Synthesis And Hydrogen Storage Characteristics Of Advanced Mg-based Composites

Magnesium-based hydrogen storage materials have some merits of high hydrogen storage capacity,low coat,light weight,and so on,but their practical applications are limited by some demerits of high hydriding/dehydriding temperature,low thermodynamic and kinetic characteristics.In this paper,the research and development of Mg-based hydrogen storage materials were exhaustively reviewed first.On this basis,the research scheme for magnesium or magnesium-based alloy modified by milling with other metal hydrides was chosen.The different magnesium-based hydrogen storage composites were prapred by mechanically milling,vacuum induction melting and other methods,and their relevant microstructures and hydriding/dehydriding characteristics were also investigated.The study on the microstructure and hydrogen storage properties of Mg+10 wt.% TiFe_1-xCr_x(x=0,0.05,0.3)composites shows that pure Mg milled with 10 wt.%TiFe_1-xCr_x (x=0-0.3)can significantly improve its hydrogen storage characteristics,and Mg-based composites containing chromium have good dehydriding dynamic performance at low temperature.At the same temperature,Mg+10 wt.%TiFe_0.7Cr_0.3composite exhibits the best hydriding/dehydriding characteristics with a hydrogenation capacity of 7.14 wt.%and a dehydrogenation capacity of 6.91 wt.%at 340℃.The further research on MgH₂+x wt.% TiFe_0.7Cr_0.3(x=5,10,15,20)composites shows that the composites are modified by milling, exhibiting a good hydriding kinetics at 180-220℃.It is found that the multi-phase structure in the melted TiFe_0.7Cr_0.3and the nano-crystalline structures of milled composites improve the hydriding kinetics of Mg-based materials.The study on Mg+x wt.%TiCr_1.8(x=5,10,15,20, 30)composite shows that the hydriding performances of Mg are improved by milling with TiCr_1.8,such as the hydrogenation capacity of beyond 4.02 wt.%in 7000s at 180℃for Mg+15 wt.%TiCr_1.8.It is found that lots of defects formed by TiCr_1.8in the milling process make hydrogen molecular react with the alloys first,playing catalytic role on the hydriding/dehydriding process of Mg,and TiCr_1.8itself has a good hydriding/dehydriding kinetics,so the hydrogen storage properties of magnesium-based alloys are improved.The research on the milled Mg+x wt.%LaMg₂Ni(x=5,10,20,30)composites under hydrogen atmosphere shows that all composites have better hydrogen storage properties than milled pure Mg,for the composites reach hydrogenation capacity of beyond 3.0 wt.%in 1000s at 120℃,and Mg+10 wt.%LaMg₂Ni has a good comprehensive hydrogen storage properties at 120℃.It is found that mechanically milling increases substantially the specific surface area of the composites and the accessible scope of the Mg-based alloy and hydrogen.The existence of more phase borders and micro-defects in the composites also provide catalytic activities for hydriding and dehydriding reaction.On this basis,Mg+10 wt.%LaMg₂Ni was chosen for modification by milling with Nb,and the structure and properties of Mg+10wt.%LaMg₂Ni+ x wt.%Nb(x=0,5,10,15)were investigated.The results show that,metallic Nb in the composites disappears after milling and forms new fcc and bcc phase with Mg/MgH₂.The hydrogen storage properties of the composites milled with Nb go down,because the catalysis of Nb is covered by that of LaMg₂Ni,and the percentage of the added Nb makes the original hydrogen storage capacity decrease.The research on the as-cast RE₂Mg₁₇Ni_1+x(x=0,0.5,1.0,1.5)alloys shows that all alloys can be activated in first cycling,and absorb hydrogen beyond 2.8 wt.%in 500s at 120-180℃(maximum hydrogenation capacity is 3.2 wt.%).Their reversible hydrogen storage capacities reach 4.2 wt.%at 300℃,and RE₂Mg₁₇Ni_2.5has a good dehydriding kinetics.On this basis,the hydrogen storage properties of as-cast RE₂Mg₁₇Ni_2.5and milled RE₂Mg₁₇Ni_2.5+x wt.%Mg(x =0,20,40)composites were investigated.The results show that the as-cast RE₂Mg₁₇Ni_2.5has better hydriding properties than the milled one,and yet RE₂Mg₁₇Ni_2.5+x wt.%Mg(x=20,40) composites have better hydriding properties than the as-cast RE₂Mg₁₇Ni_2.5alloy.RE₂Mg₁₇Ni_2.5 +40 wt.%Mg has the best hydriding properties with maximum hydrogenation capacity of beyond 4.4 wt.%at high temperature(120-180℃),and RE₂Mg₁₇Ni_2.5+20 wt.%Mg has better hydriding properties at 60℃and dehydriding properties at 280-300℃.It is found that the free Mg are packaging or attaching to RE₂Mg₁₇,Mg₁₂RE,Mg₂Ni and other alloys,which forms many phase boundaries and defects,playing catalysis roles on hydriding kinetics of the composites.