Synthesis, Dehydrogenation Properties And Reaction Mechanisms Of Magnesium Borohydride Ammoniates

The development of safe, high-efficiency and economical solid-state hydrogen-storage technologies is critical to achieve the practical large-scale utilization of hydrogen energy. Magnesium borohydride (Mg(BH4)2), which possesses a high hydrogen capacity and moderate dehydrogenation enthalpy, is regarded as one of the most promsing hydrogen storage materials. However, Mg(BH4)2suffers from relatively high dehydrogenation temperatures, poor reaction kinetics and limited reversibility under moderate conditions, hindering its practical applications. In order to improve the dehydrogentiaon properties of Mg(BH4)2, NH3is adducted to Mg(BH4)2to generate Mg(BH4)2-xNH3. In this work, the synthesis, crystal structure, thermal decomposition and reaction mechanisms of Mg(BH4)2·xNH3are investigated. In addition, their hydrogen-storage thermodynamics and kinetics are significantly improved via F doping, compositing with metal hydrides, nanocomfinement and forming derivatives. The corresponding mechanisms are also evaluated.A novel "ammonia-redistribution" strategy is proposed for the solid-state synthesis of Mg(BH4)2-xNH3at room temperature. It is discovered that there are four magnesium borohydride ammoniates, viz., Mg(BH4)2·NH3, Mg(BH4)2·2NH3, Mg(BH4)2·3NH3and Mg(BH4)2·6NH3. Specifically, Mg(BH4)2·NH3is obtained for the first time based on the "ammonia-redistribution" strategy, and the structural characteristics of Mg(BH4)2·3NH3are also demonstrated in this work. The results show that the dehydrogenation properties of Mg(BH4)2·xNH3are closely related to the coordination number of NH3. On one hand, the purity of hydrogen released from Mg(BH4)2·xNH3, which is determined by both the Hδ-/Hδ+ratio and N:â†'Mg2+bond strength, is increased with the decrease in coordination number of NH3. On the other hand, the magnitudes of charge on Hδ-and Hδ+, which are also related to the coordination number of NH3, are critical for the dehydrogenation temperatures of Mg(BH4)2·xNH3(x=1,2,3).Thermolysis mechanisms of Mg(BH4)2·6NH3are studied in-depth and a six-step decomposition process is proposed. It is founded that, at the initial stage, Mg(BH4)2·6NH3decomposes to evolve3equiv. of NH3and generate Mg(BH4)2·3NH3. Subsequently,1equiv. of NH3and3equiv. of H2are released from Mg(BH4)2·3NH3to produce a [MgNBHNH3][BH4] polymer. The decomposition of [MgNBHNH3][BH4] proceeds via three steps, resulting in the release of one equiv. of H2for each step and formation of [MgNBNH2][BH4], MgNBNH2BH2and MgNBNHBH for the first, second and third decomposition step, respectively. Finally, at temperatures higher than400℃, an additional one equiv. of H2is liberated from the decomposition of MgNBNHBH to yield Mg and BN as the resultant products. This decomposition of Mg(BH4)2·3NH3is sensitive to the heating rate, which is responsible for variation of the composition of gases released from Mg(BH4)2·6NH3with the heating rates. At a low heating rate, the breakdown of N:â†'Mg2+bond and subsequent ammonia release is allowed to proceed more sufficiently. Whereas at a high heating rate, the local combination of Hδ-and Hδ+is more favored which induces a higher dehydrogenation amount.The effect of fluorine doping on the dehydrogenation properties of Mg(BH4)2·2NH3is investigated and the corresponding mechanisms are evaluated. Based on the interaction of [BH4]-and [BF4]-anions, the F-doped Mg(BH4)2·2NH3are successfully prepared. Hydrogen release from the F-doped Mg(BH4)2·2NH3initiates at approximate70℃with enhanced dehydrogenation kinetics. More importantly, the ammonia release is depressed completely. Mechanistic investigations reveal that the Hδ+-Hδ-interactionis are enhanced, thus resulting in the decreased dehydrogenation temperatures and the enhanced dehydrogenation kinetics. Moreover, the more favorable Hδ+-Hδ-local combination in the F-doped ammoniates is also responsible for the strengthing of N:â†'Mg2+bond and depressed ammonia release.Hydrogen storage properties and mechanisms of the Mg(BH4)2-2NH3-xMgH2composites are investigated systematically. After introducing MgH2, the dehydrogenation temperature of Mg(BH4)2-2NH3is distinctly decreased and ammonia release is absent. Hydrogen release from the Mg(BH4)2·2NH3-xMgH2composites initiates at approximate70℃and more than12wt%of hydrogen is desorbed. Mechanistic investigations reveal that the Hδ-in MgH2react with Hδ+in NH3more readily than the Hδ-in [BH4]-anions, thus leading to more favorable Hδ-Hδ+combination and hydrogen release. The modified Hδ--Hδ+combination also strengthens the N:â†'Mg2+bond and depresses subsequent ammonia release. Moreover, a novel MgBH4NH2compound is formed during the decomposition of Mg(BH4)2-2NH3-MgH2composite, which provides a feasible method for the low-temperature and controllable synthesis MgBH4NH2. The effects of nanoconfinement on the decomposition behaviors of Mg(BH4)2-6NH3are investigated. It is observed that hydrogen release from the nanoconfined Mg(BH4)2·6NH3occurs at the temperature range of40-175℃, much lower than that of the bulk Mg(BH4)2·6NH3. Inspiringly, hydrogen release from the nanoconfined Mg(BH4)2·6NH3is an endothermic process although it is exothermic in nature for the bulk Mg(BH4)2-6NH3. On one hand, the enhanced H8δ--Hδ+combination on high-energy surfaces and the shortened diffusion distances contribute to the improved dehydrogenation properties of the nanoconfined Mg(BH4)2·6NH3. On the other hand, the enhanced Hδ--Hδ+combination and change in decomposition pathway of Mg(BH4)2-6NH3caused by the constraining effect of inner wall of micropores are responsible for the change in the decomposition thermodynamics.The syntheses and formation mechanisms of Mg-based mixed-cation borohydride ammoniates are investigated. A series of mixed-cation borohydride ammoniates, including Li9Mg(BH4)11·6NH3, Li2Mg(BH4)4·6NH3, Li2Mg(BH4)4·3NH3, LiMg(BH4)3·2NH3and MgCa2(BH4)6·6NH3, are synthesized successfully. It is revealed that the NH3group acts as a "stabilizing agent" in the formation of mixed-cation borohydride ammoniates. And only the metal cations, whose borohydrides are ammoniate-forming species, can coexist in mixed-cation borohydride ammoniates. Li2Mg(BH4)4·6NH3crystalizes in a tetragonal P43212structure with very short dihydrogen bond of1.8360A which is the shortest in all the known borohydride ammoniates. Li2Mg(BH4)4·6NH3possesses a low onset temperature for dehydrogenation of approximately80℃and its full decomposition would give11.01equiv. of H2(equivalent to11.1wt%) and3.07equiv. of NH3. Moreover, a reversible hydrogen storge capacity of approximate4wt%could be achieved under450℃and an initial hydrogen pressure of100bar. Investigations on the decomposition mechanisms of Li2Mg(BH4)4·6NH3indicate that, at the initial stage, Li2Mg(BH4)4·6NH3decomposes to generate Li2Mg(BH4)4·3NH3, accompanying with the release of3equiv. of NH3. Subsequently, the decomposition of Li2Mg(BH4)4·3NH3reulsts in the formation of L1BH4, MgB2N3and H2. Finally, LiBH4reacts with MgB2N3to release the remaining hydrogen and generate the resultant products of metallic Mg, BN, LiH and elemental B. The N:â†'Mg2+coordination bond in Li2Mg(BH4)4·6NH3is stronger than that in Mg(BH4)2·6NH3, which is responsible for the depressed ammonia release in Li2Mg(BH4)4·6NH3. In addition, the hydrogen release from Li2Mg(BH4)4·6NH3at low temperatures is due to the existence of very short dihydrogen bonds, which, on the other hand, also facilitate the formation of Li2Mg(BH4)4-3NH3.