Improving Hydrogen Storage Properties Of LiBH₄-MgH₂ System By Doping Different Catalyst

With the rapid development of the global economy and growing energy demand, our society is faced with the problems of fossil energy depletion and ecological environment deterioration. Thus, developing new green energy and researching materials for energy conversion have great significance. Hydrogen is the ideal energy in the future. However, hydrogen storage technology is a prerequisite for large-scale applications and infrastructure of hydrogen energy. In this paper, the history and the current state of the research on LiBH4 and MgH2 have been summarized. LiBH4 and MgH2, as the representations of complex hydrides and metal hydrides, respectively, are promising hydrogen storage materials due to their higher hydrogen storage capacity. However, the harsh condition and poor kinetics of their hydriding and dehydriding are need to be further studied.Improving the kinetics of LiBH4 and MgH2 is the significance of this thesis. Firstly, high energy ball milling and catalyst doping are used to obtain LiBH4-MgH2 composite materials. Secondly, the effects of ball milling process and catalysts on the hydrogen storage properties were investigated. Furthermore, the X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the structure of LiBH4-MgH2 system after ball milling and dehydriding.It was found that with increase of the milling time, the particle size of LiBH4-MgH2 was decreased markedly while the dehydrogenation capacity and kinetics were also improved. However, the sample milled more than 10 hours could not lead to smaller particle size and better dehydrogenation properties. It was also found that vibration ball milling have the better effect than the planetary milling on the dehydriding kinetics of LiBH4-MgH2 with the same milling time and ratio of ball to powder.With respect to the LiBH4-MgH2 composite with different mole ratios, the capacity and the kinetics of dehydrogenation was enhanced with increase of the MgH2 content. The dehydrogenation was composed of two steps while the first step is decomposition of LiBH4 and the second is MgH2. However, the decomposition of LiBH4 is not fully carried out in the first step.It was found that the transition metal elemental Ni and Nb, oxides TiO2 and Nb2O5, chlorides NiCl2, FeCl2 and YCl3 all can improve the dehydrogenation property of LiBH4-MgH2. Nb2O5 and YCl3 have the best catalytic effect, which lead to the simultaneous decomposition of MgH2 and LiBH4. The catalytic effects of Ni and Nb on dehydriding capacity of LiBH4-MgH2 were not obvious. The TiO2 and Nb2O5 have similar catalytic effect. However, the YCl3 has better catalytic effect, which increased 2wt% of the dehydrogenation capacity.LaCl3, CeCl3, NdCl3 and SmCl3 all can enhance the dehydriding capacity of LiBH4 and MgH2, in which the NdCl3 has the best effect to enhance the dehydrogenation capacity of MgH2. With the temperature increasing, the dehydriding capacity of MgH2 increased. The dehydrogenation capacity reached 7.5wt% at 350℃, which closes to the theoretical hydrogen storage capacity of MgH2. Dispersion of NdCl3 in activate carbon increase the contact area of NdCl3 and MgH2 and thus enhance the catalytic effect of NdCl3 on dehydriding kinetics of MgH2.The addition of 20wt% of NdCl3 has better catalytic effects on the LiBH4-MgH2 composite with different mole ratios. LiBH4-MgH2(molar ratio 1:2) can release 80% hydrogen of their theoretical capacity at 330℃. Moreover, with the increasing of the content of MgH2, the dehydriding capacity of LiBH4 increased.