Perovskite-type Ceramic Membranes Research Articles

Direct evidence of high oxygen ion conduction of perovskite-type ceramic membrane under hydrogen atmosphere in solid oxide fuel cell

The ionic conduction of perovskite-type oxides remains a fundamental and important issue in the research of solid oxide fuel cells (SOFCs). In this research, a thin perovskite-type ceramic membrane was fabricated in situ at anode side attached to the surface of Gd0.2Ce0·8O1.9 (GDC20) electrolyte membrane. The single cell working between H2 and static air showed good stability (over 50 h), high open circuit voltages (above 1.0 V) as well as high peak power densities (749-264 mW cm−2) from 600 to 500 °C. Detailed analyses of current research demonstrated that the thin perovskite film mainly possessed the oxygen ion conductivity under reducing atmosphere, while the proton conductivity was severely suppressed, showing the high flexibility in ionic conductivity of perovskite oxide. This work also implies that the oxygen ion and proton conduction may be in high correlation with each other, which provides important information to unveil the nature of the ionic conduction of perovskite-type oxides.

The Role of Metal Catalyst on Water Permeation and Stability of BaCe0.8Y0.2O3-δ

Perovskite type ceramic membranes which exhibit dual ion conduction (proton and oxygen ion conduction) can permeate water and can aid solving operational problems such as temperature gradient and carbon deposition associated with a working solid oxide fuel cell. From this point of view, it is crucial to reveal water transport mechanism and especially the nature of the surface sites that is necessary for water incorporation and evolution. BaCe0.8Y0.2O3-α (BCY20) was used as a model proton and oxygen ion conducting membrane in this work. Four different catalytically modified membrane configurations were used for the investigations and water flux was measured as a function of temperature. In addition, CO was introduced to the permeate side in order to test the stability of membrane against water and CO/CO2 and post operation analysis of used membranes were carried out. The results revealed that water incorporation occurs on any exposed electrolyte surface. However, the magnitude of water permeation changes depending on which membrane surface is catalytically modified. The platinum increases the water flux on the feed side whilst it decreases the flux on the permeate side. Water flux measurements suggest that platinum can block water permeation on the permeate side by reducing the access to the lattice oxygen in the surface layer. Keywords: BCY20, Water permeation, Perovskites, Fuel cells

Silver-doped strontium niobium cobaltite as a new perovskite-type ceramic membrane for oxygen separation

In this work, we report the silver doping as a novel strategy for developing single-phase mixed conducting ceramic membranes with perovskite-type structure for the efficient oxygen separation from air. Specifically, perovskite-type oxide with a nominal composition of Sr0.95Ag0.05Co0.9Nb0.1O3-δ (SANC) was designed, synthesized and investigated, and a comparative study with the Ag-free SrCo0.9Nb0.1O3-δ (SNC) perovskite-type membrane was also conducted. The incorporation of silver into the perovskite lattice of SNC has been confirmed even after the sintering at 1175 °C using Ag2O as the silver source. An improved sintering feature, enhanced mechanical strength, and an increased electrical conductivity of the SANC membrane when compared to the Ag-free SNC have been observed. Furthermore the chemical bulk diffusion and surface exchange coefficients increased through the silver doping. Above all higher oxygen permeation fluxes were achieved for the SANC membrane when compared with the SNC membrane under the corresponding conditions. Therefore, SANC turned out to be a highly promising ceramic membrane material for oxygen separation.

Experimental investigation and mathematical modeling of oxygen permeation through dense Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) perovskite-type ceramic membranes

Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) perovskite powder was synthesized via EDTA/citrate complexation method. BSCF membranes were formed by pressing powder at 400MPa and sintering at 1100°C for 10h. XRD patterns showed that a high pure powder with cubic structure was obtained. SEM micrographs revealed that the membranes are dense with large grains. Effects of temperature, feed and permeate side oxygen partial pressures, flow rates and membrane thickness on oxygen permeation flux were studied experimentally. A Nernst–Planck based mathematical model, including surface exchange kinetics and bulk diffusion, was developed to predict oxygen permeation flux. Considering non-elementary surface reactions and introducing system hydrodynamics into the model resulted in an excellent agreement (RMSD=0.0617, AAD=0.0487 and R2=0.985) between predicted and measured fluxes. The results showed that oxygen permeation flux increases with temperature, feed side oxygen partial pressure and flow rates, however decreases with permeate side oxygen partial pressure and membrane thickness. Contribution of feed side surface exchange reactions, bulk diffusion and permeate side surface exchange reactions resistances in the total resistance are in the range of 8–32%, 10–81% and 11–59%, respectively. Permeation rate-limiting step was determined using the membrane dimensionless characteristic thickness.

Research progress and materials selection guidelines on mixed conducting perovskite-type ceramic membranes for oxygen production

Oxygen production by air separation is of great importance in both environmental and industrial processes as most large scale clean energy technologies require oxygen as feed gas. Currently the conventional cryogenic air separation unit is a major economic impediment to the deployment of these clean energy technologies with carbon capture (i.e. oxy-fuel combustion). Dense ceramic perovskite membranes are envisaged to replace the cryogenics and reduce O2 production costs by 35% or more; which can significantly cut the energy penalty by 50% when integrated in oxy-fuel power plant for CO2 capture. This paper reviews the current progress in the development of dense ceramic membranes for oxygen production. The principles, advantages or disadvantages, and the crucial problems of all kinds of membranes are discussed. Materials development, optimisation guidelines and suggestions for future research direction are also included. Some areas already previously reviewed are treated with less attention.

Effect of Zirconium Doping on Hydrogen Permeation through Strontium Cerate Membranes

SrCe0.95Tm0.05O3−δ perovskite-type ceramic membranes offer high hydrogen selectivity, thermal stability, mixed protonic−electronic conductivity, and mechanical strength at temperatures above 600 °C...

A novel zincum-doped perovskite-type ceramic membrane for oxygen separation

Zincum-doped ceramic membrane materials based on BaCo 0.4Fe 0.4Zn x Zr (0.2− x) O 3− δ with 0 ≤ x ≤ 0.2 were synthesized by combining citric acid and ethylene-diamine-tetraacetic acid (EDTA) complexing method. X-ray diffraction (XRD) patterns show that the BaCo 0.4Fe 0.4Zn 0.2O 3− δ ceramic oxide exhibits a pure cubic perovskite structure. Oxygen temperature-programmed desorption (O 2-TPD) profile indicates that BaCo 0.4Fe 0.4Zn 0.2O 3− δ possesses a good phase reversibility. An oxygen permeation flux of 0.65 ml/min cm 2 was obtained at 950 °C and a single activation energy of 67 kJ/mol was observed for the oxygen permeation in the temperature range of 600–950 °C. No decline was found during more than 100 h oxygen permeation.

A semi-empirical equation for oxygen nonstoichiometry of perovskite-type ceramics

Explicit equations correlating oxygen nonstoichiometry to oxygen partial pressure and temperature are important for applications of perovskite-type ceramics as membranes, adsorbents and catalysts in various chemical reaction and separation processes. A semi-empirical equation for oxygen nonstoichiometry on perovskite-type ceramics is reported in this paper. Though derived from the results of a point defect model on a perovskite-type ceramic material, La 0.1Sr 0.9Co 0.5Fe 0.5O 3− δ , this equation describes very well the experimentally measured oxygen nonstoichiometry data for two perovskite-type ceramics measured in this work and three perovskite-type ceramics reported in the literature. The major advantage of this semi-empirical equation lies in its simplicity, explicitness and accuracy. This equation is coupled with oxygen permeation equation to predict oxygen permeation current density through two perovskite-type ceramic membranes. The predicted data agree very well with the results reported in the literature using a complex defect reaction model.

Investigation of ideal zirconium-doped perovskite-type ceramic membrane materials for oxygen separation

Zirconium-doped perovskite-type membrane materials of BaCo 0.4Fe 0.6− x Zr x O 3− δ ( x=0–0.4) with mixed oxygen ion and electron conductivity were synthesized through a method of combining citric and EDTA acid complexes. The results of X-ray diffraction (XRD), oxygen temperature-programmed desorption (O 2-TPD) and hydrogen temperature-programmed reduction (H 2-TPR) showed that the incorporation of proper amount of zirconium into BaCo 0.4Fe 0.6O 3− δ could stabilize the ideal and cubic structure of perovskite. Studies on the oxygen permeability of the as-synthesized membrane disks under air/He gradient indicated that the content of zirconium in these materials had great effects on oxygen permeation flux, activation energy for oxygen permeation and operation stability. The high oxygen permeation flux of 0.90 ml cm −2 min −1 at 950 °C, the single activation energy for oxygen permeation in the range of 600–950 °C and the long-term operation stability at a relatively lower operational temperature of 800 °C under air/He gradient were achieved for the BaCo 0.4Fe 0.4Zr 0.2O 3− δ material. Meanwhile, the effect of carbon dioxide on structural stability and oxygen permeability of this material was also studied in detail, which revealed that the reversible stability could be attained for it.

Perovskite-type ceramic membrane: synthesis, oxygen permeation and membrane reactor performance for oxidative coupling of methane

La 1− x Sr x Co 1− y Fe y O 3− δ perovskite-type oxides are typical of mixed-conducting ceramic membrane materials with very high oxygen semipermeability. In this study, several different synthesis methods were compared for the preparation of La 0.8Sr 0.2Co 0.6Fe 0.4O 3− δ (LSCF) powders. The coprecipitation method was found most suitable for preparation of the LSCF powder in terms of processibility into dense ceramic membranes. The oxygen permeation flux through 1.85 mm LSCF membrane exposed to O 2/N 2 mixture and helium is about 1×10 −7 mol/cm 2 s at 950°C. The oxygen permeation flux increases sharply around 825°C due to an order–disorder transition of the oxygen vacancies in the membrane. Oxidative coupling of methane (OCM) was performed in the LSCF membrane reactor with one membrane surface exposed to O 2/N 2 mixture stream and other to CH 4/He mixture stream. At temperatures higher than 850°C, high C 2 selectivity (70–90%) and yield (10–18%) were achieved with a feed ratio (He/CH 4) of 40–90. The C 2 selectivity dropped dramatically to less than 40% as the He/CH 4 ratio decreased to 20. The surface catalytic properties for OCM of LSCF membranes strongly depend on the oxygen activity of the membrane surface exposed to methane stream.

Catalytic Properties of Oxygen Semipermeable Perovskite-Type Ceramic Membrane Materials for Oxidative Coupling of Methane

The catalytic properties for the oxidative coupling of methane (OCM) of La0.8Sr0.2CoO3(LSC) and SrCo0.8Fe0.2O3(SCF) in solid solution were studied and compared with those of 5 wt% Li/MgO, using a steady/unsteady state packed-bed reactor and a transient microbalance. The results of the steady-state cofeed experiments show that LSC possesses OCM catalytic properties similar to those of Li/MgO in terms of C2yield and selectivity at temperatures of around 800°C. The former gives a larger C2space time yield than the latter. SCF exhibits poor OCM catalytic properties at 700–850°C. To further examine the suitability of LSC as a membrane material for use in a dense membrane reactor for OCM, the instant OCM selectivity and activity and oxygen consumption rate for LSC and 5% Li/MgO on exposure to pure methane in cyclic feed mode were measured respectively at 850°C and 800°C. For both materials, the unsteady-state cyclic feed operation gives smaller initial OCM activity and larger initial C2selectivity than the cofeed steady state operation. Li/MgO quickly loses its OCM activity and selectivity in the unsteady state operation due to rapid consumption of the active sites. Up to 5 min of methane run time, LSC maintains appreciable OCM activity with poorer C2selectivity as compared to the steady state cofeed operation. The surface of LSC membrane at low oxygen partial pressure may become nonselective for OCM in membrane reactor applications.