Oxide Ion Conductivity Research Articles

Preparation and Properties of Ce0.8Sm0.16Y0.03Gd0.01O1.9-BaIn0.3Ti0.7O2.85 Composite Electrolyte

Samarium, gadolinium, and yttrium co-doped ceria (Ce0.8Sm0.16Y0.03Gd0.01O1.9, CSYG) and BaIn0.3Ti0.7O2.85 (BIT07) powders were prepared by sol-gel and solid-state reaction methods, respectively. CSYG-BIT07 composite materials were obtained by mechanically mixing the two powders in different ratios and calcining at 1300 °C for 5 h. Samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), as well as electrical properties and thermal expansion coefficient (TEC) measurements. A series of CSYG-BIT07 composite materials with relative densities higher than 95% were fabricated by sintering at 1300 °C for 5 h. The performance of the CSYG-BIT07 composite electrolyte was found to be related to the content of BIT07. The CSYG-15% BIT07 composite exhibited high oxide ion conductivity (σ800°C = 0.0126 S·cm−1 at 800 °C), moderate thermal expansion (TEC = 9.13 × 10−6/K between room temperature and 800 °C), and low electrical activation energy (Ea = 0.89 eV). These preliminary results indicate that the CSYG-BIT07 material is a promising electrolyte for intermediate-temperature solid oxide fuel cells (IT-SOFCs).

Oxide ion conduction and transporting mechanism in the layered perovskite-related material Sr2Nb2O7

Ionic conduction has been well-documented in three typical layered perovskite materials, namely Ruddlesden-Popper-type, Dion-Jacobson-type, and Aurivillius-type. In the present work, the oxide ion conduction was reported in the Sr2Nb2O7 material which also adopts a layered perovskite-related structure but does not fall into any of the above-mentioned three typical structures. The results revealed that the oxide ion conduction originated from the slight reduction of Nb5+ ions during the high-temperature synthesis process, as the sample prepared under a pure oxygen atmosphere showed more than one order of magnitude lower conductivity than the sample prepared under an ambient atmosphere. The bond-valence-based method was applied to investigate the oxide ion migration mechanism, and revealed a two-dimensional pathway within the perovskite slabs. The discovery of this new oxide ion conductor will stimulate extensive exploitation and fundamental research on other layered perovskite-related materials.

Electrochemical Impedance Spectroscopy for the determination of oxide ion conductivity

The properties of oxide ion conductors depend upon the structure, compositions, nature of interface, dopant and defects dissemination. The electrochemical impedance spectroscopy (EIS) is emerged as strong tool for disentangling the intricacies of oxide ion conductors, and their capacities by using the different frequencies reliance for the isolations into their constituent’s parts. Consequently, the electrical homogeneities in oxide ion conductors used for SOFC applications like anode/electrolyte/cathode, grain, grain boundary (GB) conductivity, area specific resistance of fuel cell and temperature coefficient of resistance can generally be examined effectively by utilizing the EIS.

Electrochemical Impedance Spectroscopy for the determination of oxide ion conductivity

The properties of oxide ion conductors depend upon the structure, compositions, nature of interface, dopant and defects dissemination. The electrochemical impedance spectroscopy (EIS) is emerged as strong tool for disentangling the intricacies of oxide ion conductors, and their capacities by using the different frequencies reliance for the isolations into their constituent’s parts. Consequently, the electrical homogeneities in oxide ion conductors used for SOFC applications like anode/electrolyte/cathode, grain, grain boundary (GB) conductivity, area specific resistance of fuel cell and temperature coefficient of resistance can generally be examined effectively by utilizing the EIS.

Ge-Containing Oxide-Ion Conductors with CaEu2Ge3O10-Type Structure Discovered by the Bond-Valence Method and Experiments.

In the present work, we have discovered the first example of a CaEu2Ge3O10-type oxide-ion conductor, Ca1.05Sm1.95Ge3O9.975. The CaEu2Ge3O10-type structure was selected by screening 624 Ge-containing materials by the bond-valence-based-energy calculations. CaEu2Ge3O10-type CaEu2Ge3O10, CaGd2Ge3O10, and a new material CaSm2Ge3O10 were synthesized. CaSm2Ge3O10 showed the highest electrical conductivity among these three materials. Ca1+xSm2-xGe3O10-x/2 (x = 0.05, 0.1, and 0.2) were also synthesized, and we found that Ca1.05Sm1.95Ge3O9.975 exhibited the highest conductivity of 1.2 × 10-5 S cm-1 at 1373 K. Oxygen transport numbers in Ca1.05Sm1.95Ge3O9.975 were determined to be 0.64(5) at 1073 K and 0.65(8) at 1123 K, which indicates that the major carrier is the oxide ion. Therefore, CaEu2Ge3O10-type Ca1.05Sm1.95Ge3O9.975 is a new structure family of oxide-ion conductors. The crystal structures of the new materials CaSm2Ge3O10 and Ca1.05Sm1.95Ge3O9.975 were successfully analyzed by the CaEu2Ge3O10-type structure (space group P21/c) using the single-crystal X-ray diffraction data. The bond-valence-based-energy calculation for the refined crystal structure of Ca1.05Sm1.95Ge3O9.975 suggested that oxide ions migrate along the [2 0 1], [0 1 0], and [12.88 6.43 1] directions with energy barriers of 0.88, 0.92, and 1.1 eV, respectively, which indicates three-dimensional oxide-ion diffusion in Ca1.05Sm1.95Ge3O9.975.

Pyrochlore-like ZrO2-PrOx compounds: The role of the processing atmosphere in the stoichiometry, microstructure and oxidation state

The object of this work is to study the relation between composition, microstructure and oxidation state of Pr2±xZr2∓xO7±y materials produced by the laser–floating zone (LFZ) technique. Three compositions are studied, nominally Pr1.7Zr2.3O7+y, Pr2Zr2O7+y and Pr2.24Zr1.76O7±y, all within the pyrochlore field in the ZrO2–PrOx phase diagram. Samples have been processed under four different atmospheres (O2, air, N2 and 5%H2(Ar)), so as to vary the environmental conditions from oxidising to reducing. Sample colouration ranged from dark brown to bright green, owing to varying Pr4+ content. A close correlation is found between the phase homogeneity, the microstructure and the Pr content. Pr–deficient samples present a homogeneous microstructural aspect and composition, whereas Pr–rich compositions always break into 5–25 µm–sized grains with pyrochlore phases at the grain centre and ill–crystallised, Pr–rich oxidised phases at the grain–boundaries. Raman spectroscopy shows that different types of oxygen disorder occur depending on composition and processing atmosphere: in Pr–poor samples oxygen interstitials are created to compensate for Zr4+ excess charge, whereas in Pr–rich samples oxygen disorder occurs around the Pr3+ or Pr4+ ions substituting for Zr4+, because of size–mismatch. Magnetic measurements showed a high Pr4+ content, which has been attributed to several factors: the highly oxidised state of the feedstock material, the segregation of Pr and O–rich grain boundaries in compositions with praseodymium molar rate> 0.5, and the lower oxide–ion conductivity for PZO compositions, compared to either Pr–poor or Pr–rich compositions. Post–processing thermal annealing in a vacuum at 1000 °C enabled total Pr reduction, with the exception of the Pr–rich P2.24 samples, where some Pr4+ ions remained in the oxidised state.

Novel molten phase route for composite CO2 separation membranes

Alkali metal nitrates were successfully used as starting chemicals in the production of composite CO2 separation membranes. The easy conversion of nitrates into carbonates after high temperature annealing in CO2 is confirmed using techniques like FTIR (Fourier transform infrared) or selective NO3− electrodes. This conversion shows a step like increase at temperatures in the order of 600 °C, consistent with target utilization temperatures for these membranes. A second but equally relevant consequence of this novel route is the formation of significant concentrations of oxide ions in the molten phase, able to boost the membrane performance, where the ceramic phase modest oxide ion conductivity is rate determining. At 650 °C, a CO2 separation flux enhancement in the order of 60% was reached using nitrates as starting chemicals. A deep analysis of the oxide ion content in molten phases using a variety of analytical techniques, confirmed the specific features of the nitrates as starting chemicals but also showed that even when carbonates are used as precursors, the role of oxide ions in the molten phase can increase significantly the membrane performance.

Boosting the Performance of La0.8Sr0.2MnO3‐δ Electrodes by The Incorporation of Nanocomposite Active Layers

AbstractThe use of active layers is a promising strategy to improve the properties of air electrodes for solid oxide fuel cells (SOFCs). In this work, La0.8Sr0.2MnO3‐δ is combined with different oxide ion conductors Ce0.9Gd0.1O1.95, Bi1.5Y0.5O3 and Pr6O11 to form highly efficient nanocomposite active layers in a single step by spray‐pyrolysis deposition. One of the main advantages of these nanocomposite layers is that the nanoscale microstructure is retained at relatively high sintering temperatures. As a consequence, the interfacial area between the electrode and the electrolyte increases significantly, leading to fast oxide ion transport at the interface, as well as more active sites for the electrochemical reactions. An exhaustive structural, microstructural, and electrochemical characterization of the layers are performed to evaluate their potential use in SOFCs. The incorporation of a La0.8Sr0.2MnO3‐δ‐Ce0.9Gd0.1O1.95 (LSM‐CGO) interlayer decreases the polarization resistance of LSM cathode from 1.71 to 0.46 Ω cm2 at 700 °C. Moreover, a single cell with LSM‐CGO interlayer generates a peak power density of 1.20 W cm–2 at 800 °C compared to 0.79 W cm–2 for the same cell without active layer. These results demonstrate the great benefits of using nanocomposite layers to improve the properties of air electrodes.

Theoretical and Experimental Studies of Gallate Melilite Electrides from Topotactic Reduction of Interstitial Oxide Ion Conductors.

A nonstoichiometric La1.5Sr0.5Ga3O7.25 melilite oxide ion conductor features active interstitial oxygen defects in its pentagonal rings with high mobility. In this study, electron localization function calculated by density functional theory indicated that the interstitial oxide ions located in the pentagonal rings of gallate melilites may be removed and replaced by electron anions that are confined within the pentagonal rings, which would therefore convert the melilite interstitial oxide ion conductor into a zero-dimensional (0D) electride. The more active interstitial oxide ions, compared to the framework oxide ions, make the La1.5Sr0.5Ga3O7.25 melilite structure more reducible by CaH2 using topotactic reduction, in contrast to the hardly reducible nature of parent LaSrGa3O7. The topotactic reduction enhances the bulk electronic conduction (σ ∼ 0.003 S/cm at 400 °C) by ∼ 1 order of magnitude for La1.5Sr0.5Ga3O7.25. The oxygen loss in the melilite structure was verified and most likely took place on the active interstitial oxide ions. The identified confinement space for electronic anions in melilite interstitial oxide ion conductors presented here provides a strategy to access inorganic electrides from interstitial oxide ion conductor electrolytes.

Improved oxide-ion and lower proton conduction of hexagonal perovskite-related oxides based on Ba<sub>7</sub>Nb<sub>4</sub>MoO<sub>20</sub> by Cr<sup>6+</sup> doping

Improved oxide-ion and lower proton conduction of hexagonal perovskite-related oxides based on Ba<sub>7</sub>Nb<sub>4</sub>MoO<sub>20</sub> by Cr<sup>6+</sup> doping

Borates as a new direction in the design of oxide ion conductors

Lowering the operating temperature of solid oxide fuel cells (SOFCs) has extensively stimulated the development of new oxide ion conductors. Here, inspired by the structural commonalities of oxide ion conductors, the inability to accommodate oxygen vacancies in the rigid, isolated, 3-fold tetrahedral rings of SrSi/GeO3-based materials, and the considerable flexibility of BOn polyhedra in terms of coordination number, rotation, deformation, and linkage, we report the first borate-base family of oxide ion conductors, (Gd/Y)1−xZnxBO3−0.5x, through combined computational prediction and experimental verification. The oxygen vacancies in (Gd/Y)BO3 can be accommodated by forming B3O8 units in isolated, 3-fold, tetrahedral rings of B3O9 and transported through a cooperative mechanism of oxygen exchange between the B3O9 and B3O8 units, which is assisted by the intermediate opening and extending of these units. This study opens a new scientific field of the borate system for designing and discovering oxide ion conductors.

Simultaneous Reduction of Proton Conductivity and Enhancement of Oxide-Ion Conductivity by Aliovalent Doping in Ba7Nb4MoO20.

Hexagonal perovskite-related oxides have garnered a great deal of research interest because of their high oxide-ion conductivity at intermediate temperatures, with Ba7Nb4MoO20 being a notable example. However, concomitant proton conduction in Ba7Nb4MoO20 may cause a decrease in power efficiency when used as the electrolyte in conventional solid oxide fuel cells. Here, through investigations of the transport and structural properties of Ba7Nb4-xWxMoO20+x/2 (x = 0-0.25), we show that the aliovalent substitution of Nb5+ by W6+ not only increases the oxide-ion conductivity but also dramatically lowers proton conductivity. The highest conductivity is achieved for x = 0.15 composition, with 2.2 × 10-2 S cm-1 at 600 °C, 2.2 times higher than that of pristine Ba7Nb4MoO20. The proton transport number of Ba7Nb3.85W0.15MoO20.075 is smaller compared with Ba7Nb4MoO20, Ba7Nb3.9Mo1.1O20.05, and Ba7Ta3.7Mo1.3O20.15. The structure analyses of neutron diffraction data of Ba7Nb3.85W0.15MoO20.075 at 25 and 800 °C reveal that the aliovalent W6+ doping introduces interstitial oxide ions in the intrinsically oxygen-deficient c' layers, thereby simultaneously increasing the carrier concentration for oxide-ion conduction and decreasing oxygen vacancies responsible for dissociative absorption of water. Neutron scattering length density distribution was examined using the maximum-entropy method and neutron diffraction data at 800 °C, which indicates the interstitialcy oxide-ion diffusion in the c' layers of Ba7Nb3.85W0.15MoO20.075. Ba7Nb3.85W0.15MoO20.075 exhibits extremely high chemical and electrical stability in the wide oxygen partial pressure P(O2) region [ex. 10-23 ≤ P(O2) ≤ 1 atm at 903 °C]. The present results offer a strategy for developing pure oxide-ion conducting hexagonal perovskite-related oxides for possible industrial applications.

A‐site acceptor‐doping strategy to enhance oxygen transport in sodium–bismuth–titanate perovskite

AbstractSodium–bismuth–titanate (NBT) has recently been shown to contain high levels of oxide ion conductivity. Here we report the effect of A‐site monovalent ions, M+ = K+ and Li+, on the electrical conductivity of NBT. The partial replacement of Bi3+ with monovalent ions improved the ionic conductivity by over one order of magnitude without an apparent change of the conduction mechanism, which is attributed to an increase in the oxygen vacancy concentration based on an acceptor‐doping approach. The 18O tracer‐diffusion coefficient (D*) determined by the isotope exchange depth profile method in combination with secondary ion mass spectrometry confirmed that oxygen ions are the main charge carriers in the system. Among these acceptor‐doped samples, 4% Li doping provides the highest total conductivity, leading to a further discussion of doping strategies for NBT‐based materials to enhance the electrical behavior, is discussed. Comparisons with other oxide‐ion conductors and an oxygen‐vacancy diffusivity limit model in perovskite lattice suggested that the doped NBT‐based materials might already have achieved the optimization of the ionic conductivity.

Tailoring the electrolyte and cathode properties for optimizing the performance of symmetrical solid oxide fuel cells fabricated by one-step co-sintering method

ABSTRACT Symmetrical solid oxide fuel cells (S-SOFCs) are prepared via a facile one-step co-sintering method with highly reactive Sm0.2Ce0.8O1.9 (SDC) electrolyte and Sr2Fe1.5Mo0.5O6-δ (SFM) electrodes. The phase compositions and microstructures are studied by XRD, TEM, and FESEM techniques, and the electrolyte-supported single cells with gas-tight SDC electrolyte and porous SFM-SDC electrode are prepared after one-step co-sintering at 1100 °C or higher temperatures for 4 h. In particular, the AC impedance spectra reveal that the SDC electrolyte sintered at 1200 °C exhibits the highest oxide ionic conductivity of ~35 mS/cm at 700 °C, while the electrode polarization resistance achieves the lowest value of 0.32 Ω·cm2 at 700 °C in air for the sample co-sintered at 1100 °C. Accordingly, the output measurements are carried out on the single cells co-sintered at 1100 °C and 1200 °C. It is found that the single cell co-sintered at 1200 °C possesses the highest peak power density of 216 mW/cm2 at 700 °C, demonstrating the potential application of the one-step co-sintering method in low-cost manufacturing of S-SOFCs.

Oxide Ion Conductivity, Proton Conductivity, and Phase Transitions in Perovskite-Derived Ba3-x Sr x YGa2O7.5 0 ≤ x ≤ 3 Materials.

We report the synthesis, structural characterization, and oxide ion and proton conductivities of the perovskite-related Ba3–xSrxYGa2O7.5 family. Single-phase samples are prepared for 0 ≤ x ≤ 3 and show a complex structural evolution from P2/c to C2 space groups with an increase in x. For 1.0 ≲ x ≲ 2.4, average structures determined by X-ray and neutron powder diffraction show metrically orthorhombic unit cells, but HAADF-STEM imaging reveals this is caused by microstructural effects due to intergrowths of the Ba- and Sr-rich structure types. Variable-temperature powder diffraction studies suggest that 0 ≲ x ≲ 2.4 compositions undergo a phase transition upon being heated to space group Cmcm that involves disordering of the oxygen substructure. Thermal expansion coefficients are reported for the series. Complex impedance studies show that the Ba-rich samples are mixed proton and oxide ion conductors under moist atmospheres but are predominantly oxide ion conductors at high temperatures or under dry atmospheres. Sr-rich samples show significantly less water uptake and appear to be predominantly oxide ion conductors under the conditions studied.

Mechanically strong and highly ion conductive graphene oxide liquid crystal film containing the poly(amic acid) salt

Mechanically strong and highly ion conductive graphene oxide liquid crystal film containing the poly(amic acid) salt

Contact angle screening and asymmetric dual-phase CO2 separation membranes

Dual-phase CO2 separation membrane consisting of molten carbonates confined in a solid matrix can separate CO2 at high temperatures. The contact angle of molten carbonates to different oxides that can potentially serve as membrane supports was screened between 450 and 650 °C. These oxides have different electrical transport properties, including oxide ion, mixed, and electronic conducting. The contact angles vary between 80° and 10° for different materials. Asymmetric membranes were fabricated using wettable oxide ion conductors BTM and CGO (Bi0.8Tm0.2O1.5 and Ce0.8Gd0.2O2-δ) infiltrated with molten carbonates supported by the most "non-wetting" oxide BPR (Bi0.8Pr0.2O1.5) selected in the contact angle screening. The membranes show CO2 flux in the range of 0.035–0.35 ml/min cm2 at temperatures from 500 to 650 °C. Compared to a symmetric membrane with similar total membrane thickness, the asymmetric architecture significantly reduces the effective membrane thickness and increases CO2 flux. After the CO2 flux measurements, the membrane was examined with SEM and EDS mapping, showing that the molten carbonates were mainly confined within the top membrane and sealing area without penetrating the support layer.

Improved oxide-ion conductivity by substitution of Sr for Bi in Dion-Jacobson phase CsBi2Ti2NbO10

Oxide ion conducting ceramics have attracted much attention due to their various applications such as solid oxide electrolysis cells (SOECs) and solid-oxide fuel cells (SOFCs). Herein, we present the first report on the preparation, electrical and structural properties of oxide-ion conducting Dion-Jacobson phases CsBi1.9M0.1Ti2NbO9.95 (M = Mg, Ca, Sr and Ba). It was found that partial substitution of Bi3+ by Sr2+ improves the conductivity of CsBi2Ti2NbO10. The electrical conductivity of CsBi1.9Sr0.1Ti2NbO9.95 is approximately 2 times higher than that of CsBi2Ti2NbO10 at 600–900 °C and is 1.65 × 10−2 S cm−1 at 900 °C. The improvement of oxide-ion conductivity in CsBi1.9Sr0.1Ti2NbO9.95 can be attributed to the increase of the carrier (oxygen vacancy) concentration and small size mismatch between the dopant Sr and host Bi cations. The electrical conductivities of CsBi1.9Sr0.1Ti2NbO9.95 are almost constant in the oxygen partial pressure region from 10−25 to 1 atm at 600 °C, indicating the predominant oxide-ion conduction and high chemical and electrical stability. Bond-valence-based energy landscapes of a test oxide ion in CsBi1.9Sr0.1Ti2NbO9.95 suggest that oxide ions migrate along the edges of the octahedra in the inner perovskite layers, leading to two-dimensional oxide-ion diffusion. This study demonstrates the successful improvement of oxide-ion conductivity by the substitution in Dion-Jacobson phase, which would develop the science and technology of Dion-Jacobson type oxide-ion conductors.

Synthesis and characterization of Ga-doped Ba3MoNbO8.5 electrolytes for intermediate temperature-solid oxide fuel cells

Ba3MoNb1-xGaxO8.5-δ (BMNG, 0 ≤ x ≤ 0.2) powders are successfully prepared by sol-gel autoignition method. The effects of acceptor-type Ga3+ doping on Ba3MoNbO8.5 (BMN) are characterized by thermogravimetric analysis, X-ray diffraction, scanning electron microscope, Raman spectroscopy, and electrochemical impedance spectroscopy. All BMNG samples crystallize as a single phase in the R-3m space group and show great phase stability at various environments. Doping a small amount of gallium can effectively improve bulk conductivity and sintering density of BMN. The ionic conductivity of Ba3MoNb0.9Ga0.1O8.5-δ (BMNG10) is the highest, which can reach 2.05 × 10−2 S cm−1 at 800 °C. The enhanced ionic conductivity is primarily related to the increase of oxygen vacancy concentration and the number of tetrahedral units within the structure. In addition, through successfully assembling and evaluating a single cell supported by the BMNG10 electrolyte, it is proved that the practical utilization of BMNG10 in intermediate temperature-solid oxide fuel cells (IT-SOFCs) is feasible. In short, hexagonal perovskite derivative BMNG10 is a promising oxide ion conductor for IT-SOFCs.

On the mechanism of Mn(II)-doping in Scandia stabilized zirconia electrolytes

Cubic Scandia-stabilized zirconia (ScSZ) is an attractive electrolyte material for solid oxide cells due to its significant ionic conductivity, provided that the phase transition to its rhombohedral polymorph upon cooling is suppressed. The latter is achieved with addition of a secondary co-dopant, albeit it may be at the detriment of its ionic conductivity Here, we thoroughly investigate how MnO2 (0.5–10 mol%) as a co-dopant impacts on the sinterability, thermal expansion, crystal structure and ionic conductivity of ZrO2 doped with 10 mol% Scandia (10ScSZ), and we provide new insight on the chemistry of dissolved manganese in the fluorite lattice. Reactive sintering of 2 mol% MnO2 mixed with 10ScSZ enables to produce dense electrolyte with significant reduction of the peak sintering temperature and stabilisation of the cubic structure down to room temperature. Combined density functional theory and X-ray photoelectron spectroscopy analyses reveal that manganese predominantly enters the structure as Mn2+ during reactive sintering, with a prevalence of higher valence states at the surface and grain boundaries. The highest oxide ion conductivity is achieved for 2 mol% doped 10ScSZ (120 mScm−1 at 800 °C) and it decreases with increasing Mn concentration. For all compositions, the bulk conductivity remains independent of pO2 – corroborating a limited electronic conductivity contribution from Mn-doping. The grain boundary conductivity is found to decrease with sintering time and pO2, which is attributed to the chemistry and concentration of segregated manganese at the surface and grain boundaries, yielding depletion of oxygen vacancies in the space charge layer.