Oxygen ionic conduction in brownmillerite CaAl0.5Fe0.5O2.5+δ

The oxygen permeability of CaAl0.5Fe0.5O2.5+δ brownmillerite membranes at 1123-1273 K was found to be limited by the bulk ionic conduction, with an activation energy of 170 kJ/mol. The ion transference numbers in air are in the range 2 × 10-3 to 5 × 10-3. The analysis of structural parameters showed that the ionic transport in the CaAl0.5Fe0.5O2.5+δ lattice is essentially along the c axis. The largest ion-migration channels are found in the perovskite-type layers formed by iron-oxygen octahedra, though diffusion in tetrahedral layers of the brownmillerite structure is also possible. Heating up to 700-800 K in air leads to losses of hyperstoichiometric oxygen, accompanied with a drastic expansion and, probably, partial disordering of the CaAl0.5Fe0.5O2.5+δ lattice. The average thermal expansion coefficients of CaAl0.5Fe0.5O2.5+δ ceramics in air are 16.7 × 10-6 and 12.6 × 10-6 K-1 at 370-850 and 930-1300 K, respectively

Development of CO2 Protective Layers by Spray Pyrolysis for Ceramic Oxygen Transport Membranes

[EN] Ceramic mixed ionic¿electronic conducting (MIEC) membranes enable very selective oxygen separation from air at high temperatures. Two major potential applications of oxygen-transport membranes are: i) oxygen production for oxyfuel power plants, and, ii) integration within high-temperature catalytic membrane reactors for methane or alkane upgrading by selective oxidative conversions. However, these applications involve contact with carbon-bearing atmospheres and most state-of-the-art highly permeable MIEC membranes do not tolerate operation under CO 2 -rich environments due to carbonation processes. The present contribution shows our ¿ rst attempts in the development of ceria-based protective thin layers on monolithic LSCF membranes. Gd-doped ceria (CGO) deposition is carried out by air blast spray pyrolysis on mirror-polished LSCF disc membranes. The layer thickness is maintained below 0.4 ¿ m in order to prevent the formation of cracks during thermal cycling and minimize limitations caused by the reduced oxygen permeability through the ceria layer. After optimization of the spraying process, smooth crack-free dense coatings are obtained with high crystallinity in the as-deposited state. The layers are characterized by XRD, SEM, AFM, DC-conductivity measurements, interferometry and optical microscopy. Oxygen separation is studied on coated LSCF using air as the feed and argon/CO 2 mixtures as the sweep gas in the temperature range 650¿1000 ° C. The protected membrane exhibits a higher stability than the uncoated LSCF membrane, although the nominal oxygen ¿ ux is slightly reduced at temperatures below 850 ° C due to the limited ambipolar conductivity of doped ceria in the range of oxygen partial pressures investigated. Moreover, the protective layer (250 nm thickness) remains stable after the permeation testing.Financial support by the Spanish Ministry for Science and Innovation (Project ENE2008-06302 and FPI Grant JAE-Pre 08-0058), the EU through FP7 NASA-OTM Project (NMP3-SL-2009-228701), and the Helmholtz Association of German Research Centers through the Helmholtz Alliance MEM-BRAIN (Initiative and Networking Fund) is kindly acknowledged.García Torregrosa, I.; Lobera González, MP.; Solis Díaz, C.; Atienzar Corvillo, PE.; Serra Alfaro, JM. (2011). Development of CO2 Protective Layers by Spray Pyrolysis for Ceramic Oxygen Transport Membranes. Advanced Materials. 1(4):618-625. https://doi.org/10.1002/aenm.201100169S6186251

Oxygen intercalation in Ruddlesden-Popper type Sr3LaFe3O10-δ

The oxygen nonstoichiometry of Ruddlesden-Popper type Sr3LaFe3O10-δ, a promising parent material for mixed-conducting electrodes and membranes, was studied in the oxygen partial pressure range from 10−24 to 0.5 atm at 973–1223 K by coulometric titration and thermogravimetry. The p(O2)-T-δ diagram can be described using a statistical thermodynamic model relating the non-ideal behavior in oxidizing and strongly reducing atmospheres to the coulombic interaction between oxygen vacancies and electronic charge carriers. The results confirm a strong energetic affinity of anion vacancies to the central perovskite-like layers of the Ruddlesden-Popper structure, whilst the difference between crystallographic iron sites has no significant effects on the oxygen intercalation thermodynamics at elevated temperatures. No indications of long-range ordering in the oxygen sublattice were observed.publishe

Perovskite-related oxide materials for oxygen-permeable electrochemical membrans

This brief review is focused on the studies of mixed ionic-electronic conductors on the basis of lanthanum gallate doped with transition metal cations in the В sublattice. The substitution of gallium with iron, cobalt or nickel results in greater electronic conductivity, simultaneously keeping high level of the oxy-gen ionic transport. In particular, La0 90Sr0 10Ga0 65Ni0 20Mg0 1503d perovskite exhib-its attractive oxygen permeability, which is quite similar to that of La2Ni04- and (La,Sr)Co03-based phases The combination of appropriate transport and thermomechanical properties with sufficiently high thermodynamic stability en-ables to use Ni- or Fe-substituted LaGa03-based mixed conductors for the mem-brane electrocatalytic reactors for partial oxidation of light hydrocarbons

Significance enhancement in the conductivity of core shell nanocomposite electrolytes

Today, there is great demand of electrolytes with high ionic conductivities at low operating temperatures for solid-oxide fuel cells. Therefore, a co-doped technique was used to synthesize a highly ionically conductive two phase nanocomposite electrolyte Sr/Sm–ceria–carbonate by a co-precipitation method. A significant increase in conductivity was measured in this co-doped Sr/Sm–ceria–carbonate electrolyte at 550 °C as compared to the more commonly studied samarium doped ceria. The fuel cell power density was 900 mW cm−2 at low temperature (400–580 °C). The composite electrolyte was found to have homogenous morphology with a core–shell structure using SEM and TEM. The two phase core–shell structure was confirmed using XRD analysis. The crystallite size was found to be 30–60 nm and is in good agreement with the SEM analysis. The thermal analysis was determined with DSC. The enhancement in conductivity is due to two effects; co-doping of Sr in samarium doped ceria and it's composite with carbonate which is responsible for the core–shell structure. This co-doped approach with the second phase gives promise in addressing the challenge to lower the operating temperature of solid oxide fuel cells (SOFC)

Mixed conductivity of zircon-type Ce1-xAxVO 4±δ (A = Ca, Sr)

Incorporation of alkaline-earth cations into the zircon-type lattice of Ce1-xAxVO4+δ (A = Ca, Sr; x = 0 - 0.2) was found to significantly increase the p-type electronic conductivity and to decrease the Seebeck coefficient, which becomes negative at x ≥ 0.1. The oxygen ionic conductivity is essentially unaffected by doping. The ion transference numbers of Cea-xAxVO4+δ in air, determined by the faradaic efficiency measurements, are in the range from 2 × 10-1 to 6 × 10-3 at 973-1223 K, increasing when temperature increases or alkaline-earth cation content decreases. The results on the partial conductivities and Seebeck coefficient suggest the presence of hyperstoichiometric oxygen, responsible for ionic transport, in the lattice of doped cerium vanadates. The activation energies for the electron-hole and ionic conduction both decrease on doping and vary in the ranges 39-45 kJ/mol and 87-112 kJ/mol, respectively

In-Situ catalytic surface modification of micro-structured La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) Oxygen Permeable Membrane Using Vacuum-Assisted technique

This paper aims at investigating the means to carry out in-situ surface modification of La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) oxygen permeable membrane by using vacuum assisted technique. The unique structure of the LSCF hollow fibre membrane used in this study, which consists of an outer dense oxygen separation layer and conical-shaped microchannels open at the inner surface has allowed the membrane to be used as oxygen separation membrane and as a structured substrate for where catalyst can be deposited. A catalyst solution of similar material, LSCF was prepared using sol-gel technique. Effects of calcination temperature and heating rate were investigated using XRD and TGA to ensure pure perovskites structure of LSCF was obtained. It was found that a lower calcination temperature can be used to obtain pure perovskite phase if slower heating rate is used. The SEM photograph shows that the distribution of catalyst onto the membrane microchannels using in-situ deposition technique was strongly related to the viscosity of LSCF catalytic sol. Interestingly, it was found that the amount of catalyst deposited using viscous solution was slightly higher than the less viscous sol. This might be due to the difficulty of catalyst sol to infiltrate the membrane and as a result, thicker catalyst layer was observed at the lumen rather than onto the conical-shaped microchannels. Therefore, the viscosity of catalyst solution and calcination process should be precisely controlled to ensure homogeneous catalyst layer deposition. Analysis of the elemental composition will be studied in the future using energy dispersive X-ray Spectroscopy (EDX) to determine the elements deposited onto the membranes. Once the elemental analysis is confirmed, oxygen permeation analysis will be carried out