Effect Of Alkali Metal Ion Doping On The Performance Of La₂Ce₂O_7-δ-Based Hydrogen Separation Membranes

In order to solve the problems of environmental pollution and energy shortage caused by the long-term large use of fossil fuels,hydrogen as a clean energy has been widely concerned,and the separation and purification of hydrogen is extremely important.In separation and purification technology,mixed conductor hydrogen separation membranes have good application prospects due to their simple structure,good mechanical performance and low cost.At present,the most studied membrane material is the proton conductor of the perovskite type Ba Ce O₃ system.Although they have high hydrogen permeability,due to their higher alkalinity,their stability in CO₂ and H₂O-containing atmosphere is poor and it is ease to generate corresponding carbonates and alkalis,which limits the application of such membranes.In recent years,fluorite structure cerium oxide doped with high lanthanum,especially La₂Ce₂O_7-δ,has been proved to have the proton conduction properties,and has attracted the attention of many researchers because of its excellent chemical stability.However,its proton conductivity is still lower than that of Ba Ce O₃-based materials.Some research shows that ion doping can effectively improve the conductivity and sintering performance of the system.In this paper,La₂Ce₂O_7-δwas doped by a series of alkali metal ions and applied to the hydrogen separation membrane.The effect of ion doping on the electrochemical performance of La₂Ce₂O_7-δwas investigated.Firstly,a series of La_1.85M_0.15Ce₂O_7-δ（M=Li,Na,K,Rb,Cs）oxides doped with alkali metal ions were synthesized by the sol-gel method,and the oxygen vacancies which has an important effect on the conductivity were characterized.The results show that ion doping not only increases the oxygen vacancy concentration to varying degrees,but also breaks the order of the original oxygen vacancies in the material,which are conducive to proton transport.Therefore,Rb⁺doped La₂Ce₂O_7-δachieves the highest conductivity due to the highest oxygen vacancy concentration in conductivity measurements.To further explore the proton conductivity of the sample,Pt-La_1.85Rb_0.15Ce₂O_7-δmembrane was prepared by coating platinum on the sample surface and its hydrogen permeation flux was tested.The proton conductivity was calculated with Wagner equation which is more than one order of lower than the actual measured conductivity.It can be confirmed that there are other conductive behaviors in the proton conductor.Secondly,doping of alkali metal ions can also improve the micro-morphology of La₂Ce₂O_7-δ.The hydrogen permeation flux of ion-doped hydrogen separation membranes is higher than that of Pt-La₂Ce₂O_7-δmembranes,except for Pt-La_1.85Cs_0.15Ce₂O_7-δmembrane,in which Pt-La_1.85Rb_0.15Ce₂O_7-δmembrane obtains the highest hydrogen permeation flux.In addition,the increase of temperature and hydrogen partial pressure,as well as water inflow,both increase the hydrogen permeation flux of the membranes.At the same time,the thickness of the membrane also has different effects on the permeability of hydrogen.When the thickness of the membrane is greater than 0.55 mm,the hydrogen permeation is mainly controlled by bulk diffusion,whereas,it begins to be controlled by both bulk diffusion and surface exchange processes.In the stability performance test,La_1.85M_0.15Ce₂O_7-δmaterials have good resistance to water and CO₂.Finally,the electrochemical and hydrogen separation properties of La₂Ce₂O_7-δmaterials with different concentration of Rb⁺were investigated.The experimental results show that the concentration of oxygen vacancy increases first and then decreases with the increase of doping amount.The conductivity measurements show that at low temperatures,La_1.7Rb_0.3Ce₂O_7-δhas a slightly higher conductivity,while at high temperatures,highest conductivity is obtained by La_1.85Rb_0.15Ce₂O_7-δ.Similarly,the increase in temperature and partial pressure of hydrogen,and the addition of water will increase the hydrogen permeation flux.The Pt-La_2-xRb_xCe₂O_7-δmembrane shows good stability after 20%CO₂ was added to the feed gas,and the structure of the membrane remains unchanged after 3 h in boiling water and 100%CO₂,indicating good chemical stability.