| LiBH4, which owns high gravimetric and volumetric hydrogen densities (18.5 wt.% and 121 kg·H2/m3), has been regarded as one of the most promising hydrogen storage materials. However, LiBH4 is thermodynamically stable and kinetically slow, and it is required for extremely rigorous temperature and pressure conditions to reverse. So, it is limited for practical use. Based on the comprehensive review of the research and development of LiBH4 complex hydrides as hydrogen storage materials,2LiBH4-MgH2 system was selected as main research object. In this paper, dehydrogenation pathway under various conditions and formation mechanism of MgB2 in 2LiBH4-MgH2 system during dehydrogenation process were investigated carefully. In addition, effects of fluoride additives on dehydrogenation behaviors of 2LiBH4-MgH2 system and catalytic mechanism were systematically studied.The study on dehydrogenation pathway and formation mechanism of MgB2 in 2LiBH4-MgH2 system shows that, there are two different dehydrogenation pathways, (DPL and DPH) during dehydrogenation process. Under relative lower hydrogen back-pressure (DPL), LiBH4 and MgH2 are decomposed independently, resulting in the formation of LiH,B,Mg and H2 rather than MgB2. However, under relative higher hydrogen back-pressure (DPH), MgH2 decomposes firstly to produce Mg; then, LiBH4 reacts with Mg to produce LiH,H2,MgB2 instead of direct decomposition. As increasing the initial hydrogen back-pressure at constant temperature, the relative content of MgB2 to Mg in the products increases, while the decomposition of MgH2 is inhibited. Under a finite hydrogen back-pressure, elevated temperature could significantly enhance the formation of MgB2, but the ability to suppress the decomposition of LiBH4 is deteriorated. Experimental results show that an applied initial hydrogen back-pressure of at least 0.4 MPa is potentially appropriate to obtain comprehensive ability in yielding MgB2 following DPH at 450℃in 2LiBH4-MgH2 system. And analysis results suggest that the formation process of MgB2 consisting of incubation period and nucleus growth process.Various fluoride additives such as NbF5, TiF3, CeF3, LaF3 and, FeF3 were used to dope 2LiBH4-MgH2 system, and their dehydriding properties and microstructure were investigated systematically. Among the different additives, NbF5 exhibited the most prominent behavior in terms of fast kinetics and lower the dehydrogenation temperature of 2LiBH4-MgH2 system. Differential scanning calorimetry (DSC) shows that 2LiBH4-MgH2 system milled with 5 mol% NbF5 lowered the dehydrogenation temperature by 30℃and 58℃for the two-step dehydrogenation compared to undoped 2LiBH4-MgH2 system in DPL. In DPH, the completed dehydrogenation temperature of 2LiBH4-MgH2+5 mol% NbF5 is lower 50℃than undoped 2LiBH4-MgH2, and the doped 5 mol% NbF5 greatly shortened the dehydriding plateau in the dehydrogenation curves of 2LiBH4-MgH2. With increasing the addition amount of NbF5, additionally, the dehydrogenation kinetics of 2LiBH4-MgH2 is improved, while hydrogen desorption amount decreases obviously. Microstructure analysis shows that the formation amount of LiF and relative content of Mg to MgB2 increases after dehydrogenation by increasing addition amount of NbF5. By Kissinger plot calculation, the two dehydrogenation reaction activation energies of 2LiBH4-MgH2+5 mol% NbF5 in DPL are 100.3 kJ-mol-1 and 127.5 kJ-mol-1 respectively, which are lower 80.3 kJ-mol-1 and 29.2 kJ-mol-1 than undoped 2LiBH4-MgH2. Take consideration into hydrogen release amount and dehydrogenation kinetics,2LiBH4-MgH2+5 mol% NbF5 displays the best comprehensive hydrogen storage properties.Investigation results demonstrates that,2LiBH4-MgH2+5 mol% NbF5 have excellent reversible hydrogen storage properties and cycling hydrogen capacity holding ability. Dehydrogenation of 2LiBH4-MgH2+5 mol% NbF5 entirely followed DPH to produce only MgB2 at 400℃under 0.4 MPa hydrogen back-pressure in 2.5h-3.5 h. At 400℃under 7 MPa hydrogen pressure, dehydrogenated 2LiBH4-MgH2+5 mol% NbF5 rehydrogenates quickly to yield 2LiBH4 and MgH2 in 10 min. Up to 20 dehydrogenation/rehydrogenation cycles have been carried out for 2LiBH4-MgH2+5 mol% NbF5, and no trend of hydrogen capacity deterioration appears, with maintaining between 8 wt.%-9 wt.% and 8.5 wt.%-9.5 wt.% for dehydrogenation and rehydrogeantion individually. According to analysis, it is believed that NbF5 not only catalyze the decomposition of MgH2 but also react with LiBH4 to produced NbB2 playing a key role to promote the formation of MgB2, result in the significant catalytic effect on improving the hydrogen storage properties of 2LiBH4-MgH2 system.
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