Solid State Transport and Hydrogen Permeation in the System Nd_5.5W_1–<i>x</i>Re_<i>x</i>O_11.25−δ

Nd_5.5WO_11.25−δ is a mixed proton–electron conducting oxide, which shows an important mixed conductivity and stability in moist CO₂ environments. However, the H₂ fluxes obtained with this material are not high enough in order to apply them as H₂ separation membranes in industrial applications. Re⁶⁺ cation presents similar ionic radii than W⁶⁺ and Re⁶⁺ can be reduced to different oxidation states under the operating conditions typical for hydrogen membrane separation. This fact leads to the improvement of the electronic conductivity and produce the generation of oxygen vacancies with the subsequent increase in the ionic conductivity. This work presents the synthesis as nanosized powders as well as the structural and electrochemical characterization of mixed conducting materials based on the system Nd_5.5W_1–<i>x</i>Re_<i>x</i>O_11.25−δ where <i>x</i> = 0, 0.1, 0.5 and 1. The evolution of the crystalline structure and the shrinkage behavior are studied as a function of the sintering temperature. Total conductivity in reducing and oxidizing environments is studied systematically for samples sintered at 1350 °C. The H/D isotopic effect and the hydration influence are also analyzed by means of DC-electrochemical measurements. H₂ permeation is carried out for the selected compound, Nd_5.5W_0.5Re_0.5O_11.25−δ, in the range of 700–1000 °C, obtaining a peak H₂ flux value of 0.08 mL·min^–1·cm^–2. The reduction of the Re cation in this compound under reducing conditions is investigated by TPR and XPS. Finally, the stability of this material under CO₂-rich gas stream was evaluated by measuring H₂ permeation using a CO₂ containing atmosphere as sweep gas

Catalytic Layer Optimization for Hydrogen Permeation Membranes Based on La_5.5WO_11.25‑δ/La_0.87Sr_0.13CrO_3‑δ Composites

(LWO/LSC) composite is one of the most promising mixed ionic–electronic conducting materials for hydrogen separation at high temperature. However, these materials present limited catalytic surface activity toward H₂ activation and water splitting, which determines the overall H₂ separation rate. For the implementation of these materials as catalytic membrane reactors, effective catalytic layers have to be developed that are compatible and stable under the reaction conditions. This contribution presents the development of catalytic layers based on sputtered metals (Cu and Pd), electrochemical characterization by impendace spectroscopy, and the study of the H₂ flow obtained by coating them on 60/40-LWO/LSC membranes. Stability of the catalytic layers is also evaluated under H₂ permeation conditions and CH₄-containing atmospheres

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