Structural–Transport Properties Relationships on Ce_1–<i>x</i>Ln_<i>x</i>O_2−δ System (Ln = Gd, La, Tb, Pr, Eu, Er, Yb, Nd) and Effect of Cobalt Addition

A large series of doped cerias have been prepared by the coprecipitation method combined with impregnation and completely characterized in order to have an overall understanding of the structural, oxygen vacancy concentration, and transport properties relationships. Several lanthanides were incorporated in the fluorite structure, and the effects of the final sintering temperature (1073 and 1573 K) and the addition of cobalt oxide on the structural properties were studied. The chosen lanthanides (Gd, La, Tb, Pr, Eu, Er, Yb and Nd) included a large range of ionic radii and different metals exhibiting variable oxidation states under the typical operating conditions for these materials. The materials have been characterized by powder XRD, high-temperature XRD, micro-Raman spectroscopy, helium pycnometry, and dc conductivity. Transport properties were correlated with structural features induced by the different ionic radii and variable oxidation state of the dopants. The highest ionic conductivity was obtained for the less distorted cells (Gd- and Nd-doped ceria) which represent the optimum balance between Coulomb interactions, steric effects, and vacancy distribution. The lowest <i>E</i>_a value was found for materials with long cell parameters

Fast Oxygen Separation Through SO₂- and CO₂‑Stable Dual-Phase Membrane Based on NiFe₂O₄–Ce_0.8Tb_0.2O_2‑δ

Composite membranes with enhanced oxygen permeability and unprecedented stability in <i>oxyfuel</i>-like gas environments are reported. Specifically, 60 vol% NiFe₂O₄ - 40 vol% Ce_0.8Tb_0.2O_2‑δ (NFO-CTO) composite has been successfully obtained by one-pot fabrication method showing both spinel and fluorite pure phases. Narrow grain size distribution centered around 1 μm and homogeneous distribution of grains is attained, as well as percolative pathways from side to side of the dual-phase membranes. The composite resisted a stability test in wet SO₂ and CO₂ containing gas at 800 °C for 170 h, which represents a step forward toward its use in <i>oxyfuel</i> power plants. The conductivity of both phases is investigated as a function of temperature and oxygen partial pressure (<i>pO</i>_<i>2</i>). Oxygen separation in this kind of NFO-doped-ceria composite membranes occurs via the separate ambipolar transport through the two distinct percolating networks. Oxygen permeation flux values of 0.17 mL·min^–1·cm^–2 and 0.20 mL·min^–1·cm^–2 are achieved at 1000 °C when argon and pure CO₂ are used as sweep gas, respectively, through a 0.68 mm-thick membrane. Experiments at 900 °C showed that the material is stable and effective in pure CO₂ atmospheres and the oxygen permeation is even improved after 76 h on CO₂ stream

Particular Transport Properties of NiFe₂O₄ Thin Films at High Temperatures

NiFe₂O₄ (NFO) thin films were deposited on quartz substrates by rf magnetron sputtering, and the influence of the deposition conditions on their physic-chemical properties was studied. The films structure and the high temperature transport properties were analyzed as a function of the deposition temperature. The analysis of the total conductivity up to 800 °C in different <i>p</i>O₂ containing atmospheres showed a distinct electronic behavior of the films with regard to the bulk NFO material. Indeed, the thin films exhibit p-type electronic conductivity, while the bulk material is known to be a prevailing n-type electronic conductor. This difference is ascribed to the dissimilar concentration of Ni³⁺ in the thin films, as revealed by XPS analysis at room temperature. The bulk material with a low concentration of Ni³⁺ (Ni³⁺/Ni²⁺ ratio of 0.20) shows the expected n-type electronic conduction via electron hopping between Fe³⁺–Fe²⁺. On the other hand, the NFO thin films annealed at 800 °C exhibit a Ni³⁺/Ni²⁺ ratio of 0.42 and show p<i>-</i>type conduction via hole hopping between Ni³⁺–Ni²⁺

Proton Transport through Robust CPO-27-type Metal Organic Frameworks

In this work we studied the robustness of Ni-CPO-27 and Mg-CPO-27 metal organic frameworks (MOFs) upon cold uniaxial pressing and thermal cycling in dry and wet Ar/H₂. The preparation and operation limits for each material are found to be 225 and 150 MPa and temperatures of 250 and 150 °C for Ni-CPO-27 and Mg-CPO-27, respectively. The electrochemical alternating current conductivity measurements performed as high as 250 °C showed conductivity values ranging from 10^–6 to 10^–8 S/cm, depending on the material, temperature, and atmosphere. The protonic nature of the electrochemical transport phenomena was unambiguously confirmed via proton/deuteron isotopic and transient hydration studies. The study reveals high reproducibility and stability of the electrochemical measurements upon cycling in different atmospheres. Meanwhile, the crystallinity of the sample was preserved after the conductivity study and three weeks on stream, which demonstrates the midterm stability and robustness of this MOF

Synthesis and Characterization of Nonsubstituted and Substituted Proton-Conducting La_6–<i>x</i>WO_{12–<i>y</i>}

Mixed proton–electron conductors (MPEC) can be used as gas separation membranes to extract hydrogen from a gas stream, for example, in a power plant. From the different MPEC, the ceramic material lanthanum tungstate presents an important mixed protonic–electronic conductivity. Lanthanum tungstate La_6–<i>x</i>WO_{12–<i>y</i>} (with <i>y</i> = 1.5<i>x</i> + δ and <i>x</i> = 0.5–0.8) compounds were prepared with La/W ratios between 4.8 and 6.0 and sintered at temperatures between 1300 and 1500 °C in order to study the dependence of the single-phase formation region on the La/W ratio and temperature. Furthermore, compounds substituted in the La or W position were prepared. Ce, Nd, Tb, and Y were used for partial substitution at the La site, while Ir, Re, and Mo were applied for W substitution. All substituents were applied in different concentrations. The electrical conductivity of nonsubstituted La_6–<i>x</i>WO_{12–<i>y</i>} and for all substituted La_6–<i>x</i>WO_{12–<i>y</i>} compounds was measured in the temperature range of 400–900 °C in wet (2.5% H₂O) and dry mixtures of 4% H₂ in Ar. The greatest improvement in the electrical characteristics was found in the case of 20 mol % substitution with both Re and Mo. After treatment in 100% H₂ at 800 °C, the compounds remained unchanged as confirmed with XRD, Raman, and SEM