Influence Of Ion Doping On The Properties Of Ni-La₂Ce₂O₇ Based Hydrogen Separation Membranes

Industrial hydrogen is generally a mixture of hydrogen,which needs to be separated to produce pure hydrogen.The mixed conducting hydrogen separation membranes not only have the characteristics of simple structure and good mechanical properties,but also have a theoretical selectivity of 100%to hydrogen.The hydrogen separation membranes represented by perovskite-structure BaCeO₃ materials have high hydrogen permeability,but their chemical stability is poor,and they are very easy to react with CO2 and H2O to form carbonate,which affects the hydrogen permeability.In recent years,fluorite-structure La₂Ce₂O₇ materials have received a lot of attention,because they can be stable in the atmosphere containing CO2 and H2O,but the proton conductivity of the materials is lower than traditional Ba CeO₃ materials.It has been reported that a small amount of ion doping can increase the proton conductivity of La2Ce2O7 materials.Therefore,in this thesis,the La2Ce2O7 compound with good chemical stability is selected as the membrane material,and ion-doped La₂Ce₂O₇-based mixed conducting hydrogen separation membranes are studied.Na⁺,K⁺and Mg²⁺doped La₂Ce₂O₇ oxides were prepared by sol-gel method,and their conductivity and stability were studied.After the ceramic oxides are treated in boiling water and CO₂ atmosphere at 900?for 3 h,the phase structure do not change at all,indicating that they have excellent stability against CO2 and boiling water.In addition,the doping of low-valent elements of Na⁺,K⁺,and Mg²⁺destroys the ordered arrangement of oxygen vacancies in the La₂Ce₂O₇ structure,thereby improving the electrical conductivity of the system.Since Mg²⁺doping can obtain an appropriate amount of oxygen vacancies,La_1.85Mg_0.15Ce₂O_7-?proton conductor exhibits the highest conductivity performance under wet H₂ atmosphere.In order to further analyze the proton conductivity of La₂Ce₂O₇ oxides,the hydrogen permeability of La_1.85Mg_0.15Ce₂O_7-?membrane coated with Pt slurry was tested,and the proton conductivity of the sample was calculated by Wagner equation.The calculated proton conductivity is about an order of magnitude lower than the total conductivity measured under wet H₂ atmosphere.The dense cermet membranes were prepared by mixing Na⁺,K⁺,and Mg²⁺doped La₂Ce₂O₇ proton conductors with Ni powder and then sintering them at 1450?under 5%H₂+95%Ar atmosphere.The doping of Na⁺,K⁺,and Mg²⁺all improve the hydrogen permeability of the membranes,among which the doping of Mg is the best.In a wet 20%CO₂ atmosphere,the hydrogen penetration fluxes of Ni-La₂Ce₂O_7-?and Ni-La_1.85Mg_0.15Ce₂O_7-?membranes remain stable for 50 hours without attenuation,while the hydrogen penetration fluxes of Ni-La_1.85Na_0.15Ce₂O_7-?and Ni-La_1.85K_0.15Ce₂O_7-?membranes decrease slightly.According to the phase structure of the membrane surface,it is known that non-conductive impurities are generated on the surfaces of the Ni-La_1.85Na_0.15Ce₂O_7-?and Ni-La_1.85K_0.15Ce₂O_7-?membranes during the test,which affects the hydrogen permeability.However,no reaction takes place in the Mg²⁺doped membrane.The influence of Mg²⁺doping on the electrochemical properties of La₂Ce₂O₇materials was investigated.The experimental results showed that among La_2-xMg_xCe₂O_7-?samples,the sample exhibits the highest conductivity when x=0.15.As the hydrogen penetration flux is proportional to the proton conductivity,the Ni-La_1.85Mg_0.15Ce₂O_7-?membrane possesses the highest hydrogen penetration flux.Moreover,the Ni-La_2-xMg_xCe₂O_7-?membranes with different Mg²⁺content have good long-term stability under wet atmosphere of 20%CO₂.The hydrogen penetration flux increases with the decrease of film thickness,indicating that the hydrogen permeation process is controlled by the bulk diffusion.However,when the thickness is less than0.54 mm,the hydrogen permeation process starts to be controlled by surface exchange.In order to improve the membrane surface exchange rate,the 0.68 mm thick Ni-La_1.85Mg_0.15Ce₂O_7-?membrane surface was modified with Pt and Ni paste,respectively.Due to the good catalytic properties of Pt and Ni,the hydrogen penetration flux is significantly increased,and the hydrogen permeation process of the Pt-modified sample is even increased by about 5 times.