Oxygen-Ionic Conductivity in Isovalent-Doped Layered BaLaInO4-Based Perovskites

In this paper, we analyze the effect of acceptor doping (Zn2+ and Mg2+) in the indium sublattice on the transport properties of the BaLaInO4 phase with the Ruddlesden–Popper structure. The doping is shown to cause an increase in both the oxygen ion and proton conductivities of the material. The highest oxygen ion and proton conductivities are offered by the BaLaIn0.9Mg0.1O3.95 material.

In this work, a high-density ceramics Ln2Hf2O7 (Ln = La, Nd, Sm, Eu, Gd) were synthesized by mechanical activation followed by high-temperature synthesis at 1600°C (3–10 h) and their transport properties were compared with those of Ln2.1Hf1.9O6.95 (Ln = La, Nd, Sm, Eu) doped solid solutions. The total conductivity of ceramics was studied using impedance spectroscopy and dc four-probe method; for Ln2Hf2O7 (Ln = Sm, Eu), by determining the total conductivity as a function of oxygen partial pressure. The maximum oxygen-ion conductivity was observed for Gd2Hf2O7 (~1 × 10–3 S/cm at 700°C); it was shown to approach the conductivity of Gd2Zr2O7 (~2 × 10–3 S/cm at 700°C) for the first time. Thus, the gadolinium hafnate can be a promising material for further doping in order to obtain highly conductive electrolytes. Among pure rare-earth hafnates, the proton conductivity was reliably observed for Nd2Hf2O7 only; however, ac measurements detected low-temperature proton conductivity in the Gd2Hf2O7 up to 450°С as well. With a decrease in the lanthanide ionic radius, the oxygen-ion conductivity increased in the Ln2Hf2O7 (Ln = La, Nd, Sm, Gd) series. Although the conductivity of samarium hafnate is an order of magnitude lower than that of Gd2Hf2O7, it has a wide range of oxygen-ion conductivity (~10–18–1 atm at 700, 800°C); there is no contribution from hole conductivity in air, in contrast to Eu2Hf2O7. Among doped Ln2.1Hf1.9O6.95 pyrochlore solid solutions (Ln = La, Nd, Sm, Eu), the proton conductivity of ~8 × 10−5 S/cm at 700°C was shown in Ln2.1Hf1.9O6.95 (Ln = La, Nd). With a decrease in the lanthanide ionic radius, the proton conductivity disappeared; the oxygen-ion one, increased.

The new phases BaLa0.9M0.1InO3.95 (M = Ca2+, Sr2+, Ba2+) with a Ruddlesden-Popper structure were obtained. It was established that all investigated samples were capable for the water uptake from the gas phase. The ability of water incorporation was due to not only by the presence of oxygen vacancies, but also due to the presence of La-O blocks in the structure. The degree of hydration of the samples was much higher than the concentration of oxygen vacancies and the composition of the samples appear to be BaLaInO3.42(OH)1.16, BaLa0.9Ca0.1InO3.25(OH)1.4, BaLa0.9Sr0.1InO3.03(OH)1.84, BaLa0.9Ba0.1InO2.9(OH)2.1. The degree of hydration increased with an increase in the size of the dopant, i.e., with an increase in the size of the salt blocks. It was proven that doping led to the increase in the oxygen ionic conductivity. The conductivities for doped samples BaLa0.9M0.1InO3.95 were higher than for undoped composition BaLaInO4 at ~1.5 order of magnitude. The increase in the conductivity was mainly attributed to the increase of the carrier concentration as a result of the formation of oxygen vacancies during doping. The proton conductivities of doped samples increased in the order Ca2+–Sr2+–Ba2+ due to an increase in the concentration of protons. It was established that all doped samples demonstrated the dominant proton transport below 450 °C.

Sm2−xCaxZr2O7−x/2 (x = 0, 0.05, 0.1) and Gd2−xCaxZr2O7−x/2 (x = 0.05, 0.1) mixed oxides in a pyrochlore–fluorite morphotropic phase region were prepared via the mechanical activation of oxide mixtures, followed by annealing at 1600 °C. The structure of the solid solutions was studied by X-ray diffraction and refined by the Rietveld method, water content was determined by thermogravimetry (TG), their bulk and grain-boundary conductivity was determined by impedance spectroscopy in dry and wet air (100–900 °C), and their total conductivity was measured as a function of oxygen partial pressure in the temperature range: 700–950 °C. The Sm2−xCaxZr2O7−x/2 (x = 0.05, 0.1) pyrochlore solid solutions, lying near the morphotropic phase boundary, have proton conductivity contribution both in the grain bulk and on grain boundaries below 600 °C, and pure oxygen–ion conductivity above 700 °C. The 500 °C proton conductivity contribution of Sm2−xCaxZr2O7−x/2 (x = 0.05, 0.1) is ~ 1 × 10−4 S/cm. The fluorite-like Gd2−xCaxZr2O7−x/2 (x = 0.1) solid solution has oxygen-ion bulk conductivity in entire temperature range studied, whereas proton transport contributes to its grain-boundary conductivity below 700 °C. As a result, of the morphotropic phase transition from pyrochlore Sm2−xCaxZr2O7−x/2 (x = 0.05, 0.1) to fluorite-like Gd2−xCaxZr2O7−x/2 (x = 0.05, 0.1), the bulk proton conductivity disappears and oxygen-ion conductivity decreases. The loss of bulk proton conductivity of Gd2−xCaxZr2O7−x/2 (x = 0.05, 0.1) can be associated with the fluorite structure formation. It is important to note that the degree of Ca substitution in such solid solutions (Ln2−xCax)Zr2O7−δ (Ln = Sm, Gd) is low, x < 0.1. In both series, grain-boundary conductivity usually exceeds bulk conductivity. The high grain-boundary proton conductivity of Ln2−xCaxZr2O7−x/2 (Ln = Sm, Gd; x = 0.1) is attributable to the formation of an intergranular CaZrO3-based cubic perovskite phase doped with Sm or Gd in Zr sublattice.

The effect of doping potassium on the A-site of BaZrO3 for application as a high temperature proton conductor was investigated. The synthesis of K-doped BaZrO3 and its characterization by XRD, SEM and EDX is described. Four-probe dc conductivity was measured on dense and porous (44% porosity) samples in varying partial pressures of oxygen, , and water vapor pressure, , over a temperature range from 500 to 700oC. The dependence of total conductivity in a dry atmosphere on oxygen partial pressure could be described by the equation . The experimental data were fitted to this equation with good accuracy, from which the electron-hole and oxygen-ion conductivities were determined. The activation energies for oxygen-ion and electron-hole conduction were estimated to be 0.85 and 1.14 eV, respectively. A significant increase in total ionic conductivity was observed in humid atmospheres, which is attributed to high protonic contribution.

The effect of doping potassium on the A-site of for application as a high-temperature proton conductor was investigated. The synthesis of K-doped and its characterization by X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray analysis is described. Potassium-doped material was stable at high water-vapor pressures and up to , the maximum temperature of the autoclave treatment. Four-probe dc conductivity was measured on dense and porous (44% porosity) samples in varying partial pressures of oxygen, , over a temperature range from . Conductivity was measured as a function of atmosphere and temperature on porous samples to ensure attainment of equilibrium conditions in a reasonable time period. The dependence of total conductivity in a dry atmosphere on oxygen partial pressure could be described by the equation . The experimental data were fitted to this equation with good accuracy, from which the electron-hole and oxygen-ion conductivities were determined. The activation energies for oxygen-ion and electron-hole conduction were estimated to be 0.85 and , respectively. Conductivity measurements were also performed in -containing atmospheres as a function of water-vapor pressure, . A significant increase in total ionic conductivity was observed in humid atmospheres, which is attributed to high protonic contribution. Samples doped with potassium exhibit significantly higher ionic conductivity in humid atmosphere as compared to samples doped with yttrium. The enhanced ionic conductivity, which is mainly protonic, is attributed to the higher basicity of potassium. At high and low , potassium-doped exhibits predominantly electron-hole conduction, while at low and high it exhibits predominantly protonic conduction.

Investigation of the influence of zirconium substitution on the properties of neodymium-doped barium cerates

Nonporous Inorganic Membranes

ABSTRACTThe ceramic Sr-Fe-Co-O has potential use as a membrane in gas separation. This material exhibits high conductivity of both electrons and oxygen ions. It allows oxygen to penetrate at high flux rates without other gas components. Electrical properties are essential to understanding the oxygen transport mechanism and defect structure of this material. By using a gas-tight electrochemical cell with flowing air as the reference environment, we were able to achieve an oxygen partial pressure (P02) as low as 10−16 atm. Total and ionic conductivities of Sr-Fe-Co-O have been studied as a function of P02 at elevated temperature. In air, both total and ionic conductivities increase with temperature, while the ionic transference number is almost independent of temperature, with a value of ≈0.4. Experimental results show that ionic conductivity decreases with decreasing P02 at high P02 (≥10−6 atm). This suggests that interstitial oxygen ions and electron holes are the dominant charge carriers. At 800°C in air, total conductivity and ionic conductivity are 17 and 7 S/cm, respectively. Defect dynamics in this system can be understood by means of the trivalence-to-divalence transition of Fe ions when P02 is reduced. By using the conductivity results, we estimated oxygen penneation through a ceramic membrane made of this material. The calculated oxygen permeability agrees with the experimental value obtained directly from an operating methane conversion reactor.

Materials with the A2B2O7 pyrochlore structure have interesting ionic transport properties because of their crystallographic structure, which can be described as a stable array of corner-shared BO6 octahedra that is penetrated by a 3-dimensional tunnel configuration that is partly filled by the A2O sublattice. The pyrochlore stochiometry means that there are built-in intrinsic oxide ion vacancies in the crystal structure in comparison to the related fluorite type structure. These are in the A2O sublattice, so that the tunnels are only 75% occupied. The presence of these tunnels leads to the possibility of significant changes in the composition, and some ionic species in this sublattice exhibit high mobility. The cubic pyrochlore Gd2Ti2O7 was doped in various ways to change its ionic and electronic transport properties. The total conductivity and partial ionic and electronic contributions were investigated by ac impedance and EMF measurement techniques. The influence of either A or B site doping with aliovalent ions that occupy sites in the A2O and B2O6 sublattices was investigated. The results of these experiments are presented and discussed in relation to the crystal structure and defect chemistry of this family of oxides.

Novel proton-conducting hexagonal perovskites Ba7In6–xYxAl2O19 for solid oxide fuel cells

This paper reports an electrochemical study on terbia and yttria stabilized zirconia. (Tb, Y)-ZrO2 solid solutions with a fluorite type structure were prepared by using citrate synthesis and sintered at 1500°C. They are mixed electronic and oxygen ionic conductive, as well as oxygen semipermeable materials at elevated temperatures. The total electrical conductivity reaches 3.8 S m−1 at 900°C and the oxygen permeation is about 6.5 × 10−6 mol m−2 s−1 at 1100°C. The electrical conductivity of zirconia doped with a total of 30 mol % of terbia and yttria increases with oxygen partial pressure when P O 2 is below 10−2 atm, and decreases as the oxygen partial pressure rises above 10−2 atm. This is ascribed to the defect association and the predomination of oxygen ionic conductivity. The oxygen semipermeation is proportional to the square root of the oxygen partial pressure, and is not influenced by the variation of oxygen ionic or electronic conduction. The results indicate that the surface exchange reaction is most probably the rate limiting step for the oxygen semipermeation.

Structure and transport properties of donor-doped barium strontium cobaltites

Solid oxide fuel cells (SOFCs) require high operation temperatures (up to 800°C) in order to allow oxygen ion conduction through the ceramic electrolyte. Lowering SOFC operation temperature implies enhancing the transport and electrochemical properties of both electrolyte and cathode. In this work, the electrochemical performance of nanostructured La0.4Sr0.6Co0.8Fe0.2O3-d (LSCFO)/Ce0.8Gd0.2O2-d(CGO)/LSCFO and LSCFO/Ce0.8Nd0.2O2-d (CNO)/LSCFO symmetrical cells was investigated. The bulk conductivity is similar for both electrolytes, while the grain boundary and total conductivities is higher for the CGO electrolyte. Nevertheless, CGO electrolyte exhibits large pores that compromise its mechanical properties. In addition, cathode performance degrades in LSCFO/CGO/LSCFO cells.

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.