| With the gradual depletion of traditional petrochemical energy sources and increasing environmental pollution,traditional fuel vehicles are gradually replaced by new energy vehicles.Hydrogen fuel cell vehicles have attracted much attention due to their high energy conversion rate,low noise and zero emissions.Magnesium hydride(MgH2),as a solid hydrogen storage material,is considered to be one of the most promising on-board hydrogen storage materials due to its high hydrogen storage capacity,low cost and light weight.However,the high thermodynamic stability and slow hydrogen ab/desorption kinetics of MgH2 lead to the high de/hydrogenation temperature and low de/hydrogenation rate.It was found that MgH2 is easily oxidized during the preparation or hydrogen de/absorption cycles of MgH2 to form a small amount of magnesium oxide(MgO).However,the influence mechanism of this type of oxides on the hydrogen storage properties of MgH2 is still fully unclear.In order to clarify the effect and mechanism of magnesium-based oxides on the hydrogen storage properties of MgH2,MgO and transition metal TM(TM=Ti,V,Nb,Fe,Co,Ni)doped Mg(TM)O magnesium-based oxides are selected as additives to study the effects of magnesium-based oxides on the hydrogen storage properties of MgH2 by using theoretical and experimental methods.Firstly,the first-principles calculation method is used to systematically study the dehydrogenation reaction behavior of single MgH2 molecule on the surfaces of pure MgO and Mg(TM)O,as well as the confinement effect of oxides on single MgH2 molecule.The results show that all magnesium-based oxides increase the Mg-H bond length,resulting in weakened stability and reduced hydrogen desorption energy of MgH2.Meanwhile,the effect of MgO,Mg(Fe)O,Mg(Ni)O,Mg(Co)O,Mg(Ti)O,Mg(V)O and Mg(Nb)O oxides on the hydrogen dehydrogenation properties of MgH2 is sequentially enhanced.Additionally,the Mg(Nb)O exhibits the most excellent confinement effect on single MgH2 molecule.The electronic structure analysis shows that the more charge transfer from the Mg(Nb)O oxide layer to single MgH2 molecule is the intrinsic reason for the optimal hydrogen desorption properties of the MgH2/Mg(Nb)O composite system.Secondly,the dehydrogenation reaction behavior and confinement effect of magnesium-based oxides on MgH2 slab are further studied by constructing the MgO/MgH2 and Mg(TM)O/MgH2 interface models.The results show that all the MgO and Mg(TM)O oxides possess the catalytic effect on the hydrogen desorption properties of MgH2 slabs.Among them,the weakening of Mg-H bond strength by Mg(Nb)O is the most obvious.Additionally,the higher interfacial separation work between Mg(Nb)O and MgH2 slab and the lower brittleness of Mg(Nb)O itself make Mg(Nb)O have a long acting and stable confinement effect on MgH2.The electronic structure analysis shows that the presence of the Mg(TM)O oxide layer reduces the total number of bonding electrons of MgH2 slab,weakens the Mg-H bond strength,and enhances the hydrogen desorption properties of MgH2.In addition,the optimal confining effect of Mg(Nb)O for MgH2 lies in the strong interactions between O within Mg(Nb)O and Mg within MgH2.Finally,the Mg(Nb)O solid solution oxide with the optimal modification effect is synthesized by high energy ball milling method based on the theoretical calculation results.The effect of oxides on the microstructures and hydrogen desorption properties of MgH2 hydrogen storage system is studied experimentally.The results show that,as compared with pure milled MgH2 system,the addition of Mg(Nb)O not only effectively refines the particles and grains of MgH2,but also significantly reduces the hydrogen desorption temperature.The experimental results are in good agreement with the theoretical calculations.The results provide the theoretical basis and guidance for the catalytic activity regulation of MgO and the development of high-performance magnesium-based hydrogen storage materials. |
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