Preparation,Characterization And Performance Of Homologous Dual-phase Mixed-conductive Ceramic Membranes With Extremely High Hydrogen Permeation

Hydrogen is a clean and new energy in the 21 st century, it has gained much attention today. Dense ceramic hydrogen permeable membrane is mixed conductor dense inorganic ceramic membrane, which shows a mixed conducting of protonic and electronic. Dense ceramic hydrogen permeable membrane is so popular because it can efficiently separate the hydrogen from hydrogen mixture gases. Nowadays, the mixed protonic and electronic conducting(MPEC) ceramic membranes are gaining more and more attention for their potential applications in hydrogen separation, hydrogen sensor, fuel cells and catalytic membrane reactors. Today, the hydrogen permeation fluxes through these single-phase membranes are still very low because the hydrogen permeation rate is still limited mainly by electronic conduction even after metal doping, and the thermo-chemical stability and compatibility between metal and ceramic turned out to be critical. Fortunately, the ceramic-ceramic dual-phase membrane has drawn much attention because it can get good ambipolar conductivity and good thermo-chemical stability. BaCeO3 system is selected due to its high protonic conductivity and BaCe0.15Fe0.85O3-δ(BCF1585) was also reported to exhibit good electronic conductivity. Herein, we propose a novel concept for the dual-phase membrane materials design: two phases with the same elemental composition but in different concentration ratios and different structures. A MPEC-MIEC composite dual-phase membrane based on the same elements for hydrogen separation is developed as follows: the MPEC oxide of BaCe0.85Fe0.15O3-δ(BCF8515) near the critical composition of orthorhombic perovskite was chosen as the main protonic conductive phase, and the mixed ionic and electronic conductive(MIEC) oxide of BCF1585 near the critical composition of cubic perovskite was chosen as the main electronic conductive phase in order to get better ambipolar conductivity and higher hydrogen permeation flux, then the dual-phase membranes are prepared and investigated.Firstly, we studied the hydrogen permeation flux of the single phase BCF8515, BCF1585 and different proportions of dual-phase membranes. The BaCe1-xFexO3-δ(x = 0.1, 0.15, 0.2, 0.3, 0.47, 0.7, 0.8, 0.85, 0.9) powders were synthesized by solid phase method. For the BaCe1-xFexO3-δ series of oxides, pure perovskite structure could be obtained with the content of Fe is below 0.15 or above 0.8. Besides, in the range of 0.15 to 0.8, both orthorhombic perovskite and cubic perovskite are observed by the XRD. The BCF8515-BCF1585 dual-phase powders were prepared by mixing the different amounts of BCF8515 and BCF1585. Both the single-phase and mixed dual-phase powders were uniformly pressed into disk-shaped membranes, the black pellets were sintered at high temperature to be dense membranes. After the hydrogen permeation test, the result shows that the mixed 50 wt% BCF8515-50 wt% BCF1585 dual-phase membrane has an extreme high H2 permeation flux due to the improved ambipolar conductivity and the highest bulk phase homogenization of the two phases in this ratio, which can be observed in the BSEM and dyeing BSEM images. The orthorhombic perovskite BCF8515 phase and the cubic perovskite BCF1585 phase can be observed in the 50 wt% BCF8515-50 wt% BCF1585 dual-phase membrane by the XRD.Secondly, we mainly studied the 50 wt% BCF8515-50 wt% BCF1585 dual-phase membrane prepared by mixed method. From the result, we can find that the 50 wt% BCF8515-50 wt% BCF1585 membrane prepared by mixed method has a certain stability at high temperature in the reducing atmosphere. We found that the overall hydrogen permeation is limited by the bulk diffusion and the ambi-polar conductivity of 50 wt % BCF8515-50 wt % BCF1585 dual-phase membrane is much higher than that of BCF8515 and BCF1585 single-phase membranes. The 50 wt% BCF8515-50 wt% BCF1585 dual-phase membranes has an extreme high H2 permeation flux of 0.86 ml/min?cm2, which is obtained at 950 oC with 0.5 mm thickness. The hydrogen permeation flux of the 50 wt% BCF8515-50 wt% BCF1585 dual-phase membrane kept stable at 0.60 ml/min?cm2 for at least 2000 min in hydrogen permeation, which indicates a good stability of the dual-phase membrane under reducing atmosphere. SEM, BSEM, EDS, TEM, STEM, HRTEM and EELS are used for microstructure observation. The BCF8515, BCF1585 and the 50 wt% BCF8515-50 wt% BCF1585 dual-phase powder has not a sufficient total conductivity to enlarge the TPB for H+ + e-?1/2 H2 reaction, it shows poor catalytic activity towards the hydrogen exchange, so the H2 permeation flux of the membrane with oxide catalytic layer has not distanct promotion, but the membrane with a high catalytic layer Pt can reach a higher H2 permeation.Finally, in order to investigate the effect of different synthesis methods on the 50 wt% BCF8515-50 wt% BCF1585 membrane, we investigated the hydrogen permeation of 50 wt% BCF8515-50 wt% BCF1585 membrane prepared by one-pot method. When the 50 wt% BCF8515-50 wt% BCF1585 membrane was prepared by one-pot method, the orthorhombic perovskite BCF8515 phase and the cubic perovskite BCF1585 phase can be observed in the membrane by the XRD. A good two-phase structure can be observed in the SEM and BSEM images. The 50 wt% BCF8515-50 wt% BCF1585 membrane prepared by one-pot method has a certain stability at high temperature in the reducing atmosphere. After the hydrogen permeation test, the result shows that the mixed 50 wt% BCF8515-50 wt% BCF1585 dual-phase membrane has an higher H2 permeation flux than the one-pot method. The hydrogen permeation fluxes increase when the membrane thickness decreases. And the dual-phase membrane can be also used for oxygen permeation flux. For further study, we also found that H2 flux of the one-pot dual-phase membrane will degrade during long hydrogen permeation test.