Oxide Ion Conductivity Research Articles

Oxygen Vacancy Distribution in Yttrium-Doped Ceria from 89Y-89Y Correlations via Dynamic Nuclear Polarization Solid-State NMR.

Comprehending the oxygen vacancy distribution in oxide ion conductors requires structural insights over various length scales: from the local coordination preferences to the possible formation of agglomerates comprising a large number of vacancies. In Y-doped ceria, 89Y NMR enables differentiation of yttrium sites by quantification of the oxygen vacancies in their first coordination sphere. Because of the extremely low sensitivity of 89Y, longer-range information was so far not available from NMR. Herein, we utilize metal ion-based dynamic nuclear polarization, where polarization from Gd(III) dopants provides large sensitivity enhancements homogeneously throughout the bulk of the sample. This enables following 89Y–89Y homonuclear dipolar correlations and probing the local distribution of yttrium sites, which show no evidence of the formation of oxygen vacancy rich regions. The presented approach can provide valuable structural insights for designing oxide ion conductors.

Tailoring triple charge conduction in BaCo0.2Fe0.1Ce0.2Tm0.1Zr0.3Y0.1O3−δ semiconductor electrolyte for boosting solid oxide fuel cell performance

Introducing multiple-ionic transport through a semiconductor-electrolyte is a promising approach to realize the low-temperature operation of SOFCs. Herein, we designed and synthesized a single-phase Ce-doped BaCo0.2Fe0.3-xTm0.1Zr0.3Y0.1O3-δ semiconductor-electrolyte possessing triple-charge (H+/O2−/e−) conduction ability. Two different compositions are synthesized: BaCo0.2Fe0.3-xCexTm0.1Zr0.3Y0.1O3-δ [x = 0.1–0.2]. The 20% doped Ce composition exhibits an outstanding oxide-ion and protonic conductivity of 0.193 S cm−1 and 0.09 S cm−1 at 530 °C and the fuel cell utilizing BaCo0.2Fe0.2Ce0.2Tm0.1Zr0.3Y0.1O3-δ as an electrolyte yields an excellent power density of 873 mW cm−2 at 530 °C. Moreover, the fuel cell performed reasonably well (383 mW cm−2) even at a low temperature of 380 °C. Furthermore, the 10% Ce-doped utilized in fuel cell device illustrates lower performance (661 mW cm−2 at 530 °C and 260 mW cm−2 at 380 °C). Successful doping of Ce supports the formation of oxygen-vacancies at the B-site of perovskite and adjusting the ratio of Fe in the compositions. Moreover, the presence of Tm also assist in the creation of oxygen vacancies. Furthermore, the boosting of electrochemical performance and ionic conductivity of applied materials are enlightened by tuning the energy-band structure via employing the UPS and UV–Vis. The physical characterizations and verification of dual-ions (H+/O2−) in the semiconductor materials are performed via different electrochemical, spectroscopic, and microscopic techniques. A systematic study revealed triple charge conduction in this promising material, which helps in boosting the electrochemical performance of the LT-SOFC.

Synthesis and ion transport properties of RE3GaO6 (RE = rare earth) oxide ion conductors

First-principles calculations were conducted, which proposed that members of the RE3GaO6 (RE = rare earth) system were oxide ion conductors. This study experimentally verified oxide ion conduction in Dy3GaO6, Er3GaO6, and Nd3GaO6. The sintered bodies were synthesized by a solid-state reaction method, and their properties were characterized. The samples with dopants were observed to be mixed electron and oxide ion conductors. Dy2.85Ca0.15GaO6-δ exhibited oxide ion conductivities of 2.1 × 10−4 S/cm at 973 K, with an oxide ion transport number of 21 % under O2 gas flow. Additionally, the Rietveld refinement suggested that oxide ion migration might occur via the oxide ion vacancy between the O2 sites. Overall, the oxide ion conductivities of RE3GaO6 increased in the following order: Nd > Dy > Er, which was in good agreement with that predicted by using the first-principles calculations. The discrepancy between the experimentally measured and predicted conductivities was caused by the solid-solution limit at the RE site for the dopants.

Phase Evolution, Electrical Properties, and Conduction Mechanism of Ca12Al14-xGaxO33 (0 ≤ x ≤ 14) Ceramics Synthesized by a Glass Crystallization Method.

Mayenite Ca12Al14O33, as an oxide-ion conductor, has the potential of being applied in many fields, such as solid-oxide fuel cells. However, its relatively low oxide-ion conductivity hinders its wide practical applications and thus needs to be further optimized. Herein, a new recently developed glass crystallization route was used to prepare a series of Ga-doped Ca12Al14-xGaxO33 (0 ≤ x ≤ 14) materials, which is not accessible by the traditional solid-state reaction method. Phase evolution with the content of gallium, the corresponding structures, and their electrical properties were studied in detail. The X-ray diffraction data revealed that a pure mayenite phase can be obtained for 0 ≤ x ≤ 7, whereas when x > 7, the samples crystallize into a melilite-like orthorhombic Ca5Ga6O14-based phase. The electrical conduction studies evidence no apparent enhancement in the total conductivity for compositions 0 ≤ x ≤ 7 with the mayenite phase, and therefore, the rigidity of the framework cations and the width of the windows between cages are not key factors for oxide-ion conductivity in mayenite Ca12Al14O33-based materials, and changing the free oxygen content through aliovalent cation substitution may be the right direction. For compositions with a pure melilite-like orthorhombic phase, the conductivities also mirrored each other and are all slightly higher than those of the mayenite phases. These melilite-like Ca5Ga6O14-based materials show mixed Ca-ion, oxide-ion, and electron conduction. Furthermore, the conduction mechanisms of Ca ions and oxide ions in this composition were studied by a bond-valence-based method. The results suggested that Ca-ion conduction is mainly due to the severely underbonded Ca3 ions and that the oxide ions are most likely transported via oxygen vacancies.

High oxide-ion conductivity through the interstitial oxygen site in Ba7Nb4MoO20-based hexagonal perovskite related oxides

Oxide-ion conductors are important in various applications such as solid-oxide fuel cells. Although zirconia-based materials are widely utilized, there remains a strong motivation to discover electrolyte materials with higher conductivity that lowers the working temperature of fuel cells, reducing cost. Oxide-ion conductors with hexagonal perovskite related structures are rare. Herein, we report oxide-ion conductors based on a hexagonal perovskite-related oxide Ba7Nb4MoO20. Ba7Nb3.9Mo1.1O20.05 shows a wide stability range and predominantly oxide-ion conduction in an oxygen partial pressure range from 2 × 10−26 to 1 atm at 600 °C. Surprisingly, bulk conductivity of Ba7Nb3.9Mo1.1O20.05, 5.8 × 10−4 S cm−1, is remarkably high at 310 °C, and higher than Bi2O3- and zirconia-based materials. The high conductivity of Ba7Nb3.9Mo1.1O20.05 is attributable to the interstitial-O5 oxygen site, providing two-dimensional oxide-ion O1−O5 interstitialcy diffusion through lattice-O1 and interstitial-O5 sites in the oxygen-deficient layer, and low activation energy for oxide-ion conductivity. Present findings demonstrate the ability of hexagonal perovskite related oxides as superior oxide-ion conductors.

Structural and Electrochemical Properties of Lanthanum Silicate Apatites La10Si6−x−0.2AlxZn0.2O27−δ for Solid Oxide Fuel Cells (SOFCs)

An excellent oxide ion conductivity with high oxygen transportation of lanthanum silicate apatite at the solid oxide fuel cell (SOFC) can be achieved through the solid-state reaction method. The doped La10Si6−x−0.2AlxZn0.2O27−δ (x = 0.2 and 0.4) materials sintered at 1600°C accomplished crystallinity and crystal structure of apatite-type. The structural and electrochemical characterizations of La10Si6−x−0.2AlxZn0.2O27−δ (x = 0.2 and 0.4) were executed using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and electrochemical impedance spectroscopy (EIS) measurements. The total oxide ion conductivities of La10Si6−x−0.2AlxZn0.2O27−δ (x = 0.2 and 0.4) were measured from low to intermediate operating temperature range (450 to 800°C) using electrochemical impedance spectroscopy. Room temperature XRD patterns of La10Si6−x−0.2AlxZn0.2O27−δ (x = 0.2 and 0.4) exhibited La10Si6O27 apatite phase with space group P63/m as the main phase with the minor appearance of La2SiO5 as an impurity phase. The highest total oxide ion conductivity of 3.24 × 10−3 Scm−1 and corresponding activation energy of 0.30 eV at 800°C were obtained for La10Si5.6Al0.2Zn0.2O26.7 which contains a low concentration of Al3+ dopant.

Yttrium‐Doped Barium Zirconate‐Cerate Solid Solution as Proton Conducting Electrolyte: Why Higher Cerium Concentration Leads to Better Performance for Fuel Cells and Electrolysis Cells

AbstractProton conducting Y‐doped BaZrO3, BaCeO3 and their solid solutions are receiving increasing attention due to their promising application as electrolytes in ceramic fuel cells and electrolysis cells. However, the literature indicates a clear tendency that the performance of the cells increases with increasing Ce content in the electrolyte. In this work, to elucidate this phenomenon, a systematic work is performed on investigating the phase, hydration, and transport behaviors of BaZr0.8−xCexY0.2O3−δ (BZCY20). The results reveal that in the temperature range between 500 and 700 °C, with increasing Ce content, the dehydration temperature elevates and the proton concentration increases, showing that the Ce component favors the stabilization of protons. Furthermore, the transport number of hole conduction decreases, whereas the transport number of ionic conduction increases with the increasing Ce content. By further separating the contribution of oxide ions and protons, it is found that the oxide ion conductivity increases with the increasing Ce content at higher temperatures of 600 and 700 °C. Such decreased hole conductivity and increased oxide ion conductivity result in the enhancement of the ionic conduction of BZCY20 with increasing Ce content, and therefore improve the performance of fuel cells and electrolysis cells.

Determination of Oxide Ion Conductivity in Ba-Doped LaYbO3 Proton-Conducting Perovskites via an Oxygen Isotope Exchange Method

Proton-conducting oxides have garnered much attention as electrolyte materials of electrochemical devices operating at intermediate temperatures ranging 400–700 °C. For the implementation, quantifying the transport number of oxygen ions is crucial to predict the fuel utilization efficiency of solid oxide fuel cells (SOFCs). The acceptor-doped LaYbO3 proton conductors are one of the candidates for the electrolyte material of electrochemical devices such as SOFCs because of their high pure proton conduction. However, the oxide ion conductivity in this material is not quantified yet. Here, we evaluate the tracer diffusion coefficient (D*) of 5 mol % Ba-doped LaYbO3 by the oxygen isotope exchange and depth profile technique using secondary ion mass spectrometry. It is found that the oxygen ion conductivities in this material are lower than 10–4 S cm–1 below 800 °C, less than 2% to the total conductivity. We also visualize the domain of dominant conduction in this material. It reveals that proton conduction is dominant in fairly wide areas of the atmospheric condition, in particular below 600 °C, suggesting that the Ba-doped LaYbO3 is suitable for the SOFC electrolyte in terms of transport number.

Rare earth elements based oxide ion conductors

Rare-earth-elements-based oxide ion conductors with various structures and their structure-property relationships were systematically presented and summarized, which can provide new insight and guidance for the development of new oxide ion conductors.

High oxide-ion conductivity in acceptor-doped Bi-based perovskites at modest doping levels.

A combination of impedance spectroscopy, time-of-flight secondary ion mass spectrometry and literature data are used to show that, (i) the bulk oxide ion conductivity of A-site, alkaline earth-doped BiFeO3 (BF) is independent of the ionic radius of the alkaline earth ion (Ca, Sr, Ba) and, (ii) despite very different A-site environments in (Na1/2Bi1/2)TiO3 and BF, similar high levels and optimisation of bulk oxide ion conductivity in these Bi-based tilted perovskites is achieved at modest acceptor doping levels of ∼1-10%. These results clearly demonstrate that optimisation of oxide ion conductivity in these materials requires concepts beyond a simple crystallochemical approach based on matching the ionic radii of acceptor dopant and host lattice ions.

Spontaneous formation of a core–shell structure by a lithium ion conductive garnet-type oxide electrolyte for co-sintering with the cathode

The Sr–LLZ formed a core–shell structure in which Sr and La on the surface were concentrated and diluted, respectively, thus suppressing the chemical reaction with NCM and the battery worked at almost its theoretical capacity.

Study of Electrical Conductivity Superionic Conductor

The possibility of analytical determination of the type of electric current conductivity by a solid oxide ion conductor based on stabilized zirconium dioxide (solid electrolyte, superionic) under the influence of a DC electric field and high temperature in the range of 500°-900° degrees is considered. The research objectives of this work are to mathematically interpret the phenomenal phenomenon of substance transfer and irreversible changes in a solid oxide electrolyte (or superionics) of an oxygen concentration cell with mixed electrolyte conductivity. The connection with the need to expand the temperature range of the method and the range of ceramic materials under study is shown. It is confirmed that the sample has a mixed conductivity due to O^2- anions, Me+ cations and electrons with non-zero concentrations of n_a,k,e, charges q_a,k,e and mobility in the field of electric forces. A mathematical formula for determining the mobility of anions and cations in a solid oxide superionic conductor based on stabilized zirconium dioxide is derived.

Electrical properties of yttria-stabilised hafnia ceramics.

Cubic, yttria-stabilised hafnia, YSH, ceramics of general formula, YxHf1-xO2-x/2: x = 0.15, 0.30 and 0.45 were sintered at 1650-1750 °C and characterised by impedance spectroscopy. All three compositions are primarily oxide ion conductors with a small amount of p-type conductivity that depends on atmospheric conditions and appears to increase with x. The electronic conductivity is attributed to hole location on under-bonded oxide ions and the absorption of oxygen molecules by oxygen vacancies, both of which occur on substitution of Hf4+ by Y3+. Composition x = 0.15 has the highest total conductivity and shows curvature in the Arrhenius plot at high temperatures, similar to that of the most conductive yttria-stabilised zirconia.

Enhancing the oxide-ionic conductivity of Ba3Mo1+xNb1-2xGexO8.5 at intermediate temperatures: the effect of site-selective Ge4+-substitution.

Significant oxide-ionic conductivity has been recently reported for a family of cation-deficient hexagonal perovskite derivatives Ba3M2O8.5 (M = Mo/W6+ and Nb5+/V5+). Herein, strong 4-fold coordination geometry preferring Ge4+ ions are doped into Ba3Mo1+xNb1-2xGexO8.5 to manipulate the oxygen distribution within palmierite-like layers for the enhancement of oxide-ionic conductivity. Rietveld refinement of the neutron diffraction data of Ba3Mo1.2Nb0.6Ge0.2O8.5 reveals that Ge4+-ions are selectively incorporated into the palmierite-like layers, owing to their strong 4-fold coordination environment preference. Such a site-selective doping behavior leads to an increase in the occupation proportion of the O3 site and a concomitant decrease in the occupancy factor for O2. Ionic conduction measurements show that the bulk conductivity of Ba3Mo1.2Nb0.6Ge0.2O8.5 is about twice higher than that of the parent compound at intermediate temperatures (300-500 °C). Furthermore, bond-valence site energy (BVSE) landscape analysis reveals that the oxygen ionic conduction of Ba3Mo1+xNb1-2xGexO8.5 is dominated by the two-dimensional pathways along the palmierite-like layers, despite the three-dimensional (3D) oxygen diffusion pathways being present in the hybrid structure, which strongly confirms that the enhancement in ionic conductivity at intermediate temperatures is attributed to the site-selective Ge4+-substitution-induced redistribution of oxygen ions within the palmierite-like layers.

Electrical properties of Mg-doped and Mg, Si co-doped alumina

Impedance measurements on Mg-doped alumina ceramics, over the temperature range 400−910 °C, showed a combination of oxide ion conductivity and p-type electronic conductivity, depending on temperature and oxygen partial pressure. The oxide ion conductivity is attributed to oxygen vacancies, introduced as charge compensation for the Mg dopant. The p-type conductivity is attributed to hole creation on under-bonded oxide ions and is the dominant conductivity for measurements in air at high temperatures. In samples co-doped with Mg and Si, impedance measurements showed a reduction in oxide ion conductivity and therefore, number of oxide ion vacancies because the self-compensation mechanism of co-doping lead to direct replacement of Al3+by Mg2+ and Si4+; the level of p-type conductivity was also reduced.

Electronic structures of trivalent cations doped bulk and cubic La2Mo2O9 oxide ion conductors

Effect of intrinsic oxygen vacancies and different dopants such as Bi, Er and Y on the electronic structure of La2Mo2O9 have been studied in the framework of density functional theory using plane-wave pseudopotential method. First principles electronic structure calculations are performed using GGA exchange correlation functional to estimate the density of states and bands structures. Our calculated electronic density of states and band structures show that these systems are semiconductors with an indirect band gap ~ 2 ​eV. Electronic band gap increases with the increasing concentration of Bi, unlikely with that of Er and Y. O-2p and Mo-4d orbitals are mainly responsible for tuning the band gap. Bi remains the most electron donating character, whereas both Er and Y play the role of electron acceptors as evident from partial charge distribution plots and Bader charge analyses. Ionic bonds are observed for La, Bi, Er, and Y with O, but the bonds between Mo and O are covalent in nature for all these compounds.

Polyol-mediated synthesis of Bi-deficient Mg2+-doped sodium bismuth titanate and study of oxide ion migration behavior with functional properties

Sodium Bismuth Titanate, Na0.5Bi0.5TiO3 (NBT) is considered as a probable lead-free piezoelectric material. But its acceptor-doped perovskite turns out as an outstanding oxide-ion conducting system with possible utilization in intermediate-temperature solid oxide fuel cells (IT-SOFCs). Also, the Bi-deficit Sodium Bismuth Titanate (Na0.5Bi0.5−xTiO3-δ) exhibits a notable oxide-ion conductivity. In the present investigation, the Bi-deficient and Mg2+-doped Sodium Bismuth Titanate (Na0.5Bi0.49Ti1−xMgxO3-δ; x = 0.00, 0.01, 0.02, 0.03) were first time synthesized via polyol mediated synthesis route at lower temperature with better surface area and good conductivity. The Mg2+ doping at Ti4+ site improved the sinterability and augmented the grain size. The structural, microstructural, textural and most importantly electrical properties were analyzed using XRD, Raman, FTIR, SEM, BET, TGA, and EIS techniques to gain understanding about effects of substitution of Mg2+ on structural behavior and electrical conductivity. We also explored the influence of Bi-deficient Mg2+-substituted NBT compositions on the oxygen vacancies and ion migration behaviors. A correlation among the phase formation, conduction behavior and ion diffusion mechanism has been established for the Mg2+ substituted Bi-deficit NBT derived compositions. The conductivity was found to be maximum for NBT4902 composition over the entire temperature range. Though at the higher doping concentration of Mg2+ (x > 0.02), conductivity was found to slightly decrease.

Understanding the Interplay of Vacancy, Cation, and Charge Ordering in the Tunable Sc2VO5+δ Defect Fluorite System.

We report the synthesis, structure, and redox behavior of the cation-ordered tetragonal Sc2VO5+δ defect fluorite superstructure previously thought to be the oxygen precise A3+2B4+O5 phase. Four synthesis routes in oxidative, reductive, and inert atmospheres are demonstrated. Ex situ and in situ powder X-ray and neutron diffraction analyses reveal vanadium disproportionation reactions. The structure-reaction map illustrates the oxygen-dependent competition between the tetragonal cation and anion ordered Sc2VO5+δ and the disordered cubic Sc2VO5+δ' (δ < δ' ≤ 0.5) phases as a function of temperature. Oxidation states and oxide stoichiometries were determined with DC magnetometry and XANES experiments. The tetragonal cation ordered Sc2VO5+δ phase with δ = -0.15(2) for as-synthesized samples reveals vanadium charge ordering. V3+ and V4+ cations occupy octahedral sites, whereas V5+ predominantly occupies a tetrahedral site. The paramagnetic 8g{V3+/4+}4 clusters are isolated by diamagnetic 2cV5+ cations. At temperatures below 500 °C the 8g{V3+/4+}4 clusters can be topotactically fine-tuned with varying V3+/V4+ ratios. Above 600 °C the tetragonal structure oxidizes to the cubic Sc2VO5+δ' fluorite phase-its disordered competitor. The investigation of the cation- and anion-ordered Sc-V-O phases, their formation, and thermal stability is important for the design of low-temperature solid state oxide ion conductors and vacancy structures.

Deterministic Influence of Substrate-Induced Oxygen Vacancy Diffusion on Bi2WO6 Thin Film Growth

In oxide epitaxy, the growth temperature and background oxygen partial pressure are considered as the most critical factors that control the phase stability of an oxide thin film. Here, we report an unusual case wherein diffusion of oxygen vacancies from the substrate overpowers the growth temperature and oxygen partial pressure to deterministically influence the phase stability of $Bi_{2}WO_{6}$ thin film grown by the pulsed laser deposition technique. We show that when grown on an oxygen-deficient $SrTiO_{3}$ substrate, the $Bi_{2}WO_{6}$ film exhibits a mixture of (001) and (100)/(010)-oriented domains alongside (001)-oriented impurity $WO_{3}$ phases. The (100)/(010)-oriented $Bi_{2}WO_{6}$ phases form a self-organized 3D nanopillar-structure, yielding a very rough film surface morphology. Oxygen annealing of the substrate or using a few monolayer-thick $SrRuO_{3}$ as the blocking layer for oxygen vacancy diffusion enables growing high-quality single-crystalline $Bi_{2}WO_{6}$ (001) thin film exhibiting an atomically smooth film surface with step-terrace structure. We propose that the large oxide-ion conductivity of $Bi_{2}WO_{6}$ facilitates diffusion of oxygen vacancies from the substrate during the film growth, accelerating the evaporation of volatile Bismuth (Bi), which hinders the epitaxial growth. Our work provides a general guideline for high-quality thin film growth of Aurivillius compounds and other oxide-ion conductors containing volatile elements.

Crystal Growth Mechanism of Highly c-Axis-Oriented Apatite-Type Lanthanum Borosilicate Using B2O3 Vapor.

Apatite-type lanthanum silicate (LSO) exhibits high oxide-ion conductivity and has recently garnered attention as a potential solid electrolyte for high-temperature solid oxide fuel cells and oxygen sensors that operate in the low- and intermediate-temperature ranges (300–500 °C). LSO exhibits anisotropic oxide-ion conduction along with high c-axis-oriented oxide-ion conductivity. To obtain solid electrolytes with high oxide-ion conductivity, a technique for growing crystals oriented along the c-axis is required. For mass production and upscaling, we have thus far focused on the vapor-phase synthesis of c-axis-oriented apatite-type LSO and successfully grew polycrystals of highly c-axis-oriented boron-substituted apatite-type lanthanum silicate (c-LSBO) using B2O3 vapor. Here, we investigated the mechanism of c-LSBO crystal growth to determine why the utilization of B2O3 vapor resulted in such a strong c-axis crystal orientation. The synthesis of c-LSBO by the B2O3 vapor-phase method results in crystal growth accompanied by the diffusion of B2O3 supplied from another new compound that formed on the surface of the La2SiO5 disk, LaBO3. In addition, c-LSBO crystals are formed not only by vapor–solid reactions but also by solid–solid and liquid–solid reactions. The increase in the c-axis orientation degree might be due to the increase in the amount of the liquid-phase interface.