Modification And Characterization Of Lanthanide Tungstate Mixed Proton-electron Conducting Membrane

Mixed proton-electron conducting(MPEC)ceramic membrane is a type of oxides with high H2 permeation selectivity at elevated temperatures.This type of membrane possesses inherent proton and electron conductivity,therefore,an additional external electrical circuit is not required in the permeation process.MPEC membranes can be applied in the hydrogen separation process in the industrial applications,such as the gasification of fossil fuels,the transition of biomass,the CO2 capture and storage of power plants.Moreover,they could also be integrated in membrane reactors for higher selectivity and productivity by in situ removal of hydrogen from reactions.They could offer several advantages over traditional technologies,including simplification of operating units,saving of energy and material,and improvement of separation efficiency.MPEC membranes with good chemical and thermal stability,high permeability and mechanical strength are needed to meet the operating requirements for the above applications.Many efforts have been made to develop MPEC membranes.Perovskite compounds based on SrCeO3 and BaCeO3 are the most studied.But they possess low stability when they are exposed to CO2 and H2 O,because carbonates are easy to form during the hydrogen permeation process.Recently,the family of rare-earth tungstate has received significant attention since it is stable in the presence of CO2 and H2 O.Therefore,we studied the lanthanide tangstates with high chemical stablity in this work,focusing on improving the hydrogen permeation flux of the selected MPEC membranes by enhancing the bulk diffusion and surface reaction.First,we introduced metal Ni into the membrane as electron conductor.Ni formed channels for electron conducting in the membrane bulk to improve the rate of bulk diffusion,therefore enhancing the hydrogen permeation flux.Lanthanide tungstate based on Nd5.5W0.5Mo0.5O11.25-?(NWM)was chosen for its good chemical stability.Different volume ratio of Ni and NWM was mixed to fabricated daul-phase Ni-NWM ceramic membranes.The experimental results indicated that the highest hydrogen permeation flux can be achieved when 40 vt% Ni was introduced,and all the Ni-NWM membranes obtained good chemical stablity.Then,referring to the method of doping different valence cations,we introduced electron conductor by in-situ synthesis.We introduced Ni2+ into La5.5W0.6Mo0.4O11.25-?(LWM)by a citric acid–nitrate gel combustion method,then Ni2+ was de-oxidized to Ni to fabricate Ni-LWM membranes.Meanwhile,Ni-LWM membranes were prepared by traditional mixing method,in order to study the influence of different synthesis methods on the Ni-LWM membranes.The results revealed that the Ni-LWM membrane fabricated by in-situ method was more homogeneous,obtaining higher hydrogen permeation flux and better chemical stability.Remarkably,Ni-LWM membrane sustained relatively high hydrogen permeation flux when exposed to CO2.Third,in order to improve the bulk diffusion and surface reaction at the same time,we fabricated LWM asymmetric membranes with a thin dense layer and a large surface porous layer.The integrated asymmetric membrane without a support layer avoided the mismatch of dense layer and support layer.We studied the conditions for asymmetric fabrication by phase-inversion process and investigated the hydrogen permeation properties of synthesized membranes.Compared with the conventional dense membrane,the hydrogen permeation flux through the asymmetric membrane was generally increased by two times.A hydrogen permeation flux of-2-10.3316 ??cmminmL was achieved at 1000 oC with a mixture of 80 % H2-20 % He as feed gas when steam was added to sweep gas.Finally,we adopted surface modification to accelerate the surface exchange reaction and further improve the hydrogen flux through the LWM asymmetric membrane.Pt catalyst was coated on different surfaces of the membrane.We analysed the phase structure and micro-structure of the Pt layer,and detaily studied the hydrogen permeation properties of the membrane with Pt-coated on the dense layer.The results showed that catalyst coating was helpful to lower the activation energies and motivate the hydrogen permeation reaction.The coating of Pt catalyst can protect the Mo-cation from reduction,thus improve the stability of the hydrogen permeation flux.