碳载硼氢化物催化增强LiMgN储氢材料的吸放氢反应机理及循环稳定性研究

In the Li-Mg-N-H system, LiMgN has high hydrogen storage capacity and low enthalpy of reaction. At present, the synthesis of LiMgN system is complicated, which leads to different microstructures and reaction mechanisms, higher absorption- desorption temperature, as well as poor cycle stability, they have become one of the urgent problems to be solved. In the present project, LiMgN materials are prepared by the microwave sintering a simple system Li3N-MgH2 focusing on obtaining controllable microstructure with uniform nano particles. Because the addition of carbon nanotubes has a spillover effect, and the strong affinity between H- in the [BH4]- and H+ in [NH2]- promotes the release of hydrogen and decreases the temperature of hydrogen desorption, carbon supported borohydride is one of the effective methods to improve the hydrogen storage performance of LiMgN. The emphasis of the project is to study the thermodynamics, kinetics and cycle stability of LiMgN enhanced by catalyst of carbon supported borohydrides. The morphologies of the samples are observed by a transmission electron microscopy (TEM), and the hydrogen storage performance and hydrogen absorption- desorption rate are studied by using the pressure- composition- temperature (PCT) and simultaneous thermal analyzer (STA). The phases structure of the hydrogenated and the dehydrogenated samples are studied by X-ray diffraction (XRD), combined with in-situ synchrotron radiation X-ray diffraction (SR-XRD) and Fourier transform infrared spectroscopy (FTIR), the relationship between the microstructure and cycle stability of hydrogen absorption- desorption is established. The formation mechanism of LiMgN, phase transformation and catalytic effects during hydrogen absorption-desorption process are studied.

在Li-Mg-N-H体系中，LiMgN储氢容量高，反应焓值低。目前，合成LiMgN的体系纷繁复杂，微观组织与反应机理迥异，吸放氢温度较高，循环稳定性较差，是亟待解决的问题之一。本项目采用微波烧结Li3N-MgH2合成LiMgN储氢材料，体系简单，微观组织可控，纳米尺寸均匀。碳纳米管的添加具有溢流效应，硼氢化物中[BH4]-基团的H-和[NH2]-基团的H+之间具有强的吸引力，促进氢气的释放，降低放氢温度。因此，碳载硼氢化物是改善LiMgN储氢性能有效手段之一。项目重点研究碳载硼氢化物催化增强LiMgN吸放氢热力学、动力学和循环稳定性。利用透射电镜观察微观形貌；采用压力-组分-温度及同步热分析仪测试储氢性能；采用X射线衍射、原位同步辐射X射线衍射和傅里叶变换红外光谱仪分析吸放氢过程中的物相结构；建立微观组织与吸放氢循环稳定性之间的模型；揭示LiMgN合成机制、吸放氢过程相结构转变和催化机理。

Li-Mg-N-H是一种新发现的复杂氢化物，其在储氢中的应用引起了广泛关注。然而，迟缓的放氢动力学和热力学性能，阻碍了其在储氢中的实际应用。本项目选用同为储氢材料的Mg(BH4)2作为主要添加剂，以改善Li-Mg-N-H的储氢性能。通过添加不同量（0.05、0.1、0.2、0.3 at.%）Mg(BH4)2探究了其对2LiNH2-MgH2体系的储氢性能的改善状况。其中，原位生成的Li4(BH4)(NH2)3作为中间体的催化作用促进了氢的释放。添加量为0.05Mg(BH4)2的样品在较低温度(240℃)的脱氢/加氢循环中具有良好的循环稳定性。此外，研究了Ti3C2 MXene添加剂对Mg(BH4)2储氢性能的影响。综合考虑放氢温度、放氢容量和放氢动力学，发现当Ti3C2 MXene添加量为40 wt%时，复合材料放氢性能表现最佳。Mg(BH4)2-40wt%Ti3C2复合材料两步放氢的活化能降低至151.3 kJ/mol和178.0 kJ/mol。初始放氢温度从210℃降低至相对较低的82℃。Ti3C2表面的F基团在材料放氢过程中起到重要作用，MgF2的生成表明Ti3C2 MXene表面的F终端基团促进反应进行，Ti3C2 MXene特殊的二维层状结构可以提供更多的氢扩散通道，球磨处理可以使纳米级的Mg(BH4)2粘附在其表面和层间，协同催化促进放氢反应。在探究Li-Mg-N-H材料的放氢机理时，我们发现，Mg能够调节LiNH2i的放氢反应。在LiNH2发生热解时，Mg可以吸收NH3、N2等气态副产物，最终使得氢以H2的形式释放，而N则被以Mg3N2的形式固定下来。与此同时，LiNH2也可以与Mg直接反应生成LiMgN并释放H2。