Preparation And Characteristic Of Mixed Conducting Membranes For Hydrogen Separation

The mixed-conducting perovskite hydrogen permeable membranes are dense ceramicmembranes with both protonic and electronic conductivities under hydrogen containingatmosphere at elevated temperatures. Such membranes have great potential applications in gasseparation. One of the demands for proton conductor for practical application is intrinsic highprotonic conductivity; on the other hand, chemical stability is also a prerequisite forsuccessful application. Strontium cerates generally exhibit high proton conductivities,however, these materials are unstable at high temperatures in the presence of CO2and steam.Because the higher lattice acidity from the electronegativity of atom in the B site wouldredound to the chemical stability. With that in mind, partial replacement of Ce⁴⁺by highelectronegative Nb⁵⁺can increase the chemical stability.In this paper, SrCe_0.95-xNb_xTm_0.05O_3-δpowders were synthesized by a modified sol-gelmethod using citrate as a chelating agent, and the membranes of SrCe_0.95-xNb_xTm_0.05O_3-δwereprepared by pressing followed by sintering. The stages of SrCe_0.95-xNb_xTm_0.05O_3-δformationand its characterization were examined using thermogravimetric（TG）, X-ray diffraction（XRD）, scanning electron microscopy（SEM）. The pure perovskite-type structure and goodsintered morphologies according to XRD and SEM results demonstrate that the modifiedsol-gel method to prepare dense membrane is feasible. A improvement in the chemicalstability against CO2and/or boiling water of SrCe_0.95-xNb_xTm_0.05O_3-δceramics was certifiedwith introduction of Nb into doped strontium cerate. And it was found that the Nbintroduction improved significantly higher chemical stability of the grain than grain boundaryin boiling water atmosphere. Electrical conductivities of these solid solutions measured by thefour-point DC method under different atmospheres were reported: in O2/N₂atmosphere, in10%H₂/He and CO/He gas mixture atmospheres. Hydrogen permeation through theSrCe_0.95-xNb_xTm_0.05O_3-δmembranes was carried out using gas permeation setup under variousoperating conditions. Hydrogen permeation flux for SrCe_0.94Nb_0.01Tm_0.05O_3-δmembranereaches0.0322mL·min^-1·cm^-2at upstream50%H₂/He at900℃. The average activationenergy for hydrogen permeation is73kJ·mol^-1in the temperature range between700℃and900℃. The activation energy of hydrogen permeation is close to the activation energy ofelectronic conduction of the materials, confirming that the hydrogen permeation is determinedby the electronic conductivity of the material. Nb⁵⁺doping can not only enhance the stabilityof the materials, but also decrease the electrical conductivity and the hydrogen permeationflux.