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

Computational discovery of fast interstitial oxygen conductors

New highly oxygen-active materials may enhance many energy-related technologies by enabling efficient oxygen-ion transport at lower temperatures, for example, below ~400 °C. Interstitial oxygen conductors have the potential to realize such performance but have received far less attention than vacancy-mediated conductors. Here we combine physically motivated structure and property descriptors, ab initio simulations and experiments to demonstrate an approach to discover new fast interstitial oxygen conductors. Multiple new families were found, which adopt completely different structures from known oxygen conductors. From these families, we synthesized and studied oxygen kinetics in La4Mn5Si4O22+δ, a representative member of the perrierite/chevkinite family. We found that La4Mn5Si4O22+δ has higher oxygen-ion conductivity than the widely used yttria-stabilized ZrO2, and among the highest surface oxygen exchange rates at the intermediate temperature of known materials. The fast oxygen kinetics is the result of simultaneously active interstitial and interstitialcy diffusion pathways. We propose that the essential features for forming an effective interstitial oxygen conductor are the availability of electrons and structural flexibility, enabling a sufficient accessible volume. This work provides a powerful approach for understanding and discovering new interstitial oxygen conductors.

Cation Distribution and Anion Transport in the La3Ga5-xGe1+xO14+0.5x Langasite Structure

Exploration of compositional disorder using conventional diffraction-based techniques is challenging for systems containing isoelectronic ions possessing similar coherent neutron scattering lengths. Here, we show that a multinuclear solid-state Nuclear Magnetic Resonance (NMR) approach provides compelling insight into the Ga3+/Ge4+ cation distribution and oxygen anion transport in a family of solid electrolytes with langasite structure and La3Ga5-xGe1+xO14+0.5x composition. Ultrahigh field 71Ga Magic Angle Spinning (MAS) NMR experiments acquired at 35.2 T offer striking resolution enhancement, thereby enabling clear detection of Ga sites in different coordination environments. Three-connected GaO4, four-connected GaO4 and GaO6 polyhedra are probed for the parent La3Ga5GeO14 structure, while one additional spectral feature corresponding to the key (Ga,Ge)2O8 structural unit which forms to accommodate the interstitial oxide ions is detected for the Ge4+-doped La3Ga3.5Ge2.5O14.75 phase. The complex spectral line shapes observed in the MAS NMR spectra are reproduced very accurately by the NMR parameters computed for a symmetry-adapted configurational ensemble that comprehensively models site disorder. This approach further reveals a Ga3+/Ge4+ distribution across all Ga/Ge sites that is controlled by a kinetically governed cation diffusion process. Variable temperature 17O MAS NMR experiments up to 700 °C importantly indicate that the presence of interstitial oxide ions triggers chemical exchange between all oxygen sites, thereby enabling atomic-scale understanding of the anion diffusion mechanism underpinning the transport properties of these materials.

High Conductivity and Diffusion Mechanism of Oxide Ions in Triple Fluorite-Like Layers of Oxyhalides

Oxide ion conductors are attractive materials because of their wide range of applications, such as solid oxide fuel cells. Oxide ion conduction in oxyhalides (compounds containing both oxide ions and halide ions) is rare. In the present work, we found that Sillén oxychlorides, Bi2-xTexLuO4+x/2Cl (x = 0, 0.1, and 0.2), show high oxide ion conductivity. The bulk conductivity of Bi1.9Te0.1LuO4.05Cl reaches 10-2 S cm-1 at 431 °C, which is much lower than 644 °C of yttria-stabilized zirconia (YSZ) and 534 °C of La0.8Sr0.2Ga0.83Mg0.17O2.815 (LSGM). Thanks to the low activation energy, Bi1.9Te0.1LuO4.05Cl exhibits a high bulk conductivity of 1.5 × 10-3 S cm-1 even at a low temperature of 310 °C, which is 204 times higher than that of YSZ. The low activation energy is attributed to the interstitialcy oxide ion diffusion in the triple fluorite-like layer, as evidenced by neutron diffraction experiments (Rietveld and neutron scattering length density analyses), bond valence-based energy calculations, static DFT calculations, and ab initio molecular dynamics simulations. The electrical conductivity of Bi1.9Te0.1LuO4.05Cl is almost independent of the oxygen partial pressure from 10-18 to 10-4 atm at 431 °C, indicating the electrolyte domain. Bi1.9Te0.1LuO4.05Cl also exhibits high chemical stability under a CO2 flow and ambient air at 400 °C. The oxide ion conduction due to the two-dimensional interstitialcy diffusion is considered to be common in Sillén oxyhalides with triple fluorite-like layers, such as Bi1.9Te0.1RO4.05Cl (R = La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu) and Bi6-2xTe2xO8+xBr2 (x = 0.1, 0.5). The present study opens a new field of materials chemistry: oxide ion-conducting Sillén oxyhalides with triple fluorite-like layers.

Novel NASICON-Type Na-V-Mn-Ni-Containing Cathodes for High-Rate and Long-Life SIBs

Partial substitution of V by other transition metals in Na3 V2 (PO4 )3 (NVP) can improve the electrochemical performance of NVP as a cathode for sodium-ion batteries (SIBs). Herein, phosphate Na-V-Mn-Ni-containing composites based on NASICON (Natrium Super Ionic Conductor)-type structure have been fabricated by sol-gel method. The synchrotron-based X-ray study, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) studies show that manganese/nickel combinations successfully substitute the vanadium in its site within certain limits. Among the received samples, composite based on Na3.83 V1.17 Mn0.58 Ni0.25 (PO4 )3 (VMN-0.5, 108.1 mAh g-1 at 0.2 C) shows the highest electrochemical ability. The cyclic voltammetry, galvanostatic intermittent titration technique, in situ XRD, ex situ XPS, and bond valence site energy calculations exhibit the kinetic properties and the sodium storage mechanism of VMN-0.5. Moreover, VMN-0.5 electrode also exhibits excellent electrochemical performance in quasi-solid-state sodium metal batteries with PVDF-HFP quasi-solid electrolyte membranes. The presented work analyzes the advantages of VMN-0.5 and the nature of the substituted metal in relation to the electrochemical properties of the NASICON-type structure, which will facilitate further commercialization of SIBs.

High proton conductivity within the 'Norby gap' by stabilizing a perovskite with disordered intrinsic oxygen vacancies

Proton conductors are attractive materials with a wide range of potential applications such as proton-conducting fuel cells (PCFCs). The conventional strategy to enhance the proton conductivity is acceptor doping into oxides without oxygen vacancies. However, the acceptor doping results in proton trapping near dopants, leading to the high apparent activation energy and low proton conductivity at intermediate and low temperatures. The hypothetical cubic perovskite BaScO2.5 may have intrinsic oxygen vacancies without the acceptor doping. Herein, we report that the cubic perovskite-type BaSc0.8Mo0.2O2.8 stabilized by Mo donor-doing into BaScO2.5 exhibits high proton conductivity within the 'Norby gap' (e.g., 0.01 S cm-1 at 320 °C) and high chemical stability under oxidizing, reducing and CO2 atmospheres. The high proton conductivity of BaSc0.8Mo0.2O2.8 at intermediate and low temperatures is attributable to high proton concentration, high proton mobility due to reduced proton trapping, and three-dimensional proton diffusion in the cubic perovskite stabilized by the Mo-doping into BaScO2.5. The donor doping into the perovskite with disordered intrinsic oxygen vacancies would be a viable strategy towards high proton conductivity at intermediate and low temperatures.

Disorder and Oxide Ion Diffusion Mechanism in La1.54Sr0.46Ga3O7.27 Melilite from Nuclear Magnetic Resonance

Layered tetrahedral network melilite is a promising structural family of fast ion conductors that exhibits the flexibility required to accommodate interstitial oxide anions, leading to excellent ionic transport properties at moderate temperatures. Here, we present a combined experimental and computational magic angle spinning (MAS) nuclear magnetic resonance (NMR) approach which aims at elucidating the local configurational disorder and oxide ion diffusion mechanism in a key member of this structural family possessing the La1.54Sr0.46Ga3O7.27 composition. 17O and 71Ga MAS NMR spectra display complex spectral line shapes that could be accurately predicted using a computational ensemble-based approach to model site disorder across multiple cationic and anionic sites, thereby enabling the assignment of bridging/nonbridging oxygens and the identification of distinct gallium coordination environments. The 17O and 71Ga MAS NMR spectra of La1.54Sr0.46Ga3O7.27 display additional features not observed for the parent LaSrGa3O7 phase which are attributed to interstitial oxide ions incorporated upon cation doping and stabilized by the formation of five-coordinate Ga centers conferring framework flexibility. 17O high-temperature (HT) MAS NMR experiments capture exchange within the bridging oxygens at 130 °C and reveal coalescence of all oxygen signals in La1.54Sr0.46Ga3O7.27 at approximately 300 °C, indicative of the participation of both interstitial and framework oxide ions in the transport process. These results further supported by the coalescence of the 71Ga resonances in the 71Ga HT MAS NMR spectra of La1.54Sr0.46Ga3O7.27 unequivocally provide evidence of the conduction mechanism in this melilite phase and highlight the potential of MAS NMR spectroscopy to enhance the understanding of ionic motion in solid electrolytes.

In Situ Formation of Ba3CoNb2O9/Ba5Nb4O15 Heterostructure in Electrolytes for Enhancing Proton Conductivity and SOFC Performance

The in situ formation of a heterostructure delivers superior electrochemical properties as compared to the mechanical mixing, which shows great promise for developing new electrolytes for solid oxide fuel cells (SOFCs). Herein, in an SOFC constructed by the Ba5Nb4O15 electrolyte and Ni0.8Co0.15Al0.05LiO2-δ anode, an in situ formation of Ba3CoNb2O9/Ba5Nb4O15 heterostructure is designed by Co-ion diffusion from the anode to the electrolyte during cell operation, resulting in improved ion conductivity and fuel cell performance. An abnormal phenomenon is observed that the SOFC based on the Ba3CoNb2O9/Ba5Nb4O15 electrolyte delivered a peak power density of 703 mW/cm2 at 510 °C, which is higher than that at 550 °C. Characterization in terms of X-ray photoelectron spectroscopy and X-ray diffraction verifies that the operating temperature affected the Co doping concentrations, leading to different conducting behaviors of the heterostructure. Furthermore, it is found that the heterojunction of Ba3CoNb2O9 and Ba5Nb4O15 can restrict the electron migration to avoid current leakage of the cell and simultaneously enhance the proton conductivity. These findings manifest the developed in situ Ba3CoNb2O9/Ba5Nb4O15 heterostructure as a promising electrolyte for SOFCs.

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.

Chemical Compatibility and Electrochemical Performance of Ba7Ta3.7Mo1.3O20.15 Electrolytes for Solid Oxide Fuel Cells

Hexagonal perovskite-related oxides Ba₇Ta_3.7Mo_1.3O_20.15 (BTM) have recently been reported as promising electrolyte materials for intermediate-temperature solid oxide fuel cells (IT-SOFCs). In this work, sintering properties, thermal expansion coefficient, and chemical stability of BTM were studied. In particular, the chemical compatibilities of (La_0.75Sr_0.25)_0.95MnO_3±δ (LSM), La_0.6Sr_0.4CoO₃ (LSC), La_0.6Sr_0.4Co_0.2Fe_0.8O_3+δ (LSCF), PrBaMn₂O_5+δ (PBM), Sr₂Fe_1.5Mo_0.5O_6-δ (SFM), BaCo_0.4Fe_0.4Zr_0.1Y_0.1O_3-δ (BCFZY), and NiO electrode materials with the BTM electrolyte were evaluated. The results show that BTM is highly reactive with these electrodes, in particular, BTM tends to react with Ni, Co, Fe, Mn, Pr, Sr, and La elements in the electrodes to form resistive phases, thus deteriorating the electrochemical properties, which has not been reported before.

Hidden chemical order in disordered Ba7Nb4MoO20 revealed by resonant X-ray diffraction and solid-state NMR

The chemical order and disorder of solids have a decisive influence on the material properties. There are numerous materials exhibiting chemical order/disorder of atoms with similar X-ray atomic scattering factors and similar neutron scattering lengths. It is difficult to investigate such order/disorder hidden in the data obtained from conventional diffraction methods. Herein, we quantitatively determined the Mo/Nb order in the high ion conductor Ba₇Nb₄MoO₂₀ by a technique combining resonant X-ray diffraction, solid-state nuclear magnetic resonance (NMR) and first-principle calculations. NMR provided direct evidence that Mo atoms occupy only the M2 site near the intrinsically oxygen-deficient ion-conducting layer. Resonant X-ray diffraction determined the occupancy factors of Mo atoms at the M2 and other sites to be 0.50 and 0.00, respectively. These findings provide a basis for the development of ion conductors. This combined technique would open a new avenue for in-depth investigation of the hidden chemical order/disorder in materials.

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 CsM₂Ti₂NbO_10-δ (M = Bi and lanthanoids; δ represents oxygen-vacancy content) and their solid solutions. CsBi₂Ti₂NbO_10-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.

Synthesis, Hydration Processes and Ionic Conductivity of Novel Gadolinium-Doped Ceramic Materials Based on Layered Perovskite BaLa2In2O7 for Electrochemical Purposes

The search for novel highly effective materials with target properties for different electrochemical purposes is active for now. Ceramic materials with high levels of ionic conductivity can be applied as electrolytic materials in solid oxide fuel cells and in electrolyzers. Layered perovskites are a novel class of ionic conductors demonstrating almost-pure proton transportation at mid-temperatures. Gadolinium-doped ceramic materials based on layered perovskite BaLa2In2O7 were obtained and investigated for the first time in this study. The effect of the dopant concentrations on the hydration processes and on ionic conductivity was revealed. It was shown that compositions 0 ≤ x ≤ 0.15 of BaLa2–xGdxIn2O7 exhibited proton conductivity when under wet air and at mid-temperatures (lower than ~450 °C). Gadolinium doping led to an increase in the conductivity values up to an order of magnitude of ~0.5. The protonic conductivity of the most conductive composition BaLa1.85Gd0.15In2O7 was 2.7∙10−6 S/cm at 400 °C under wet air. The rare earth doping of layered perovskites is a prospective approach for the design of ceramics for electrochemical devices for energy applications.

Novel Nb5+-doped hexagonal perovskite Ba5In2Al2ZrO13 (structure, hydration, electrical conductivity)

The new phase Ba5In2Al2Zr0.9Nb0.1O13.05 with hexagonal perovskite structure was obtained. The substitution of Zr4+ by smaller Nb5+ was accompanied by the incorporation of the oxygen interstitials and did not lead to a significant change in the lattice parameters. It was established that the investigated sample was capable for water incorporation from the gas phase, the hydration degree value was 0.24 mol H2O. IR-spectroscopy analysis defined the presence of OH−-groups with different thermal stability, which participate in different hydrogen bonds. The new phase Ba5In2Al2Zr0.9Nb0.1O13.05 demonstrates the predominant protonic conductivity at pH2O = 2·10−2 atm and Т600 °C.

Advanced Proton-Conducting Ceramics Based on Layered Perovskite BaLaInO4 for Energy Conversion Technologies and Devices

Production of high efficiency renewable energy source for sustainable global development is an important challenge for humans. Hydrogen energy systems are one of the key elements for the development of sustainable energy future. These systems are eco-friendly and include devices such as protonic ceramic fuel cells, which require advanced proton-conducting materials. In this study, we focused on new ceramics with significantly improved target properties for hydrogen energy purposes. Neodymium-doped phase based on layered perovskite BaLaInO₄ was obtained for the first time. The ability for water intercalation and proton transport was proved. It was shown that the composition BaLa_0.9Nd_0.1InO₄ is the predominant proton conductor below 400 °C under wet air. Moreover, isovalent doping of layered perovskites AA'BO₄ is the promising method for improving transport properties and obtaining novel advanced proton-conducting ceramic materials.

Ion Hopping: Design Principles for Strategies to Improve Ionic Conductivity for Inorganic Solid Electrolytes

Solid electrolytes are considered as an ideal substitution of liquid electrolytes, avoiding the potential hazards of volatilization, flammability, and explosion for liquid electrolyte-based rechargeable batteries. However, there are significant performance gaps to be bridged between solid electrolytes and liquid electrolytes; one with a particular importance is the ionic conductivity which is highly dependent on the material types and structures. In this review, the general physical image of ion hopping in the crystalline structure is revisited, by highlighting two main kernels that impact ion migration: ion hopping pathways and skeletons interaction. The universal strategies to effectively improve ionic conductivity of inorganic solid electrolytes are then systematically summarized: constructing rapid diffusion pathways for mobile ions; and reducing resistance of the surrounding potential field. The scoped strategies offer an exclusive view on the working principle of ion movement regardless of the ion species, thus providing a comprehensive guidance for the future exploitation of solid electrolytes.

Proton and Oxide Ion Conductivity in Palmierite Oxides

Solid proton and oxide ion conductors have key applications in several hydrogen-based and energy-related technologies. Here, we report on the discovery of significant proton and oxide ion conductivity in palmierite oxides A₃V₂O₈ (A = Sr, Ba), which crystallize with a framework of isolated tetrahedral VO₄ units. We show that these systems present prevalent ionic conduction, with a large protonic component under humidified air (t _H ∼ 0.6-0.8) and high protonic mobility. In particular, the proton conductivity of Sr₃V₂O₈ is 1.0 × 10^-4 S cm^-1 at 600 °C, competitive with the best proton conductors constituted by isolated tetrahedral units. Simulations show that the three-dimensional ionic transport is vacancy-driven and facilitated by rotational motion of the VO₄ units, which can stabilize oxygen defects via formation of V₂O₇ dimers. Our findings demonstrate that palmierite oxides are a new promising class of ionic conductors where stabilization of parallel vacancy and interstitial defects can enable high ionic conductivity.

Materials Research Directions Toward a Green Hydrogen Economy: A Review

A constellation of technologies has been researched with an eye toward enabling a hydrogen economy. Within the research fields of hydrogen production, storage, and utilization in fuel cells, various classes of materials have been developed that target higher efficiencies and utility. This Review examines recent progress in these research fields from the years 2011-2021, exploring the most commonly occurring concepts and the materials directions important to each field. Particular attention has been given to catalyst materials that enable the green production of hydrogen from water, chemical and physical storage systems, and materials used in technical capacities within fuel cells. The quantification of publication and materials trends provides a picture of the current state of development within each node of the hydrogen economy.

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, Ca_1.05Sm_1.95Ge₃O_9.975. 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. Ca_1+xSm_2-xGe₃O_10-x/2 (x = 0.05, 0.1, and 0.2) were also synthesized, and we found that Ca_1.05Sm_1.95Ge₃O_9.975 exhibited the highest conductivity of 1.2 × 10^-5 S cm^-1 at 1373 K. Oxygen transport numbers in Ca_1.05Sm_1.95Ge₃O_9.975 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 Ca_1.05Sm_1.95Ge₃O_9.975 is a new structure family of oxide-ion conductors. The crystal structures of the new materials CaSm₂Ge₃O₁₀ and Ca_1.05Sm_1.95Ge₃O_9.975 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 Ca_1.05Sm_1.95Ge₃O_9.975 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 Ca_1.05Sm_1.95Ge₃O_9.975.

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 Ba₇Nb_4-xW_xMoO_20+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 Ba₇Nb_3.85W_0.15MoO_20.075 is smaller compared with Ba₇Nb₄MoO₂₀, Ba₇Nb_3.9Mo_1.1O_20.05, and Ba₇Ta_3.7Mo_1.3O_20.15. The structure analyses of neutron diffraction data of Ba₇Nb_3.85W_0.15MoO_20.075 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 Ba₇Nb_3.85W_0.15MoO_20.075. Ba₇Nb_3.85W_0.15MoO_20.075 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.

High Oxide-Ion Conductivity in a Hexagonal Perovskite-Related Oxide Ba7 Ta3.7 Mo1.3 O20.15 with Cation Site Preference and Interstitial Oxide Ions

Solid oxide-ion conductors are crucial for enabling clean and efficient energy devices such as solid oxide fuel cells. Hexagonal perovskite-related oxides have been placed at the forefront of high-performance oxide-ion conductors, with Ba₇ Nb_{4- x} Mo_{1+ x} O_{20+ x /2} (x = 0-0.1) being an archetypal example. Herein, high oxide-ion conductivity and stability under reducing conditions in Ba₇ Ta_3.7 Mo_1.3 O_20.15 are reported by investigating the solid solutions Ba₇ Ta_{4- x} Mo_{1+ x} O_{20+ x /2} (x = 0.2-0.7). Neutron diffraction indicates a large number of interstitial oxide ions in Ba₇ Ta_3.7 Mo_1.3 O_20.15 , leading to a high level of oxide-ion conductivity (e.g., 1.08 × 10^-3 S cm^-1 at 377 °C). The conductivity of Ba₇ Ta_3.7 Mo_1.3 O_20.15 is higher than that of Ba₇ Nb₄ MoO₂₀ and conventional yttria-stabilized zirconia. In contrast to Ba₇ Nb_{4- x} Mo_{1+ x} O_{20+ x /2} (x = 0-0.1), the oxide-ion conduction in Ba₇ Ta_3.7 Mo_1.3 O_20.15 is dominant even in highly reducing atmospheres (e.g., oxygen partial pressure of 1.6 × 10^-24 atm at 909 °C). From structural analyses of the synchrotron X-ray diffraction data for Ba₇ Ta_3.7 Mo_1.3 O_20.15 , contrasting X-ray scattering powers of Ta⁵⁺ and Mo⁶⁺ allow identification of the preferential occupation of Mo⁶⁺ adjacent to the intrinsically oxygen-deficient layers, as supported by DFT calculations. The high conductivity and chemical and electrical stability in Ba₇ Ta_3.7 Mo_1.3 O_20.15 provide a strategy for the development of solid electrolytes based on hexagonal perovskite-related oxides.

Simultaneous Hetero- and Isovalent Doping as the Strategy for Improving Transport Properties of Proton Conductors Based on BaLaInO4

This work focused on the novel electrochemical energy material with significantly improved electrical properties. The novel complex oxide Ba_1.1La_0.9In_0.5Y_0.5O_3.95 with layered perovskite structure was obtained for the first time. It was proven that the simultaneous introduction of barium and yttrium ions in the structure of BaLaInO₄ leads to the increase in the unit cell volume of up to 4% and water uptake by about three times. The increase in the proton conductivity values was both due to an increase in the proton concentration and their mobility. The sample Ba_1.1La_0.9In_0.5Y_0.5O_3.95 was a nearly pure proton conductor below 400 °C. The co-doping strategy allowed us to increase the protonic conductivity values up to two orders of magnitude and it is the successful method for the design of novel protonic conductors based on the layered perovskites.

Electrical Properties, Defect Structures, and Ionic Conducting Mechanisms in Alkali Tungstate Li2W2O7

Discovery of new high-conductivity solid-state ionic conductors has been a long-lasting interest in the field of solid-state ionics for their important applications in solid-state electrochemical devices. Here, we report the mixed oxide-ion and Li-ion conductions, together with their conducting mechanisms in the Li₂W₂O₇ material with triclinic symmetry. The process for the ionic identity is supported by several electrochemical measurements including electrochemical impedance spectroscopy, DC polarization, oxygen concentration cell, and theoretical analysis of neutron diffraction data and bond-valence-based energy landscape calculations. We show from electrochemical measurements strong evidences of the predominating oxide-ion conducting and minor Li-ion chemistry in Li₂W₂O₇ at high temperatures, while the bond-valence-based energy landscape analysis reveals possible multidimensional ionic migration pathways for both oxide-ions and Li-ions. Thus, the presented results provide fundamental insights into new mixed ionic conduction mechanisms in low-symmetry materials and have implications for discoveries of new ionic conductors in years to come.