| Hydrogen, as a kind of alternative energy source, has attracted a lot of attention due to its environmentally benign, high heat of combustion and reversible. The achievement of compact, light safe and affordable hydrogen storage is one of the key challenges in the implementation of hydrogen-based energy society. Among all the discovered hydrogen storage systems, solid hydrogen storage system, which is high energy density and safe, is regarding as potential candidate for hydrogen storage. In this thesis, we introduced a novel method consisting of a metathesis reaction between metal chloride ammoniates and lithium or sodium borohydride by mechanical milling to obtain ammine transition metal borohydrides. A series of characterization methods, i.e. high-resolution powder X-ray diffraction (HRXRD), temperature programmed decomposition (TPD), thermogravimetric analysis (TGA), mass spectrometry (MS), differential scanning calorimetry (DSC), UB solid state nuclear magnetic resonance (11NMR) and Fourier transform infrared(FTIR) were carried out to investigate the structure, hydrogen storage properties and dehydrogenation mechanism of ammine transition metal borohydrides. The main findings are below:(1) The ammine complex of yttrium borohydride Y(BH4)3-4NH3, which contains a theoretical hydrogen capacity of11.9wt%, has been successfully synthesized via a simple ball milling of YC13-4NH3and LiBH4. The structure of Y(BH4)3·4NH3, determined by high resolution powder X-ray diffraction, crystallizes in the orthorhombic space group Pc21n with lattice parameters a=7.1151(1) A, b=11.4192(2)A, c=12.2710(2)A and V=997.02(2)A3, in which the dihydrogen bonds with distances in the range of2.043to2.349A occurred between the NH3and BH4-units contribute to the hydrogen liberation via the combination reaction of N-H…H-B. Thermal gravimetric analysis combined with mass spectrometer results revealed that the decomposition of Y(BH4)3·4NH3consists of three steps with peaks at86℃,179℃and279℃, respectively, in which the first and second steps mainly release hydrogen accompanied by a fair amount of ammonia emission, while the third one accounts for a pure hydrogen release. Isothermal dehydrogenation results revealed that over8.7wt%hydrogen was released for Y(BH4)3·4NH3at200℃. The favorable dehydrogenation properties, presented by the Y(BH4)3·4NH3, i.e. lower dehydrogenation temperature and higher nominal hydrogen contents than that of Y(BH4)3, enable it to be a promising candidate for hydrogen storage.(2) Three ammine titanium borohydrides (denoted as ATBs), Ti(BH4)3·5NH3, Li2Ti(BH4)5·5NH3, and Ti(BH4)3·3NH3were successfully synthesized via metathesis reaction of metal chloride ammoniates (TiCl3·5NH3and TiCl3·3NH3) and lithium borohydride. These ATBs present favorable stability, owing to the coordination with NH3groups, compared to the unstable Ti(BH4)3at room temperature. Bragg peaks in the room temperature diffraction pattern from Li2Ti(BH4)5·5NH3can be indexed to an orthorhombic unit cell with lattice parameters a=18.283A,b=10.216A, c=7.954A and V=1485.64A3. Analysis of systematic extinction shows the structure has a space group of P222or P2221. Dehydrogenation results revealed that Ti(BH4)3·5NH3, which theoretically contains15.1wt%hydrogen, is able to release13.4wt%H2plus a small amount of ammonia. This occurred via a single-stage decomposition process with a dehydrogenation peak at130℃upon heating to200℃. For Li2Ti(BH4)5·5NH3, a three-step decomposition process with a total of15.8wt%pure hydrogen evolution peaked at105,120, and215℃was observed until300℃. In the case of Ti(BH4)3·3NH3, a release of14wt%pure hydrogen via a two-step decomposition process with peaks at109and152℃can be achieved in the temperature range of60-300℃. Isothermal TPD results showed that over9wt%pure hydrogen was liberated from Ti(BH4)3·3NH3and Li2Ti(BH4)5·5NH3within400min at100℃. Preliminary research on the reversibility of this process showed that dehydrogenated ATBs could be partly recharged by reacting with N2H4in liquid ammonia. These aforementioned preeminent dehydrogenation performances make ATBs very promising candidates as solid hydrogen storage materials.(3) Two ammine vanadium (Ⅲ)borohydride, i.e. V(BH4)3·5NH3and V(BH4)3·3NH3, were successfully synthesized via ball-milling of metal chloride ammoniates (VC13·5NH3and VC13·3NH3) and LiBH4·Dehydrogenation results revealed that14.3wt%hydrogen with high purity was released from V(BH4)·3NH3until300℃. V(BHU)3·5NH3was shown to release11.5wt%hydrogen with a H2-purity of85mol%by350℃. To improve the dehydrogenation purity of V(BH4)3·5NH3, Mg(BH4)2with various molar ratios was mixed with V(BH4)3·5NH3to synthesize expected ammine metal-mixed borohydrides. Dehydrogenation results revealed that the Mg(BH4)2modified V(BH4)3·5NH3system presents significantly enhanced dehydrogenation purity. For example, in the case of V(BH4)3·5NH3/2Mg(BH4)2sample,12.4wt%pure hydrogen can be released upon heating to300℃. Structure analysis show that is identified into a cubic structure with the lattice parameters of a=10.78060(25) A and space group of F23. VMg(BH4)5·5NH3was indexed to be a monoclinic unit cell with lattice parameters of a=19.611A, b=14.468A, c=6.261A, β=93.678°and V=1772.75A3. Further investigation on the dehydrogenation mechanism of the VMg(BH4)5·5NH3system by isotope tagging revealed that the interactions of homo-polar BH units also participated throughout the dehydrogenation process (onset at75℃) as complementary to the prime combination of BH…HN.(4) Magnesium borohydride ammoniate ammonia borane (denoted as MBAAB), i.e. Mg(BH44)2·(NH3)2(NH3BH3), has been successfully synthesized via the reaction of Mg(BH4)2·2NH3and NH3BH3. The structure of Mg(BH4)2-(NH3)2(NH3BH3), determined by high resolution powder X-ray diffraction, crystallizes in the tetragonal space group P4bm with lattice parameters a=9.4643A, c=5.5230A and V=485.643. Compared to ammine magnesium borohydride and AB, Mg(BH4)2·(NH3)2(NH3BH3) can release13.8wt%pure hydrogen with negligible borazine. In addition, The structure of Mg(BH4)2·6NH3, determined by high resolution powder X-ray diffraction, crystallizes in the cubic space group Fm3with lattice parameters a=10.7806(3)A.。... |
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