Preparation And Hydrogen Permeation Of Nd Doped BaCeO₃ Membranes

The mixed-conducting perovskite hydrogen permeable membranes are dense ceramic membranes with both protonic and electronic conductivities under hydrogen containing atmosphere at elevated temperatures. Such membranes have great potential applications in gas separation and alkaline conversion.In this thesis, a mixed proton-electron conducting perovskite made of BaCe_0.95Nd_0.05O_3-δ (BCNd5) was prepared by EDTA/citric acid complexing method. The precursors calcined at different temperatures were investigated by differential scanning calorimetry (DSC), thermogravimetry (TG), and X-ray diffraction (XRD). In order to learn the perovskite formation process during the calcination, the intermediate, i.e. the sample calcined at 750 oC for 5 hours, was investigated by scanning transmission electron microscopy (STEM), energy-filtered transmission electron microscopy (EFTEM), and high-resolution transmission electron microscopy (HRTEM) as well as electron energy-loss spectroscopy (EELS). The results revealed that the perovskite structure was formed via a solid-state reaction between barium - cerium mixed carbonate and cerium - neodimium mixed oxide particles.Dense mixed conducting BaCe_0.95Nd_0.05O_3-δ (BCNd5) membranes were made by pressing BCNd5 powder followed by sintering. The hydrogen permeation through the BCNd5 membrane was studied using a high-temperature permeator combined a gas chromatography (GC). Hydrogen permeation properties of the membrane were studied under different conditions. Hydrogen permeation fluxes increase with temperatures. At 925 oC, the hydrogen permeation flux reaches 0.017 ml/min·cm2 under dry conditions. However, the hydrogen permeation fluxes under wet conditions are higher than those under dry conditions, which is due to increased proton concentrations for the H⁺ hopping via OH groups. The hydrogen permeation increased with increasing the hydrogen and steam concentrations in the feed. For a steam concentration of 15 vol.%, the hydrogen permeation flux reaches 0.026 ml/min·cm2. The microstructure of the membranes before and after permeation was investigated by scanning electron microscopy (SEM). The results showed that the membrane became porous after hydrogen permeation.In order to improve the stability of the material, Zr-doping on the hydrogen permeation and chemical stability of barium cerate ceramic membranes were investigated. The hydrogen permeation ability of BaCe_0.85Nd_0.05Zr_0.10O_3-δ (BCNZ10) was studied and the phase structure and microstructure of the membrane were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive x-ray spectrometry (EDXS) technology. It was found that the Zr was introduced into the perovskite-type oxide. SEM showed that the BCNZ10 membrane was still dense even after hydrogen permeation, which demonstrated that Zr-doping can improve the stability of BCNd5.