Effects And Mechanisms Of Doping On The Dehydrogenation Properties Of Mg-based Hydrogen Storage Materials

Magnesium hydride(MgH2) is hailed as one of the most promising hydrogen storage material because of its high hydrogen storage capacity(7.6wt%), abundant resources and low cost. If the Mg-based hydrogen storage material want to be the wide applied, sluggish absorption kinetics and high thermal stability should be better solved. The sluggish absorption kinetics due to the hydrogen atom is very difficult to dissociate from H2 on the surface of Magnesium. And the high dehydrogenation temperature of system is resulted by its high thermodynamic stability. In recent years,several studies have been carried out to solve these obstacles; some significant progresses have been achieved to accelerate the decomposition kinetics by mechanical alloying and the doping of different additives or catalysts. While we have had to wrestle with the big challenges of the high thermodynamic stability. In this thesis, we studied the effect and mechanism of transition metal and carbon material separate/multi-doped on the dehydrogenation properties of MgH2 by high energy mechanical ball mill experimental methods and first principles calculation. Our works offer useful information and theoretical basis for improving the commercialization of fuel-cell-powered vehicles. The primary coverage of this thesis is as follows:(1) The effects and mechanisms of Ni and Ti separate/multi-doping on dehydrogenation properties of MgH2 were analyzed. For the Ti or Ni doping system,the particles of MgH2 are refined and homogeneous in size relative to pure MgH2 system, and the agglomerations of smaller particles are weakened. The initial dehydrogenation temperature of MgH2 is decreased by 136℃ significantly due to the lattice deformation and the weakened structural stability of MgH2 matrix where some Mg atoms were replaced by Ni or Ti atoms. Comparatively, the solid solution effects of Ni-doped MgH2 system are relatively remarkable. However, for the Ni and Ti multi-doped system, the particles refinement effects of MgH2 are weakened relative to Ti or Ni doping system due to the emergence of Ni Ti phase. The initial dehydrogenation temperature is further decreased by 161℃ with respect to Ti or Ni doping system, which achieves the synergistic effects of solid solution by both Ni and Ti.(2) In order to relief the decline of the hydrogen storage capacity in the MgH2 system by doping Transition metal, we choose the carbon materials(such as Graphite,Graphene) as dopant which have the physical hydrogen storage properties by itself.And the effects and mechanisms of Graphite or Graphene separate-doping on dehydrogenation properties of MgH2 were analyzed. It turned out that the initial hydrogenation temperature of MgH2 doped by Graphite or Graphene decreased by about 10℃ or 33℃ as a result of the effect of grain refinement and uniform size of MgH2 particles suffered from the structure confinement effect by Graphite or Graphene in the ball-milling process. The effects of Graphene-doped MgH2 system are relatively remarkable. The mechanism of doping is that the effect of grain refinement of carbon weakened the reaction between Mg and H in MgH2, so that the initial hydrogenation temperature of MgH2 doped by Graphite or Graphene were decreased.(3) Basing on the research results of(1) and(2), we choose Ni and Graphene as dopant which have the excellent doping effect. And the effects and mechanisms of Ni and Graphene multi-doping on dehydrogenation properties of MgH2 were analyzed. It turned out that the doping order played an important role in the Graphene and Ni multi-doped MgH2 system. When Graphene and Ni doped in the MgH2 system at the same time, the initial dehydrogenation temperature of MgH2 system did not reduce because the buffer function of MgH2 particle coated by Graphene made it difficult for the Ni atoms solid-soluted into the MgH2 matrix. While when Graphene doped in the MgH2 system after Ni atom, the initial dehydrogenation temperature further decreased by closely 175℃ with respect to pure MgH2 system due to both the effect of the realization of solid solution by Ni atom and the structure confinement effect by Graphene.