| With the stress of global climate change and the development of fuel cells, hydrogen is expected to be the clean fuel which can substitute oil-based fuels in the future.The high cost of hydrogen production is a choke point in its commercialization process as a fuel.As a low energy cost process,membrane technology is promising in the hydrogen separation from product gases which are generated by reforming or gasification of fossil fuels and biomass.As the gas mixtures usually contain H2O,CO2 and H2S,the membranes must be chemically stable in these atmospheres.Dense cermets(ceramic-metal composite)composing of Ni and high temperature proton conductors such as doped barium cerates(BaCeO3)and doped ceria have been developed as cost-effective materials for hydrogen separation membranes.This thesis concerns the hydrogen permeation performance and chemical stability of Ni-BCY/CeO2 based hydrogen separation membrane.Chanpter 1 describes applications of high-temperature proton conductors in hydrogen economy and petrochemical industry,the progress of research on the high-temperature proton conductors,the mechanism of proton incorporation and conduction in perovskite-type proton conductors.In chapter 2,the preparation condition of Ni-BaCeO3 is investigated.When pellets are sintered in a gas flow,BaCeO3 is ready to react with Al2O3 powder brought by the gas flow from the bubble stick made of alumina.The reaction forms BaAl2O4 and doped CeO2,reducing the relative densities of the pellets.The results of hydrogen permeation measurement show that a membrane containing 30%Ni in volume possesses the highest performance.Besides,the double doping at the Ce site turn to be useless or even harmful.Unlike the increase of the oxide ion conductivity caused by co-doping,no obvious increase in proton conductivity is observed in BaCeO3.In chapter 3,the hydrogen permeation performance and chemical stability of Ni—BaCe0.9Y0.1O3-δ(Ni—BCY)and Ni—BaZr0.05Ce0.85Y0.1O(3-δ)(Ni—BZCY)in moisture between 600 and 750℃is investigated.For Ni—BCY,when the feed gas contains a H2O level of less than 1.2 kPa,the increase in H2O concentration is beneficial to the hydrogen permeation performance.However,when the H2O concentration is above the value,the increase in H2O concentration takes the opposite effect.After the experiments,the analysis on microstructure and phase composition show the formation of Ba(OH)2 and CeO2,which is caused by the reaction between H2O and BaCeO3.Results of thermodynamic calculations based on the data supplied by Tanner show that BaCeO3 reacts with H2O at a H2O level of about 1.1 kPa below 750℃,and is consistent with the experimental results.When BCY is doped with 5% Zr,its stability in moisture is improved.When Ni-BZCY samples are exposed to H2O, their performance increase with the H2O level after the iniatial drop.In chapter 4,the hydrogen permeation performance and chemical stability of Ni -BaZr0.1Ce0.7Y0.2O3-δ(Ni-BZCY)in H2S is investigated.At 900℃,performance loss of hydrogen permeation starts when the H2S concentration is over 50 ppm and increase with increasing H2S levels.The performance loss in 60,100,300 ppm H2S is 3%,19%and 45%,respectively.X—ray diffraction results show that BaS,doped CeO2,Ce2O2S,Ni3S2 is formed.With the increase in performance loss,the degree of reaction between H2S and BaCeO3 increases.Only a little BaCeO3 reacts in 60 ppm H2S,but all BaCeO3 on the surface disappear in 100 ppm H2S,causing volume expansion and fracture on the surface of the sample.CeO2 reacts with H2S and H2 forming Ce2O2S and H2O in 300 ppm H2S,causing severer volume expansion and surface fracture.Thus H2S can reacts with more BaCeO3 and continue to decrease the hydrogen permeation flux.When H2S in the feed gas is removed,the hydrogen permeation flux recovers completely,with an unexpected increase(2%)which is related with the porosity of the surface.Besides,results of thermodynamic calculations show that BaCeO3 begins to react with H2S at a level of 55 ppm in an 1.5%H2O-containing atmosphere at 900℃,which also proves that the reaction between BaCeO3 and H2S is the cause of the performance loss.It is likely that the tolerance of Ni-BZCY to H2S can be improved by increasing the H2O level in the feed gas.At 700℃,the performance loss of Ni-BZCY in 30,60 ppm H2S is 20%and 30%, respectively.XRD analysis show the formation of slight BaS,doped CeO2 and Ni3S2.The adsorption of sulfur on Ni surface should be the cause of performance loss in this condition.In chapter 5,hydrogen separation membranes composed of doped ceria and nickel(Ni—LaX-YCaYCe1-XO2-δare fabricated.The effects of La,Ca doping at the Ce sites are investigated.The highest performance is achieved when X=0.5 and Y=0.0125.The dependence of hydrogen permeation flux on membrane thickness is also studied.When the thickness is above 0.51 mm,the hydrogen permeation process is controlled by the bulk diffusion step.When the thickness is 0.32 mm,the hydrogen permeation process is affected by the surface exchange step.In chapter 6,the chemical stability and hydrogen permeation performance of Ni—La0.4875Ca0.0125Ce0.5O2-δ(Ni—LDC)in H2O,CO2 and H2S is investigated.Ni-LDC show high resistance in moisture,but its performance decreased with the increase of H2O levels.It also show high stability in CO2,and its performance maintains in 40% CO2 at 700℃.However,due to the reaction between H2 and CO2 at 900℃,the concentration of H2 decreases while the concentration of H2O increases.Both of them are harmful to the hydrogen permeation.Thus the hydrogen permeation flux reduces 4%and 10%at 900℃in 20%and 40%CO2,respectively.When CO2 in the feed gas is removed,the hydrogen permeation flux recovers completely in one hour.Ni-LDC does not show high resistance to H2S.It is found that LDC reacts with H2S forming Ce2O2S and La2O3,causing the performance loss.The hydrogen permeation flux decreases 5,15 and 30%in 75,150 and 300 ppm H2S,respectively.
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