Study Of High Performance And High Stability Dense Ceramic Hydrogen Separation Membrane

Hydrogen is clean energy materials and important industrial chemicals. Particularly with the environmental degradation in recent years, people desire that the fossil fuel can be substituted by a clean energy. There are two main sources for hydrogen:one is the stream reforming of natural gas; the other is the coal or biomass gasification. The primary products after reactions are a mixing gas containing H2, CO and CO2etc, so hydrogen separation is an indispensable process to obtain the high-purity hydrogen gas. Currently, hydrogen can be purified through one (or a combination) of three major processes:(1) pressure swing adsorption (PSA), fractional/cryogenic distillation, or membrane separation. PSA and fractional/cryogenic distillation systems are in commercial operation, while membrane separation is currently considered to be the most promising because of low energy consumption, possibility for continuous operation, dramatically lower investment cost, its ease of operation and ultimately cost effectiveness. Hydrogen separation membranes also have various types, and the dense ceramic membrane is one of them. People have carried on a large amount of research on dense ceramic membranes and yielded a rich harvest:the hydrogen separation efficiency remarkably increased when previous single-phase membranes developed into present metallic-ceramic membranes; the badly chemical stability of cerate has been improved significantly through doping high-electronegativity element. These developments bring a bright prospect for application of hydrogen separation membranes, but some problems have to be settled urgently. This thesis focuses on hydrogen permeation performance and chemical stability of dense ceramic hydrogen separation membrane.Chapter1is the literature review. Firstly, it briefly describes proton conductors and its potential applications as well as hydrogen permeation principle. Next, the current developing situation of ceramic hydrogen separation membrane is introduced in detail, specially for SrCeO3, BaCeO3, La2Ce2O7and La6WO12material system. Finally, the thesis presents development direction and burning questions of ceramic membrane.In chapter2, Ni-Ba(Zr0.1Ce0.7)Y0.2O3-δ(BZCY) metal-ceramic asymmetric membranes were prepared via a method to combine co-pressing technique and two-step sintering process, and developed as hydrogen permeation membrane for the first time. The key process is one-step synthesis of NiO-BZCY powders using a citrate-nitrate combustion method. The obtained powders are very fluffy, and in which NiO and BZCY keep well chemical compatibility and mixing homogeneity. Another key factor is two-step sintering process, sintering in air to remove the pore-forming agent and sintering in5%H2/Ar to obtain a dense top membrane. Hydrogen permeation flux through a30-μm-thickness asymmetric membrane achieves1.37×10-7mol cm-2s-1at900℃when using20%H2/N2(with3%of H2O) as feed gas and dry high purity Ar as sweep gas. At last, the relationship between hydrogen permeation fluxes and membrane thicknesses were investigated.Chapter3showed the study on preparation technology and permeation properties of Ni-La0.5Ce0.5O2-δ(LDC) asymmetrical membranes. The asymmetrical membrane was fabricated by one-step sintering process by adopting inorganic pore former NH4HCO3. The permeation fluxes respectively increases with increasing of temperature and hydrogen partial pressure gradient; at900℃the value is6.8×10-8mol cm-2s-1under20%H2/N2(with3%H2O) as feed gas and dry Ar as sweep gas, which has a great growth in comparison with1.57×10-8mol cm-2s-1of symmetric membrane. The effect of water vapor on both sides of membrane was studied:adding water vapor into feed gas or sweep gas all result in a increasing of hydrogen fluxes, particularly sweep gas, increasing by1.5-2.0times. A long-term testing operated in CO2-containing atmosphere demonstrates Ni-LDC membrane is a stable membrane for hydrogen separation.In chapter4, the hydrogen permeation performance of single-phase mixed electronic-protonic conducting membrane was investigated. The testing of fuel cell with LDC electrolyte ascertained the electronic conductivity existing in LDC. Therefore, LDC asymmetric membrane was fabricated and then its properties were investigated. The LDC membrane was dense sintering at1350℃for5h. Permeation fluxes increased from3.27×10-9to2.67×10-8mol cm-2s-1with the temperature increasing from700℃to900℃under20%H2/N2(with3%H2O) as feed gas. As the hydrogen partial pressure in feed gas increased from0.2to0.8atm, permeation fluxes increased from2.67×10-8to4.51×10-8mol cm-2s-1at900℃. The water vapor in feed gas had a negative effect because it caused the decreasing of LDC electronic conductivity; the water vapor in sweep gas had a positive influence because water splitting reaction produced extra hydrogen. In chapter5, Ni-Ba(Zr0.7Pr0.1)Y0.203-δ(BZPY) mixed electronic-protonic conductor was employed as the hydrogen separation membrane for the first time. Dense samples were obtained by combining dry-press and5%H2/Ar-atmosphere sintering process. Hydrogen permeation properties were systemically studied. The hydrogen fluxes of400μm-thickness membrane increase with increasing temperature in range of700-950℃and the value was1.21×10-8mol cm-2s-1at950℃under humid40%H2/N2as feed gas and dry Ar as sweep gas. Water vapor efficiently increased the hydrogen permeation performance and reduced the corresponding activity energy. Thickness dependence of hydrogen permeation through Ni-BZPY composite membranes revealed that the bulk diffusion was the rate determining step of hydrogen permeation throughout the investigated thickness range. The Ni-BZPY membrane maintained steady output of hydrogen permeation under humid and CO2-containing atmosphere during40-hours testing.Chapter6reported the BZCY asymmetric membrane with an external short circuit for hydrogen permeation, which was fabricated by covering Ni-BZCY supported BZCY membrane surface and flank using a porous Pt layer. Due to increase the volume of proton conducting phase, the permeation flux further increased correspondingly. Permeation flux reached1.71×10-7mol cm-2s-1at900℃when using20%H2/N2(with3%of H2O) as feed gas. The influence of hydrogen partial pressure in feed gas and flow rate of sweep gas was researched and the results indicated that the bulk diffusion controlled the hydrogen permeation process. The long-term stability testing indicated that the asymmetric membrane remained steady output under3%CO2atmosphere, but under20%CO2the permeation performance degraded by about8%.