The Preparation And Characterization Of Anion-doped Mixed-conducting Hydrogen Permeable Membrane

Mixed protonic-electronic conducting hydrogen permeable membrane has gained much attention in the hydrogen separation with 100%selectivity from hydrogen-containing mixture gases.For industrial applications,a dense ceramic MPEC membrane has to show considerably high hydrogen permeability and stability under the practical operation conditions.In this paper,La_5.5W_0.6Mo_0.4O_11.25-??LWMO?was chosen to study.The effect of anion doping at the oxygen site on the crystal structure,hydrogen permeability properties and the stability of lanthanum tungstate were investigated in detail.The asymmetric membrane with optimized proportion was prepared and its hydrogen permeation fluxes and stability were also studied.Firstly,we had prepared fluoride anion-doped lanthanum tungstate membranes La_5.5W_0.6Mo_0.4O_11.25-?F_x?LWMF_x,x=0⁰.50?for the first time,and the effects of fluorine substitution on the lattice structure,redox stability and hydrogen permeability were investigated.It turned out that the cubic fluorite structures were retained when lower quantities of F^-were doped,namely x?0.2.The corresponding hydrogen permeation fluxes first increased and then decreased as a function of the fluorine concentration at the same temperature,and the peaks all appeared at x=0.05.Additionally,we synthesized chloride anion-doped lanthanum tungstate membranes La_5.5W_0.6Mo_0.4O_11.25-?Cl_x?LWMCl_x,x=0⁰.20?and studied the influence of chloride anion-doping on the crystal structure,hydrogen permeation performance and stability.The difference of hydrogen permeability between F-doping and Cl-doping was compared.The results showed that LWMCl_x remained the cubic fluorite structure when x?0.2.With the increase of Cl^-doping amount,LWMCl_x membranes showed a trend similar with LWMF_x,and the peaks appeared at x=0.10.The comparation of hydrogen permeability of LWMO,LWMCl_0.10 and LWMF_0.05 showed that LWMF_0.05 owned the best performance amog them.Finally,to further improve the hydrogen permeablity through LWMF_0.05 membrane,we prepared asymmetric LWMF_0.05 membrane by co-pressing and high-temperature sintering technology and studied the impacts of asymmetric structure in detail.The data showed that the hydrogen permeability of asymmetric LWMF_0.05 was significantly improved,compared with the symmetric one.The porous support reduced the in-suit hydrogen chemical potential on the surface of dense layer,and then protected the dense layer from being reduced directly by hydrogen to some extent,thus improved the redox stability of membrane.