| With advantages of light weight,large reserves,low price and high hydrogen storage density,Mg-based hydrogen storage materials are become promising in the field of hydrogen storage.However,the poor kinetics and thermodynamic properties in the process of hydrogen absorption and desorption have restricted the development of them.To improve the hydrogen storage performance of Mg-based materials,In this thesis,Mg-based nanocomposite hydrogen storage materials were designed and prepared with various nano transition metals?Ni,Co,Cu,Pd,Nb,Zn?and large surface area materials?MOFs,porous carbon,graphene,carbon nanotubes,acetylene black?as catalysts by means of liquid deposition reduction,hydrothermal/solvothermal,heat treatment reduction and high-energy mechanical ball milling.SEM,TEM,EDS,XRD,XPS,BET,DSC,PCT and other techniques were used to characterize the microstructure,phase transformation and hydrogen absorption/desorption temperature,kinetics,thermodynamics,cycle performance of the samples.And the synergistic modification mechanism of the added catalyst was further investigated.And the effects of microstructure,catalytic doping and preparation on the hydrogen storage properties of Mg-based materials were systematically studied in this paper.Mg/MOFs?MOFs=ZIF-8,ZIF-67,MOF-74?nanocomposites were prepared by deposition-reduction method.The hydrogen absorption/desorption kinetic properties of Mg/ZIF-67 are better than those of Mg/ZIF-8 and Mg/MOF-74composites.The results show that the Co Mg2 phase formed in the hydrogen absorption/desorption process can significantly improve the hydrogen storage kinetics and thermodynamics performance of Mg/Mg H2.The dehydrogenation apparent activation energy of Mg H2 phase decreases from 204.9 k J/mol H2 to161.7 k J/mol H2 under the catalytic action of ZIF-67 nanocomposite.In addition,due to the stable core-shell structure of Mg/ZIF-67,the Mg nanoparticles cannot be agglomerated during the hydrogen absorption and desorption process,resulting in the capacity of Mg/ZIF-67 composite has almost no attenuation after 100 cycles of hydrogen absorption and desorption and showing a good comprehensive hydrogen storage performanceBased on the proparation method of Mg/ZIF-67,Mg TM/ZIF-67?TM=Ni,Cu,Pd,Nb?nanocomposites were prepared by one-step reduction method.The Ni-doped and Nb-doped sample have the lowest hydrogen absorption/desorption temperature among the four samples.When absorbs/desorbs 1.0wt.%H2,the corresponding temperature for them is 80 and 247?,respectively,which is 82?and 42?lower that of Mg/ZIF-67 prepared by the same method.It is found that the main hydrogenation/dehydrogenation phases of Mg Nb/ZIF-67 are Mg/Mg H2and Co Mg2/Co Mg2H5.Both Co Mg2 and polyvalent Nb paly an important role in the improvement of the dehydrogenation kinetic and thermodynamic properties of Mg.The capacity retention rate of Mg Nb/ZIF-67 is enhanced by hindering the growth of Mg nanoparticles during the hydrogen absorption-desorption cycle.On the basis of the study mentioned above,MOF-74 with different active metal sites?Ni,Co,Cu?were prepared by solvothermal method.The Graphene?G?supported with transition metal?TM=Ni,Co,Cu?nanoparticles?TM/G?were obtained by mean of heat treatment and reduction of the obtained MOF-74composites.Mg@TM/G nanocomposites were prepared by deposition-reduction Mg nanoparticles on TM/G composites.The results show that the transition metals in Mg@TM/G are in the form of Mg2Ni,Co Mg2 and Cu Mg2,respectively.Mg@Ni/G sample has the lowest hydrogenated temperature and the best kinetics/thermodynamics performance among the three samples.The temperature of 1.0 wt.%hydrogen absorption and desorption for it is just only 80?and233?,respectively.The apparent activation energy and the enthalpy of hydrogenation/dehydrogenation are 29.09/65.74 k J/mol H2 and 61.0/74.4 k J/mol H2 respectively.The Mg@TM/G?BM nanocomposites were further study by mechanical milling.The results show that mechanical milling can reduce the agglomeration of TM/G nanocomposites,increase the specific surface area and enchance the catalytic effect of catalysts,so can further reduce the initial de/hydrogenation temperature of the sample.The Mg@TM/G?BM nanocomposites can absorb hydrogen even at room temperature.Additionally,it releases 4.1 wt.%H2 at 250?just only need 500s.Compared with the samples prepared by deposition-reduction or ball-milling alone,the Mg@TM/G?BM samples prepared by deposition-reduction combined with ball-milling have the best comprehensive hydrogen storage performance.Mechanism analysis found that the Ni nanoparticles encapsulated in graphene can be well-diversified in Mg nanoparticles of Mg@TM/G?BM samples which prepared by the methods of deposition-reduction and ball-milling.In the process of hydrogen absorption and desorption,Ni nanoparticles released by opening of the graphene capsules,and reacting with Mg rapidly and forming Mg2Ni nanoparticles.Graphene coating on the surface of Ni can effectively inhibit the agglomeration of Ni,improve the dispersion of Ni and inhibit the oxidation of Ni.In order to further investigate the effect of carbon and nitrogen on magnesium hydrogen storage materials.ZIF-8 precursor was synthesized by hydrothermal method,and three kinds of nitrogen doped carbon nanoparticles?NC?were obtained by high temperature heat treatment and acid treatment.Afterwards,three kinds of Mg/NC nanocomposites were prepared by depositing and reducing nano-Mg on NC nanoparticles.The results show that NC nanoparticles can significantly reduce the dehydrogenation temperature,activation energy and enthalpy of Mg nanoparticles,and improve the dehydrogenation kinetics/thermodynamic performance and cycle stability of nanocomposites.The increase of the ratio of graphitic N in NC can enhance electron mobility of the particle surface,thereby promoting the combination and decomposition of H in the process of de/hydrogenation.To further simplify the preparation process and improve the hydrogen storage capacity and stability of Mg-based materials as discussion above,commercial graphene with large specific surface area was selected as the raw material.Mg nanocomposites doped with transition metal elements?Mg TM?TM=Ni,Cu,Pd,Nb??were prepared by deposition-reduction method,and then Mg TM/G nanocomposites were further prepared by milling Mg TM with commercial graphene.The results show that the doping of transition metal elements?Ni,Cu,Pd,Nb?can improve the de/hydrogenation properties of Mg H2/Mg,and the composite of Mg TM and monolayer graphene by ball-milling can synergistically catalyze the dehydrogenation of Mg H2 to obtain a lower dehydrogenation temperature.Mg Ni and Mg Ni/G nanocomposites exhibited the lowest de/hydrogenation temperature and the fastest de/hydrogenation rate in Mg TM and Mg TM/G samples.Moreover,Mg Ni/G also shows the lowest dehydrogenation activation energy and good cyclic stability.For Ni and Pd that can significantly improve the hydrogen storage performance of Mg,Mg Ni Pd composites were prepared by one-step reduction method,and then the obtained Mg Ni Pd composites were compounded with low-dimensional carbon nanomaterials?monolayer graphene,single-walled carbon nanotubes and acetylene black,respectively?by in-situ formation and ball-milling methods.The products were marked as Mg Ni Pd+G,Mg Ni Pd+CNTs and Mg Ni Pd+AC,respectively.The results show that Mg Ni Pd can release 4.0 wt%H2 at 255?.And the dehydrogenation activation energy value of Mg Ni Pd is124.15 k J/mol H2,which is lower than the as-prepared binary alloy.The Mg Ni Pd+CNTs BM samples which prepared by milling Mg Ni Pd with carbon nanotubes shows the lowest dehydrogenation temperature among nine kinds of Mg Ni Pd C nanocomposites.The dehydrogenation activation energy of Mg Ni Pd+CNTs BM is just only 52.44 k J/mol H2,showing the excellent dehydrogenation kinetics and cycle performance. |
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