钾催化氨基镁-氢化锂体系储氢性能研究

Mg(NH2)2-LiH system received extensive investigations in the past decade due to its suitable thermodynamics (△H = 38.9 kJ/mol-H2 and 1 bar equilibrium pressure can be reached at temperature as low as 90 ℃ ), acceptably high cycling capacity (5.5 wt%) and low cost for synthesis (less than 150 RMB/gram). Recently, catalytic enhancement of KH on Mg(NH2)2-LiH system led to a further reduction of cycling temperature to 107 ℃ which is very close to the operation temperature of PEM fuel cell. Inspired by this, several developed countries set Mg(NH2)2-LiH the target material in their efforts for practical hydrogen storage system. In this study, mechanism of hydrogen desorption from Mg(NH2)2-LiH system will be systematically investigated by dense collection of reaction intermediates and resolving their structures. Understanding on the structural transformation following hydrogen absorption/desorption will lead us to the identification of step that has the highest kinetic barrier. Then, KH will be introduced to this reaction step and promote the desorption rate, which may help to elucidate the mechanism of K taking effect on the dehydrogenation of Mg(NH2)2-LiH. Compounds containing BH4- anion such as KBH4 and Mg(BH4)2 will later be introduced to KH catalyzed Mg(NH2)2-LiH system, in hope that some M(NH2)(BH4) (M=Li, Mg, K) complex having low melting point would be formed to further improve the kinetic performance and meanwhile to restrict the growth of KH particle during sample hydrogenation. The ultimate purpose of this study is to achieve a modified Mg(NH2)2-LiH with superior storage performance that enables the material applicable in a demonstration device comprising a hydrogen generator and a PEM fuel cell.

氨基镁（Mg(NH2)2）-氢化锂（LiH）储氢体系因其极佳的热力学性能（△H = 38.9 kJ/mol-H2，一大气压平台压力对应脱氢温度仅为90 ℃）、可观的循环吸氢量（5.5 %重量比）以及低廉的制备成本（实验室合成成本150元/克）而成为目前储氢研究的热点。最近，KH催化的Mg(NH2)2-LiH首次实现107 ℃下可逆吸氢，更是将该体系推向实用，成为各国开发实用储氢系统的目标材料。本项目立足过去十几年的研究经验，采用多点收集反应中间态并结合结构解析及多种表征手段，缜密研究Mg(NH2)2-LiH及KH促进体系的具体脱氢反应途径，筛选出动力学阻力最大的反应步骤并将K作用于该步骤，达成对K促进机制的认识提升。通过添加BH4-阴离子到K促进体系，制造低熔点物种，利用熔融媒介稳定KH晶粒尺寸，达成提升操作稳定性，为本研究项目建立氢源-燃料电池联用演示装置做好材料准备。

Mg(NH2)2-LiH复合储氢体系是最具应用潜力的储氢材料之一, 但存在较高的动力学能垒。前期工作发现了一种有效的非过渡金属催化剂-KH，可以使其吸、放氢过程在低至107 °C的条件下完成，使其成为实用储氢系统的目标材料。本课题研究揭示了Mg(NH2)2-LiH复合体系反应机理，通过多种添加剂优化和调控了其的吸脱氢动力学和热力学性能。Mg(NH2)2与LiH的界面反应存在严重的动力学能垒。KH首先与Mg(NH2)2反应生成K2Mg(NH2)4，这有效地活化Mg(NH2)2，进而K2Mg(NH2)4与LiH置换反应再生KH，显著降低了该界面反应的能垒。KH修饰的Mg(NH2)2-2LiH体系在放氢过程中至少涉及三种钾的存在状态，即K2Mg(NH2)4、KLi3(NH2)4和KH。它们的产生与相互转化催化了Mg(NH2)2与LiH的反应。通过引入LiBr、LiI和LiBH4，与脱氢产物LiNH2形成较为稳定的Li2NH2Br、Li3(NH2)2I和Li4BN3H10，将2Mg(NH2)2–3LiH体系的脱氢反应热力学焓值降低，在一个大气平衡氢压下的热力学允许的操作温度分别为47、60和64 °C。更为重要的是Mg(NH2)2–3LiH-4LiBH4体系脱/吸氢反应热力学性质进一步明显改变，脱氢反应焓变为24 kJ mol-1 H2。多种谱学表征说明LiBH4可以和中间产物和最终产物形成低熔点的复合物，改变了反应机理，从而显著地改变了反应热力学和动力学。Mg(NH2)2–3LiH-4LiBH4样品可在98°C恒温条件下可逆放3摩尔氢气，这是目前Mg(NH2)2–LiH体系能实现可逆储氢的最低温度。这保证了Mg(NH2)2–LiH材料应用于氢源-燃料电池联用演示装置。