A novel dual phase composite oxygen transport membrane (Al0.02Ga0.02Zn0.96O1.02-Gd0.1Ce0.9O1.95-δ) was successfully prepared and tested. This membrane shows chemical stability against CO2 and SO2, and stable oxygen permeation over 300 hours in CO2 was demonstrated. ZnO is cheap and non-toxic and is therefore highly advantageous compared to other common materials used for this purpose.

Ag/Pr6O11 catalysts supported by either Pr6O11 nanorods (Pr6O11-NRs) or nanoparticles (Pr6O11-NPs) were prepared by conventional incipient wetness impregnation. The nanocomposite of Ag/Pr6O11-NRs demonstrated a higher catalytic activity for CO oxidation than Ag/Pr6O11-NPs at lower temperatures. This improved performance may be ascribed to the mesoporous features and resultant oxygen vacancies of the Pr6O11 nanorods support, as well as the large surface area and homogeneous loading of Ag species. As a result, 98.7 and 100% CO conversions were achieved at 210 and 240 °C for Ag/Pr6O11-NRs, while Ag/Pr6O11-NPs require a temperature of 320 °C to obtain the 100% CO conversion rate. These findings reveal that Pr6O11-NRs is the preferable support, comparative to Pr6O11-NPs, for Ag/Pr6O11 catalysts, for CO oxidation.

A microchanneled asymmetric dual phase composite membrane of 70 vol % Gd(0.1)Ce(0.9)O(1.95-δ)-30 vol % La(0.6)Sr(0.4)FeO(3-δ) (CGO-LSF) was fabricated by a "one step" phase-inversion tape casting. The sample consists of a thin dense membrane (100 μm) and a porous substrate including "finger-like" microchannels. The oxygen permeation flux through the membrane with and without catalytic surface layers was investigated under a variety of oxygen partial pressure gradients. At 900 °C, the oxygen permeation flux of the bare membrane was 1.6 (STP) ml cm(-2) min(-1) for the air/He-case and 10.10 (STP) ml cm(-2) min(-1) for the air/CO-case. Oxygen flux measurements as well as electrical conductivity relaxation show that the oxygen flux through the bare membrane without catalyst is limited by the oxygen surface exchange. The surface exchange can be enhanced by introduction of catalyst on the membrane surface. An increase of the oxygen flux of ∼1.49 (STP) mL cm(-2) min(-1) at 900 °C was observed when catalyst is added for the air/He-case. Mass transfer polarization through the finger-like support was confirmed to be negligible, which benefits the overall performance. A stable flux of 7.00 (STP) ml cm(-2) min(-1) was observed between air/CO/CO2 over 200 h at 850 °C. Partial surface decomposition was observed on the permeate side exposed to CO, in line with predictions from thermodynamic calculations. In a mixture of CO, CO2, H2, and H2O at similar oxygen activity the material will according to the calculation not decompose. The microchanneled asymmetric CGO-LSF membranes show high oxygen permeability and chemical stability under a range of technologically relevant oxygen potential gradients.

The oxygen permeation flux of Ce 0.9 Gd 0.1 O 1.95-δ (CGO)-based oxygen transport membranes under oxidizing conditions is limited by the electronic conductivity of the material. This work aims to enhance the bulk ambipolar conductivity of CGO by partial substitution of Ce with the redox active element Pr. A series of compositions of Pr x Gd 0.1 Ce 0.9-x O 1.95-δ (x = 0, 0.02, 0.05, 0.08, 0.15, 0.25, 0.3 and 0.4) was prepared by solid state reaction. X-ray powder diffraction (XPD) indicates that Pr is completely dissolved in the fluorite structure up to 40 at.%. Pronounced nonlinear thermal expansion behavior was observed as a function of temperature, due to the simultaneous contributions of both thermal and chemical expansion. The electronic and ionic conductivities were measured as a function of temperature and oxygen partial pressure. Within the range from 10 to 15 at.% Pr, a drastic drop of the activation energy of the hole mobility and an abrupt increase of the hole conductivity at low temperature was observed. The behavior could be rationalized by a simple percolation model. Oxygen permeation fluxes through disk shaped samples fed with air on one side and N 2 on the other side were also measured. The oxygen flux through Pr 0.05 Gd 0.1 Ce 0.85 O 1.95-δ was higher than that for CGO by one order of magnitude owing to the enhanced electronic conductivity albeit the flux is still limited by the electronic conductivity. In terms of the electronic and ionic conductivity, the estimated maximum oxygen permeation flux of a 10 μm Pr 0.4 Gd 0.1 Ce 0.9 O 1.95-δ -based membrane exceeds 10 Nml cm −2 min −1 at 900 • C under a small oxygen potential gradient (0.21/10 −3 bar) which is promising for use in oxygen production and in oxy-fuel combustion. Also the material may be well applicable to SOFC/SOEC composite electrodes where mixed conductivity is also desirable. Dense ceramic oxygen transport membranes (OTMs) could potentially be applied for production of high purity oxygen for medical purposes, supply of oxygen in the steel industry, oxy-fuel combustion schemes, as well as in the cement and glass industries. Also, importantly, OTMs can beneficially be integrated with a biomass gasifier, allowing production of syngas (CO and H 2 ), which is a precursor for a variety of high value chemicals. 1 Besides being applicable for OTMs, 2 acceptor doped-ceria has been intensively studied for use in a number of other applications e.g. solid oxide fuel cells (SOFCs), 3 solid oxide electrolysis cells (SOECs) 4 and for electrocatalysis. 5 In particular, acceptor doped ceria (e.g. CGO) is interesting owing to high oxide ion conductivity (0.12 Scm −1 for Gd 0.1 Ce 0.9 O 1.95-δ at 900• C 6 ), appreciable electrocatalytic activity, high electronic conductivity under reducing conditions, and excellent chemical stability under harsh reducing and even corrosive gaseous conditions. 3,7 Kaiser et al. 8 reported that the oxygen permeation flux of a 27 μm asymmetric 10 at.% Gd-doped ceria-based membrane exceeds 10 ml cm −2 min −1 under a gradie...

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Silica impurity originated from the sealing or raw materials of the solid oxide cells (SOCs) accumulating at the Ni-YSZ triple phase boundaries (TPBs) is known as one major reason for electrode passivation. Here we report nanosilica precipitates inside Ni grains instead of blocking the TPBs when operating the SOCs at |i| ≥ 1.5 A cm(-2) for electrolysis of H2O/CO2. An electrochemical scavenging mechanism was proposed to explain this unique behavior: the removal of silica proceeded through the reduction of the silica to Si under strong cathodic polarization, followed by bulk diffusion of Si into Ni and reoxidation of Si in the Ni grain.

The application areas of oxygen transport membranes (OTM) and the potential benefits they can provide will be described. The importance of close thermal integration with the process to which the oxygen is supplied will be elucidated by balance-of-plant studies and techno-economical evaluations in two different applications; biomass gasifiers and cement production. The status of the technology will be presented with emphasis on results from several national and EU-funded membrane development projects.  There are still several challenges to overcome before use of the technology will be wide spread. One is the required upscale of the technology another one is ensuring the required reliability and durability of the membranes. Recently developed methods for characterizing the mechanical properties of OTM materials and components at operating conditions and a theoretical analysis of stresses encountered during operation will be presented and based on these strategies to ensure fail-safe design/operation. An OTM material should ideally have high ionic and electronic conductivity as well as good chemical stability under both oxidizing and reducing conditions. Further, the mechanical properties (strength, toughness, creep rate) must match the requirements of the application. Candidate materials involve a broad class of Fe-, Co- and Ti-based perovskites. Recent results on selected materials from this broad class of materials will be presented. Challenges with reaching both the required transport properties and the required mechanical reliability with such single phase perovskites has spurred renewed interest in dual phase membranes where a good ionic conductor (typically based on zirconia or ceria) is combined with a second phase providing electronic (or mixed) conductivity. Our most recent results on such dual phase membranes will be presented involving both zirconia and ceria based systems. On both tubular and planar LSF/CGO membranes high oxygen fluxes >10 Nml min-1 cm-2 (850°C) have been demonstrated between air on one side of the membrane and a strongly reducing “syngas” mixture (CO/H2/H2O/CO2) on the other side. The tubular membranes were prepared by dip coating on an extruded low cost MgO support and the planar ones by phase inversion tape casting providing a highly oriented pore structure in the support layer. The processes limiting the flux in these systems were investigated in some detail by conductivity relaxation.  Whereas the addition of CGO to LSF has the expected effect of increasing the ionic transport in the bulk it also had a more surprising effect of enhancing the surface exchange rate. Possible explanations for this will be presented.  Finally, selected results on zirconia based composite membranes optimized for high pO2 applications will be presented including ZnO-ZrO2 and (MnCO)3O4 -ZrO2 membranes. For the latter system (MnCo2O4/(Y2O3)0.01(Sc2O3)0.10(ZrO2)0.89) fluxes around 1 Nml min-1 cm-2 at 850°C have recently been demonstrated between air and a nitrogen purge (pO2~10-3 atm.) using a ~10 μm thick supported membrane.