Tungstates Ln6WO12 are proton conducting crystalline materials which show significant mixed protonic and electronic conductivity and stability in moist CO2 environments. These materials are promising ...candidates for hydrogen separation membranes at temperatures above 700°C, especially when integrated in intensified processes as catalytic membrane reactors. From this family of compounds, La6−xWOy (0.4<x<0.7) presents the highest protonic conductivity and hydrogen permeability. This contribution presents the preparation, hydrogen permeation and stability study of La5.5WO11.25−δ membranes. Hydrogen separation properties of La5.5WO11.25−δ were systematically analyzed, i.e. the influence of the H2 concentration in feed stream, humidification degree and operating temperature were investigated. The integrity of the La5.5WO11.25−δ after the hydrogen permeation experiments was confirmed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman spectroscopy. The stability of La5.5WO11.25−δ permeation when CO2 containing atmosphere was used as sweep gas was evaluated. Furthermore, the stability of this compound in CO2-rich and sulfur-containing environments was confirmed by XRD.
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•The stability at high temperature in CO2-rich and sulfur-bearing gases was confirmed by XRD.•Hydrogen separation properties of La5.5WO11.25−δ were systematically analyzed.•Stable hydrogen operation proved using CO2-rich sweep gas.•The role of oxide-ion transport in hydrogen separation was investigated.
The effects of NiO, ZnO, and CuO sintering additives (0.5, 1.0, or 2.0 wt%) on the sintering behaviour, effective acceptor concentration, and hydration thermodynamics are examined for ...BaZr0.4Ce0.4Y0.1Yb0.1O3−δ and BaZr0.4Ce0.4Y0.2O3−δ proton conducting electrolytes. Thermogravimetry of hydration shows that the sintering additives – except for 0.5 wt% CuO – lead to a decrease in the effective acceptor concentration, and the decrease per mole sintering additive is the largest for NiO. The absence of typical secondary phases such as BaY2NiO5 and the homogeneous distribution of Ni determined from elemental mapping imply that sintering additives dissolve into the perovskite lattice. Exsolution of metallic Ni and Cu upon reduction leads to a substantial recovery of the effective acceptor concentration. Subsequent oxidation is accompanied by a repeated decrease in the effective acceptor concentration as the sintering additives appear to re-dissolve. Defect chemical reactions are proposed to explain the observed results and these are supported by energetics from first principles calculations. Overall, NiO has the highest positive impact on densification and grain growth, and a relatively small amount of 0.5 wt% NiO or CuO is preferable to optimise both sintering and hydration.
•NiO, CuO, and ZnO dissolve in BaZr0.4Ce0.4(Y,Yb)0.2O2.9 and promote sintering, but suppress acceptor level.•Conclusions supported by defect chemistry, ab initio calculations, and atomic resolution STEM-EDS.•Ni and Cu cations exsolve as metal under reducing conditions and re-dissolve under oxidising; acceptor level follows.•Use of MgO over Al2O3 crucibles and Ba-rich sacrificial cover during sintering is essential to prevent Ba loss.•Low contents of NiO or CuO offer best compromise between sintering and acceptor concentration.
This work presents the structural and electrochemical characterization of mixed conducting materials based on the system Nd5.5W1−xUxO11.25−δ where x=0.1, 0.5 and 1. The evolution of the crystalline ...structure is studied as a function of the sintering temperature and the U content in the oxide lattice. The influence of hydration and pO2 as well as the H/D isotopic effect are studied by DC-electrochemical measurements under reducing environments. Hydrogen permeation is carried out for Nd5.5WO11.25−δ and Nd5.5W0.9U0.1O11.25−δ compounds in the range from 650 to 750°C. Finally, stability of the developed materials at 800°C is evaluated in contact with CO2-rich gas streams. This work completes previous reported studies on Ln6−xWO12−y based materials that show the possibility of engineering the conductivity properties of the Ln6−xWO12−y just by doping strategies.
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•The symmetry evolves from cubic to rhombohedral symmetry with increasing U contents and sintering temperatures.•Incorporation of U increases the total conductivity, becoming predominant oxygen ionic conductivity at high U%.•The enhanced oxygen ion transport is assigned to the U+6 reduction to U+4.•U additions allow reaching H2 flows due to H2 production via water splitting.•Crystalline symmetry depends on U contents and sintering temperatures.•U incorporation increases the total conductivity and the oxygen ion conductivity.
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•Mechanical long-term stability of full ceramic membranes was studied.•100 h of exposure at 750 °C in different atmospheres H2, CO2, H2 + CO2 are evaluated.•In H2 + CO2 the membrane ...shows a slight decrease in flexural strength and hardness.•In H2 + CO2 no evident morphological or structural changes.•Promising and stable hydrogen permeation flux were recorded.
This work investigates, for the first time, the hydrogen permeation of BaCe0.65Zr0.20Y0.15O3-δ-Ce0.8Gd0.2O2-δ (BCZY-GDC) asymmetric membranes for 100 h, using wet 15% CO2 in Ar as sweep gas. In the same frame, ex-situ aging tests were performed for 100 h exposure at 750 °C in different atmospheres (H2, CO2, H2 + CO2), to evaluate the phase, microstructure, and mechanical long-term stability of this system. The thermal aging in H2-atmosphere leads to lower flexural strength caused by a microstructure embrittlement of the BCZY-GDC asymmetric membrane, due to chemical expansion/contraction of the GDC cell after the aging cycle. Indeed, micro-cracking of GDC grains, that decreases the composite hardness, is observed in symmetric (pressed pellet) membranes. The aging in CO2 causes a slightly increase in flexural strength values due to the formation of sub-micrometric Zr-doped ceria-BaCO3 phases at the expense of the perovskite. Higher hardness values related to the emerging of BaCO3 islands on the symmetric membrane surface were also recorded. In H2 + CO2 atmosphere (real testing condition), the membrane shows a slight decrease in flexural strength and hardness while no evident morphological or structural changes (except the BaCO3 formation in traces) were observed. This study highlights that promising and stable hydrogen permeation flux values can be recorded using the asymmetric configuration for 100 h, using wet 15% CO2 in Ar as sweep gas. Neither structural nor morphological modification of the membrane were detected after the testing.
(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.
The tungstates Ln6WO12 are proton-conducting materials exhibiting sufficient electronic conductivity to consider them as potential candidates for the separation of hydrogen at high temperature. ...Hydrogen-permeable membranes will find application in power plants applying precombustion strategies, process intensification using high-temperature catalytic membrane reactors, and in components for electrochemical systems as proton conducting fuel cells (PCFCs) and electrolyzers. This work presents the preparation and characterization of nanocrystalline mixed conducting materials with three different nominal compositions (Nd6WO12−Eu6WO12−Er6WO12) using a sol−gel complexation synthesis method. The evolution of the crystalline structure and crystallite size is studied as a function of the sintering temperature. Generally, the nanosized oxides show a (pseudo)-cubic crystalline fluorite structure which evolves into the most stable fluorite symmetry (tetragonal for Nd and rhombohedral for Er) with increasing sintering temperatures, i.e., crystallite sizes. Shrinkage behavior was analyzed for the three compositions in the range from 1000 to 1500 °C and the nanosized Nd-based oxide showed a very high sintering activity even at relatively low temperatures (1100−1200 °C). In addition, the total conductivity in different environments has been studied systematically for samples sintered at different temperatures and the highest total conductivity was obtained for the Eu-based compound having structure with tetragonal symmetry (0.009 S/cm at 850 °C). Hydrogen permeation was studied for a disk-shaped Nd6WO12 membrane in the range of 700−1000 °C. Finally, stability of these materials at 700 and 800 °C has been evaluated in contact with a CO2-rich gas stream (dry or humidified) as well as thermochemical compatibility with yttria-stabilized zirconia in the range 1200−1500 °C.
Self-supported palladium tubular membranes are surface-functionalized and tested for the selective and controlled addition of H2 along a fixed-bed catalytic reactor for Fischer–Tropsch hydrocarbon ...synthesis (FTS). In order to avoid CO poisoning of the active sites of the metallic membrane at low working temperatures (~250 °C), a Cu-based protective layer is deposited on the outer surface of a tubular membrane by RF sputtering at room temperature. Upon thermal treatment in H2, the Cu layer alloys with Pd on the membrane surface, as confirmed by means of XRD, FESEM and TEM, while the membrane assembly is fully functional, i.e. the negative effect of CO surface adsorption is highly diminished and the membrane provides an appropriate H2 flux (e.g. 12 ml/min·cm2) under the harsh operation conditions practiced in FT synthesis. When the Cu-functionalized membrane is fully integrated in the FTS reactor (250 °C, 20 bar, 30% CO in feed), the membrane delivers a stable H2 permeation flux and enables to increase the yield of hydrocarbons in the range of gasoline (C5–C12) while reducing methane formation over a bifunctional CoRu/Al2O3-zeolite catalyst.
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•Cu-coated membrane enable high H2 permeation in presence of CO at 250 °C.•The sputtered Cu-thin film is partly alloyed with Pd upon heat treatment.•Operation in a FT-synthesis membrane reactor is done succesfully.
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