Li-Mg-N-H体系的导热性能与可逆储氢性能之间的关联及其热化学研究

Due to its higher hydrogen capacity, Li-Mg-N-H system as hydrogen storage material was paid special attention by researchers all over the world. Now it has been one of the hot research fields in hydrogen storage materials. But this kind of hydrogen storage materials still have some drawbacks, such as poor mass and heat transfer for solid-solid reaction, higher temperature for hydrogen desorption, higher kinetic barrier for hydrogen absorption/desorption, poor reversibility for hydrogen absorption/desorption, and so on. In order to overcome the aforementioned problems, some light metals with different ion radius and electronegativity (Li, Na, K, Mg, Ca) or the additives (melted when heated) with good thermal conductivity (KNH2, NH3BH3, graphene) will be introduced in Li-Mg-N-H system. Combined with theoretical calculation (first-principle or DFT), the relationship between thermal conductivity and hydrogen storage properties will be focused on and the model between thermal conductivity and hydrogen storage properties will be established in order to find the strategy for decreasing temperature and increasing the reaction rate for reversible hydrogen absorption/desorption. And the thermal conductivity, formation heat and other thermodynamic parameters (Heat capacity, Enthalpy and so on) of as-prepared samples will be investigated systematically by means of the apparatus for thermal conductivity, TG/DSC and C80 micro-calorimeter. This project will bring further insights for developing new materials for hydrogen storage.

金属氮氢储氢材料Li-Mg-N-H体系以其理论储氢容量高的特点引起了各国科研工作者的极大兴趣，是目前储氢材料研究领域的热点之一。但此类储氢材料存在着固-固相反应传质传热差、放氢反应温度高、吸放氢动力学阻力大、低温时可逆吸氢性能较差等问题，针对存在的以上问题，本项目拟以Li-Mg-N-H体系为研究对象，考察引入各种不同离子半径和电负性的轻金属及其化合物（如KNH2）或其他加热熔融（如NH3BH3）、导热性能好的添加剂（如石墨烯）对其导热性能、吸放氢动力学和热力学性能的影响，并结合理论计算重点探究材料的导热性能与储氢性能之间的相关性，建立两者之间的关系模型，以寻找降低其可逆吸放氢温度和提高吸放氢反应速率的途径。运用导热系数测试仪、差示扫描量热仪和C80微量热仪等设备对所合成的材料体系进行导热系数、热容、焓、生成热等重要热力学参数进行研究，为新型储氢材料的研发提供系统的科学理论依据。

氢气的安全高效储存是制约氢能规模化应用的瓶颈之一。Li-Mg-N-H储氢材料体系因其具有高的氢含量和较好的可逆性能，受到研究人员的广泛关注，但较高的放氢动力学阻力和放氢过程中副产物较多等缺点严重阻碍了其实用化的进程。为了进一步改善Li-Mg-N-H系储氢材料的储氢性能，本项目从催化改性入手，对其进行了系统的研究。我们首先制备了Mg(NH2)2和Ca(NH2)2等碱土金属氨基化合物，并利用微量热仪测定了它们的生成焓。此外，利用Mg(BH4)2、Li3AlH6和超级活性炭（SuperC）对2LiNH2-MgH2体系的储氢性能进行优化。2LiNH2-MgH2-0.1Mg(BH4)2样品的起始放氢温度为80℃，较2LiNH2-MgH2体系降低了50℃。放氢后的样品在180℃，48 bar氢压条件下经过2小时能够吸氢5.0 wt%，而且2LiNH2-MgH2-0.1Mg(BH4)2样品的热扩散系数在没有牺牲储氢容量的前提下较2LiNH2-MgH2提高了近一倍。2LiNH2-MgH2-0.1Li3AlH6样品在 300℃下能够放出6.43 wt%的氢气，放氢起始温度仅为78℃，同时有效抑制了氨气的释放。2LiNH2-MgH2-10 wt% SuperC具有最佳综合储氢性能，其初始放氢温度和峰值放氢温度分别为71℃和168℃，与此同时成功抑制了副产物氨气的释放。SuperC的添加有助于提高材料热扩散系数，解决材料固相反应相界面的传质传热较差的问题，从而改善体系的放氢性能。本项目的顺利完成为新型储氢材料的研发提供系统的科学理论依据。