Preparation And Properties Of In,Ta Co-doped BaCeO₃-based Hydrogen Separation Membranes

With gradual depletion of the traditional fossil fuels,the importance of new energy development has become increasingly necessary.Hydrogen has attracted a lot of attention because of its high thermal value,clean reaction product and extensive sources.The hydrogen should be separated from the mixture gas which is obtained from the raw materials.Due to no external circuit,excellent hydrogen permeation performance,good mechanical properties and low price,the proton-electron mixed conducting membrane is more efficient than the traditional separation method.Among the proton conductors,though the BaCeO₃ based proton conductors possess the highest conductivity,they show poor chemical stability,which can not meet the requirements of mixed conducting hydrogen separation membranes.As highly electronegative ions contribute to the chemical stability of BaCeO₃ base system,this thesis studied the properties of the Ni-BaCeO₃ based hydrogen separation membrane doped with In and Ta.The conductivity of BaCe_0.7In_0.2Ta_0.1O_3-??BCI20T10?proton conductor in dry and wet?3%H₂O?air,Ar and atmosphere was tested.The conductivity increases with the hydrogen partial pressure increasing in different atmospheres.In addition,the conductivity of the sample in dry atmosphere is higher than that in wet environment.The XRD analysis indicates that no significant reaction occurred.The dense mixed conductor hydrogen separation membrane was prepared by mixing BCI20T10 proton conductor powder with Ni and sintering in 5%H₂ balanced with 95%Ar atmosphere.Because the hydrogen permeation rate was proportional to the proton conductivity for membrane with large thickness,the hydrogen permeability of Ni-BCI20T10 in dry atmosphere was also higher than that in wet atmosphere.The chemical stability of Ni-BCI20T10 hydrogen separation membrane in water is lower than that of BCI20T10,which may be caused by the mutual diffusion between BCI20T10 and Ni.When 10 vol%CO₂ is introduced into the wet feed gas,the hydrogen permeation flux quickly decreases and finally remains stable.After CO₂ is removed,the hydrogen permeation flux completely returns to its initial value,investigating that the sample has good stability in 10%CO₂-containing environment.Due to higher electronegativity,doping In and Ta into the BaCeO₃ system will improve the acidity of the sample,making the hydrogen separation membrane more stable in CO₂atmosphere.When 20 vol%CO₂ is introduced,the hydrogen permeation flux decreases more and can not recovers to the initial value after CO₂ is removed.XRD and SEM results show that irreversible chemical reaction takes place and the non-conductive phases BaCO₃ and CeO₂ are generated.When the membrane thickness is larger than 0.8mm,the hydrogen permeation process is controlled by the bulk diffusion.When the membrane thickness is less than0.80 mm,the hydrogen permeation process is controlled by both the surface exchange and the bulk diffusion.In order to increase the surface exchange rate,the surface of the hydrogen separation membrane with 0.6 mm thickness is modified by Pt and Ni respectively.Due to the good catalytic performance of Pt and Ni,the hydrogen permeability is greatly improved,and the hydrogen permeation control step becomes to be the bulk diffusion for the Pt modified membrane.Ni-BCI20T10,Ni-BCI30,BCI30 hollow fiber membranes with asymmetric structure were prepared by the phase inversion method.Because the mixed conducting membrane needs to be sintered in a reducing atmosphere,the insufficient combustion of the organic additives such as binder and dispersant make the membrane more difficult to be sintered compactly.