Enhanced Oxide Ion Conductivity Research Articles

Enhanced oxide ion conductivity in sodium niobate-based ceramics

Doping of Mg2+ and Ga3+ leads to the transition of electrical conduction mechanism of NaNbO3 from mixed oxide ion and electron conduction to oxide ion dominated conduction.

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.

Revealing the Structure and Oxygen Transport at Interfaces in Complex Oxide Heterostructures via 17O NMR Spectroscopy.

Vertically aligned nanocomposite (VAN) films, comprising nanopillars of one phase embedded in a matrix of another, have shown great promise for a range of applications due to their high interfacial areas oriented perpendicular to the substrate. In particular, oxide VANs show enhanced oxide-ion conductivity in directions that are orthogonal to those found in more conventional thin-film heterostructures; however, the structure of the interfaces and its influence on conductivity remain unclear. In this work, 17O NMR spectroscopy is used to study CeO2–SrTiO3 VAN thin films: selective isotopic enrichment is combined with a lift-off technique to remove the substrate, facilitating detection of the 17O NMR signal from single atomic layer interfaces. By performing the isotopic enrichment at variable temperatures, the superior oxide-ion conductivity of the VAN films compared to the bulk materials is shown to arise from enhanced oxygen mobility at this interface; oxygen motion at the interface is further identified from 17O relaxometry experiments. The structure of this interface is solved by calculating the NMR parameters using density functional theory combined with random structure searching, allowing the chemistry underpinning the enhanced oxide-ion transport to be proposed. Finally, a comparison is made with 1% Gd-doped CeO2–SrTiO3 VAN films, for which greater NMR signal can be obtained due to paramagnetic relaxation enhancement, while the relative oxide-ion conductivities of the phases remain similar. These results highlight the information that can be obtained on interfacial structure and dynamics with solid-state NMR spectroscopy, in this and other nanostructured systems, our methodology being generally applicable to overcome sensitivity limitations in thin-film studies.

Enhanced oxide-ion conductivity of solid-state electrolyte mesocrystals.

Oxide ion-conducting porous films were produced on a substrate by evaporation-induced self-assembly of rare earth-doped CeO2 (REDC) nanocubes 4-5 nm in size and subsequent mild calcination at 400 °C. Mesocrystalline structures comprising iso-oriented REDC nanocubes were formed by ordered assembly based on strict attachment of their {100} faces. Enhanced oxide-ion conductivities in a temperature range of 250-350 °C were observed on the mesocrystalline films consisting of Sm-doped CeO2 (SDC) nanocubes. The specific electrical properties of the mesocrystalline films are ascribed to improved surface-ion conduction due to a large specific surface area and a high crystallographic connectivity of SDC nanocubes.

Influences of cation and anion substitutions on oxidative coupling of methane over hydroxyapatite catalysts

Lead substituted hydroxyapatite (Pb-HAP) has been an active catalyst for oxidative coupling of methane (OCM) reactions. CO32− substituted HAP (HAP-CO3) has showed enhanced oxide ion conductivity than bare HAP in high temperature solid oxide fuel cells. Substitutions for both cations and anions in HAP structure (Pb-HAP-CO3) are promising to integrate the catalytic property of Pb-HAP and oxide ion conductive property of HAP-CO3 into one apatite-based ceramic material that can be manufactured into membrane reactors for possessing CH4 activation and O2 permeation capabilities for efficient OCM reactions. In this work, the effects of substitutions for both cation (Pb2+) and anion (CO32−) in HAP structure on OCM reactions were studied. The composition and physicochemical properties of HAP catalysts were changed by the cation and anion substitutions, respectively, and as consequences, they influenced the catalytic performances of HAP structure in OCM reactions. The selectivity to C2 (ethylene and ethane) products increased in the order of HAP-CO3<HAP<Pb-HAP-CO3<Pb-HAP, while Pb-HAP-CO3 showed the best stability and comparable C2 yield (under optimized reaction conditions) to Pb-HAP catalyst. Under different reaction temperature and/or CH4/O2 ratio in the OCM reactions, the CH4 conversion and C2 or COx (CO and CO2) selectivity showed a strong dependence on the composition of HAP-based catalysts. The present study forms a basis for understanding of the correlations between the composition, structure, and catalytic performance of HAP and other apatite structured catalysts, which are potential membrane materials for OCM reactions in membrane reactors.

Synthesis and characterisation of La0·6Sr0·4Co0·8Fe0·2O3−δ–Gd0·2Ce0·8O1·9 composite cathode for Gd0·2Ce0·8O1·9 electrolyte SOFC

A high performance composite cathode, La0·6Sr0·4Co0·8Fe0·2O3−δ–Gd0·2Ce0·8O1·9 (LSCF–CGO), was synthesised for intermediate temperature solid oxide fuel cell. The effect of CGO incorporation on the electrical properties was studied by electrochemical impedance spectroscopy. The results show that the LSCF–CGO composite cathode shows better electrochemical performance with maximum power densities of 88 mW cm−2 at 800°C. The improved performance of LSCF–CGO is attributed to the extended triple phase boundary and enhanced oxide ion conductivity.

ChemInform Abstract: Enhanced Oxide Ion Conductivity in Stabilized Cubic BaInO2.5 by Mn5+ Doping.

AbstractMn‐doped, room temperature stabilized cubic BaInO2.5 is prepared by solid state reaction of BaCO3, In2O3, and MnCO3 with molar ratios of Ba:In:Mn = 1:(1‐x):x (0.13 ≤ x ≤ 0.33) in Al2O3 crucibles at 1200 °C for 12 h and at 1350 °C for 24 h.

Enhanced oxide ion conductivity in stabilized cubic BaInO2.5 by Mn5+ doping

Abstract The preparation of room-temperature stabilized cubic BaInO 2.5 was successfully achieved by adjusting Mn 5 + doping (0.13 ≤ x ≤ 0.33) using high-temperature solid state reactions. XPS and magnetic investigations revealed that the valence state of Mn ions in stabilized cubic BaInO 2.5 was + 5. The stabilized cubic BaInO 2.5 showed enhanced oxide ion conductivity, and the best low-temperature conductivity at 400 °C was 3.0 × 10 − 3 S cm − 1 , which was larger than previously reported highest conductivity for stabilized cubic BaInO 2.5 .

Comparison of the electrochemical properties of impregnated and functionally gradient LaNi0.6Fe0.4O3–Gd0.2Ce0.8O2 composite cathodes for Solid Oxide Fuel Cell

This study investigates the electrochemical properties of three different configurations of LaNi0.6Fe0.4O3 (LNF)-based cathodes for Solid Oxide Fuel Cell (SOFC). The results show functionally gradient LNF cathode and Gd0.2Ce0.8O2 (GDC)-impregnated LNF cathode both reveal the better electrochemical properties compared to that of 70 wt.% LNF–30 wt.% GDC cathode. Functionally gradient LNF cathode with better interface of LNF/GDC/ScSZ can be achieved by the gradual change in composition from electrolyte to cathode, resulting in the decline in charge transfer resistance and gas phase diffusion resistance, therefore the decline in specific cathode polarization resistance (Rp). Whereas, the dramatic decrease in Rp for GDC-impregnated LNF cathode is mainly attributed to the extended triple phase boundary (TPB) and enhanced oxide ion conductivity. The nano-sized GDC particle on LNF backbone with high porosity would allow gas phase molecules to easily diffuse to the LNF/GDC/ScSZ boundaries, which significantly enhances cathode activities for oxygen reduction reaction (ORR). The GDC-impregnated LNF cathode reveals the lowest Rp, the lowest activation energies of Rp and the lowest activation energy of ORR among three cathode configurations. Noticeably, the cathode performance can be significantly improved by impregnating nano-sized GDC particles into LNF porous backbone.

Influence of doping with various cations on electrical conductivity of apatite-type neodymium silicates

Abstract Apatite-type neodymium silicates doped with various cations at the Si site, Nd 10 Si 5 B O 27− δ ( B =Mg, Al, Fe, Si), were synthesized via the high-temperature solid state reaction process. X-ray diffraction and complex impedance analysis were used to investigate the microstructure and electrical properties of Nd 10 Si 5 B O 27− δ ceramics. All Nd 10 Si 5 B O 27− δ ceramics consist of a hexagonal apatite structure with a space group P 63/ m and a small amount of second phase Nd 2 SiO 5 . Neodymium silicates doped with Mg 2+ or Al 3+ cations at the Si site have an enhanced total conductivity as contrasted with undoped Nd 10 Si 6 O 27 ceramic at all temperature levels. However, doping with Fe 3+ cations at the Si site has a little effect on improving the total conductivity above 873 K. The enhanced oxide-ion conductivity in a hexagonal apatite-type structure depends upon the diffusion of interstitial oxide-ion through oxygen vacancies induced by the Mg 2+ or Al 3+ substitution to the Si 4+ site and through the channels between the SiO 4 tetrahedron and Nd 3+ cations. At 773 K, the highest total conductivity is 4.19×10 −5 S cm −1 for Nd 10 Si 5 MgO 26 ceramic. At 1073 K, Nd 10 Si 5 AlO 26.5 silicate has a total conductivity of 1.55×10 −3 S cm −1 , which is two orders of magnitude higher than that of undoped Nd 10 Si 6 O 27 .

High-performance Gd0.2Ce0.8O2-impregnated LaNi0.6Fe0.4O3−δ cathodes for intermediate temperature solid oxide fuel cell

High-performance LaNi0.6Fe0.4O3−δ (LNF) cathode for intermediate temperature solid oxide fuel cell is fabricated by the ion impregnation of oxygen ion conducting Gd-doped ceria (GDC, Gd0.2Ce0.8O2). The microstructure of the GDC-impregnated LNF cathode is characterized by the dispersion of nano-sized GDC particles in the LNF porous framework. The particle size of the impregnated GDC phase is in the range of 40–50nm. Uniform distribution of the nano-sized ionic conducting GDC phase significantly enhances the cathode activities for the O2 reduction reaction. In the case of the 21.3wt.% GDC-impregnated LNF cathode, the specific polarization resistance (Rp) is 0.13Ωcm2 at 750°C, a reduction by 81.43% compared with that of the pure LNF cathode at the same temperature. The results indicate that GDC impregnation has significant electrocatalytic effect on the O2 reduction reactions on the LNF cathodes. The improved performance of GDC-impregnated LNF cathode is attributed to the extended triple-phase boundary (TPB) and enhanced oxide ion conductivity.

Electrochemical Properties of Ln(Sr,Ca)3(Fe,Co)3O10 + Gd0.2Ce0.8O1.9 Composite Cathodes for Solid Oxide Fuel Cells

Ln(Sr,Ca)3(Fe,Co)3O10 (Ln = La and Nd) + Gd0.2Ce0.8O1.9 (GDC) composites have been investigated as cathodes for intermediate temperature solid oxide fuel cells (IT-SOFC). The effects of GDC incorporation on the thermal and electrical properties, phase stability, and catalytic activity for the oxygen reduction reaction (ORR) of the Ln(Sr,Ca)3(Fe,Co)3O10 intergrowth oxide have been studied. The Ln(Sr,Ca)3(Fe,Co)3O10 + GDC (50:50 vol %) composite cathode exhibits lower thermal expansion coefficients (TEC) of ∼ 16 × 10−6°C−1 in the range of 80–900°C as well as lower polarization resistance compared to those of the pure Ln(Sr,Ca)3(Fe,Co)3O10 cathodes. The NdSr2.5Ca0.5Fe1.5Co1.5O10 + GDC composite cathode shows the lowest polarization resistance (0.15 Ω cm2 at 750°C) and best electrochemical performance with maximum power densities of 615 mW cm−2 at 800°C (electrolyte-supported single cell with 500 μm thick LSGM electrolyte), and 357 mW cm−2 at 700°C (anode-supported single cell with 20 μm thick GDC electrolyte). The improved performance of NdSr2.5Ca0.5Fe1.5Co1.5O10 + GDC is attributed to the extended triple-phase boundary (TPB), enhanced oxide ion conductivity, and decreased TEC that provides good compatibility with those of standard SOFC electrolytes.

High temperature phase stabilities and electrochemical properties of InBaCo 4− xZn xO 7 cathodes for intermediate temperature solid oxide fuel cells

InBaCo 4− x Zn x O 7 oxides have been synthesized and characterized as cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFC). The effect of Zn substitution for Co on the structure, phase stability, thermal expansion, and electrochemical properties of the InBaCo 4− x Zn x O 7 has been investigated. The increase in the Zn content from x = 1 to 1.5 improves the high temperature phase stability at 600 °C and 700 °C for 100 h, and chemical stability against a Gd 0.2Ce 0.8O 1.9 (GDC) electrolyte. Thermal expansion coefficient (TEC) values of the InBaCo 4− x Zn x O 7 ( x = 1, 1.5, 2) specimens were determined to be 8.6 × 10 −6 to 9.6 × 10 −6/°C in the range of 80–900 °C, which provides good thermal expansion compatibility with the standard SOFC electrolyte materials. The InBaCo 4− x Zn x O 7 + GDC (50:50 wt.%) composite cathodes exhibit improved cathode performances compared to those obtained from the simple InBaCo 4− x Zn x O 7 cathodes due to the extended triple-phase boundary (TPB) and enhanced oxide-ion conductivity through the GDC portion in the composites.

The structure and ionic conductivity of the fluorite-related isostructural materials Bi 20Ca 7NbO 39.5, Bi 10.75Ca 4.375GaO 22 and quenched Bi 9ReO 17

The structure and ionic conductivity of fluorite-related Bi 20Ca 7NbO 39.5, Bi 10.75Ca 4.375GaO 22 and the high temperature form of Bi 9ReO 17, formed by quenching from 800 °C, were studied by neutron powder diffraction, X-ray powder diffraction and impedance spectroscopy. All materials formed distorted δ-Bi 2O 3–related monoclinic superstructures of a fluorite-related hexagonal subcell. The supercell, with P2 1/ m symmetry, is derived from the cubic fluorite subcell axes ( a f, b f, c f) using the transformation matrix: − 1 1 1 0.5 0.5 0 1 − 1 1 . Both Bi 20Ca 7NbO 39.5 and Bi 10.75Ca 4.375GaO 22 are shown to display good oxide ion conductivity (2.52 × 10 − 5 Ω − 1 cm − 1 and 1.02 × 10 − 5 Ω − 1 cm − 1 at 673 K with activation energies of 1.12 eV and 1.25 eV, respectively); quenched Bi 9ReO 17 has enhanced oxide ion conductivity (1.44 × 10 − 3 Ω − 1 cm − 1 at 673 K) with a lower activation energy of 0.76 eV.

Enhanced Oxide Ion Conductivity in Stabilized δ‐Bi2O3.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.