Investigations On The Hydrogen Storage Performance And Reaction Mechanism Of Amide-Hydride Systems

Metal-N-H system has been intensively investigated since2002. Among all the systems explored in the past12years, Mg(NH2)2-2LiH appears to be the most promising candidate for on-board application due to its relatively high hydrogen capacity of-5.5wt%and favorable reaction thermodynamics. However, its majority of hydrogen can only be released at temperatures above180℃even if Mg(NH2)2-2LiH composite has been milled intensively, showing the presence of severe kinetic barrier in the dehydrogenation process. In addition, Mg(NH2)2-2LiH system also needs to improve its thermodynamic properties in order to achieve near ambient operation condition. The aims of this thesis are to modify the system with additives to improve the kinetic and thermodynamic properties and to understand the dehydrogenation/re-hydrogenation mechanisms.Kinetic investigations demonstrated that the mechanism of hydrogen desorption is of conditional dependence. Upon forming Li amide bromide (Li2NH2Br), where LiNH2unit is confined in the cage of Br likely resulting in less mobility of ion, hydrogen desorption from Li2NH2Br-2LiH appears to follow the NH3mediated mechanism. On the other hand, the migrations of Li+and H+play important roles leading to H2formation from the direct combination of H+from LiNH2and H-from LiH when amide and hydride are in intimate contact.In order to improve the dehydrogenation kinetics of the Mg(NH2)2-nLiH system, the effects of Li3AlH6and NH3BH3addition on the dehydrogenation properties of the Mg(NH2)2-nLiH system were investigated systematically. The following results were obtained.1) It was found that0.1Li3AlH6-Mg(NH2)2-2LiH sample can release hydrogen at a rate ca.4.5times as fast as that of Li3AlH6-free sample at140℃. The enhancement of desorption kinetics was also reflected by the significant reduction of activation energy (Ea) from127.0kJ/mol to ca.96.0kJ/mol. The interaction of Li3AlH6and Mg(NH2)2during ball milling results in the formation of LiAl(NH)2, LiNH2and Mg3N2. LiAl(NH)2was supposed to be the active species for the enhancement of dehydrogenation/re-hydrogenation kinetics of the system.2) It was found that the dehydrogenation of Mg(NH2)2-3LiH-NH3BH3was a three-step process during ball milling, giving rise to the formation of intermediates LiNH2BH3and [LiMgBN3H3]. This stepwise reaction results in a total release of ca.9.6wt%H2at ambient temperature.Lil, LiBr and LiBHU species were chosen to modify the reaction enthalpy of the2Mg(NH2)2-3LiH system. It is encouraging to find that substantial reaction enthalpy reduction of the2Mg(NH2)2-3LiH system can be achieved, i. e., the enthalpy changes of Lil, LiBr and LiBH4doped samples are decreased to33.3,31.9and35.8kJ/mol H2, respectively, which are significantly lower than that of the pristine material (40.1kJ/mol H2). Releasing1.0bar H2is thermodynamically allowed at60,47and64℃in the Lil, LiBr and LiBH4doped systems, respectively. Such a thermodynamic improvement is due to the formation of more stable products, i. e., Li3(NH2)2I, Li2NH2Br, and Li4BN3H10. By using this strategy further optimization to the Mg(NH2)2-nLiH and other amide-hydride systems can be foreseen.