Preparation,Characterization And Performance Of CO₂-Tolerant Mixed Protonic-electronic Conducting Membrane

In recent years, mixed protonic-electronic conducting（MPEC） ceramic materials have gained considerable attention. They can be used as hydrogen separating membranes and catalytic membrane reactors（CMRs） at high temperature. Among the different proton conductors, rare-earth tungstates are regarded as one of the most promising hydrogen separating membrane materials in the future due to their excellent chemical stability against acid gases and sufficient ambipolar conductivity. The hydrogen permeation flux of MPEC membranes were mainly influenced by the membrane wall thickness, the ambipolar conductivity, the hydrogen partial pressure gradient, the catalytic activity of membrane surface and temperature. In this work, we focused on Nd_5.5W_0.5Mo_0.5O_11.25-δ（NWM） compound and aimed to improve its hydrogen permeation properties from the points of optimizing the membrane structure and increasing the conductivity.In order to enhance the hydrogen permeation flux of MPEC membranes, we prepared the U-shaped NWM hollow fiber membrane which possesses the advantages of thin wall thickness as well as large specific surface area and investigated its hydrogen separating performances. The NWM powder was synthesized by the solid state reaction method and the NWM hollow fiber precursor was fabricated through a wet-spinning phase-inversion technique. The U-shaped precursor was then sintered at 1500 oC for 10 h to get dense membrane. According to the hydrogen permeation results, the hydrogen permeation flux of the U-shaped NWM hollow fiber membrane increased with the increasing of operating temperature and hydrogen partial pressure gradient across the membrane. And the biggest hydrogen production flux was obtained with the sweep side humidified. A high hydrogen permeation flux of 1.29 m L/min·cm² was achieved at 975 oC when using 80% H₂-20% He as feed gas and humidified Ar as sweep gas.H₂ was mostly generated with CO₂ as the by-product simultaneously in the practical industry. Therefore, it is necessary for MPEC membranes to possess good chemical stability when separating H₂ from such CO₂-containing gas mixture. We studied the hydrogen permeation properties and stability of the U-shaped NWM hollow fiber membrane in the CO₂-containing atmosphere. The NWM powder was treated by CO₂ at 900 oC for 48 h and examined by TG. The results indicated no formation of carbonates. CO₂ was introduced into the feed side of the U-shaped NWM hollow fiber membrane. When the concentration of CO₂ was changed from 0 to 40%, the corresponding hydrogen permeation flux decreased from 0.58 mL/min·cm² to 0.30 m L/min·cm² as a result of the reaction of CO₂ and H₂ and the competitive adsorption of them in the membrane surface. However, the hydrogen permeation flux recoverd to 0.58 mL/min·cm² immediately after CO₂ was substituted by inert gas. The hydrogen permeation stability of the U-shaped NWM hollow fiber membrane in CO₂-containing atmosphere was also investigated. The hydrogen permeation flux of the membrane maintained at around 0.16 mL/min·cm² during 80 h when using the CO₂-containing feed gas. The XRD and SEM results of the spent NWM hollow fiber membrane showed the membrane kept the unchanged phase structure and the intact gastight microstructure, indicating the good chemical stability against CO₂ of the U-shaped NWM hollow fiber membrane.In order to get higher hydrogen permeation fluxes through the NWM-based membrane, W7+ was partially substituted by Nb5+. Aliovalent ion doping can result in the increase of oxygen vacancy concentration and oxygen ion conductivity. The Nd5.5W0.35Mo0.5Nb0.15O11.25-δ（NWMN） powder was synthesized by the solid state reaction method and the disk-shaped NWMN membrane was prepared via pressing under 20 MPa in a stainless steel module for 10 min followed by sintering at 1500 oC for 10 h. The chemical stability of the NWMN powder in CO₂-contaning atmosphere was investigated by CO₂ treatment at 900 oC for 48 h and thermogravimetry from room temperature to 800 oC. No formation of carbonates was observed in the experimental process, indicating the good CO₂-tolerant stability of the NWMN compound. The hydrogen permeativity of the NWMN and NWM membrane was performed in a home-made high temperature apparatus and the results showed that the hydrogen permeation properties of the NWMN membrane were better than that of the NWM membrane. For instance, the hydrogen permeation flux of the NWMN membrane was 1.5 times as that through the NWM membrane at 1000 oC. This result implied that the method of W7+ partially substituted by Nb5+ to improve the conductivity and hydrogen permeation flux was feasible.In conclusion, either optimizinging the membrane structure or enhancing the conductivity is able to increase the hydrogen permeation flux of the MPEC membranes. The good chemical stability of the NWM material in CO₂-containing atmosphere was also proved in our work, demonstrating that the NWM-based mixed conductors have a great potenrial for hydrogen seperation and purification in the future.