The Synthesis And Characterization Of Mg/Ti-Cr-V Composite Hydrogen Storage Materials

It is well known that hydrogen storage and transportation are the main obstacles for the practical application.High capacity hydrogen storage materials are the effective means to solve this problem.Magnesium is considered as one of the attractive hydrogen storage materials due to the advantages of high hydrogen content of 7.6wt%,low density,the abundance and relatively low cost.However,it is severely limited to the unfavorable de-/hydriding thermodynamic and kinetic properties,which leads to high decomposition temperature(>300?)for ab-/desorption process.Therefore,it is still the main drawback,which hinders the practical applications.In order to solve problems mentioned above,in this work,the hydrogen storage materials with good hydrogen storage property and lower thermal stability were introduced to Mg powders to prepare nanocrystalline Mg-based composite materials.It is expected to decrease desorption temperature and alter good kinetic properties by the muti-phase synergetic desorption effect and high content of y-MgH2.First,we optimized the chemical composition of Ti-Cr-V alloys to obtain Ti-Cr-V alloys with the matching desorption temperatures with MgH2.Then,we prepared the nano-composite of Mg and Ti-Cr-V phases by RBM to further alter the thermodynamics and kinetics properties and decrease the ab-/desorption temperature of Mg/MgH2 systems.Finally,the phases composition and micro structure of nanocomposite during ab-/desorption hydrogen processes were investigated by the scanning electron microscopy(SEM),X-ray diffraction(XRD)and transmission electron microscopy(TEM).Besides,the thermal stability,Pressure-composition-isotherms(P-C-I)curves and ab-/desorption kinetic properties also were measured.We further revealed the mechanism on improvements of thermodynamic and kinetic properties.The Mg/20wt%Ti0.45Cr0.35V0.2 powders were synthesized by high energy RBM.We optimize the ball milling conditions by the analysis of phase composition,micro-structure and DSC results.The nanocomposite,which was synthesized under the ball milling condition of the respective disk and vial rotation speed of 400 and-800 rpm(relative to disk)at room temperature after 8h RBM with a ball to powder mass ratio of 100:1,shows the desorption temperature of 260? and high content of y-MgH2.Based on the optimization of ball milling conditions,Mg/x wt%Ti0.45Cr0.35V0.2(x=5,10,20,30)nanocomposites were prepared by RBM to optimize the content of Ti0.45Cr0.35V0.2 alloys.Results show that the Mg/20wt%Ti0.45Cr0.35V0.2 composite shows lowest decomposition temperature and the better kinetic properties than others.The hydrogenation and dehydrogenation enthalpies of Mg/20wt%Ti0.45Cr0.35V0.2 composite show a noticeable decrease of 2.14 and 11.65kJ/(mol H2),respectively,compared with that of the pure MgH2 powders under the same preparation conditions.Although introducing the Ti0.45Cr0.35V0.2 alloy to Mg matrix realizes the thermodynamic destabilization of Mg-based composites,the hydrogenation/dehydrogenation enthalpies are still higher than theoretical values.Therefore,in order to improve thermodynamic properties of Mg/MgH2,it is necessary to further optimize on chemical composition of Ti-Cr-V alloys.We further study the effect of TixCr0.8-xV0.2 alloys on the hydrogen storage properties of the Mg/MgH2 systems by introducing the TixCr0.8-xV0.2(x=0.25,0.35,0.45,0.55)alloys with the temperatures ranging from 227 to 310? to Mg matrix.DSC results show that desorption temperature(Tde)of as-milled nanocomposites gradually decrease with decreasing Tde of TixCr0.8-xV0.2 alloys.The nanocomposite shows the lowest temperature of 256? for x=0.35.According to the P-C-I curves,the hydrogenation and dehydrogenation enthalpies of Mg-20wt%Ti0.35Cr0.45V0.2 are calculated to be-69.53 and 70.60kJ/(mol H2),respectively.Based on present results,we can conclude that introducing Ti0.35Cr0.45V0.2 to Mg can further improve thermodynamic properties.Herein,it sheds light on the ways to further improve the thermodynamic properties of magnesium-based composites by introducing the lower Td,of Ti-Cr-V alloys.According to the results mentioned above,in order to further decrease decomposition temperature,we adopt the Ti-Cr-Vx(x=0.4,0.6,0.8;Ti/Cr=2:3)alloys with the temperatures ranging from 170 to 220? to prepare the Mg/20wt%Ti-Cr-V,nanocomposites by RBM.According to the TEM analysis,the nanocrystalline ?-MgH2,?-MgH2 and Ti0.16Cr0.24V0.6Hy phases coexist in the as-milled samples.DSC results show that Tde of Mg/20wt%Ti-Cr-Vx composites exhibit the nonmonotonic variation tendency with the decrease on that of Ti-Cr-VxHy alloys,and the lowest Tde of 225? can be obtained for x=0.6.Mg/20wt%Ti0.16Cr0.24V0.6 composite shows the apparent activation energy of 76.32kJ/mol and lower dehydrogenation enthalpy of 54.16kJ/(mol H2)due to the synergetic desorption effect co-existing in the nano-composite ?-MgH2/?-MgH2/Ti0.i6Cr0.24V0.6Hy phases according to the TEM analysis.Besides,It can absorb 3.93wt%H2 at room temperature within 30min,and the reversible hydrogen storage capacity is 5.96wt%at 270?.Metastable ?-MgH2 shows the great potential to improve thermodynamic property of Mg-based hydrogen storage materials,but y-MgH2 is very sensitive to the applied temperature,which no longer exists when the applied temperature is higher than 270?.The content of y-MgH2 keeps on decreasing with number of cycles and the end-sample only contains 2.94wt%?-MgH2 with the number of de-/absorption cycles increasing to 8th cycle.More efforts need to be paid to further decrease the working temperature to maintain the synergetic effect among y-MgH2,Ti0.16Cr0.24V0.6Hy and ?-MgH2 phases for this composite.