Study On The Dehydrogenation Mechanism And Improvement Of BN-Based Hydrogen Storage Materials

Hydrogen, as an ideal clear energy carrier, has attracted a lot of attention due to its highly abundant, lightweight, and environmentally friendly oxidation product (water). However, it is still a great challenge to develop an ideal hydrogen storage material which would be safe, dense, light weight, inexpensive and highly reversible. B-N compounds are potential hydrogen storage materials due to their light weight and high hydrogen capacity. The direct use of B-N compounds as hydrogen energy carriers in on-board applications, however, is impeded by its sluggish dehydrogenation kinetics below 100℃ and by the concurrent release of a large amount of detrimental volatile by-products (i.e., ammonia, borazine, and diborane). Therefore, a detailed investigation of electronic structure and decomposition pathway will be helpful for understanding the dehydrogenation mechanism of B-N compounds so that to further improve and modify their hydrogen storage properties. In this thesis, a combination of density functional theory (DFT) calculation and experiment are conducted on the structure, dehydrogenation performance, and decomposition mechanism of these materials. The main findings are as below:(1) DFT calculations demonstrate that nitrogen lone pairs in nitrogen contains carbon nanomaterial (NCCN) function as hydrogen acceptors to allow metal-free hydrogen transfer from ammonia borane (NH3BH3, AB) to NCCN, resulting in facile release of pure H2 from AB. The dehydrogenation of AB-NCCN combined systems involves two key steps:First, there is a net transfer of hydrogen atoms from AB to NCCN that results in simultaneous dehydrogenation of AB and hydrogenation of the NCCN, and then, the hydrogenated NCCN further react with AB to release H2 with relatively low reaction barriers. The experimental results further confirm that the NCCN can act as effective catalyst for AB dehydrogenation at relatively low temperature and formation of polyborazylene as single spent fuel.(2) DFT calculations of the native defects in LiNH2BH3 indicate that the dominant atomic charge defects are positively charged Li interstitial (ILi+) and negatively charged Li vacancy (VLi-) with a formation energy of 0.50 eV. Further study indicates that the neutral H2 interstitial (IH2) has the lowest formation energy (0.31 eV), suggesting that the major defect in LiNH2BH3 is IH2-The IH2 defects can migrate with low energy barriers of 0.13 eV. Therefore, the low activation energy of neutral H2 interstitial probably contributes to the experimental observation of fast kinetics for the dehydrogenation process of LiNH2BH3. Our calculation results further suggest that the creation of the negatively charged H vacancy on a N-H site (V(N)H-), ILi+ and VLi- defects is the rate-limiting step for their transportation in LiNH2BH3.(3) The electronic structure and initial dehydrogenation mechanism of M(BH4)2-2NH3 (M=Mg, Ca and Zn) have been systematically studied using DFT calculations. It reveals that the metal cations in M(BH4)2-2NH3 play a crucial role in both suppressing ammonia emission and destabilizing the N-H/B-H bonds. Introducing metal cations with strong coordination of metal-NH3 can significantly improve the dehydrogenation properties of M(BH4)2·nNH3. Further experiment confirmed that introduction of Mg(BH4)2 significantly suppress the release of ammonia and tremendous improve the dehydrogenation kinetic of Ca(BH4)2·nNH3 (n=1,2 and 4). Particularly, for the Ca(BH4)2-4NH3/2Mg(BH4)2, Ca(BH4)2-2NH3/Mg(BH4)2 and Ca(BH4)2·NH3/Mg(BH4)2 samples, fairly pure hydrogen (>99 wt%) are released upon heating from room temperature to 500℃. Further investigation via introduction of isotope deuterium in the combined system reveals that the dehydrogenation reactions are mainly mediated by the combination of Hδ+…Hδ+ interactions, while Hδ+…Hδ+ interactions also contribute in a complementary way.(4) The synthesis, crystal structure and dehydrogenation performances of two new H-enriched compounds, Mg(BH4)2(NH3BH3)2 (Mg(BH4)2-2AB) and Mg(BH4)2-(NH3)2(NH3BH3) (Mg(BH4)2-2NH3-AB) are reported. The Mg(BH4)2-2AB composite shows similar decomposition behaviors of the constituent phases and give rise to release a certain amount of toxic gases, while introduction of ammonia to the Mg(BH4)2-AB composite prompts the formation of an intensive dihydrogen bonding network in Mg(BH4)2-2NH3-AB and facilitates the interaction between NH3 and AB, then allowing to release～9.6 wt% high purity hydrogen at 170℃.(5) A combined strategy via mixing Mg(BH4)2·6NH3 with ammonia borane is employed to improve the dehydrogenation properties of Mg(BH4)2·6NH3. The combined system shows a mutual dehydrogenation improvement in terms of dehydrogenation temperature and hydrogen purity compared to the individual components. The initial two-step dehydrogenation of Mg(BH4)2-6NH3-nAB is resulted from the interaction of BH groups in AB and NH groups in A further improved hydrogen liberation from the Mg(BH4)2-6NH3-6AB is achieved with the assistance of ZnCl2, which plays a crucial role in stabilizing the NH3 groups and promoting the recombination of NH5+… HB5-.Specifically, the Mg(BH4)2·6NH3-6AB/ZnCl2 (with a mole ratio of 1: 0.5) composite is shown to release over 7 wt.% high-pure hydrogen (>99 mol%) at 95℃ within 10 min, thereby making the combined system a promising candidate for solid hydrogen storage.