| Storing hydrogen in an efficient, compact and safe manner is a technical challenge of utilizing hydrogen as an alternative energy carrier. One novel branch of hydrogen storage that attracts the attention of chemists and researches is chemical hydrogen storage owning to its prominent advantages in storing density, efficiency and safety. Recently, complex hydrides are becoming attractive materials for chemical hydrogen storage due to their intrinsic high hydrogen content. However, their practical applications are blocked by sluggish kinetics and poor reversibility. Based on the review of the research development and existing problems of complex hydrides, NaAlH4 and NH3BH3 (AB) were selected as the subject of the present work, which focused on the following aspects:Firstly, an improved de-/re-hydrogenation properties was achieved by using space-confining NaAlH4 in nanoporous materials; and then the dehydrogenation of NaAlH4 cayalyzed by nanocarbon materials were studied; finally a "chemical modification" method was applied to promote the hydrogen release from AB. All the research contents and results are as follows:(1) Through thermal melting impregnation NaAlH4 was loaded partially into ordered mesoporous carbon (MC) host with a pore size of～4 nm (as denoted as NaAlH4/MC) where the larger NaAlH4 particles reside on the external surface and amorphous and/ or nanosize ones on the MC internal surface. The thermal analysis results indicate that with respect to the pristine NaAlH4, the onset temperature for dehydrogenation of NaAlH4/MC decreases from about 220℃to around 150℃and the temperature for the complete dehydrogenation is reduced from about 320℃to about 210℃. Moreover, an enhanced reversibility of NaAlH4/MC is also observed. Without a metal catalyst, the re-hydrogenation in a dehydrogenated NaAlH4/MC system can be achieved even under the temperatures of 100～150℃and the hydrogen pressures of 3.0～7.0 MPa. These improvements are attributed to the synergistic effects of both nanoconfinement and chemical catalysis caused by the MC where the nanoconfinement plays a dominant role.(2) A nanoconfinement system with NaAlH4 exclusively embedded in MC was synthesized firstly by a three-step procedure; viz., thermal melting impregnation plus de-/re-hydrogenation. Through the consecutive steps, NaAlH4 is successfully loaded by the impregnation step, and its dehydrogenated products of NaH and Al are also confined in the MC pores; while some of the NaAlH4 resided on the MC external surfaces is eliminated by the de-/re-hydrogenation steps, and its resulting products are essentially in a "dormant" state and are no longer functional in the following de-/re-hydrogenation experiments, as denoted as Space-confined NaAlH4/MC. The capacity retention for Space-confined NaAlH4/MC is>80% after fifteen cycles and higher than 50% for pristine NaAlH4 over five cycles. Furthermore, as suggested by EDX analysis, after five cycles, the resulting Al of pristine NaAlH4 grows to around 2-3μm and even up to 8μm. In contrast, for Space-confined NaAlH4/MC, after fifteen cycles, the distribution of Al elements is still uniform. It is believed that the MC pore serves as a "nano-reactor" where both NaAlH4 particles and their resulting NaH and Al products are physically limited at a nanoscale level, which facilitates the mass transfer of the solid phases by shortening the diffusion distance and thus leads to an enhanced cycling stability.(3) The Space-confined NaAlH4/MC was found to exhibit faster kinetics for dehydrogenation. For pristine NaAlH4, about 0.5 wt.% hydrogen is released at 180℃in 90 min. In contrast, the value for Space-confined NaAlH4/MC increases to about 5.0 wt.%, without an apparent induction period. Moreover, the activation energy Ea for hydrogen release is 46 kJ/mol for Space-confined NaAlH4/MC, and much lower than 116 kJ/mol for the pristine one. Furthermore, kinetic modeling studies suggest that the hydrogen release from the confined NaAlH4 system is governed by two processes; the initial process can be expressed by one-dimensional nucleation and growth, and the second process is jointly controlled by three-dimensional phase boundary migration and diffusion.(4) A "dissolution-recrystallization" method was applied to prepare nanocarbon materials supported NaAlH4. The NaAlH4 particles in Graphene supported NaAlH4 exhibit a heterogeneous matrix with layered structure, the ones in C60 supported NaAlH4 sample have a flower-like shape with size of 5～10μm, and the ones for MC supported NaAlH4 are shaped like sphere with 1～3μm in size. The thermal analysis indicates that the pristine NaAlH4 starts to release hydrogen at about 220℃, but the onset temperature for dehydrogenation of Graphene, C6o and MC supported NaAlH4 is remarkably reduced to 190,185 and 160℃, respectively. Moreover, as compared to the energies for three-step dehydrogenation of pristine NaAlH4 determined by using the Kissinger analysis, the activation energy for first-step dehydrogenation of Graphene supported NaAlH4, C6o supported NaAlH4 and MC supported NaAlH4 is reduced by 13,19 and 40 kJ/mol, the value for second-step is reduced by 77,125 and 148 kJ/mol, and the value for third-step is reduced by 59,122 and 131 kJ/mol, respectively. Furthermore,27Al NMR spectra show that the local structure of Al atom in nanocarbon materials supported NaAlH4 is different from that of prisitine one and suggests the existence of interaction between nanocarbon materials and NaAlH4, which may be the reason for these property improvements. By comparing with the above results, the destabilizing effect from high to low is in the following sequence: MC>C60>Graphene.(5) The reaction for hydrogen generation from AB via a solid-state can be promoted by alkaline earth metal chlorides. It is found that tuning the reactivity of both B-H and N-H bonds in AB by alkaline earth metal chlorides not only results in a significantly decrease in the onset dehydrogenation temperature by 60℃but also suppresses undesirable volatile by-products, such as NH3, B2H6 and N3B3H6. Moreover, alkaline earth metal chlorides such as MgCl2 and CaCl2 are found to exhibit a similar effect on improving the decomposition behaviors of AB, but the MgCl2 is more efficient than CaCl2 in suppressing the release of NH3. These improvements are attributed to the dual modification involving changing the reactivity of both B-H and N-H bonds by reaction with the additives which provides further insights into the promotion of hydrogen release from amidoboranes and related borohydride ammine complexes. |
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