| The safe, efficient and economical storage of hydrogen is widely recognized as one of the key technological challenges in the transition toward a hydrogen energy. Therefore, the research and development of new high-capacity hydrogen storage materials is of both scientific interest and practical importance. Considerable attention has been paid on LiBH4, which has the highest hydrogen storage capacity among the light metal complex hydrides. However, high dehydrogenated temperature and poor kinetics for re-/dehydrogenation prevent it from practical applications. Based on the overview of the progress in LiBH4as the hydrogen storage medium, the desorption process and kinetics performances of the LiBH4samples with and without Mg-based additive were first studied systematically. And then, the effects of the in situ formation of MgH2and LaH3on the hydrogen storage behaviors and mechanisms of LiBH4were elucidated. Thirdly, MgH2was introduced into the6LiBH4-CaH2system, and its function in improved hydrogen storage thermodynamics and kinetics was revealed. Finally, Ti(OEt)4was selected as catalyst to further improve the6LiBH4-CaH2-3MgH2system. The role played by Ti(OEt)4was systematically investigated and discussed.It is found that the thermal decomposition of LiBH4is a dehydrogenation process. Approximately9.7wt%of hydrogen, equivalent to2.1mol hydrogen atoms, was released while heating the sample from room temperature to600℃, and the products are composed of LiH and Li2B12H12. Ball milling can slightly decrease in the operating temperature for hydrogen desorption. The apparent activation energies of LiBH4with and without BM were calculated to be148and179kJ/mol, respectively. JMA analyses revealed that the dehydrogenation of LiBH4was a diffusion-controlled reaction, where the resultant products grow with finite long dimensions. Further investigations revealed that the addition of Mg and MgF2decreased the operating temperature of dehydrogenation from LiBH4. For the LiBH4-0.5Mg composite, Mg can react with LiBH4to release hydrogen and generate LiH and MgB2, which induces a thermodynamic destabilization, and consequently reduces the dehydrogenation temperatures. However, the self-decomposition of LiBH4still dominates in the dehydrogenation process of the LiBH4-0.5MgF2composite. At the same time, partial MgF2may react with LiBH4and the decomposition product of LiH, which is responsible for the slight decline of the dehydrogenation temperatures.To improve the hydrogen storage properties of LiBH4, a reactive composite of LiBH4-xLa2Mg17was prepared by means of mechanochemical reaction under40bar of hydrogen. It was found that MgH2and LaH3were readily formed in situ during ball milling the La2Mg17alloy with LiBH4under40bar of H2, and a strong dependency of hydrogen storage performance of the LiBH4-xLa2Mg17composites on the content of La2Mg17was observed. The as-prepared LiBH4-0.083La2Mg17composite under40bar of H2exhibits superior hydrogen storage properties as～6.8wt%of hydrogen can be reversibly desorbed and absorbed below400℃. It was also purposed that the self-decomposition of MgH2first occurred to convert into Mg with hydrogen release upon dehydrogenation and subsequently catalyzed the reaction of LiBH4and LaH3to liberation additional hydrogen along with the formation of LaB6and LiH. The apparent activation energy and desorption enthalpy change of the two-step reaction for LiBH4-0.083La2Mg17composite were calculated to be122,129kJ/mol and70.8±4.4,40.2±2.0kJ/mol-H2, both lower than those of the pristine sample, respectively. The in situ formed MgH2and LaH3provide a synergetic thermodynamic and kinetic destabilization on the de/hydrogenation of LiBH4, which is responsible for the distinct reduction in the operating temperatures of the as-prepared LiBH4-xLa2Mg17composites.And then, MgH2was introduced into the6LiBH4-CaH2system to improve its hydrogen storage performances.～8.0wt%of hydrogen could be reversibly stored in a6LiBH4-CaH2-3MgH2composite below400℃and100bar of hydrogen pressure with a stepwise reaction, which is superior to the pristine6LiBH4-CaH2and LiBH4samples. Upon dehydriding, MgH2first decomposed to convert to Mg and liberate hydrogen with an on-set temperature of～290℃. Subsequently, LiBH4reacted with CaH2to form CaB6and LiH in addition to further hydrogen release. Hydrogen desorption from the6LiBH4-CaH2-3MgH2composite finished at～430℃in non-isothermal model, a～160℃reduction relative to the6LiBH4-CaH2sample. JMA analyses revealed that hydrogen desorption was a diffusion-controlled reaction rather than an interface reaction-controlled process. The newly produced Mg of the first-step dehydrogenation possibly acts as the heterogeneous nucleation center of the resultant products of the second-step dehydrogenation, which diminishes the energy barrier and facilitates nucleation and growth, consequently reducing the operating temperature and improving the kinetics of hydrogen storage. Furthermore, Ti(OEt)4was selected as catalyst to improve the hydrogen storage properties of the6LiBH4-CaH2-3MgH2sample. It was found that for the6LiBH4-CaH2-3MgH2-0.15Ti(OEt)4composite, the on-set temperature of dehydrogenation was decreased from290℃to180℃, and～6.8wt%of hydrogen could be reversibly stored. The formation of the by-products of TiO2, MgO, B2H6and C2H6upon heating is responsible for the decrease in the hydrogen storage capacity. In comparison with the pristine sample, the apparent activation energy of the two-step reactions for the samples with Ti(OEt)4were reduced by25%and20%, respectively, which should be the most important reason for the decreased dehydrogenation temperatures. Moreover, a2-fold increase in the re-hydrogenation rate was achieved for the catalyst-doped sample under the conditions of400℃and100bar of hydrogen pressure. |
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