Oxide Ion Conductivity Research Articles

Oxide Ion-Conducting Materials Containing Tetrahedral Moieties: Structures and Conduction Mechanisms.

This Review presents an overview from the perspective of tetrahedral chemistry on various oxide ion-conducting materials containing tetrahedral moieties which have received continuous growing attention as candidates for key components of various devices, including solid oxide fuel cells and oxygen sensors, due to the deformation and rotation flexibility of tetrahedral units facilitating oxide ion transport. Emphasis is placed on the structural and mechanistic features of various systems ranging from crystalline to amorphous materials, which include a variety of gallates, silicates, germanates, molybdates, tungstates, vanadates, aluminates, niobate, titanates, indium oxides, and the newly reported borates. They contain tetrahedral units in either isolated or linked manners forming different polyhedral dimensionality (0 to 3) with various defect properties and transport mechanisms. The development of oxide ion conductors containing tetrahedral moieties and the elucidation of the roles of tetrahedral units in oxide ion migration have demonstrated diverse opportunities for discovering superior electrolytes for solid oxide fuel cells and other related devices and provided useful clues for uncovering the key factors directing fast oxide ion conduction.

Preparation of grain-aligned polycrystal of doped lanthanum silicate oxyapatite by templated grain growth method and evaluation of its structural and electrical properties

The disk-shaped sintered compacts consisting mainly of c-axis-aligned lanthanum silicate oxyapatite (LSO) polycrystals doped with K2O and Al2O3 were prepared by templated grain growth method. The tabular single crystals of K2O- and F-doped LSO were used as template particles, and the Al2O3-doped LSO as matrix powder. The chemical formula derived from the average chemical composition of the constituent LSO crystal grains was (La9.60K0.16□0.24)Σ=10(Si5.48Al0.38□0.14)Σ=6O26, where □ denotes vacancies in La and/or Si sites. One of the doped LSO grains was examined by single-crystal X-ray diffraction to confirm that the crystal structure was isomorphous to those previously reported. Other coexisting phases were 0.16 mol% LaAlO3 and 0.34 mol% SiO2-rich interstitial material (probably in a liquid state at high temperature). With increasing temperature from 773 to 1073 K, the total oxide-ion conductivity (σtotal) along the grain-alignment direction steadily increased from 1.31 × 10−4 to 3.21 × 10−3 S cm−1, each value of which was, at the same temperature, more than 7.7 times larger than that of the randomly grain-oriented polycrystal with the same bulk chemical composition. The larger σtotal-value of the former polycrystal would be due to the significantly higher oxide-ion conductivity along the c-axis direction of the doped LSO. A solid oxide fuel cell (SOFC) using the textured polycrystal as the electrolyte was constructed and tested for power generation. The maximum power density was satisfactorily high at 873 K, indicating that the c-axis-aligned polycrystals of doped LSO would be potentially applicable as electrolytes for the SOFCs operating at medium to low temperatures.

Correlation of oxygen defects, oxide-ion conductivity and dielectric relaxation to electronic structure and room temperature ferromagnetic properties of Yb3+ doped CeO2 nanoparticles

Ce1-xYbxO2 nanoparticles synthesized using co-precipitation route were studied for properties like structural, optical, impedance and X-ray Photoelectron Spectroscopy. Existence of cubic fluorite structure found intact with heterogeneous aggregation and spherical surface morphology. Red-shifted energy gap along with formation of defects and oxygen vacancies was confirmed with UV–Vis and Photoluminescence analyses. XPS analyses were undertaken to observe the chemical states 2+ and 3+ of Yb and 3+ and 4+ of Ce cations. The magnetization results demonstrated improved ferromagnetic ordering for varying Yb doping, explained with Bound Magnetic Polarons model and F-Centre Mechanism for creation of distinct complexes, Ce4+-Vo-Ce3+ or Yb3+-Vo-Ce3+ in the CeO2 lattice. All samples' dielectric permittivity and modulus were measured in the 120 Hz–100 kHz frequency range. The ∈′ (f) and ∈″(f) plots show two distinct regions: a plateau zone at high frequency and low frequency dielectric dispersion region. These nanoparticles could be employed for photolytic, optoelectronics and energy storage applications.

Diffusion mechanisms between La2SiO5 and SiO2 during formation of textured lanthanum silicate oxyapatite crystals

Lanthanum silicate oxyapatite (LSO) is a promising alternative electrolyte for solid oxide fuel cell (SOFC) applications. They exhibit anisotropic oxide-ionic conduction; therefore, a preferential crystal orientation along the c-axis is necessary to achieve optimal conductivity. This study is performed to understand the reaction mechanisms involved in the formation of an LSO layer at the interface of the La2SiO5/SiO2 diffusion pair during reactive sintering at high temperatures. Different La2SiO5/SiO2 bilayers were fabricated in sandwich-type structures using silica-type quartz or La2SiO5 supports obtained via electrophoresis and uniaxial pressing, respectively. These supports were pre-sintered and then coated with a suspension of the opposite component. The diffusion pairs were isothermally heated at 1500 °C at different holding times (10, 20, and 40 h) and at 1600 °C for 10 h. Their surfaces and cross-sections were investigated via X-ray diffraction, scanning electron microscopy (SEM) observations, energy-dispersive X-ray spectroscopy (EDS), and Raman spectroscopy.Experimental results show that two different driving forces were involved in the nucleation and growth of apatite crystals. The first is the chemical potential gradient in lanthanum between the components of the layers, and the second is the electric field created to preserve the electroneutrality of the system. The predominant diffusing species in this bilayer system were La+III and O-II, and their diffusion is enhanced by the formation of a glassy phase on the silica grains through the transformation of silica-type quartz into cristobalite. The formation of a La2Si2O7 intermediate phase was detected, which is vital to the interdiffusion of species and the nucleation and growth of oriented apatite crystals. The porosity gradient observed in the cross-section after reactive diffusion suggests the possibility of achieving an SOFC half-cell device via this process.

Development, characterization, and properties of LnSmZr2O7 (Ln = Dy, Ho, Yb) defect fluorite functional ceramics

LnSmZr2O7 (Ln = Dy, Ho, Yb) functional nanoceramics are prepared through a citrate-nitrate auto-combustion method. High-density pellets using the powder are obtained by sintering at 1530 °C for 2 h. The defect fluorite structure of the powder and sintered samples is established using XRD and Raman analysis. The morphology of the powder analyzed using Transmission Electron Microscopy revealed the nanocrystalline nature of the combustion powder. The microstructure of the sintered sample analyzed using Field emission Scanning Electron Microscopy showed increased grain growth with larger cationic substitution. LnSmZr2O7 (Ln = Dy, Ho, Yb) have an indirect-allowed optical wide band gap and exhibit photoluminescence due to intrinsic defect structure. LnSmZr2O7 (Ln = Dy, Ho, Yb) are pure oxide ion conductors in the temperature range from 400 °C to 900 °C, with conductivity values of 0.89 × 10−3 S/cm, 0.72 × 10−3 S/cm and 0.60 × 10−3 S/cm, respectively, at 900 °C.

Developing CeO2-CoAl2O4 Semiconductor Ionic Based Heterostructure Composite Electrolyte for Low-Temperature Solid Oxide Fuel Cells (SOFCs)

Semiconductor ionic electrolytes, especially heterostructure composites, have a significant role in enhancing oxide ion conductivity and peak power density (PPD) because of their interfacial contact. In this work, the fluorite structure CeO2 and spinel-based CoAl2O4 samples, as a heterostructure composite electrolyte, are successfully fabricated. The p-type CoAl2O4 and n-type CeO2 heterostructure (CeO2-CoAl2O4) used as an electrolyte exhibits a cell performance of 758 mW/cm2 under fuel cell H2/air conditions at 550 °C, which is quite higher than the pure CoAl2O4 and CeO2 fuel cell devices. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HR-TEM) verified the heterostructure formation including the morphological analysis of the prepared heterostructure composite. The heterostructure-based CeO2-CoAl2O4 composite achieved a higher ionic conductivity of 0.13 S/cm at 550 °C temperature, which means that the constructed device successfully works as an electrolyte by suppressing electronic conductivity. Meanwhile, the obtained results demonstrate the semiconductor ionic heterostructure effect by adjusting the appropriate composition to build heterostructure of the n-type (CeO2) and p-type (CoAl2O4) components and built-in electric field. So, this work exhibits that the constructed device can be effective for energy conversion and storage devices.

Thermally Induced Oxygen Vacancies and High Oxide Ion Conduction in K2ZnV2O7 with a Melilite-Related Structure

Thermally Induced Oxygen Vacancies and High Oxide Ion Conduction in K₂ZnV₂O₇ with a Melilite-Related Structure

SPS preparation of La10Si6O27 and the effect of sintering temperature on electrical conductivity

La10Si6O27 powder was synthesized by a high-energy ball milling method and its sintered body was prepared by spark plasma sintering (SPS) technology. Phase structure, microscopic morphology, density, porosity and oxide ion conductivity of La10Si6O27 were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), Archimedes method and alternating current (AC) impedance spectrum, respectively. XRD results showed the ball-milled powder could form apatite-type La10Si6O27 at 1400 ℃. The crystal structure of La10Si6O27 sintered body remained stable. SEM results showed that La10Si6O27 sintered body using SPS sintering method had better sintering between grains and a large reduction in open pores compared to conventional sintering. Grains of the sintered body were clearly visible and the grain size was uniform, about 1 µm. Archimedes method analysis showed relative density of La10Si6O27 sintered body is 100%, and open porosity is 0. Electrical conductivity of La10Si6O27 sintered body has an Arrhenius relationship with temperature.

Investigation on Reaction Mechanism in an SOFC Composite Cathode by Using Patterned Thin Film Electrodes

The reaction mechanism in an SOFC composite cathode was investigated by using patterned thin film model electrodes. In this work, we proposed new types of model electrodes, which could reproduce SOFCs cathodic reactions in a composite electrode of a mixed conducting electrode and an electrolyte. Then the model electrode was fabricated by choosing La0.6Sr0.4CoO3-δ (LSC) and Ce0.9Gd0.1O1.95 (GDC) as the electrode and the electrolyte, respectively. From the DC polarization measurements of the model electrode, it was found that making a composite of LSC and GDC can improve the cathodic performance especially at lower temperatures, possibly due to higher oxide ion conduction in GDC than LSC.

Correlation between the thickness of NiFe2O4 and hydrogen production performance for solid oxide electrolysis cells

This study investigates the influence of cathode thickness on the performance of solid oxide electrolysis cells with 20% H2O–80% N2 at 800 °C. NiFe2O4 (NFO) was used as the cathode material. The thickness of the cathode was controlled to be approximately 10–70 μm. NFO can be considered as a mixed electron and oxide-ion conductor because the cell performance changes with its thickness. Furthermore, the ohmic and polarization resistances decreased significantly when the cathodic thickness increased to 30 μm. The performance improvement was because of the increase in conduction networks. In addition, the electrochemical reaction area increases to lower polarization resistances. However, with a cathodic thickness of 30–70 μm, the ohmic resistance increased significantly because the conduction paths became longer from the current collector to the reaction area. In addition, the polarization resistance increased slightly owing to the difficulty in gas diffusion.

Structural analysis and properties of novel Terbium Samarium Zirconate functional ceramic

Synthesis and characterization of Terbium Samarium Zirconate (TbSmZr2O7) is reporting for the first time. XRD analysis along with Raman spectroscopy revealed that citrate-nitrate combustion synthesized TbSmZr2O7 powder and sintered sample crystallize in defect-fluorite structure with Fm3‾m space group. This composition lies at the pyrochlore-defect fluorite phase boundary. TEM analysis showed that the powder particles are in size range between 3 nm and 9 nm. FESEM image showed a well-sintered pellet surface with a grain size distribution between 0.1 μm and 2 μm. TbSmZr2O7 has an optical band gap of 1.66 eV and has an indirect allowed mechanism of optical transition. Due to intrinsic defect structure, TbSmZr2O7 exhibits photoluminescence with excitation bands at 270 nm and 313 nm and emission bands at 364 nm and 400 nm. Impedance spectroscopic analysis established that from 400 °C to 900 °C TbSmZr2O7 is a pure oxide ion conductor with a conductivity value of 1.9 × 10−3 S/cm at 900 °C.

Oxygen Vacancy Migration in Ca2Ga2+xGe1–xO7–0.5x Melilite

Compared with interstitial defects, vacancy-mediated oxide ion conduction in melilite family received little attention. Herein, melilite-type oxide ion conductors of Ga-excess compositions Ca2Ga2+xGe1–xO7–0.5x (0 ≤ x ≤ 0.15) demonstrate remarkable oxygen vacancy migration by experimental and computational verifications. Neutron powder diffraction data and defect formation energies of oxygen vacancy revealed that oxygen vacancies favor occurring at the bridging O sites shared by four-linked (Ga/Ge)O4 tetrahedra, opening the 5-fold rings coupled by the local structure relaxation around the oxygen vacancies. Molecular dynamics simulations revealed that the oxygen vacancies migration involves the cooperative rotation and deformation of adjacent tetrahedra, allowing the opening and re-forming of 5-fold rings in melilite structure. This study provides structural chemistry mechanisms on oxygen vacancy conduction in melilite structure.

Environmentally regulated intrinsic oxygen-ion transport for oxide-ion conductors

Oxide-ion conductors have been widely used as catalytic, conductive, detecting and other materials under oxidizing, reducing, inert, mixed environments and the like. However, so far the evaluation of their oxygen-ion transport (such as oxide-ion conductivity and oxygen permeability) either is extrinsic or is limited only in oxidizing or inert environment. Herein, the evaluation of intrinsic oxygen-ion transport for oxide-ion conductors in all environments seems especially important. In this work, a new test system was designed to enable the oxide-ion conductors placing in single oxidizing, reducing, inert or mixed environment separately, which also realized all the oxygen-vacancy concentrations of oxide-ion conductors are in equilibrium in all environments. The intrinsic oxide-ion conductivity and oxygen permeability were evaluated in all environments, and the influencing factors regulated by environments also were analyzed to correlate the variation of oxygen-ion transport.

Enhanced Oxide Ion Conductivity by Ta Doping of Ba3Nb1-xTaxMoO8.5.

Significant oxide ion conductivity has previously been reported for the Ba3M'M″O8.5 family (M' = Nb5+, V5+; M″ = Mo6+, W6+) of cation-deficient hexagonal perovskite derivatives. These systems exhibit considerable structural disorder and competitive occupation of two distinct oxygen positions (O3 site and O2 site), enabling two-dimensional (2D) ionic conductivity within the ab plane of the structure; higher occupation of the tetrahedral O3 site vs the octahedral O2 site is known to be a major factor that promotes oxide ion conductivity. Previous chemical doping studies have shown that substitution of small amounts of the M' or M″ ions can result in significant changes to both the structure and ionic conductivity. Here, we report on the electrical and structural properties of the Ba3Nb1-xTaxMoO8.5 series (x = 0.00, 0.025, 0.050, 0.100). AC impedance measurements show that substitution of Nb5+ with Ta5+ leads to a significant increase in low-temperature (<500 °C) conductivity for x = 0.1. Analysis of neutron and X-ray diffraction (XRD) data confirms that there is a decrease in the M1O4/M1O6 ratio upon increasing x from 0 to 0.1 in Ba3Nb1-xTaxMoO8.5, which would usually coincide with a lowering in the conductivity. However, neutron diffraction results show that Ta doping causes an increase in the oxide ion conductivity as a result of longer M1-O3 bonds and increased polyhedral distortion.

Recent developments in oxide ion conductors: focusing on Dion-Jacobson phases.

Oxide-ion conductors, also known as "oxygen ion conductors," have garnered significant attention in recent years due to their extensive applications in a variety of electrochemical devices, including oxygen concentrators, solid-oxide fuel cells (SOFCs), and solid oxide electrolysis cells. The key to improving the performance of these devices is the creation of novel oxide-ion conductors. In this feature article, we discuss the recent developments of new structural families of oxide-ion conductors and of the Dion-Jacobson-type layered oxide-ion conductors with a particular emphasis on CsM2Ti2NbO10-δ (M = Bi and lanthanoids; δ represents oxygen-vacancy content) and their solid solutions. CsBi2Ti2NbO10-δ is the first example of an oxide-ion conductor with a Dion-Jacobson-type layered perovskite structure, and the structural characteristics of these materials are extracted here. We have proposed an original concept that the large sized Cs+ cations and M3+ displacements yield the large bottlenecks for oxide-ion migration, which would facilitate the discovery of novel oxide-ion conductors. This article presents evidence that Dion-Jacobson-type layered perovskites are superior oxide-ion conductors. We also demonstrate how the information gleaned from these studies can be applied to the design of novel oxide-ion conductors.

Metal-insulator transition and resistive switching in Y-doped CeO2 ceramics.

Y-doped ceria, Y0.16Ce0.84O1.92, is an oxide ion conductor that shows n-type conductivity under a small applied voltage. With increasing voltage, resistive switching by 2-3 orders of magnitude occurs that is reversible with some hysteresis and is enhanced in atmospheres of reduced pO2. The switching is a bulk effect, is not associated with Schottky barriers or with a crystallographic transition, occurs rapidly after a premonitory onset period depending on conditions and shows characteristics of a Mott transition. This is the third known example of low field-induced resistive switching in a bulk ceramic and represents an emergent phenomenon in materials that are taken outside their zone of thermodynamic stability.

Barium cerate and its composite perovskites – synthesis techniques: a comprehensive review

Perovskites have attracted growing interest in recent eras due to their unique properties and potential applications. Barium cerate (BC) proton-conducting perovskites have been examined over decades for various applications such as hydrogen sensors, fuel cells, proton separation membranes, etc. When compared with oxide ion conductors under the same circumstances, ceramic proton conducting barium cerate can diminish the working temperature of solid oxide fuel cells to an intermediate temperature range, 673–873 K because of their higher ionic conduction. The present review carefully analyses and summarizes different synthesis approaches with optimization conditions to prepare BC-derived perovskites and the reported data have been carefully analyzed. A range of synthesis methods such as solid-state reaction method and wet chemical processes like coprecipitation, combustion, Pechini method, etc., have been systematically investigated. Even though the solid-state method has considerable disadvantages, most researchers avoid its incompetence due to the easiness of processing. Applications of the material have been briefly deliberated to exemplify its technological importance. The present review concluded with the recent signs of progress and innovative techniques employed to overcome the processing complications in these materials.

Mixed electronic and oxide ionic conduction and migration mechanisms in digermanate La2-xCaxGe2O7-x/2.

Mixed electronic and oxide ionic conduction was enabled in digermanate-type La2-xCaxGe2O7-x/2 containing Ge3O10 chains and isolated GeO4 units by substitution of La3+ with Ca2+. The structure and solid solution limit of Ca doped La2Ge2O7 were studied by systematic experiments, including rotation electron diffraction (RED), X-ray diffraction (XRD) and neutron powder diffraction (NPD) experiments, etc. The preferred occupation of Ca2+ and oxygen vacancies was investigated by Rietveld analysis of the NPD data. The obtained conducting material La1.925Ca0.075Ge2O6.963 exhibits superior thermal stability and an order of magnitude improvement in conductivity compared to the parent La2Ge2O7 (∼9 × 10-5 S cm-1 at 1000 °C). BVEL calculations reveal that the oxygen vacancies were stabilized and transported within the framework of La2Ge2O7 by sharing oxygen and oxygen exchange between the adjacent Ge3O10 chains and GeO4 units, exhibiting a three-dimensional oxide ion transport nature.

Preparation and characterization of La1.75Y0.25Mo2-xAlxO9-δ as solid electrolyte for intermediate temperatures SOFC

Yttrium and aluminium co-substitutions on lanthanum molybdate ceramics with the nominal formula La1.75Y0.25Mo2-xAlxO9-?? (LYMA, x = 0, 0.1, 0.2, 0.3 and 0.4) were synthesized by citric acid-nitrate combustion method and used for preparation of solid electrolyte for intermediate-temperature solid oxide fuel cells (ITSOFCs). Phase composition, structure, conductivity and electrical properties of LYMA have been investigated as a function of aluminium content by X-ray diffraction, scanning electron microscopy and electrochemical impedance spectroscopy, respectively. Experimental results showed that the substitution of La and Mo with appropriate amounts of Y and Al can effectively stabilize the ?-form of La2Mo2O9 at room temperature and inhibit its phase transition to ?-form. The LYMA synthesized by combustion method exhibited a better sinterability where relative density of the samples sintered at 950?C for 4 h was higher than 95%TD. The performance of the LYMA electrolyte was found to be related to Al-content. The La1.75Y0.25Mo1.8Al0.2O8.7 exhibited high oxide ion conductivity (?= 42mS/cm at 800?C) and low electrical activation energy (Ea = 1.18 eV). These preliminary results indicate that the LYMA is a promising electrolyte for IT-SOFCs.

Effect of humidified atmosphere on the physicochemical properties of S-doped La2Mo2O9 oxide-ion conductors

Effect of humidified atmosphere on the physicochemical properties of S-doped La2Mo2O9 oxide-ion conductors