| As the double pressure of energy crisis and environmental pollution, it is extremely urgent to develop a new renewable clean energy. Hydrogen is considered as a favorable energy carrier because it is the most abundant element in the universe.It possesses the highest energy density per unit mass and burns clean, producing only water. Currently, the main challenge of large-scale use of hydrogen as an energy carrier is to store it in a safe, efficient and reversible system. Solid state hydrogen storage of chemicals has attracted great attention due to the relatively safe structures of the chemicals and high hydrogen storage capacity. Among them, Ca(BH4)2is particularly of interest due to its suitable dehydrogenation enthalpy and relatively high theoretical hydrogen content (11.4wt%). However, its high desorption temperature and harsh conditions for rehydrogenation seriously hinder its application in practice. Based on the overview of the progress in Ca(BH4)2and to clarify further the hydrogen storage mechanism and further improve the hydrogen storage properties of Ca(BH4)2, the present work focuses on the following aspects:the efficient synthesis of high-purity Ca(BH4)2with low cost; the effect of ball-milling on the phase structure and morphology of Ca(BH4)2, and their effects on the hydrogen desorption temperature and kinetics of Ca(BH4)2; the synergetic effect of porous morphology and in situ formed TiO2derived from Ti(OEt)4in improving hydrogen storage performance of Ca(BH4)2; the synergetic actions of the different components on the hydrogen storage properties of Ca(BH4)2+2LiBH4+2MgH2ternary system; the effect of in situ introduced LaH3and MgH2by hydrogenating LaMg3alloy during ball milling on the de-/hydrogenation performances for the Ca(BH4)2+LiBH4composite.The synthesis methods of Ca(BH4)2, via solid-state ball milling CaB6and CaH2under H2pressure and liquid-state ball milling NaBH4, CaCl2and THF, are investigated and discussed. It is found that the precursor of Ca(BH4)2·2THF can be prepared in high yield via directly liquid ball milling NaBH4and CaCl2in THF. High-purity Ca(BH4)2in polymorph hybrid (α,β and γ) is obtained after a heat treatment of the Ca(BH4)2·2THF. Ball-milling of Ca(BH4)2·2THF facilicates the formation of γ-Ca(BH4)2, and single γ-Ca(BH4)2is successfully obtained. The hydrogen desorption temperature and kinetics of Ca(BH4)2is strongly related to its crystal structure, crystallite and particle sizes, and morphology. It is found that a short period of ball milling reduces effectively the desorption temperature of Ca(BH4)2due to the reduction of crystallite and particle sizes, however, agglomeration takes place when over period of milling is performed, and hence reduces the role of the milling in improving the hydrogen storage properties of Ca(BH4)2. The apparent activation energy of the milled Ca(BH4)2is evidently lower than that of the pristine one, possessing improved hydrogen desorption kinetics. The desorption process of Ca(BH4)2is a diffusion-contorlled one, and the kinetic mechanism is not changed by ball milling, however, the nucleation rate of the products of Ca(BH4)2is increased, resulting in faster desorption rate.With the introduction of Ti(OEt)4into Ca(BH4)2by ball milling followed by a heat treatment, porous CaB2H7with in situ introduced nano-TiO2, viz., CaB2H7-0.1TiO2, is firstly synthesized. TiO2is found to act as a catalyst and the mechanism is revealed. There is ca.5wt%H2rapidly released below300℃for the porous CaB2H7-0.1TiO2system, and its peak temperature for hydrogen desorption is reduced by50and30℃, respectively, compared with single Ca(BH4)2and the best catalyzed Ca(BH4)2system (with NbFs added) ever reported in literature. The porous morphology is partially retained after desorption, which favores the absorption properties of Ca(BH4)2. The dehydrogenated product starts to absorb H2at as low as175℃, and there is ca.4wt%H2soaked at350℃and90bar H2pressure for1h, which equals to80%of the initial desorption capacity. The activation energy and reaction enthalpy of the porous CaB2H7-0.1TiO2system is evidently lowered compared to the single Ca(BH4)2, resulting in the improved kinetics and thermodynamics. The porous morphology and the in situ formed TiO2provide synergetic action on the improvement of the hydrogen storage properties of Ca(BH4)2, which is much more significant than that of the external addition of TiO2catalyst.A novel ternary composite system of high capacity and high cyclic stability, Ca(BH4)2+2LiBH4+2MgH2, is synthesized via simply ball milling the three hydrides. The system also shows lower completion temperature. It releases ca.8.1w%H2below370℃. The reversibility and cyclic stability of the Ca(BH4)2+2LiBH4+2MgH2system are superior to the single and binary counterparts. The initial dehydrogenated product starts to soak H2at ca.75℃, and there is ca.7.6wt%H2absorbed at350℃and90bar H2pressure for18h, which is equivalent to a reversibility of94%. After10cycles, the system maintains ca.6.7wt%reversible H2content. The apparent activation energy and reaction enthalpy for the ternary system are evidently reduced compared to other single and binary counterparts, resulting in the improved de-/hydrogenation performances. A new dual-cation hydride of CaMgH3.72is formed after the initial cycle. CaMgH3.72is highly reversible and is stable below300℃. CaMgH3.72facilitates greatly the decomposition of Ca(BH4)2and MgH2, lowering the desorption temperature in the following cycles.The effect of the addition of different contents of LaMg3on the de-/hydrogenation performances of the Ca(BH4)2+LiBH4system is investigated. It is found that LaH3and MgH2formed in situ during ball milling by means of the absorption of H2into LaMg3alloy. They exhibit amorphous state and highly disperse. The de-/hydrogenation temperatures of the system was evidently lowered by in situ introducing LaH3and MgH2. The Ca(BH4)2+LiBH4+0.3LaMg3system shows onset desorption and absorption temperature of150and204℃, respectively, both lowered by100℃compared to those of the Ca(BH4)2+LiBH4composite. The system soaked ca.5.4wt%H2up to450℃, which is evidently higher than that of the Ca(BH4)2+LiBH4system (2.0wt%). The system released ca.4wt%H2in the5th cycle, which is equivalent to be70%of the initial desorption capacity. The in situ introduced LaH3and MgH2change thermodynamically the reaction process of the Ca(BH4)2+LiBH4system, and accelerate kinetially the reaction efficiency compared to the one with LaH3and MgH2externally added, lowering the apparent activation energy and reaction enthalpy, which results in an evident reduction in the operating temperatures and a significant increase of the de-/hydrogenation rates. Introduction of in situ formed active additives is proposed to be an effective approach in improving the hydrogen storage property of boron hydrides. |
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