Synthesis And Characterization Of Multi-component Mg-transition Metal Based Nano-composites

Magnesium hydride has a relatively high hydrogen storage capacity?7.6 wt%?,environmentally friendly properites and economical cost,which is able to satisfy some of the basic requirements of on-board.However,the high thermodynamic stability and poor kinetics of Mg/MgH₂,which correspond to high operating temperatures?up to about 573 K?,hinder the practical applications especially in the energy storage systems of vehicles.Thus,tuning the kinetic and thermodynamic properties of Mg-based hydrides is the key issue to overcome the problems limiting their practical applications.Nano structuring of Mg/MgH₂ and doping with various catalysts or additives are considered as effective methods to accelerate hydrogen sorption properties of Mg-based hydrides.In this work,Mg-rare earth?RE?-H?RE=La,Gd,Er?nano-composites were successfully prepared through reactive ball milling.It is found that the Mg-La-H has the best kinetics of de/re-hydrogenation among these composites.Mg-La-Fe-H nano-composite was then prepared through reactive ball milling to further improve the kinetic properties of de/re-hydrogenation of Mg/MgH₂ with the synergetic catalytic effects from both La and Fe.Finally,for improving both the thermodynamic and kinetic properties of Mg-based hydrides,coarse-grained Mg powders or ultrafine Mg powders were ball milled with the Fe?Ni?and Ni?Fe?nano-composites prepared through arc plasma method and then these mixtures were hydrognated to prepare Mg-Fe-Ni-H composites.The investigations on the Mg-RE-H?RE=La,Gd,Er?nano-composites show that:different reaction behaviors were observed between MgH₂ and the three selected RE elements.An unsaturated RE hydride-LaH_2.3 formed in the Mg-La-H composite and a saturated ErH₂ formed in the Mg-Er-H composite after milling and they are both stable during de/re-hydrogenation cycles.In contrast,Gd atoms are able to substitute Mg atoms in MgH₂ or Mg without the formation of Gd hydride or Gd-Mg intermetallic.The Mg-La-H has the best kinetics of de/re-hydrogenation among these composites.The peak dehydriding temperature of Mg-La-H is 78.4 K lower than that of pure MgH₂ and its activation energy of hydrogen desorption of Mg-La-H was reduced to 106.6 kJ/mol H₂.The Mg-La-H can absorb 5 wt%of hydrogen within 360 s at 473 K and its activation energy of hydrogen absorption was reduced to 42.87 kJ/mol H₂.However,the doping with rare earth elements through reactive ball milling are not able to alter the thermodynamic properties of Mg/MgH₂.The study of the Mg-La-Fe-H nano-composite shows that:LaH_2.3 and?-Fe in the Mg-La-Fe-H composite are stable during de-rehydrogenation cycles.Compared with the kinetic properties of de/re-hydrogenation of the Mg-La-H and Mg-Fe-H,those of the Mg-La-Fe-H can be significantly improved.This is especially promising for the Mg-La-Fe-H composite,which can absorb 5.0 wt%of hydrogen within 8 h at room temperature.The peak dehydriding temperature of Mg-La-Fe-H is 141.0 K lower than that of pure MgH₂.The activation energies for hydrogen absorption and desorption of this composite were further reduced to 23.9 and 100.3 kJ/mol H₂.However,the simultaneous addition of La and Fe to MgH₂ through reactive ball milling cannot effectively improve the thermodynamic properties of Mg/MgH₂.The investigations on the Mg-Fe-Ni-H composites show that:Mg₂Fe?Ni?H₆ was synthesized under relatively mild conditions using precursors of coarse-grained Mg powders and Fe?Ni?nano-composite containing?-Fe?Ni?nano particles.It is observed that the Mg₂Fe?Ni?H₆ in the Mg-Fe?Ni?-H composite has a“tangled nanowire”morphology.A promotion effect on the synthesis of Mg₂FeH₆ is found for the utilization of?-Fe?Ni?precursor instead of pure?-Fe.The absorption and desorption enthalpies of Mg₂Fe?Ni?H₆ were measured to be-68.8 and 69.2 kJ/mol H₂,of which absolute values are lower than the ones of Mg₂FeH₆.The activation energy for hydrogen desorption of Mg₂Fe?Ni?H₆ was reduced to144.4 kJ/mol H₂.Similarly,Mg₂Ni?Fe?H₄ was synthesized using precursors of coarse-grained Mg powders and Ni?Fe?nano-composite.The absorption and desorption enthalpies of Mg₂Ni?Fe?H₄ were measured to be-53.1 and61.4 kJ/mol H₂,of which absolute values are lower than the ones of Mg₂NiH₄.Meanwhile,the activation energy for hydrogen desorption of Mg₂Ni?Fe?H₄ was reduced to 97.2 kJ/mol H₂.Therefore,both the thermodynamic and kinetic properties of Mg-Fe-Ni-H composites are effectively improved.Furtherly,instead of the coarse-grained Mg powders,ultrafine Mg powders prepared through arc plasma method were ball milled with the Fe?Ni?and Ni?Fe?nano composites to prepare nano Mg-Fe?Ni?and nano Mg-Ni?Fe?composites.Compared with the Mg-Fe?Ni?-H and Mg-Ni?Fe?-H,the yields of Mg₂Fe?Ni?H₆ and Mg₂Ni?Fe?H₄ in the nano Mg-Fe?Ni?-H and nano Mg-Ni?Fe?-H have been increased by 49.2and 35 wt%after hydrogenated at 623 K under 3.7 MPa H₂ for 48 h,respectively.In addition,the activation energies for hydrogen desorption of the Mg₂Fe?Ni?H₆ and Mg₂Ni?Fe?H₄ in the nano Mg-Fe?Ni?-H and nano Mg-Ni?Fe?-H were also reduced to be 114.7 and 70.8 kJ/mol H₂.Therefore,the use of ultrafine Mg powders can further enhance the hydrogen sorption kinetic properties of the Mg-Fe-Ni-H composites.