Structural Modulation And Hydrogen Permeability Of Vanadium-based Solution Alloy Membrane

Hydrogen energy has advantages of zero carbon emission,no pollution and recycling,which is the strategic focus of sustainable development of national energy in the future.Hydrogen production from fossil fuels and gray hydrogen and blue hydrogen recovered from industrial by-products have low cost and wide sources,but there are impurity gases such as CO₂ and N₂,so it is necessary to use alloy membrane to separate high purity hydrogen.Pd alloy membrane has stable hydrogen permeability,good hydrogen brittleness resistance,excellent mechanical properties and high temperature stability.However,Pd,as a precious metal element,is expensive and not suitable for large-scale application in industrial and commercial hydrogen purification field.V metal membrane shows good hydrogen permeation rate in hydrogen purification,but hydrogen embrittlement is easy to occur,and hydrogen penetration stability is poor.In our previous work,we designed V-base alloys with good permeability and resistance to hydrogen embrittlement by alloying.However,when the experimental temperature is 673 K and above,the hydrogen permeability of the membrane will show an obvious downward trend due to the mutual diffusion phenomenon.In order to reduce the influence brought by the mutual diffusion,a separation membrane with good and stable comprehensive hydrogen permeability performance is also developed.The improved experiments of hydrogen permeation alloy membrane were carried out from the two perspectives of the base layer and the catalytic layer,and verified in the mixed gas permeation experiment.（1）In terms of the substrate,V₉₂Fe₈ and V₉₂Fe₄Pd₄ alloys with good comprehensive performance were used as the substrate,and 0.2%rare earth element Y was doped into the substrate.The main body of the alloy sample was V-BCC solid solution structure.The results showed that the second-phase compound segregated from the single phase solid solution and the Y-rich Y-V compounds were segregated in the V_91.8Fe₈Y_0.2 alloy.Pd-rich Pd-Y-V compounds were segregated in V_91.8Fe₄Pd₄Y_0.2 alloy.The hydrogen solubility of the alloy was reduced,and the hydrogen brittleness resistance was good.The experimental results of hydrogen permeability at high temperature show that the hydrogen permeability of V_91.8Fe₄Pd₄Y_0.2 membrane decreases with time.The hydrogen permeability of V_91.8Fe₄Pd₄Y_0.2 membrane has the best hydrogen permeability,better than V₉₂Fe₄Pd₄membrane,and the hydrogen flux remains at 8.28×10^-6mol H₂ m^-1s^-1 at 723K temperature for 10 hours.The characterization results showed that a small amount of Y element could effectively alleviate the interdiffusion between Pd layer and substrate.（2）In terms of catalytic layer,Ti N membrane has the effect similar to Pd membrane.Under certain temperature and pressure conditions,Ti N membrane can catalyze the dissociation and bonding between hydrogen molecules and hydrogen atoms.In this paper,using ternary alloy V₉₂Fe₄Pd₄ as the base,Ti N/V₉₂Fe₄Pd₄/Ti N membrane with 200 nm catalytic layer is prepared.The surface catalytic layer is a single-phase nanocrystalline structure with fine and uniform grains and high density.The electrochemical results show that V substrate<V₉₂Fe₈ substrate<V_91.8Fe₄Pd₄Y_0.2 substrate<V₉₂Fe₄Pd₄ substrate<Pd/V₉₂Fe₄Pd₄/Pd separation membrane<Ti N/V₉₂Fe₄Pd₄/Ti N separation membrane,0.2%Y doping can not improve the electrocatalytic hydrogen evolution performance of the alloy.When the initial overpotential of Ti N/V₉₂Fe₄Pd₄/Ti N alloy membrane reaches 10m A/cm²,the absolute value of the overpotential corresponding to the current density is small,the lowest Tafel slope value is 123 m V/dec,the existence of more active sites on the surface of Ti N catalytic layer can enable titaniμm ions to be coupled with hydrogen atoms to form Ti-H groups,showing good electrocatalytic hydrogen evolution performance.At the temperature of 673 K,the hydrogen permeability coefficient of Ti N/V₉₂Fe₄Pd₄/Ti N alloy membrane is 6.29×10^-8 mol H₂ m^-1s^-1 Pa^-0.5,higher than Pd/V₉₂Fe₄Pd₄/Pd that alloy membrane permeability coefficient is 5.72×10^-8 mol H₂m^-1s^-1 Pa^-0.5.In the 70 hours hydrogen permeation persistence experiment,when the experimental temperature was 573K,the permeability flux of the two alloy membranes remained stable for 70 hours.When the experimental temperature rose to 673 K,the hydrogen permeation flux of the alloy membranes decreased significantly with the increase of time.The permeation flux of Pd/V₉₂Fe₄Pd₄/Pd alloy membrane decreased from 26.28×10^-6mol H₂ m^-1s^-1 to 20.97×110^-6mol H₂ m^-1s^-1.The permeability of Ti N/V₉₂Fe₄Pd₄/Ti N alloy membrane decreased from28.78×10^-6mol H₂ m^-1s^-1 to 22.74×10^-6mol H₂ m^-1s^-1.（3）The effects of different concentrations of CO₂ and N₂ on the hydrogen penetration rate of Pd/V₉₂Fe₄Pd₄/Pd and Ti N/V₉₂Fe₄Pd₄/Ti N hydrogen separation alloy films were investigated.The hydrogen permeability coefficient of the alloy film in the gas mixture decreases to different degrees,and the decreasing trend is more obvious with the increase of the impurity gas concentration.Co MPared with the pure hydrogen experiment,the decrease of Pd/V₉₂Fe₄Pd₄/Pd alloy film reached 30.9%and that of Ti N/V₉₂Fe₄Pd₄/Ti N alloy film reached 37.4%in the 95%H₂+5%N₂ mixed atmosphere（673 K）.The permeability of Pd/V₉₂Fe₄Pd₄/Pd alloy film decreases to 2.85×10^-8 mol H₂ m^-1s^-1 Pa^-0.5 in95%H₂+5%CO₂ mixed atmosphere.The permeability of Ti N/V₉₂Fe₄Pd₄/Ti N alloy film decreased to 3.45×10^-8 mol H₂ m^-1s^-1 Pa^-0.5.The CO₂ that does not pass through the alloy film accμmulates in the upstream end of the alloy film,occupying part of the active point site originally belonging to the adsorption of H atom,and gradually forms a CO₂ barrier layer with higher concentration.The hydrogen concentration is diluted within this range,and the hydrogen penetration rate is therefore reduced.The detoxification results showed that the toxicity of CO₂ impurity gas on the hydrogen separation alloy film was greater than that of N₂.