| In the separation and purification technology of hydrogen,the membrane separation method has broad development prospects due to its simple process design and compatibility with other separation technologies.Among metal membranes purified by hydrogen permeation,Pd and its alloy membranes are common hydrogen separation alloy membranes in industrial applications for their good hydrogen permeability and selectivity.However,the main problem with precious metal Pd is that its resources are scarce and expensive,resulting in high cost in industrial applications and large-scale production.In contrast,Group VB metals(such as Nb,V,Ta)ensure high hydrogen permeability while greatly reducing material costs.Among VB group metals,Nb group has the highest hydrogen permeation performance in theory,but in known study,the hydrogen permeation performance of composite membranes prepared with Nb as the substrate is not as good as that of composite membranes with V as the substrate.This is related to the influence of the substrate material on the surface morphology of the catalytic membrane.Pure Ta is prone to hydrogen embrittlement.Therefore,in this paper,V foil is used as a substrate,and a catalytic film is plated on the surface to prepare a hydrogen dissociation catalytic composite film with good hydrogen dissociation capability and high hydrogen permeation performance.In the selection of surface catalytic membranes,transition group metal carbides have attracted much attention due to their high hydrogen catalytic activity similar to Pt group metals.Compared with Mo2 C and WC,VC has a lower density.Under the condition of loading the same weight and the same grain size,VC can provide a larger exposed surface,which also makes VC a substitute for Pd metal as a surface catalytic membrane material suitable choice.Therefore,this topic selects VB group metal V as the substrate,prepares VC/V composite membrane by magnetron sputtering process,and uses the single variable method to divide the experiment into A,B,C,and D groups.The influence of process parameters is systematically studied,such as substrate temperature,sputtering power,sputtering time and substrate negative bias,on the phase composition and structure of the surface catalytic membrane in the VC/V composite membranes.Combined with electrochemical testing methods,the hydrogen evolution catalytic performance of VC/V composite membranes prepared under different conditions were characterized from linear voltammetry(LSV),electrochemical impedance spectroscopy(EIS),cyclic voltammetry(CV)and electrochemical double-layer capacitance testing.On this basis,the temperature of the substrate is selected as a variable.Hydrogendissolution and permeation tests are performed on the composite membrane to explore the reaction rate limiting steps and reaction mechanism of the hydrogen penetration process.Combined with the surface GIXRD test results and electrochemical hydrogen evolution test results of the VC/V composite membrane,it can be seen that when the magnetron sputtering parameter is selected as the substrate temperature is room temperature,the sputtering power is 200 W,the sputtering time is 20 min,the substrate is negatively biased is 0 V,the prepared VC/V composite membrane has the best catalytic performance and stability for electrochemical hydrogen evolution.At this time,when the exchange current density j is 10 m A cm-1,the hydrogen evolution overpotential is about 580 m V.And the slope of the Tafel curve is 145 m V dec-1.The charge transfer resistance is about 1.23 k?.The double-layer capacitance Cdl is 2.85 m F cm-2.After 500 cyclic voltammetry tests,the overpotential deviation does not exceed 20 m V.Hydrogen dissolution test results show that from the thermodynamic point of view,under isothermal equilibrium,the hydrogen solubility of the composite membrane is mainly controlled by the vanadium substrate.As the temperature increases,the hydrogen solubility decreases,which is mainly related to the heat dissipation of H dissolved in V.Under the closed environment,the temperature rise is not conducive to the exothermic reaction.Therefore,the solubility of hydrogen decreases.From a kinetic point of view,the hydrogen absorption kinetic curve of the VC/V composite membrane mainly conforms to the Ginsling-Braunshtein model,which is limited by the hydrogen atom diffusion process;the kinetic hydrogen absorption process of the V conforms to two models.The VC/V composite membrane is the same,which conforms to the Ginsling-Braunshtein model,while the previous stage conforms to the nucleation growth model,which is limited by the hydride nucleation growth process.The hydrogen permeation test results show that when the substrate temperature is673 K,the prepared composite membrane has the best hydrogen permeability performance,followed by room temperature,and the worst at 473 K.This may be related to the decrease of the lattice constant of the VC/V composite membrane.When the hydrogen permeation temperature is 873 K,each composite membrane exhibits excellent hydrogen permeation flux,and as the hydrogen permeation temperature decreases,the hydrogen permeation properties of membranes with different substrate temperatures show significant differences.At the same hydrogen permeation temperature,the hydrogen permeation flux J increases with the increase of upstream pressure,which approximately meets the linear relationship.The higher the temperature,the greater the influence of upstream hydrogen pressure,that is,the greater thehydrogen permeability coefficient ?.By fitting the least square method of the Pu-J curve,we can find that when the temperature is higher,the n value is closer to 0.5,that is,the main receptor diffusion limit,as the temperature gradually decreases,the n value gradually approaches 1.It shows that the influence of surface restrictions gradually increases and occupies a dominant position. |
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