Thermodynamics and kinetics analyses of high CO2 absorption properties of Li3NaSiO4 under various CO2 partial pressures.

The properties of Li3NaSiO4 as a CO2 absorbent were evaluated by thermogravimetry (TG) and X-ray diffraction, which revealed that the CO2 absorption and desorption reaction, Li3NaSiO4 + CO2 ↔ LiNaCO3 + Li2SiO3, is reversible. In scanning-type TG under various CO2 partial pressures (P(CO2)), mass gain ascribed to CO2 absorption started from approximately 400 °C, irrespective of P(CO2). CO2 desorption was observed at a higher temperature, which was considered the approximate equilibrium temperature of the above-mentioned reaction. A pseudo-Ellingham diagram of the reaction between Li3NaSiO4 and CO2, constructed using the obtained approximate equilibrium temperature under each P(CO2), showed similar behavior to that between Li4SiO4 and CO2, especially between 10-2 and 10-1 bar of P(CO2). Kinetics analysis by isothermal TG using the Jander model revealed that the reaction rate of CO2 absorption of Li3NaSiO4 was higher than that of Li4SiO4. The rate-determining step of CO2 absorption by Li3NaSiO4 was the diffusion process at the examined temperatures and P(CO2), which was different from the rate-determining step of the reaction between Li4SiO4 and CO2 under low P(CO2).

Thermodynamic analyses of the orthorhombic-to-tetragonal phase transition in Pr2-xNdxNiO4+δ under controlled oxygen partial pressures.

The behavior of the structural orthorhombic-tetragonal phase transition of Pr2-xNdxNiO4+δ, a candidate material for solid oxide fuel cells and oxygen permeation membranes, was investigated by differential scanning calorimetry and thermogravimetry (TG), under controlled oxygen partial pressures, P(O2). The structural phase transition temperature, TP, of Pr2-xNdxNiO4+δ increased with increasing Nd content, x, or increasing P(O2). The phase transitions of all compositions involved discrete variations in the oxygen content, Δδ, which were observed in the TG curves under various P(O2) values. Δδ of Pr2-xNdxNiO4+δ with 0.5 ≤x≤ 1.5 were between those of Nd2NiO4+δ and Pr2NiO4+δ, regardless of P(O2), and were slightly increased with decreasing P(O2). It was proposed that the effect of the valence change of the Pr ion on Δδ was decreased with increasing Nd content. The standard enthalpy change, ΔH°, and entropy change, ΔS°, of the phase transition were estimated from the Ellingham diagrams and van't Hoff plots, which were prepared from the relationship between P(O2) and TP using an ideal solution model. ΔS° was decreased with increasing Nd content for the specimens with 0.0 ≤x≤ 1.5. The ΔH° of Pr2-xNdxNiO4+δ with 0.0 ≤x≤ 1.5 was almost constant for all Nd contents. The increase in the phase transition temperature of Pr2-xNdxNiO4+δ with increasing x from 0.0 to 1.5 was successfully explained using the calculated values of ΔH° and ΔS°.

Oxide-ion conduction in the Dion-Jacobson phase CsBi2Ti2NbO10-δ.

Oxide-ion conductors have found applications in various electrochemical devices, such as solid-oxide fuel cells, gas sensors, and separation membranes. Dion-Jacobson phases are known for their rich magnetic and electrical properties; however, there have been no reports on oxide-ion conduction in this family of materials. Here, for the first time to the best of our knowledge, we show the observation of fast oxygen anionic conducting behavior in CsBi2Ti2NbO10-δ. The bulk ionic conductivity of this Dion-Jacobson phase is 8.9 × 10-2 S cm-1 at 1073 K, a level that is higher than that of the conventional yttria-stabilized zirconia. The oxygen ion transport is attributable to the large anisotropic thermal motions of oxygen atoms, the presence of oxygen vacancies, and the formation of oxide-ion conducting layers in the crystal structure. The present finding of high oxide-ion conductivity in rare-earth-free CsBi2Ti2NbO10-δ suggests the potential of Dion-Jacobson phases as a platform to identify superior oxide-ion conductors.

Discovery of a Rare-Earth-Free Oxide-Ion Conductor Ca3Ga4O9 by Screening through Bond Valence-Based Energy Calculations, Synthesis, and Characterization of Structural and Transport Properties.

In this work, we have discovered Ca3Ga4O9 as a rare-earth-free oxide-ion conductor by a combined technique of bond valence (BV)-based energy calculations, synthesis, and characterization of structural and transport properties. Here, the energy barriers for oxide-ion migration (Eb) of 217 Ga-containing oxides were calculated by the BV method to screen the candidate materials of oxide-ion conductors. We chose the orthorhombic calcium gallate Ca3Ga4O9 as a candidate of oxide-ion conductors, because Ca3Ga4O9 had a relatively low Eb. Ca3Ga4O9 was synthesized by a solid-state-reaction method. Rietveld analyses of time-of-flight neutron and synchrotron X-ray powder diffraction data of Ca3Ga4O9 indicated an orthorhombic Cmm2 layered crystal structure consisting of Ca18 and (Ga4O9)6 units where the (Ga4O9)6 units form the two-dimensional (2D) corner-sharing GaO4 tetrahedral network. The electromotive force measurements with an oxygen concentration cell showed that the transport numbers of the oxide ion were 0.69 at 1073 K and 0.84 at 973 K in Ca3Ga4O9, which indicates that the major carrier of Ca3Ga4O9 is the oxide ion. The oxide-ion conductivity was estimated to be 1.03(8) × 10-5 S cm-1 at 1073 K. The total electrical conductivity and impedance spectroscopy measurements of this Ca3Ga4O9 sample indicated that the bulk conductivity was much higher than the grain-boundary conductivity and that the total conductivity was equivalent to the bulk conductivity. The bond valence-based energy landscape calculated using the refined crystal parameters of Ca3Ga4O9 indicated 2D oxide-ion diffusion in the layered tetrahedral network [(Ga4O9)6 unit]. It was found that the structural and transport properties of Ca3Ga4O9 are similar to those of LaSrGa3O7 melilite.

A new structure family of oxide-ion conductors Ca0.8Y2.4Sn0.8O6 discovered by a combined technique of the bond-valence method and experiments.

Mg3TeO6-type Ca0.8Y2.4Sn0.8O6 has been found as a new structure family of oxide-ion conductors. From bond-valence-based energy (BVE) calculations for 147 compositions, which contain tin (Sn) as an essential element, Mg3TeO6-type Ca0.8Y2.4Sn0.8O6 was found to have a low energy barrier for oxide-ion migration. Ca0.8Y2.4Sn0.8O6 was synthesized by the solid-state reaction, and its electrical conductivity and crystal structure were investigated. The total electrical conductivity at various partial oxygen pressures and band gap estimated from the UV-vis spectrum suggested that Ca0.8Y2.4Sn0.8O6 is a pure oxide-ion conductor. The activation energy for the oxide-ion conductivity of Ca0.8Y2.4Sn0.8O6 was 1.39(4) eV. Synchrotron X-ray powder diffraction data of Ca0.8Y2.4Sn0.8O6 at 300 and 1273 K were successfully analyzed with the Mg3TeO6-type structure. The BVE calculation using the refined crystal structure of Ca0.8Y2.4Sn0.8O6 at 1273 K strongly suggested three dimensional oxide-ion diffusion.