The Hydrogen Release Properties And Mechanism Of Metal Borohydrides And Their Ammoniates And Magnisium-based Hydrides

Hydrogen, as a globally accepted clean and renewable energy carrier, is one of the potential candidates for changing fossil fuels to clean energy power sources. The challenge for realizing a hydrogen economy is to develop efficient and reliable hydrogen generation/storage methods and materials. Metal borohydrides and their ammoniates and magnesium-based hydrides have received great attention as potential candidates for solid-state hydrogen generation/storage materials due to their high gravimetric hydrogen densities. The direct use of those kinds of materials as hydrogen energy carriers in on-board applications, however, are impeded due to their sluggish dehydrogenation kinetics and by the concurrent release of detrimental volatile by-products(i.e., ammonia, borazine, and diborane). In this thesis, the key scientific issues, such as reducing hydrogen release temperature, increasing hydrogen purity and capacity, accelerating the hydrogen release rate et al, were studied. The improved approaches in terms of combination, nanostructuring, alloying and catalyzing. The main contents are as below:The pure zirconium borohydride ammoniates(Zr(BH4)4·8NH3), without the impurity of LiCl, was successfully fabricated by ammonization of Zr(BH4)4 crystal. Moreover, as the highest coordination number of NH3 groups among all the known metal borohydride ammoniates, the crystal structure of Zr(BH4)4·8NH3 was determined for the first time. The dehydrogenation mechanism of Zr(BH4)4·8NH3 was the combination of N–Hδ+···B–Hδ-. This compound could quickly dehydrogenate at 130 oC with hydrogen capacity being 6.2 wt.% in 60 min. The emission of detrimental by-products, NH3, during the dehydrogenation process of Zr(BH4)4·8NH3 was ascribed to the surplus NH3 groups related to the BH4 groups and the weak metal–ligand bonding between Zr and 8NH3 in Zr(BH4)4·8NH3.In order to suppress the release of ammonia and reduce the dehydrogenation temperature, three approaches were applied in the Zr(BH4)4·8NH3 system. Approach 1, introducing NH3BH3 to consume the excessive NH3 groups; Approach 2, introducing additional BH4 groups to balance with the NH3 groups; Approach 3, decreasing the coordination number of NH3 groups to enhance the metal–ligand bonding. By optimizing the added amount of NH3BH3, LiBH4, Mg(BH4)2 and adjusting the coordination number of NH3 groups to promote the dehydrogenation by combination of N–Hδ+···B–Hδ-, and thus obtaining the composites with the low dehydrogenation temperature(81 oC), high purity of hydrogen released(> 99 mol.%), and high dehydrogenation capacity(100 oC/45 min, 7.0 wt.% H2). The decomposition pathway and mechanism were discussed.Nanostructuring was used to improve the dehydrogenation properties of LiBH4 and Zr(BH4)2. Poly(methylmethacrylate)(PMMA), with a high permeability ratio of H2/O2, was used to protect LiBH4 from oxygen and water. Furthermore, The synergetic effect in terms of nanoconfinement of LiBH4 by the fine network pore of PMMA and the interaction between the B atom in LiBH4 and the O atom in C=O of PMMA resulted in a much lower hydrogen release temperature of LiBH4. The nanoconfinement effect by mesoporous carbon scaffold(CMK3) and the interaction between Zr(BH4)4 and the pore wall of CMK3 stabilized the volatile Zr(BH4)4 in the pore of CMK3. Furthermore, the evolution of diborane during the decomposition of Zr(BH4)4 was suppressed, and the purity of hydrogen released was significantly increased to 99.9 mol.%, and the hydrogen release temperature was reduced to 164 oC.The desolvation process would led to the decomposition of Mg(B3H8)2 for the traditional wet chemical synthesis of Mg(B3H8)2. In this thesis, the solvent-free Mg(B3H8)2 was prepared by gas-solid reaction between B2H6 and Mg2NiH4 for the first time. It is found that Mg2NiH4 readily reacts with B2H6 at room temperature to form Mg(B3H8)2. By applying ball-milling, the reaction was faster and 74.5 % Mg2NiH4 was converted into Mg(B3H8)2 within 6 h. Elongated ball milling resulted in further conversion of Mg(B3H8)2 into MgB12H12.The mechanism that different hydrogenation degree affects on the hydrolysis rate and hydrolysis yield of Mg-La hydride was studied. It was observed that LaH3 could effectively promote the hydrolysis of MgH2 and Mg, especially on the promotion on the hydrolysis of MgH2, while the remained Mg inhibited the hydrolysis of LaH3. In addition, by changing the reaction temperature, Ag/Fe molar ratio, solvent polarity and surfactant, the nanoparticles(NPs) from individual Ag NPs to Ag-Fe3O4 heterdimer NPs to Ag-Fe3O4 nanoflower NPs and to Ag@Fe3O4 core-sehll NPs were prepared, and the hydrolysis of MgH2 by adding Ag-Fe3O4 heterdimer NPs was studied.