Stability Of Microstructure And Hydrogen Permeation Of Pd/Nb₃₀Ti₃₅Co₃₅/Pd Composite Membranes In Wide Temperature Range

Hydrogen is regarded as the most potential energy in the 21st century because of its large reserves,recyclable utilization and no environmental pollution.There are many methods for hydrogen production,such as electrolysis or photolysis of water,natural gas reforming and water gas shifting,etc.However,the hydrogen produced from these processes contain impurity gases,and must be further purified to obtain high purity hydrogen meeting the requirements of industrial application.It is an important way to prepare high purity hydrogen by membrane separation using hydrogen permeation materials.At present,Palladium and its alloys are the main membrane materials used in industry.However,Pd is scarce and expensive.In recent years,hydrogen permeation alloys based on 5B group metals（Nb,V,Ta）are considered to be the most promising new low-cost hydrogen purification membrane materials to replace Pd.Due to the lack of catalytic activity of hydrogen dissociation on the surface of 5B group metals and their alloys,it is often necessary to coat Pd with nanometer thickness as catalyst layer to form Pd/5B group metal（alloy）/Pd composite membranes to carry out hydrogen permeation and purification.Considering the differences of hydrogen production methods,the composite membranes need excellent hydrogen permeability and long-term stability in a wide temperature range.However,the interdiffusion between the Pd on the surface and the substrate metal of the composite membranes occurs at high temperature above 400°C,which will affect the hydrogen permeability.It is a critical problem in this field to understand the interdiffusion behavior and mechanism of composite membrane at high temperature and its influence on hydrogen permeability.In this paper,Nb₃₀Ti₃₅Co₃₅ eutectic alloy developed by our research group was selected to systematically study the microstructure and performance stability of Pd/Nb₃₀Ti₃₅Co₃₅/Pd composite membrane during hydrogen permeation process in wide temperature range.In the low temperature region,which is below 400°C,there is no interfacial interdiffusion and the dissociation of Pd is not the rate limiting step of hydrogen permeation.The influence of morphology,distribution and volume fraction of biphasic structure in Nb₃₀Ti₃₅Co₃₅substrate on the hydrogen permeability of the composite membrane was investigated.The quantitative relationship between the microstructure and the hydrogen permeability was constructed by using the effective thermal conductivity model in heat transfer theory,to optimize the hydrogen permeability and stability of the composite membrane.In the high temperature region,which is above400°C,the interdiffusion kinetics between Pd and Nb₃₀Ti₃₅Co₃₅substrate of the composite membrane and its effect on the hydrogen permeability were investigated in the high temperature region.Furthermore,Hf N interlayer was introduced between the Pd and the substrate alloy,and the effects of the different Hf N thickness on interdiffusion and hydrogen permeability at high temperature was systematically investigated.For the Pd/Nb₃₀Ti₃₅Co₃₅/Pd composite membrane,which is suitable for low temperature range（≤400°C）,four typical eutectic structures of Nb₃₀Ti₃₅Co₃₅substrate alloy were obtained by integral solidification,directional solidification,rolling and heat treatment.Among them,the"random"structure of bcc-（Nb）and B2-Ti Co with disordered distribution is obtained under the condition of integral solidification;the"parallel"structure with orderly distribution of two phases is obtained form directional solidification;the"parallel"structure of two-phase laminated distribution is obtained form rolling condition;and the"isolated"structure of bcc-（Nb）isolated in B2-Ti Co is obtained after high temperature annealing of as-rolled alloy.The effect of Nb₃₀Ti₃₅Co₃₅substrate with different microstructure on the hydrogen permeability of the composite membrane was systematically studied.The quantitative relationship model between the microstructure and the hydrogen permeability was established by using the effective thermal conductivity model,which was correlated well with the experimental results.The results show that the composite membrane has excellent hydrogen permeability when the two phases in Nb₃₀Ti₃₅Co₃₅substrate are parallel to the direction of hydrogen permeation.The hydrogen permeation coefficient is 4.26×10^-8 mol H₂ m^-1s^-1Pa^-0.5 at 400°C,which is1.6 times of that of as-cast Nb₃₀Ti₃₅Co₃₅,and 2.7 times of that of pure Pd.It shows excellent hydrogen embrittlement resistance and hydrogen permeation stability during hydrogen permeation process for 120 h.The stability of microstructure and properties of Pd/Nb₃₀Ti₃₅Co₃₅/Pd composite membrane during hydrogen permeation at high temperature（>400°C）was systematically studied.It is found that the hydrogen permeation rate decreases slightly at 450°C,and the hydrogen permeation flow rate is 88.5%of the initial steady state value after 330 h.At 500°C,the hydrogen permeation rate declines greatly,and the hydrogen permeation flow rate after 330 h is 27%of the initial value.When the temperature reaches or exceeds550°C,the hydrogen permeation flow declines sharply,and the flow rate decreases to 0within 150 h.The flow rate degradation of the composite membrane in the high temperature region is closely related to the interdiffusion of Pd and Nb₃₀Ti₃₅Co₃₅.EDS results show that the interdiffusion layer formed by interdiffusion increases with the hydrogen permeation process,and the thickening rate increases with the increase of temperature.In addition,when the temperature exceeds 550°C,aggregation occurred on the surface Pd layer,which leads to the decrease of Pd coverage on the alloy substrate and the effective area of hydrogen dissociation.Based on the diffusion couple model,a quantitative relationship model between the kinetic parameters of interfacial interdiffusion and hydrogen permeation flux is established,and the factor f_b（t）of the influence of interdiffusion on the flow rate is obtained.Using software COMSOL,the mathematical model of the influence of Pd particle distribution on hydrogen permeation flux after Pd layer agglomeration is calculated,and the factor f _Pd（t）of the effect of Pd aggregation on hydrogen permeation flux is obtained.Combining the two factors,a quantitative prediction model of the flow rate degradation with time in the high temperature region of the composite membrane is obtained,which correlated well with the experimental results.In order to solve the problem of interdiffusion and hydrogen permeation flow degradation of the Pd/Nb₃₀Ti₃₅Co₃₅/Pd composite membranes during hydrogen permeation at high temperature region.Hf N diffusion barrier layer was introduced between the surface Pd and the substrate Nb₃₀Ti₃₅Co₃₅ alloy.And the influence of Hf N boundary layer with the thickness of 50 nm,100 nm and 200 nm on the microstructure and properties of the composite membranes were systematically studied.The results show that at 500°C,50 nm HFN can effectively inhibit the interfacial diffusion between Pd and the substrate alloy.The Pd/Hf N/Nb₃₀Ti₃₅Co₃₅/Hf N/Pd composite membrane shows excellent hydrogen permeation stability,and there is no flow decline phenomenon during100 h hydrogen permeation.However,the introduction of Hf N reduces the absolute hydrogen permeation rate of the composite membrane.At 550°C,the introduction of Hf N can alleviate the interface diffusion between Pd and substrate alloy.The larger the Hf N thickness is,the weaker the interdiffusion degree is.The hydrogen permeation flow decline degree of Pd/HFN/Nb₃₀Ti₃₅Co₃₅/Hf N/Pd composite membrane is lower,but the absolute hydrogen permeation flow is also smaller.At 600°C,even with the introduction of HFN of 200 nm thickness,there are still serious interfacial interdiffusion,aggregation of Pd on the surface layer and severe degradation of hydrogen permeation flow rate.The hydrogen permeation flow rate after 100 h is only 12%of the initial value.Based on the prediction model of high temperature hydrogen permeation flux decline of composite membrane,the influence of diffusion barrier on interfacial interdiffusion coefficient and hydrogen permeation flow rate is coupled,which provides a basis for strengthening the high temperature hydrogen permeation stability of composite membrane.