Synthesis Of Mg-based Hydrogen Storage Material And Studies On Hydrogenation Properties

In this thesis, the literatures on Mg-based hydrogen storage materials were exhaustively reviewed firstly. The high hydriding / dehydriding temperature and low kinetics rate of the Mg-based alloys were unanimously regarded as the chief obstacles for the commercialization of the Mg-based alloys in spite of their many attractive merits. The object of this study is to improve the hydriding / dehydriding reaction kinetics and to lower the reaction temperature of the Mg-based hydrogen storage material. On the basis of the literature review, the research scheme for lowering the hydriding / dehydriding temperature of the ball-milled Mg-Ni-Cr ternary alloys by means of multi-component alloying was chosen. In addition to the finding of some alloys with better hydrogenation properties, Mg-Ti-Ni-Cr alloy, Mg₂Ni_0.8Cr_0.2 with addition of surface catalyst and Mg-based multi-phase compositse were optimized step by step in order to obtain the effective method for improving hydrogenation properties of the Mg-based hydrogen storage material.For Mg₂Ni_1-xCr_x (x= 0 0.30) alloys, as their stoichiometry changed, their phase structure and hydrogen storage properties varied accordingly. For the sintered Mg₂Ni_1-xCr_x alloys, a single Mg₂Ni phase was obtained. The addition of Cr, which was in a solid solution, increased the cell parameters of Mg₂Ni phase. However, (Mg₂Ni-Ni) multi-phase formed because of the unstable structure of Mg₂Ni_1-xCr_x during high energy ball milling. Mg_2-xTi_xNi_0.8Cr_0.2 (x= 0.05- 0.20) alloys were further investigated. The addition of Ti resulted in more than one phase existing in the sintered alloys. The hydrogen desorption kinetics properties of Mg_2-xTi_xNi_0.8Cr_0.2 alloys were improved with addition of Ti.TiO₂ nanoparticles and CoO / Al₂O₃ composite as surface catalysts added in Mg₂Ni_0.8Cr_0.2 alloy, and the reaction mechanism of oxides as surface catalyst for improving the hydrogenation properties were studied. For ball milled Mg₂Ni_0.8Cr_0.2 alloy with addition of TiO₂ nanoparticles, Ni and TiO₂ nanoparticles dispersed on the surface of Mg₂Ni based alloy. The Mg₂Ni_0.8Cr_0.2-1.5 wt.% TiO₂ composite showed the most favorite hydrogenation properties. The enthalpy (â–³H°) of hydrides forming of Mg2Ni_0.8Cr₍0.2_-1.5 wt.% TiO₂ composite was -58 kJ /mol H₂ estimated from P-C-T curves and the pressure of hydriding reaction plateau increased with increasing the content of TiO₂ nanoparticles. When the content of CoO /Al₂O₃ multi-phase catalyst changed in 0.5 wt.% 5.0 wt.%, the content of catalyst played few effects on the phase structure of Mg₂Ni_0.8Cr_0.2-CoO /Al₂O₃ composite. The addition of CoO /Al₂O₃ catalyst improved the hydriding / dehydriding kinetics properties of Mg₂Ni_0.8Cr_0.2 alloy. The maximum hydrogen capacity of the composite decreased with increasing the content of CoO /Al₂O₃.On the bases of above researches, the hydrogenation properties of Mg-Mg₂Ni_0.8Cr_0.2-TiO₂ composites with different contents of Mg₂Ni_0.8Cr(0.2) alloy and TiO₂ nanopartiecles were studied respectively. In case of x =1.5 wt.%, the composite indicated best hydrogenation properties for Mg-15 wt.% Mg₂Ni_0.8Cr(0.2)-x wt.% TiO₂ (x = 0, 0.5, 1.5,2.5) composites. The hydrogen storage properties of Mg-x wt. % Mg2Nio.8Cro.2-l-5 wt.% TiO2 (x= 20, 25, 30, 50) composites were subsequently studied. The maximum capacity of composites decreased, but the hydrogenation properties at lower temperatures were improved when the content of Mg2Nio.sCro.2 increased. The favorite performance of Mg-Mg2Nio.8Cro.2-Ti02 composites was resulted from the cooperated function of catalyst of both Mg2Nio.8Cro.2 and TiO2 nanoparticles in addition with the fine structure formed during ball milling.Mg with doped La or Ce through melting was investigated and the hydrogen storage properties of optimized Mg-2.0 at.% Ce alloy with addition of Ni as surface catalyst were discussed. La2Mgn or Ce2Mgn phase existed in the Mg matrix instead of La or Ce accordingly. By means of hydriding firstly with subsequent ball milling, Ce hydrides indicated the catalytic effect on the hydriding / dehydriding reaction of Mg-Ce /Ni composite. The enthalpy (AH0) of hydrides forming of Mg-Ce /Ni composite was -70.58 kJ /mol H2 estimated from P-C-T curves.For further improving the maximum hydrogen capacity, the hydrogen storage properties of Mgi7Alj2 alloy with low density and Mg-10 wt.% MgnAl^ composite were studied. Ball milled Mgi7Ali2 alloy can be hydrided at 473 K. The maximum capacity and hdyriding kinetics properties increased with increasing temperature. For the Mg-10 wt.% composite, the MgpAlu alloy showed mainly catalytic function. The brittle intermetallic compound could be inlayed on the surface of soft Mg particles during ball milling, which can accelerate the diffusion of hydrogen in formed hydride layer.The preparation and hydrogen storage properties of organometallic Pt-C6o compound were studied. It was found that the Pt-C6o compound with weak interaction between Pt and C60 molecule could be obtained by a chemical method. The maximum capacity of Pt-C6o compound was 1.6 wt.%. The hydrogenation properties were related to the electron structure of transition metals. To some extent, this result suggested the potential hydrogen storage in TM-C60 (TM= transition metal) compounds.