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Hydrogen (H2)-selective membrane is an enabling technology for high purity H2 production from natural gas. Here, we demonstrate the feasibility to apply BaCo0.4Fe0.4Zr0.1Y0.1O3-δ (BCFZY), a widely studied MPEC composition with high electronic conductivity as H2 permeation membrane by sputtering thin Palladium (Pd) films on its feed and permeate surfaces as a protective layer to prevent BCFZY reduction upon long-term exposure to reducing H2. Pd thin film also acts as a catalytic layer to enhance H2 dissociation in the feed surface and H association in the permeate surface. Pd-sputtered BCFZY (Pd|BCFZY|Pd) membrane displayed exceptionally high H2 flux that varies from 0.101 rise to 0.213 mL min−1 cm−2 (450 °C–650 °C) under the 10% H2-90% N2/Argon (Ar) gradient. The attainment of significant H2 permeability below 500 °C shown here indicates the possibility to lower the operating temperature for H2 permeation using MPEC materials. Pd|BCFZY|Pd membrane also demonstrated stable, high H2 flux that fluctuates more or less 0.138 mL min−1 cm−2 during the continuous operation over 225-h duration.

BaZr 0.8 Y 0.2 O 3 (BZY) may be one of the most promising proton conductors for hydrogen separation membranes and proton-conducting solid oxide fuel cells. Doping strategies have been widely applied to improve the sintering activity and conductivity of BZY ceramics. In this study, the phase structure, sinterability, chemical stability and conductivity of BaZr 0.6 M 0.2 Y 0.2 O 3− δ (BZMY) compositions with various M cations of Zr 4+ , Ce 4+ , Pr 3+ , Nd 3+ , Sm 3+ and Gd 3+ with different ionic radii were comparatively studied. All the rare-earth dopants could improve the sinterability of BZY. As the ionic radii of the dopants decreased, the oxides are more and more stable in the CO 2 atmosphere. EIS results indicated that a greater ionic conductivity was achieved for BZMY with a larger M dopant. Among the various composites, BZNY and BZCY showed favorable protonic conductivity at 600 °C in a wet 10% H 2 –Ar atmosphere, with good sinterability and resistance to CO 2 attack. A peak power density of 153 mW cm −2 was achieved at 700 °C with a fuel cell that contained BZCY as an electrolyte and BSCF as a cathode.

BaZr 0.4 Ce 0.4 Y 0.2 O 3− δ (BZCY4) has been widely considered to be a promising electrolyte material for H + -SOFC, but it is restricted to commercial applications due to its poor densification behavior. A dense BZCY4 pellet was obtained by sintering at 1250 °C after impregnating the material with a zinc nitrate solution. The dilatometer curves and scanning electron microscopy (SEM) images indicated that the sinterability of the BZCY4 material is effectively improved by impregnating the green membrane with 4 wt% Zn. Moreover, EDX mapping indicated that the Ba, Zr and Ce elements were homogeneously distributed in the BZCY4+4 wt% Zn sample sintered at 1250 °C. In addition, an integrated SOFC employing a BZCY4+4 wt% Zn electrolyte was successfully fabricated without any cracks and delamination by impregnating the BZCY4 electrolyte membrane with zinc nitrate as a sintering aid. This single cell with a 25 mm thick BZCY4+4 wt% Zn electrolyte membrane exhibited power densities as high as 360 and 276 mW cm −2 at 700 and 600 °C, respectively. Electrical conductivity measurements demonstrated that the total conductivities of BZCY4+4 wt% Zn were 0.46×10 −2 S cm −1 , 0.56×10 −2 S cm −1 , 0.20×10 −2 S cm −1 and 0.40×10 −2 S cm −1 at 600 °C in air, wet air, 10% H 2 –Ar and wet 10% H 2 –Ar, respectively.

Chemically stable BaZr 0.8 Y 0.2 O 3− δ (BZY) oxide is limited to applications as an electrolyte for solid oxide fuel cells (SOFCs) because of its poor sintering behavior. This study attempts to improve the sinterability and conductivity of BZY using the partial substitution of Zr 4+ in BZY with Nd 3+ . An oxide with the nominal composition of BaZr 0.7 Nd 0.1 Y 0.2 O 3− δ (BZNY) is specifically investigated. Results from X-ray diffraction (XRD) demonstrate Nd 3+ is successfully doped into the lattice as anticipated and transmission electron microscopy (TEM) characterizations verify the morphology and crystal structure of the BZNY powder. Dilatometric measurement and scanning electron microscopy (SEM) observations provide verification that the sinterability of the oxide is effectively improved by introducing Nd 3+ . XRD and CO 2 -TPD results demonstrate that BZNY is relatively stable with respect to the CO 2 atmosphere. The total conductivity of BZNY in wet H 2 is 2.76×10 −3 S cm −1 at 600 °C. An anode-supported thin-film BZNY electrolyte (∼30 μm) cell is fabricated, and the electrolyte layer is found to be well densified after co-sintering with the anode substrate at 1450 °C for 5 h. The cell delivers a peak power density of 142 mW cm −2 at 700 °C, higher than the reported values for a similar cell with BZY electrolyte.

The effect of high-energy ball milling (HEBM) of the starting material of crystalline Ba0.5Sr0.5Co0.5Fe0.5O3-delta (BSCF) powders on the sintering and oxygen permeability of the corresponding ceramic membrane was systematically investigated. Two different methods of dry milling and wet milling in a liquid alcohol medium were investigated along with three ball milling times (1,2 and 3 h), two different types of starting powders, and three different sintering temperatures (1000, 1050 and 1100 degrees C). XRD, SEM and oxygen permeation measurements were performed on as-prepared membranes. The experimental results showed that HEBM is an effective way to improve the sintering, microstructure and oxygen permeability of BSCF membranes. By optimizing the HEBM process, the relative density of BSCF membranes improved significantly; as a result, the oxygen permeation flux of BSCF membranes improved by about 20% in comparison to BSCF membranes whose starting powders were not ball milled. Thus, HEBM is a promising way to increase the performance of BSCF membranes for oxygen separation. (C) 2010 Elsevier B.V. All rights reserved.

The potential application of combustion synthesis Of La(0.6)Sr(0.4)Co(0.2)Fe(0.8)O(3-delta) (LSCF) based on a modified ethylenediaminetetraacetic acid (EDTA)-citrate complexing method with NH(4)NO(3) as combustion aid for ceramic oxygen separation membrane was systematically investigated. Depending on the relative amount of NH(4)NO(3) applied during the synthesis, the combustion can proceed in three different modes: self-propagating combustion, volumetric combustion and smothering combustion. As a whole, electrical conductivity and oxygen permeation fluxes of derived LSCF membrane increased with sintering temperature (1000-1300 degrees C). Among three different combustion modes, LSCF from self-propagating combustion showed the highest sintering ability, electrical conductivity and oxygen permeability. Under optimized conditions, the derived membrane exhibited the permeation fluxes comparable to that of LSCF membrane prepared from the normal EDTA-citrate complexing method. (C) 2008 Elsevier B.V. All rights reserved.

SrCo1-yNbyO3-delta (y = 0.025-0.4) were synthesized for oxygen separation application. The crystal structure, phase stability, oxygen nonstoichiometry, electrical conductivity, and oxygen permeability of the oxides were systematically investigated. Cubic perovskite, with enhanced phase stability at higher Nb concentration, was obtained at y = 0.025-0.2. However, the further increase in niobium concentration led to the formation of impurity phase. The niobium doping concentration also had a significant effect on electrical conductivity and oxygen permeability of the membranes. SrCo0.9Nb0.1O3-delta exhibited the highest electrical conductivity and oxygen permeability among the others. It reached a permeation flux of similar to 2.80 x 10(-6) mol cm(-2) s(-1) at 900 degrees C for a 1.0-mm membrane under an air/helium oxygen gradient. The further investigation demonstrated the oxygen permeation process was mainly rate-limited by the oxygen bulk diffusion process. (c) 2008 Elsevier B.V. All rights reserved.

(Ba 0.5 Sr 0.5 )(Co 0.8 Fe 0.2 ) y O 3− δ ((BS)(CF) y , 1.00 ≥ y ≥ 0.77) oxides were investigated for oxygen separation application with emphasis on long-term operational stability. Pure phase cubic perovskite was formed at y ≥ 0.83. The oxygen nonstoichiometry increased while electrical conductivity and permeability decreased with the decrease of y . However, the (BS)(CF) 0.97 membrane still displayed an attractive oxygen flux as high as 2.4 × 10 −6 mol cm −2 s −1 at 900 °C, as compared to 2.5 × 10 −6 mol cm −2 s −1 for a cation stoichiometric BSCF membrane. The B-site deficiency greatly restrained the A-site cation diffusion and stabilized the perovskite structure and permeation properties of the membranes. During the long-term operation of the (BS)(CF) 0.97 membrane at 850 °C for more than 300 h, a stable permeation flux of (1.8 ± 0.3) × 10 −6 mol cm −2 s −1 was achieved. Further investigation demonstrated that the improved importance of oxygen bulk-diffusion regime in the rate-determination of oxygen permeation through the membranes.

(Ba 0.5 Sr 0.5 ) 1− x Co 0.8 Fe 0.2 O 3− δ (BSCF) ( x = 0–0.3) oxides were prepared by a combined EDTA–citrate complexing method. The crystal structure, electrical conductivity, sintering behavior and oxygen desorption property of the oxides were studied by X-ray diffraction (XRD), four-probe direct current (DC) conductivity, environmental scanning electron microscopy (ESEM) and oxygen temperature-programmed desorption (O 2 -TPD) technologies, respectively. At x = 0.0–0.15, BSCF had a cubic perovskite structure, the lattice parameter and electrical conductivity both decreased steadily with increasing A-site deficiency. The A-site deficiency resulted in the increasing oxygen vacancy concentration in the lattice structure and improved the sintering of the oxides. BSCF ( x = 0.03) showed the highest oxygen permeability with the flux reaching 3.5 × 10 −6 mol cm −2 s −1 [STP: standard temperature and pressure] at 900 °C through a 1.0 mm thickness membrane. Long-term stability test of BSCF ( x = 0.03) membrane at 850 °C indicated a slight deterioration of the permeation flux, which was attributed in part to the increased A-site deficiency of the bulk from Ba 2+ and Sr 2+ segregation over the membrane surface.

In this work, the effects of sintering temperature on the phase structure, oxygen nonstoichiometry, microstructure, electrical conductivity, and oxygen permeation behavior of perovskite La0.6Sr0.4Co0.2Fe0.8O3-delta membranes were systematically studied. The sintering temperature has negligible effect on the bulk properties of the phase structure and the oxygen nonstoichiometry, but has significant influence on the microstructure and electrical conductivity of the membranes, and therefore imposes large impact on its oxygen permeability. With an increase in the sintering temperature, the grain size increased steadily from 0.3 mu m at 1000 degrees C to 3.5 mu m at 1300 degrees C, accompanied with the substantial enhancement of electrical conductivity but the decrease of activation energy for the electrical conduction. Results indicate that the grain boundary had a much lower electrical conductivity than that of the bulk. The oxygen permeation process was mainly rate-determined by the slow oxygen diffusion over the grain boundary. Higher sintering temperature lowers the activation energy for oxygen permeation and accordingly gives rise to higher oxygen permeation flux. The oxygen permeation flux could be improved more than 10 times if the sintering temperature increased from 1000 to 1300 degrees C. (c) 2007 Elsevier B.V. All rights reserved.

A configuration of dense mixed ionic and electronic conducting (MIEC) membrane with layered morphological structure for oxygen separation, which combines the benefits of high oxygen permeation flux of cobalt-based membrane, high chemical stability of iron-based perovskite and high mechanical strength of thick membrane, was studied. The membrane is normally composed of two layers; each layer is a dense MIEC oxide. The substrate layer is a thick dense membrane with high oxygen permeability but relatively lower chemical stability. The feasibility of dense thick Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCF5582) membrane as the substrate layer and Ba0.5Sr0.5Co0.2Fe0.8O3-delta (BSCF5528) as the thin-film layer was mainly experimentally investigated. Both the BSCF5582 and the BSCF5528 show the same cubic perovskite structure and the similar lattice constant with no detrimental reaction products formed. By optimizing fabrication parameters of a simple dry pressing process, dual-layered membrane, free of cracks, was successfully fabricated. The oxygen permeation flux of a dual-layered membrane with the thin-film BSCF5528 layer facing to the sweep gas reached 2.1 mL cm(-2) min(-1) [STP] (1.56 x 10(-6) mol cm(-2) s(-1)) at 900 degrees C, which is about 3.5 times higher than that of the BSCF5528 membrane (0.6 mL cm(-2) min(-1), [STP] (4.46 x 10(-7) mol cm(-2) s(-1)) under the same conditions. (C) 2007 Elsevier B.V. All rights reserved.