The Microstructure And Hydrogen Permeation Properties Of Pd-Mo₂N And Nb₅₆Ti₂₃Ni₂₁ Composite Membranes

Nb₅₆Ti₂₃Ni₂₁ alloy has high hydrogen permeability coefficient and excellent hydrogen embrittlement resistance.It is a low-cost hydrogen permeation membrane material that attracts much attention at present.In order to accelerate the dissociation of hydrogen molecule and the binding of hydrogen atom,a thin Pd coating on Nb₅₆Ti₂₃Ni₂₁is needed to act as catalyst.However,the hydrogen permeation composite membranes composed of Pd and Nb alloys will occur interdiffusion at higher temperature,resulting in the serious deterioration of hydrogen permeation flux.In this paper,Mo₂N barrier layer is inserted between Pd and Nb₅₆Ti₂₃Ni₂₁ alloy to repress interfacial diffusion.In addition,Mo₂N has a certain catalytic property for hydrogen evolution.By adjusting the microstructure of Mo₂N,it is expected that the high efficient hydrogen transport of Mo₂N barrier layer can be achieved,which can significantly enhance the high temperature hydrogen permeability of Pd-Mo₂N and Nb₅₆Ti₂₃Ni₂₁ composite membranes.In this paper,Mo₂N layers were prepared on single crystal Si by magnetron sputtering.The effects of atmosphere?5%,10% and 30% N₂?,power?100W,200W and300W?,substrate temperature?298K?423K and 573K?and negative bias voltage?0V?150V and 300V?on the phase and micro-morphology of Mo₂N were analyzed by XRD and SEM,respectively.Mo₂N layers were prepared on Nb₅₆Ti₂₃Ni₂₁ alloy films and their hydrogen evolution activity was investigated.With the increase of nitrogen partial pressure and power,the diffraction peak shifts to the left,and the number of N atoms dissolved in Mo₂N increases.With the increase of temperature and bias voltage,the diffraction peak shifts to the right,and the number of N atoms dissolved in Mo₂N decreases.The increase of nitrogen partial pressure will make the micro-morphology of Mo₂N more smooth.The increase of power and temperature will agglomerate the Mo₂N grains and reduce the specific surface area.The increase of bias voltage will make the surface density of Mo₂N film increase first and then decrease.The results of electrochemical tests showed that the catalytic activity of Mo₂N prepared at 10%N₂?200W?423K?150V was the highest:the potential was-248mV at 10mA cm^-2 current density,and the slope of Tafel was 82mV dec^-1.Mo₂N thin films with 50,150 or 300nm were deposited on both sides of Nb substrate using the best electrochemical parameters,and hydrogen permeation test of Mo₂N-Nb₅₆Ti₂₃Ni₂₁-Mo₂N composite membranes with three different thickness catalytic layers at 773K temperature.The results show that the pressure exponent n=1 of the three composite membranes,which indicates the"surface dissociation limit".The relationship between their hydrogen permeability coefficient is?_50nm<?_300nm<?_150nm.It is concluded that the grain size of Mo₂N prepared under current conditions is between30 and 100nm,and the micro-scale irregularity of the substrate due to the different hardness of primary and eutectic phases during polishing process makes the surface of the substrate covered by Mo₂N with 50nm thickness incomplete,thus the hydrogen permeation of the composite membrane is relatively low.150nm Mo₂N just covers the surface of the substrate.The lower diffusion coefficient and dissolution coefficient of H atom in the sample of 300nm catalytic layer results in the decrease of hydrogen permeability coefficient.The hydrogen permeation of the three catalytic membranes is stable and the hydrogen permeation coefficient remained near the initial test value after6h.Therefore,150nm catalyst layer is an ideal thickness in this paper.Hydrogen permeability of Pd-Mo₂N-Nb₅₆Ti₂₃Ni₂₁-Mo₂N-Pd membranes with four different thicknesses of intermediate layer?0,50,150 or 300nm?was investigated.The results show that the pressure exponent n=0.5 of the three composite membranes,which indicates the"bulk diffusion limit".The relationship between their hydrogen permeability coefficient is?_300nm<?_150nm<?_50nm<?_0nm,and become?_300nm<?_0nm<?_50nm<?_150nm50nm after 6h,in which the sample with 0nm and 50nm intermediate layer occur significant decline in hydrogen permeability.The film after heat treatment was analyzed,and it was found that surface of Pd has agglomerated at high temperature.The EDS linear scanning of the elements in the cross section shows that the Pd atom content in the surface layer of the sample without intermediate layer decreases to 70%.,and the thickness of Pd decreases due to the interdiffusion in the local micro-region of the sample with 50nm intermediate layer.The composite film with 150 and 300nm intermediate layer does not show obvious interdiffusion phenomenon.It can be seen that Mo₂N,as an intermediate layer,can play an active role in repressing the interdiffusion between Pd and Nb,but on the other hand,too thick intermediate layer can also reduce hydrogen permeability.Therefore,the intermediate layer of 150nm is an ideal thickness in this paper.