 and Ca1-xKxWO4-x/2 (x = 0.2 for both systems) were prepared by solid state reaction method from the starting oxides. Calcining and sintering temperatures are selected as 800oC and 900oC, respectively for Pb1-xLaxWO4+x/2, and 800oC and 1000oC for Ca1-xKxWO4-x/2. The phase singularities of the obtained compounds are confirmed by X-ray diffraction. About 10 g of the sample pellets were inserted into a vanadium holder through a thin quartz tube and neutron powder diffraction experiments were performed in the temperature range between 200°C and 800°C by using SHRPD diffractometer in J-PARC. After the Rietveld refinements using Z-Rietveld code, oxide ion conduction paths at high temperatures were estimated by using maximum entropy method (MEM) using Z-MEM. Typically, 50 reflections are served for the MEM analyses. Results and Discussions Figs.1 and 2 represent the nuclear density distribution for Pb1-xLaxWO4+x/2 and Ca1-xKxWO4-x/2, respectively, obtained by MEM based on the neutron diffraction data collected at 800oC. Regular atomic positions obtained by the Rietveld analysis are indicated by spheres in the figures. For Pb1-xLaxWO4+x/2, nuclear density seems to extend from regular site to interstitial ones, suggesting the oxide ion diffusion path occurs parallel to the a-b plane. On the other hand, Ca1-xKxWO4-x/2 without oxide ion interstitials did not show such diffusion path parallel to the a-b plane, but the neutron density distribution extends toward c-directions. Defect structure would largely attribute to the diffusion mechanism in these two scheelite systems.

The electrochromic behavior of Bi2O3 thin film prepared by sol–gel technique

Bi2O3 is a polymorphic material with δ phase most known for its highest oxide ion conductivity. In this work, Bi2O3 thin films were prepared by sol–gel technique with annealing temperature of 450 and 550 °C. Electrochromic performances of the films were inspected. It discovered that the color state of the Bi2O3 thin films switched between black and light yellow transparent when applying a voltage of −2 ~ 2 V. Electrochromic degradation was further observed in the thin films. However, steadier electrochromic efficiency was found in the film annealed at higher temperature. This could be related to the larger δ-Bi2O3 phase fraction which may contribute to higher conductivity and show positive electrochromic effect. Bi2O3 is a polymorphic material with δ phase most known for its highest oxide ion conductivity. In this work, Bi2O3 thin films were prepared by sol–gel technique with annealing temperature of 450 and 550 °C. Electrochromic (EC) performances of the films were inspected. It discovered that the color state of the Bi2O3 thin films switched between black and light yellow transparent when applying a voltage of −2 ~ 2 V. EC degradation was further observed in the thin films. However, steadier EC efficiency was found in the film annealed at higher temperature. This could be related to the larger δ-Bi2O3 phase fraction which may contribute to higher conductivity and give a positive EC result.

Reducing the Gadolinium Dopant Content by Partial Substitution with Yttrium in a Ce0.9Gd0.1O1.95-based Oxide-Ion Conductor

Because of its high oxide-ion conduction and the chemical inactivity, Ce1-xGdxO2-δ fluorites are used as a reaction protection layer between the electrode and the ZrO2 electrolyte and addition materials to electrode elements for intermediate-temperature SOFCs. The rare-earth ores have 17 kinds of rare-earth elements including Sc and Y. However, Gd exists a low mineral content in the ores. Gadolinium element is used as magnetic devices, magnetooptic disk device, and control rod of the nuclear reactor. Under the present conditions, demand for Gd increases. The conductivity and sintering characteristics are very important as SOFC components and it is well known that two typical compositions, Ce0.9Gd0.1O1.95 and Ce0.8Gd0.2O1.9, have be commercially available. Although many papers for the properties have been reported, the appropriate compositions of Gd are unclear. The reason for this is that the properties are strongly affected by synthesis methods and sintering conditions. In the present study, the appropriate compositions of Gd-doped cerias with a Gd content of 0 ≤ x ≤ 0.2, which were prepared using the conventional co-precipitation method, have been discussed from the viewpoint of sintering characteristics, electrical conductivity and its long term stability. Sintering characteristics of the doped ceria samples have a tendency to decrease with increasing Gd content. The maximum conductivity of 2.56×10-2 S/cm at 600ºC was observed for the sample with 10 mol% Gd dopant. During the annealing process at 600ºC, the conductivity decreased with annealing time due to the grain-boundary resistance. The synchrotron X-ray diffraction analysis showed to be Gd2O3 phase in the fluorite and the decrease of lattice parameter. These results suggest that the decrease of the conductivity should be related to the segregation of Gd element at the grain-boundary of the Ce0.9Gd0.1O1.95 fluorite.

High oxide ion conductivity below 500∘C in the garnets LaxY3−xFe5O12+δ

We address the issue of achieving high oxide ion conductivity at low temperatures $(\ensuremath{\le}500{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C})$ in stable electrolytes for solid oxide fuel cell (SOFC) techonology. We report here a high oxide ion conductivity $(\ensuremath{\sim}{10}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}\mathrm{S}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1})$ in the La substituted yttrium iron garnets $\mathrm{L}{\mathrm{a}}_{x}{\mathrm{Y}}_{3\text{\ensuremath{-}}x}\mathrm{F}{\mathrm{e}}_{5}{\mathrm{O}}_{12+\ensuremath{\delta}}$ at low temperatures $(\ensuremath{\sim}450{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C})$. The judicious substitution of high electropositive $\mathrm{L}{\mathrm{a}}^{3+}$ ions in the yttrium iron garnet $(x\ensuremath{\le}0.45)$ affords continuous and easy oxide ion conduction pathways through polyhedral networks even at a low temperature of $\ensuremath{\sim}127{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. The conduction pathways have been visualized from the maximum entropy method analyzed soft bond valence sum distribution, obtained from neutron diffraction data. Our study also shows that these iron garnets have high structural stability over a broad range of temperature (at least over $27\text{--}800{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C})$, and compatible thermal expansion coefficient $(\ensuremath{\sim}10.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}{\mathrm{C}}^{\ensuremath{-}1})$ with that of the commonly used interconnect and electrode SOFC materials. We reveal that the oxide ion conduction in the present garnet is based on an excess oxide ion concentration mechanism. The observed high oxide-ion conductivity at low temperatures in the present trivalent substituted garnets is a remarkable finding for the development of an efficient SOFC technology.

Bulk- and Powder Surface Exchange Coefficients of (La,Sr)CoO3-d

LSC64 have been recognized as a promising cathode for SOFC. Their high oxide ion conductivity makes oxygen reduction reaction possible to occur not only at triple-phase boundary but also on two-phase boundary of an electrode and gas phase. Thus, it is important to study the surface exchange kinetics to understand the surface oxygen reduction process. In this study, we attempted to determine the surface exchange kinetics by pulse isotope exchange. This is innovative approach to determine k by observe the sample response when 16O exchanged by the isotope. The response and k will be discussed in this study.

Oxygen Insertion Reactions within the One-Dimensional Channels of Phases Related to FeSb2O4.

The structure of the mineral schafarzikite, FeSb2O4, has one-dimensional channels with walls comprising Sb3+ cations; the channels are separated by edge-linked FeO6 octahedra that form infinite chains parallel to the channels. Although this structure provides interest with respect to the magnetic and electrical properties associated with the chains and the possibility of chemistry that could occur within the channels, materials in this structural class have received very little attention. Here we show, for the first time, that heating selected phases in oxygen-rich atmospheres can result in relatively large oxygen uptakes (up to ∼2% by mass) at low temperatures (ca. 350 °C) while retaining the parent structure. Using a variety of structural and spectroscopic techniques, it is shown that oxygen is inserted into the channels to provide a structure with the potential to show high one-dimensional oxide ion conductivity. This is the first report of oxygen-excess phases derived from this structure. The oxygen insertion is accompanied not only by oxidation of Fe2+ to Fe3+ within the octahedral chains but also Sb3+ to Sb5+ in the channel walls. The formation of a defect cluster comprising one 5-coordinate Sb5+ ion (which is very rare in an oxide environment), two interstitial O2- ions, and two 4-coordinate Sb3+ ions is suggested and is consistent with all experimental observations. To the best of our knowledge, this is the first example of an oxidation process where the local energetics of the product dictate that simultaneous oxidation of two different cations must occur. This reaction, together with a wide range of cation substitutions that are possible on the transition metal sites, presents opportunities to explore the schafarzikite structure more extensively for a range of catalytic and electrocatalytic applications.

Highly active oxide cathode of La(Sr)Fe(Mn)O3 for intermediate temperature CO2 and CO2-H2O co-electrolysis using LSGM electrolyte

Electrochemical conversion of carbon dioxide to carbon monoxide (CO2→CO) is a potential step in the desirable transformation of the greenhouse gas to fuel or useful chemicals. In this study, the electrochemical conversion of CO2 to CO and H2O to H2 on an oxide cathode was studied in a solid oxide electrolysis cell (SOEC) by using a LaGaO3-based electrolyte (La0.9Sr0.1Ga0.8Mg0.2O3, LSGM) at elevated temperature (i.e. 973–1173K), which is a good candidate as electrolyte for electrolysis cell because of the high oxide ion conductivity at intermediate temperatures. Although metal, in particular, Ni is widely used as cathode of CO2 electrolysis cell, it was found that a LaFeO3−δ–based perovskite material as cathode (La0.6Sr0.4Fe0.8Mn0.2O3−δ, LSFM6482) shows much higher activity and selectivity to CO2 electrolysis and a current density of CO2 electrolysis is achieved with a value of 0.52A/cm2 at 1.6V and 1173K. In contrast, substitution of Co, Cu, and Ni decrease the anodic performance of LaFeO3 resulting in the decrease in electrolysis current. A cell using the LSFM6482 oxide cathode has a high selectivity for the electrolysis of CO2 to CO without carbon formation. In this study, application of LSGM6482 to CO2-H2O co-electrolysis was also studied and it was found that CO2-H2O co-electrolysis was successfully proceeded and CO/H2 syngas with ca. 1/1 ratio was obtained from CO2/H2O=1/1.

Synthesis of Gadolinium Doped Cerium Oxide at Low Temperature for Ion Transport Membrane

Recently, Ion transport membrane (ITM) or solid oxide membrane (SOM) technologies have been developed for new metal reduction technology because traditional extraction process emitted massive CO and CO2 gas and consumed large amount of electricity. The membrane based extraction process is now a promising candidate for resolving previous problems. Using high oxide ion conductivity such as yttria-stabilized zirconia (YSZ) or gadolinium-doped cerium oxide (GDC) for solid electrolytes between the anode and liquid electrolyte. Here we successfully were synthesized gadolinium doped ceria (GDC) below 100oC by urea-mediated routes in two steps. Gadolinium nitrate hydrates and Cerium nitrate hydrates were used as reagents. Hydrolysis reactions were induced by various concentration of Urea. The Collected powders were calcined to transform from cerium carbonate hydrates to cerium oxide at 700oC for 2 hours in air. The GDC powders were analyzed by X-ray diffractometer (XRD) for reveal its phases. The amount of Gadolinium dopants were measured by X-ray fluorescence (XRF) and Energy dispersive X-ray spec troscopy (EDX). Scanning Electron Microscopy (SEM) and Transmission electron microscopy (TEM) were exhibits its size and morphology. Finally, its ion conductivity were measured by electrochemical impedance spectroscopy in various temperature for comparison with commercial YSZ. Figure 1

High Temperature Oxide Ion Conduction Path for Pb1-XLaxWO4+x/2

Introduction Partially substituting lanthanum ions for the Scheelite-type structured PbWO4, oxide ion interstitials are formed as Pb1-xLaxWO4+x/2 and high oxide ion conduction appears at high temperatures [1]. While most of the oxide ion conductors including zirconias, cerias, or LSGM employ the oxide ion vacancy for the high-oxide ion conduction, the present system utilize the slight amount of oxide ion interstitials. Since there is no sufficient space for the oxide ion to migrate through only the interstitial sites, oxide ion diffusion should be made using regular and interstitial sites. However, unlike the vacancy diffusion mechanism, actual diffusion path is hardly estimated without direct experiment such as neutron diffraction. We have previously measured the neutron diffraction on Pb1-xLaxWO4+x/2(x = 0.1 and 0.2) at room temperature to evaluate the diffusion path from the interatomic distances and direction of anisotropic temperature factors of regular oxide ions [2]. Nevertheless, the estimated model only support the 2 D diffusion path, and oxide ion distribution at high temperature would be different from that at room temperature.We therefore carried out the neutron diffraction experiments on Pb0.8La0.2WO4.1in the temperature range from 200°C to 800°C and estimated the high temperature conduction path of oxide ions by means of maximum entropy method (MEM). Experimental Pb0.8La0.2WO4.1 sample has been prepared by solid state reaction method. Stoichiometric mixtures of PbO, H2WO4 and La2O3 were at first calcined at 800°C in air for 10 h, finely ground, pressed into pellets under a hydrostatic pressure of 100 MPa, and finally sintered at 900°C in air for 10 h. The base material of PbWO4 was also prepared for comparison. Crystalline phase of sintered sample was identified by X-ray diffraction measurement. About 10 g of the sample pellets were placed in vanadium holder through a thin quartz tube and the neutron powder diffraction experiments were performed in the temperature range between 200°C and 800°C by using SHRPD diffractometer in J-PARC and the Rietveld refinements were carried out by using Z-Rietveld code, and the oxide ion conduction path at high temperatures was estimated with maximum entropy method using Z-MEM. Results and Discussions Fig.1 represent the refined lattice parameters, a and c, of Pb1-xLaxWO4+x/2 (x = 0 and 0.2), both of which grow larger with increasing temperature. On the other hand, the interatomic distances between W and O, d W-O, gradually decreases with temperature in x = 0.2, while it grows larger for x = 0 as shown in Fig. 2. This indicates that the regular oxide ion is distorted by the diffusing oxide ions through the interstitial sites at high temperature. As for the oxide ion interstitials, two possible sites are deduced for the present high-temperature diffraction data in x = 0.2. The fractional coordinates of Oi(1) and Oi(2) are (0 0 0.208) and (0.667 0.333 0.238), respectively, which are represented in Fig. 3. The former corresponds to the previously reported one obtained from the room temperature neutron diffraction data. On the other hand, the latter was found by the present high-temperature diffraction experiments. Therefore, a new diffusion route through Oi(2) site is expected at high temperature in comparison with the previous route estimated at room-temperature. Fig.4 shows the nuclear density diagram of Pb0.8La0.2WO4.1 at 800°C. It is indicated that oxide ions migrate through Oi(2) as well as Oi(1) in Pb0.8La0.2WO4.1. It is indicated that actual diffusion path through Oi(1) (black arrow) is rather complex in comparison with that estimated from previous room temperature data (red arrow). In addition, additional new diffusion path is found through both Oi(1) and Oi(2) in the present high temperature experiment. Using these two typical conduction paths, oxide ion can migrate to achieve 3 D diffusion network. On the other hand, PbWO4doesn’t have such diffusion path even at high temperature.

Defect-Structure Analysis on La2(Ni,Cu)O4+ d with Reverse Monte Carlo Simulation Using Neutron and Synchrotron X-Ray Total Scatterings

Recently, it is highly expected to lower an operating temperature of the solid oxide fuel cell (SOFC). To satisfy this demand, a development of a novel air-electrode material can be considered as an important issue. Materials with high mixed oxide-ion and electronic conduction can be regarded as a promising candidate for the electrode since the materials enable to expand area for the oxygen-reduction reaction. As such materials, La2NiO4+δ with the layered perovskite structure has drawn much attention nowadays. In the oxides, interstitial oxide ions are introduced into the crystal due to a charge compensation of Ni oxidation from 2+ to 3+. This change of the defect structure leads to high mixed oxide-ion and electronic conduction and thus superior electrode property. It is also found that a partial substitution of Cu for Ni enhances electrochemical property of La2NiO4+δ [1]. Considering the mechanism for the mixed conduction mentioned above, the Cu substitution must affect the defect structure in the crystal: that is, an extra oxygen amount, a distribution and/or a local situation of the interstitial oxide ion. From such background, some previous works have carried out actively the crystal structure analysis using the Bragg profile and discussed the average position of the extra oxide ions. As is well known, however, the structure analysis (a so-called average-structure analysis) cannot give deep insight on the defect structure unfortunately. In this work, we focused on La2NiO4+δ and La2Ni0.6Cu0.4O4+δ with the layered perovskite structure, and investigated their defect structures by the reverse Monte Carlo (RMC) simulation using the Bragg profiles and the pair distribution functions (PDFs) simultaneously [2, 3]. Since PDF data include information on the atomic configurations without periodicity, we could clarify the defect structures of the samples on the basis of the atomic-configuration snapshots of the RMC simulations. La2NiO4+δ and La2Ni0.6Cu0.4O4+δ could be synthesized by a citric-acid method, according to literature [1]. The obtained products were identified by laboratorial X-ray diffraction measurements, and their metal compositions were evaluated by the inductively coupled plasma spectrometry. Average structures of the samples were determined by the Rietveld analysis (Z-Rietveld) using the neutron Bragg profiles which were recorded with iMATERIA installed at J-PARC. Neutron and synchrotron X-ray total scattering patterns were also measured with NOVA at J-PARC and BL04B2 at SPring-8, respectively. By using all the data, we performed the RMC simulations with RMCProfile for the specimens. In the analyses, we used super cells with 1152 atoms which were constructed based on the refined unit cells. The Rietveld analysis using neutron Bragg profiles confirmed that both the La2NiO4+δ and La2Ni0.6Cu0.4O4+δ had a single phase of the layered rock-salt structure with a space group of Fmmm. In order to determine distributions of the interstitial oxide ions, we analyzed atomic configurations of the samples by the RMC modeling using neutron PDF, neutron and synchrotron X-ray structure factors in addition to the neutron Bragg profile. As a result, it was found that a distance of Cu-O was longer than that of Ni-O, and the distribution of the distance became wider around Cu. Such a change in the local situation may be one of the reasons for higher oxide-ion diffusion in the Cu-substituted specimen. It was also indicated from the La-Ni and La-Cu distances that La2Ni0.6Cu0.4O4+δ had larger interstitial cavity volume in the rock-salt layer compared with La2NiO4+δ. Since the rock-salt layer forms the diffusion pathway of the interstitial oxide ion, the larger interstitial volumes in the Cu-substituted sample should contribute to better electrode property. From the analysis on cavities around interstitial oxide ions, it was suggested that an oxide-ion diffusion accompanied a significant change in the cavity volume around the mobile oxide ion in the case of La2NiO4+δ, whereas such a change was suppressed in La2Ni0.6Cu0.4O4+δ. This should be one of the reasons why the oxide-ion diffusion was enhanced by substituting Cu for Ni partially.

Synthesis of Al-Substituted Lanthanum Silicate with Oxyapatite Structure By Hydrothermal Method and Local Structural Analysis

INTRODUCTION The solid oxide fuel cell (SOFC) can be regarded as one of the most promising power generation systems because of the high efficiency and low emission of pollutants. At present, however, SOFC must be operated at temperature as high as 1000 oC, and thus it is expected to decrease the operating temperature to the intermediate temperature range around 500 oC for the wide-spread use. Since La9.33Si6O26-based oxide-ion conductors have high oxide-ion conductivity and low activation energy even at the intermediate temperature, the materials are considered as a promising candidate for an electrolyte of SOFC. In addition, it was also reported that the conductive properties were improved by Al substitution1). However, it is well-known that both the synthetic and sintering processes of the lanthanum silicates need extremely high temperature, and this demerit obstruct the practical use. In this work, we synthesized nanopowders of the lanthanum silicates substituted by Al for Si with a hydrothermal process at 180 oC, and then investigated effects of the substitution on conductive properties and crystal structure. We also examine the substitution effect on the local structure in lanthanum silicates by means of the reverse Monte Carlo (RMC) simulation. EXPERIMENTAL La9.33Si6-xAlxO26- d(x=0~1) were synthesized by hydrothermal method. In the hydrothermal synthesis, LaCl3・6H2O and SiO2, Al(OH)3 were mixed in the 4.8mol/l NaOH aqueous solution. The mixture was stirred for 1 hour at room temperature and then transferred into a Teflon lined autoclave. Synthesis was carried out at 180 oC for 96h. The powder after washing with distilled water was dried at 100 oC for a day. The product with PVA addition was pressed into a pellet and then sintered at 1250 oC for 12 h. The sample was identified by X-ray diffraction. Particle morphologies of the synthesized power, cross section and surface morphologies of the bulk samples were observed by the scanning electron microscopy (SEM). The densities of the sintered pellets were evaluated by the Archimedes method. The conductivity measurements were carried out between 300-900oC by the AC impedance method. In order to analyze crystal structures of the samples, synchrotron X-ray diffraction measurements were performed. The crystal structures were refined with Rietveld technique (REITAN-FP) using the data. Atomic arrangements (local structures) were also investigated in detail by the RMC simulation (RMC Profile) using the results of synchrotron X-ray (BL02B2, SPring-8) and neutron total scatterings (NOVA, J-PARC). RESULTS AND DISCUSSION La9.33Si6-xAlxO26- d(x=0~0.3) prepared by hydrothermal method had a single phase of the oxyapatite structure. However, La9.33Si6-xAlxO26- d(x=0.35~1) contained a secondary phase just after the hydrothermal process. By sintering the samples at 1250 oC, a single phase was obtained for La9.33Si6-xAlxO26- d(x=0.35~0.75). Fig.1 shows an SEM image of the powder of La9.33Si5.25Al0.75O26- d after hydrothermal method. An average particle size of the powder was 150 nm. From the measurements results of Archimedes method, relative densities of the sintered sample were more than 90 % regardless of the Al substitution amount. As reported in literatures, the Al-substituted samples showed higher conductivity and lower activation energies compared to the sample without the Al substitution. Since the conductivity did not depend on oxygen partial pressure, it can be considered that the oxygen ion conduction is dominant in the samples. In the Rietveld analysis, four possible space groups (P63/m, P63 P-3 and P21) were examined. From the refined unit cell, an initial model of 3 × 3 × 4 super cell was constructed for RMC simulation. In the simulation, we used synchrotron X-ray diffraction for the average structure, neutron and synchrotron X-ray total scattering for the local structure, simultaneously. As a result, it was suggested that the local structure around La vacancies was different significantly from that around La. REFERENCES 1)S. Chefi, A. Madani, H. Boussetta, C. Roux, A. Hammou, J. Power Sources, 177, 464 (2008) Figure 1

Local Structure Analysis of Na0.5Bi0.5TiO3 based Oxide Ion Conductor By DFT Calculations and RMC Method

INTRODUCTION In recent decades, oxide-ion conducting ceramics are expected to be applied for various electrochemical devices, such as the solid oxide fuel cells (SOFC), and play an important role in the field of the solid-state electrochemistry. Among the devices, SOFC has attracted much attention because it realizes high power generation efficiency and low environmental load. The operating temperature of commercially available SOFC is too high, i.e. 1000 ˚C, at present and thus lowering the operating temperature to around 500 ˚C (intermediate temperature) is highly desired to put them into practical use. Recently, it has been found that Na0.5Bi0.5TiO3 based materials exhibit high oxide-ion conductivity and low activation energy, and the materials become potential candidates for electrolytes of SOFC at the intermediate temperature. In this work, in order to investigate effects of Bi and O deficits on atomic configurations of Na0.5Bi0.5TiO3-based materials and local environment of these deficits, we prepared Na0.5Bi0.5-xTiO3-δ and Na0.5+yBi0.5-yTiO3-δ. The atomic configurations of the samples were investigated by analyzing neutron and synchrotron X-ray total scatterings with the density functional theory (DFT) and the reverse Monte Carlo (RMC) simulation. EXPERIMENTAL Na0.5Bi0.5-xTiO3-δ and Na0.5+yBi0.5-yTiO3-δ were synthesized by solid-state method. In the solid–state method, mixture of Na2CO3, TiO2 and Bi2O3 was calcined at 850 oC for 3 h in air, and then sintered at 1100 oC for 2 h in air. The samples were identified by X-ray diffraction. The metal compositions were evaluated by inductively coupled plasma spectroscopy (ICP) and atomic absorption spectrometry(AA). Cross-section morphologies of the bulk samples were observed by scanning electron microscopy (SEM). The densities of the sintered pellets were evaluated by the Archimedes method. The conductivity measurements were carried out between 300-900 oC by the AC impedance method. Neutron powder diffraction experiments were performed at BL20B2 (J-PARC, Japan). The data were refined using Rietveld technique (Z-code). In order to discuss the effect of Bi and O deficits on the atomic configurations, Na0.5Bi0.5-xTiO3-δ and Na0.5+yBi0.5-yTiO3-δ were also investigated by the RMC simulation (RMCProfile) using total scattering data measured by synchrotron X-ray (BL04B2, SPring-8) and neutron (BL21, J-PARC) sources in addition to the Bragg profile. RESULTS AND DISCUSSION It was found from XRD measurements that Na0.5Bi0.5-xTiO3-δ and Na0.5+yBi0.5-yTiO3-δ could be attributed to the perovskite-type structure with a space grope of Cc. The analytical compositions of Na0.5Bi0.5-xTiO3-δ and Na0.5+yBi0.5-yTiO3-δ evaluated by ICP and AA analysis were able to be controlled, and thus essentially equal to the nominal values. From the results of the Archimedes method, relative densities of the samples were more than 94 %. Figure 1 shows conductivities of the samples as a function of temperature. As seen in this figure, the conductivities of Na0.5Bi0.5-xTiO3-δ were increased with decreasing the Bi amount. In addition, by comparing the conductivity of Na0.5Bi0.5-xTiO3-δ and Na0.5+yBi0.5-yTiO3-δ, it was demonstrated that better electrochemical property could be achieved in the case of Na0.5+yBi0.5-yTiO3-δ. Conductivities measured under conditions of oxygen and argon showed similar behavior against temperature, indicating dominant oxygen-ion conduction. In the Rietveld analysis, we assumed the space group as Cc, and then could refine the average structure successfully. In addition, in order to examine local structures and cation arrangements around the Bi vacancies around and O vacancies in detail, the synchrotron radiation X-ray and neutron total scatterings were measured for Na0.5Bi0.5-xTiO3-δ and Na0.5+yBi0.5-yTiO3-δ. By using the data, the local structure analysis by the RMC method and the DFT was carried out. As a result, considerable differences in the bond distance between the each oxygen and bismuth was observed, and the effect on the conductive properties were suggested. REFERENCES 1)M.Li, M.J.Pietrowski et al., Nature Materials, 13, 31 (2014). Figure 1

Investigations on acceptor (Pr3+) and donor (Nb5+) doped cerium oxide for the suitability of solid oxide fuel cell electrolytes

Nanocrystalline acceptor (Pr3+) and donor (Nb5+) doped cerium oxide are synthesized by sol-gel method via hydrolysis process and evaluated for the suitability of applying as an electrolyte for the intermediate temperature solid oxide fuel cells. Phase purity and crystallite size of the synthesized materials are ascertained by powder X-ray diffraction studies. Introduction of Pr3+ ions in the cerium lattice exhibited a lower crystallite size than the Nb5+, which exposes the probability to attain high oxide ion conductivity of Pr3+ doped cerium oxide. Fourier transform infrared and Raman spectra confirm the functional groups and formation of Pr3+ and Nb5+ ions in the cerium lattice. Absorbance spectra exhibit the charge-transfer transition from O2− (2p) to Ce4+ (4f) orbital in cerium oxide. Pr3+ and Nb5+ ions doped cerium lattice create the oxygen vacancies and favor the formation of Ce3+ from Ce4+. Valence band transition of Ce3+ ions from the 5d to 4f levels are examined by photoluminescence studies. The morphological features of Ce-Pr-O and Ce-Nb-O are investigated by scanning and transmission electron microscopy. Electrochemical impedance spectroscopy is used to analyze the conductivity properties of solid electrolytes. Ce-Pr-O shows the high oxide ion conductivity of 0.1 S/cm at 600 °C with an activation energy of 0.73 eV. Electrolytes with specific conductivities higher than 10−2 S/cm at intermediate temperatures (∼400–600 °C) are required for solid oxide fuel cells to operate with less maintenance. Hence, Ce-Pr-O can be a suitable electrolyte material for intermediate temperature solid oxide fuel cells.