Oxide Ion and Proton Conductivity in a Family of Highly Oxygen-Deficient Perovskite Derivatives

Self-assembly of water-filled molecular saddles to generate diverse morphologies and high proton conductivity

The design of single-component organic compounds acting as efficient solid-state proton conduction (SSPC) materials has been gaining significant traction in recent times. Molecular design and controlled self-assembly are critical components in achieving highly efficient SSPC. In this work, we report the design, synthesis, and self-assembly of an organic macrocyclic aza-crown-type compound, P2Mac, which complements synthetic ease with efficient SSPC. P2Mac is derived from the pyridine-2,6-dicarboxamide (PDC) framework and contains polar amide and amine residues in its inner region, while aromatic residues occupy the periphery of the macrocycle. The crystal structure analysis revealed that P2Mac adopts a saddle-shaped geometry. Each P2Mac molecule interacts with one water molecule that is present in its central polar cavity, stabilized by a network of five hydrogen bonds. We could self-assemble P2Mac in a variety of unique, aesthetically pleasing morphologies such as micron-sized octahedra, hexapods, as well as hollow nanoparticles, and microrods. The water-filled polar channels formed through the stacking of P2Mac allow attaining a high proton conductivity value of 21.1 mS cm-1 at 27 °C under a relative humidity (RH) of 95% in the single crystals of P2Mac, while the as-prepared P2Mac pellet sample exhibited about three-orders of magnitude lower conduction under these conditions. The low activation energy of 0.39 eV, calculated from the Arrhenius plot, indicates the presence of the Grotthus proton hopping mechanism in the transport process. This report highlights the pivotal role of molecular design and self-assembly in creating high-performance SSPC organic materials.

MEMS Resonant Beam with Outstanding Uniformity of Sensitivity and Temperature Distribution for Accurate Gas Sensing and On-Chip TGA

Micromechanical resonators have aroused growing interest as biological and chemical sensors, and microcantilever beams are the main research focus. Recently, a resonant microcantilever with an integrated heater has been applied in on-chip thermogravimetric analysis (TGA). However, there is a strong relationship between the mass sensitivity of a resonant microcantilever and the location of adsorbed masses. Different sampling positions will cause sensitivity differences, which will result in an inaccurate calculation of mass change. Herein, an integrated H-shaped resonant beam with uniform mass sensitivity and temperature distribution is proposed and developed to improve the accuracy of bio/chemical sensing and TGA applications. Experiments verified that the presented resonant beam possesses much better uniformity of sensitivity and temperature distribution compared with resonant microcantilevers. Gas-sensing and TGA experiments utilizing the integrated resonant beam were also carried out and exhibited good measurement accuracy.

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.

Perovskite-related structures of Ba2YAlO5 and the β and α phases of Ba6Y2Al4O15 containing AlO4 tetrahedra

Single crystals of Ba2YAlO5 and of the α and β phases of Ba6Y2Al4O15 suitable for X-ray structure analysis were obtained via grain growth of polycrystalline samples prepared by solid-state reactions. Ba2YAlO5 was found to have a monoclinic crystal structure, with lattice parameters a = 7.2333 (7), b = 6.0254 (5), c = 7.4294 (7) Å and β = 117.249 (3)°, and to belong to the space group P21/m, while α-Ba6Y2Al4O15 was determined to be monoclinic, with a = 5.9019 (2), b = 7.8744 (3), c = 9.6538 (3) Å and β = 107.7940 (10)°, and the space group Pm, and β-Ba6Y2Al4O15 was found to be monoclinic, with a = 7.8310 (2), b = 5.8990 (2), c = 18.3344 (6) Å and β = 91.6065 (11)°, and the space group P2/c. In each of these compounds, BO6 octahedra in ABO3 perovskite-type structures were replaced by AlO4 tetrahedra and YO6 octahedra. Polycrystalline samples in which some Y atoms were replaced with Eu exhibited orange-red luminescence in the range 580-730 nm in response to exposure to radiation having a wavelength of approximately 250 nm.

New Triclinic Perovskite-Type Oxide Ba5CaFe4O12 for Low-Temperature Operated Chemical Looping Air Separation

We present the discovery of Ba5CaFe4O12, a new iron-based oxide with remarkable properties as a low-temperature driven oxygen storage material (OSM). OSMs, which exhibit selective and rapid oxygen intake and release capabilities, have attracted considerable attention in chemical looping technologies. Specifically, chemical looping air separation (CLAS) has the potential to revolutionize oxygen production as it is one of the most crucial industrial gases. However, the challenge lies in utilizing OSMs for energy-efficient CLAS at lower temperatures. Ba5CaFe4O12, a cost-competitive material, possesses an unprecedented 5-fold perovskite-type A5B5O15-δ structure, where both Fe and Ca occupy the B sites. This distinctive structure enables excellent oxygen intake/release properties below 400 °C. This oxide demonstrates the theoretical daily oxygen production rate of 2.41 mO23 kgOSM-1 at 370 °C, surpassing the performance of the previously reported material, Sr0.76Ca0.24FeO3-δ (0.81 mO23 kgOSM-1 at 550 °C). This discovery holds great potential for reducing costs and enhancing the energy efficiency in CLAS.

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.

An Engineering Method for Resonant Microcantilever Using Double-Channel Excitation and Signal Acquisition Based on LabVIEW

Resonant microcantilevers have the advantages of ultra-high heating rates, analysis speed, ultra-low power consumption, temperature programming, and trace sample analysis when applied in TGA. However, the current single-channel testing system for resonant microcantilevers can only detect one sample at a time, and need two program heating tests to obtain the thermogravimetric curve of a sample. In many cases, it is desirable to obtain the thermogravimetric curve of a sample with a single-program heating test and to simultaneously detect multiple microcantilevers for testing multiple samples. To address this issue, this paper proposes a dual-channel testing method, where a microcantilever is used as a control group and another microcantilever is used as an experimental group, to obtain the thermal weight curve of the sample in a single program temperature ramp test. With the help of the LabVIEW's convenient parallel running method, the functionality of simultaneously detecting two microcantilevers is achieved. Experimental validation showed that this dual-channel testing system can obtain the thermogravimetric curve of a sample with a single program heating test and detect two types of samples simultaneously.

Temperature-driven order-disorder structural transition in the oxygen sub-lattice and the complex superstructure of the high-temperature polymorph of CaSrZn2Ga2O7

Structural order-disorder plays a decisive role in the physical properties of materials, such as magnetism, second-order harmonic generation, and ionic conductivity, and it is thus widely utilized to manipulate the crystal structure and understand structure-property correlations. Herein, we report the structural polymorphism, complex crystal structure and temperature-driven irreversible order-disorder phase transition of the polar oxides (Sr_1-xCa_x)SrZn₂Ga₂O₇. The low-temperature (LT) structure crystallizes in Pna2₁ with partial Zn/Ga ordering. Upon heating, (Sr_1-xCa_x)SrZn₂Ga₂O₇ undergoes an irreversible phase transition from orthorhombic Pna2₁ to hexagonal P6₃. Interestingly, the high-temperature (HT) P6₃ structure possesses an unexpected 3/2-fold superstructure rather than a substructure of the low-temperature (LT) Pna2₁ structure, which is a rare structural phenomenon in solid-state chemistry. This new HT superstructure is the most complex one in this series of oxides with 21 crystallographically independent sites determined accurately by a combination of the maximum entropy method and Rietveld refinement against high-resolution neutron powder diffraction data. In terms of the mechanism, this is a temperature-driven order-to-disorder transition in the oxygen sublattice. A careful structural analysis revealed that the oxygen disordering mainly occurs in the [SrO₃] layers of the HT structure and it can be understood as respective clockwise and anticlockwise rotations of distinct GaO₄-tetrahedra along the c-axis. Alternating current electrochemical impedance spectroscopic analysis revealed that the oxygen disordering in the HT structure is incapable of giving rise to oxide ionic conductivity but does lead to increased electronic conduction compared to the LT structure. The optical properties of the CaSrZn₂Ga₂O₇ and Sr₂Zn₂Ga₂O₇ representatives are also investigated in-depth via diffuse reflectance spectroscopy and theoretic calculations.

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.

Thermogravimetric Analysis on a Resonant Microcantilever

Thermogravimetric analysis (TGA) is a widely applied classic method for material characterization. However, the existing TGA method has reached its technical ceiling in regard to its sensitivity, efficiency, application scope, and extensibility. With temperature-programming and a picogram (10^-12 g) mass resoluble measurement function, an integrated resonant microcantilever is proposed and developed into micro-electromechanical system-based TGA (MEMS TGA) technology to satisfy the significantly higher TGA requirements. With only a nanogram (10^-9 g) level of a loaded sample, the microcantilever can conduct ultrasensitive mass-loss analysis along with ultrafast and controllable heating up to 1200 °C. Experiments have verified that MEMS TGA can improve heating efficiency by at least one order of magnitude compared to conventional TGA while maintaining measurement accuracy. The trace sample requirement also enables MEMS TGA to directly test explosive substances in an air atmosphere, while conventional TGA with a larger amount of sample has difficulty in avoiding explosion-induced equipment damage during heating. The silicon MEMS TGA microchip also exhibits functional combination (even integration) with other analytical techniques such as Raman spectroscopy to realize operando characterization.

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 Ba_3-x Sr _x YGa₂O_7.5 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.