Study On Synthesis And Hydriding-dehydriding Behaviors Of Advanced Mg-based Composites

Among metal hydrides studied as possible solid-state hydrogen storage materials, magnesium hydride is one of the most promising candidates for hydrogen storage due to its high hydrogen storage capacity(7.6 wt.%in MgH₂),low cost and abundance. However,its practical application for hydrogen storage is hampered by its high thermodynamic stability and sluggish hydriding/dehydriding kinetics.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 hydride modified by milling with Ni₂P or transition metal fluorides was chosen.The different magnesium-based hydrogen storage composites were prepared by mechanical ball-milling under hydrogen atmosphere.The influences of dopants on the hydrogen storage behaviors and microstructures of the composites was been investigated systemically.Beside,the effects of technical parameters of vapor-transport deposition on the particle size and morphology of Mg was been investigated.The investigation on the microstructure and hydrogen storage behaviors of Mg+χwt.%Ni₂P(χ=0,5,10,15) composites shows that the hydriding/dehydriding kinetics of Mg can be markedly improved by doping of Ni₂P.The pure magnesium only absorbs 2.02 wt.%hydrogen within 600 s at 493 K and then desorbs 1.40 wt.% hydrogen within 3600 s at 573 K.As for the Ni₂P doped Mg,the hydrogenation processes can be almost fulfilled within 30 s at a moderate temperature of 433 K. Based on the analysis of both the absorption and desorption performances,the optimal catalyst concentration is around 10 wt.%.The Mg+10 wt.%Ni₂P composite can absorb 5.02 wt.%hydrogen within 60 s at 433 K and desorb 5.62 wt.%hydrogen with 20 min at 573 K.The hydriding/dehydriding kinetics is increasing with the ball-milling time increased.The Mg+10 wt.%Ni₂P composite with 60 h milling time absorbs 5.71 wt.%hydrogen within 60 s at 453 K and desorbs 5.08 wt.%hydrogen with 300 s at 573 K.XRD analyses show that the size of crystal grains of the composites is reduced by ball-milling.SEM analyses represent that Ni₂P is effective in inhibiting the aggregation of Mg during the de-/hydrogenation processes.DSC measurement indicates that the doping of Ni₂P can lower the dehydriding temperatures of the hydrided Mg-Ni₂P composites.The study on the microstructure and hydrogen storage behaviors of MgH₂+10 wt.%MF_n(MF_n=TiF₃,FeF₃,NbF₅,CeF₃) composites shows that the hydriding/dehydriding kinetics of MgH₂ is markedly improved by doping transitional mental fluorides.The composites can absorb hydrogen rapidly at moderate temperature range of 313-473 K.At 573 K,it can absorb 5.22 wt.%hydrogen for TiF₃-doped composite,5.57 wt.%hydrogen for FeF₃-doped composite and 5.67 wt.% hydrogen for NbF₅-doped composite within 60 s.The MgH₂ + 10 wt.%(TiF₃,FeF₃, NbF₅) composites can desorb 6.05 wt.%,6.47 wt.%and 6.14 wt.%hydrogen within 10 min,respectively.The further research on MgH₂+5 wt.%TiF₃+5 wt.%NbF₅ composite and MgH₂ + 5 wt.%TiF₃ + 5 wt.%FeF₃ composite shows that the hydriding/dehydriding kinetics of composites are modified by ball-milling.The composites can absorb 5.61 wt.%hydrogen and 5.73 wt.%hydrogen within 10 min at 473 K,respectively.The MgH₂+5 wt.%TiF₃ + 5 wt.%NbF₅ composite and MgH₂+ 5 wt.%TiF₃+5 wt.%FeF₃ composite can desorb 5.95 wt.%hydrogen and 6.47 wt.% hydrogen with 10 min at 573 K.Combining XRD,SEM and DSC analysis,the results show that the size of crystal grains of the composites is reduced and the abundant defects are formed by ball-milling under hydrogen atmosphere,which can create more fresh surfaces and active locations to accelerate the diffusion of hydrogen.Some MgF₂ and TiH₂ or Fe or Nb active particles formed in the hydriding/dehydriding processes of the transitional mental fluorides-doped composites,acting as the active species in the processes of the hydrogen absorption/desorption of MgH₂..Besides,the doping of transitional mental fluorides can lower the dehydriding temperature of MgH₂-transitional mental fluorides composites.So the hydrogen storage behaviors of magnesium are improved.The research of the Vapor-transport deposition technology on Mg micro/nano scale hydrogen storage materials shows that the gas flow rate,deposition temperature,and evaporation temperature are the important parameters influencing the particle size and morphology of Mg.The shape of Mg presents a trend of transformation from hexagonal prisms to sphericity and the particle size of Mg increases with the gas flowing rate increasing from 50 ml/min to 150 ml/min.As the deposition temperature decreases from 473 K to 373 K,the average particle size of magnesium declines correspondingly.The particle size of Mg in the vapor temperature of 1173 K is the smallest compared with the size measured in the vapor temperature of 1123 K and 1223 K.The particle size of Mg prepared with the gas flowing rate of 50 ml/min,the heating rate of 10 K/min,the evaporation temperature of 1173 K,and the deposition temperature of 373 K is the smallest,which is about 50 nm to 120 nm.Furthermore, the materials prepared under the conditions mentioned above will exhibit the best capacity with fast hydriding/dehydriding kinetics.