Investigation On The Hydrogen Storage Properties Of LiBH₄+Mg-based Hydride Systems

Complex hydrides have been investigated intensely as new hydrogen storage materials just because of their high hydrogen storage capacity, such as LiBRt owns high gravimetric hydrogen density of 18.5 wt.%and volumetric hydrogen density of 121 kg-H2/m3. However, LiBH4 is thermodynamically stable and kinetically slow, so it is required for extremely rigorous temperature and pressure conditions to reverse. Based on the summary of researches about LiBHU as hydrogen storage materials, the L1BH4+Mg-based hydride composites were investigated in this paper. The effect of premilling of MgH2 on the dehydrogenation behavior of 2LiBH4+MgH2 composite was investigated firstly. Then, the effect of stoichiometry x on the dehydrogenation properties of LiBH4+xMg2NiH4 (x=0.25-1.25) and the effect of preparation methods of Mg2NiH4 on the dehydrogenation behavior of LiBH4+0.25Mg2NiH4 composite were investigated systemically. In addition, the hydrogen storage performance of 5LiBH4+Mg2FeH6 composite was also studied.The investigation on the effect of premilling of MgH2 on the dehydrogenation behavior of the 2LiBH4+MgH2 composite show that, Mg can destabilize the B-H bond and this effect is enhanced with the premilling of MgH2. The good dispersivity of micro-/nano-structure MgHb shortens the diffusion distance between the solid phase (MgH2) and liquid phase (LiBH4). Under lower initial hydrogen back-pressure, the kinetics of isothermal hydrogen desorption (under 0.2 bar initial hydrogen back-pressure) and continuous hydrogen desorption (under 0.001 bar initial hydrogen back-pressure) of the composite are enhanced with the premilling of MgH2, however, the dehydrogenation pathway does not change. Under higher initial hydrogen back-pressure (4 bar), the kinetics of the first and second step dehydrogenations are enhanced with the premilling of MgH2, but that of the third step dehydrogenation is impressed, however, the total dehydrogenation capacity is increased. Meanwhile, more B element exists as MgB2 in the dehydrogenation products after the premilling of MgH2, leading to the improvement of rehydrogenation behavior of the composite.The study on the dehydrogenation properties of LiBH4+χMg2NiH4 composites demonstrates that, the stoichiometry x has a great effect on the dehydriding kinetics of LiBH4+xMg2NiH4 composites. With the increase of x value, the decrease of the dehydrogenation temperature and the improvement of the dehydriding kinetics are observed with a loss of dehydrogenation capacity. Both the stoichiometry x and hydrogen back-pressure can influence the wetting between liquid LiBHU and solid Mg2NiHU during the dehydrogenation, and have a great effect on the dehydrogenation pathway and kinetics furthermore. The dehydrogenation properties and mechanism of LiBH4+0.25Mg2NiH4 (i.e.4LiBH4+Mg2NiH4) composite have been investigated by changing the preparation methods of Mg2NiH4. Compared with the Mg2NiH4 prepared by ingot metallurgy, the Mg2NiH4 prepared by hydriding combustion synthesis (referred as Mg2NiH4-HCS) has finer crystal size, and 4LiBH4+Mg2NiH4-HCS composite gains a better dehydrogenation property. Finally, to confirm whether the effect of preparation methods of Mg2NiH4 on the 4LiBH4+Mg2NiH4 composite results from the difference of crystal size, the dehydrogenation properties of the 4LiBH4+Mg2NiH4 composite has been also studied by premilling treatment of Mg2NiH4. With the increase of premilling time of Mg2NiH4, the crystal size of Mg2NiH4 gets finer, and the wetting between liquid LiBH) and solid Mg2NiH4 has been increased, so that the dehydrogenation properties and dehydrogenation capacity has been improved gradually.The hydrogen storage performance of 5LiBH4+Mg2FeH6 composite was studied systemically by compared with 2LiBH4+MgH2 composite in the last section. The results demonstrate that, the dehydrogenation kinetics of 5LiBH4+Mg2FeH6 composite is better than that of 2LiBBH4+MgH2 and LiBH4+Fe composites. The self-decomposition of Mg2FeH6 leads to the in situ formation of Mg and Fe particles on the surface of LiBH4, resulting in a well dispersion between different reacting phases, and realizing the favorable co-doping of Mg and Fe. The investigation on the hydriding-dehydriding cycling of 5LiBH4+Mg2FeH6 composite shows that, the boron element exists as FeB and MgB2 phases in the dehydrogenation products, and the FeB and MgB2 can be regenerated to LiBH4 by reacting with LiH and H2 under moderate condition. The crystallize size of FeB and MgB2 doesn’t grow during the cycling, which accounts for the good hydriding-dehydriding kinetics properties. The loss of hydrogen storage capacity and de-/rehydrogenation kinetics are attributed to the incomplete generation of Mg2FeH6 during the rehydrogenation process.