Preparation And Characterization Of CO₂-Tolerant Mixed Protonic-Electronic Conducting Membrane

Mixed protonic-electronic conducting(MPEC) ceramic is a kind of ceramic materials which possess proton and electron conductivities at high temperature. And MPEC oxides exhibit infinite selectivity to H2 in principle. These MPEC membranes can be used as hydrogen separation membranes, hydrogen sensors and catalytic membrane reactors. In order to meet the requirement of industrialization, the MPEC membranes must exhibit good chemical and thermal stability and high hydrogen permeation flux. Currently, perovskite-type oxides are the most commonly studied mixed protonic-electronic conducting ceramic materials, particularly doped SrCeO3 and BaCeO3 materials. However, the perovskite-type membranes are alkaline-earth metal-containing oxides, which are instable in the exposure to CO2 due to the reaction between CO2 and the alkaline-earth metal ions(e.g., Ba2+, Sr2+). Therefore, the present works focused on lanthanum tungstate, which have quite high mixed protonic-electronic conductivity and high tolerance to acidic gas(CO2 and H2S). We concentrate on development of innovative membrane geometries, new composition by doping techniques and surface modification to improve the hydrogen permeation flux of lanthanum tungstate by increase of bulk diffusion rate, ambi-polar conductivity and surface exchange rate.Firstly, materials of La5.3WO11.25-δ(LWO53), La5.4WO11.25-δ(LWO54), La5.5WO11.25-δ(LWO55) and La5.6WO11.25-δ(LWO56) were prepared through a solid state reaction. The phase structures of the powders at different temperatures were characterized by XRD, which indicate that the calcined temperature for necessary to obtain a single phase increases with the increase of La/W ratio. The oxygen absorption of LWO53, LWO54, LWO55 and LWO56 powders was studied by TG and O2-TPD. The powder of LWO56 possesses the highest capacity of oxygen absorption. Moreover, the hydrogen permeation properties through the membranes with different La/W ratios were investigated, and the hydrogen permeation fluxes of lanthanum tungstates with different La/W ratio were very small, although LWO56 showed a higher hydrogen permeation flux.Secondly, in order to improve the bulk diffusion rate of lanthanum tungstate, we chosen hollow fibre as membrane geometry. A Mo-substituted lanthanum tungstate mixed protonic-electronic conductor, La5.5W0.6Mo0.4O11.25-δ(LWM04), was synthesized. And dense U-shaped LWM04 hollow-fiber membranes were successfully prepared using wet-spinning phase-inversion and sintering method. The stability of LWM04 in a CO2-containing atmosphere was investigated using in situ XRD and TG, and the results demonstrate that LWM04 powder possesses good stability under pure CO2 atmosphere. The effects of temperature, steam, hydrogen concentration and CO2 on hydrogen permeation flux through LWM04 hollow-fiber membrane were studied in detail. A high hydrogen permeation flux of 1.36 mL/min?cm2 was obtained for the U-shaped LWM04 hollow-fiber membrane at 975 °C when a mixture of 80%H2-20%He was used as the feed gas and the sweep side was humidified. Moreover, the hydrogen permeation flux did not significantly decrease over 70 h of operation when fed with a mixture containing 25% CO2, 50% H2, and 25% He, indicating that the LWM04 hollow-fiber membrane has good stability under a CO2-containing atmosphere.Thirdly, for the enhancement of ambi-polar conductivity of lanthanum tungstate, Mo and Nb ions were doped into W site. La5.5W0.45Nb0.15Mo0.4O11.25-δ(LWNM) has been developed by partial co-substitution of W with Nb and Mo in La5.5WO11.25-δ aiming to improve the hydrogen permeation property. The XRD and SEM results reveal that the doping of Mo ions decreases the sintering temperature and diminishes grain size of LWN. For LWNM disk membrane, a hydrogen permeation flux of 0.195 m L/min?cm2 was obtained at 1000 °C with a mixture of 50%H2-50%He as the feed gas under dry condition. More importantly, there was no obvious decline of the hydrogen flux during 80 h operation even with CO2 in the feed gas, and the phase structure and densification were unchanged after the hydrogen permeation test, indicating that the LWNM membrane possesses high chemical stability and hydrogen permeation stability under CO2 atmosphere.Finally, in order to improve the surface exchange rate of lanthanum tungstate, surface modification was used by Pt coating. The effects of Pt layers on feed side, sweep side and both sides on the hydrogen permeation flux of LWNM membrane, apparent activation energy and rate controlling step were studied, respectively. When the Pt layer was on the feed side, the hydrogen permeation flux increased slightly; When the Pt layer was on the sweep side, the hydrogen permeation flux increased a lot; When the Pt layers were on both sides, the hydrogen permeation flux was even higher than that of the Pt layer was on the sweep side. Compared to the LWNM membrane without Pt layer, the apparent activation energy of hydrogen permeation process decreased after surface modification. The Wagner Theory was used to calculate the rate controlling step. When the Pt layer was on the sweep side or both sides of LWNM disk membrane, the hydrogen permeation is influenced by the bulk diffusion together with the surface exchange reaction. Moreover, the bulk diffusion is predominant. The above results indicated that the existence of Pt layers on the membrane surface is beneficial to hydrogen permeation, and for LWNM disk membrane, a highest hydrogen permeation flux can be achieved by surface modification with Pt on both sides of the membrane.