金属催化剂改进络合硼氢化物储氢性能及其机理的研究

New hydrogen storage materials have become one of the hot topics of modern research.Nowadays, the main problems for hydrogen storage materials are their low capacity of H2 storage, as well as high temperature and slow rate during hydrogen adsorbtion/desoption process. As our preliminary studies have demonstrated that the hydrogen storage performance of LiBH4 can be effectively improved by metal catalytic agent with NH4+ and F- (such as (NH4)xMFy)doping, in this project we propose, for the first time, the research on improving the hydrogen storage properties of borohydrides by (NH4)xMFy doping based on the mechanisms of H+/H- coupling and in situ catalysis. Main contents of this project will focus on synthesis of the borohydrides/(NH4)xMFy composites by mechanical ball milling and clarification of the processes of solid state reaction, generation of new substances during the milling. Then comprehensive characterization of the hydrogen storage properties of the composites will be performed in terms of thermodynamics and kinetics. After that the dehydrogenation mechanisms of these composites, especially in respects of chemical reactions and material phases will be investigated, providing in depth understanding of the influence of H+/H- coupling and in situ catalysis to the hydrogen storage performance of the composites. In addition, the project will also study the microscale change of morphology, elements distribution and valence state of the composites during the dehydrogenation process. Finally, we will summarize the principles and internal mechanisms of the improved hydrogen storage performance of borohydrides by (NH4)xMFy doping by combining with first principle calculation. This project will be of great importance for guiding further R&D of novel hydrogen storage materials.

新型轻质高容量储氢材料是当今的研究热点之一。针对当今储氢材料存在的储氢容量低、储放氢温度高、吸放氢速度缓慢等问题，在前期研究发现(NH4)2TiF6基于H+/H-偶合和原位催化的机制可以有效改进LiBH4储氢性能的基础上，本项目首次提出利用含铵、含氟的金属催化剂改进络合硼氢化物储氢性能。拟运用机械球磨方法合成络合硼氢化物/(NH4)xMFy复合材料，阐明球磨过程中的固相反应以及新物质的生成机制。对复合材料的储氢热力学和动力学性能进行全面的表征，对其脱氢机理，特别是脱氢过程中发生的化学反应、物相变化进行深入的研究，考察H+/H-偶合和原位催化对于复合材料储氢性能的影响。从微观尺度分析研究复合材料在脱氢过程中形貌，元素分布以及价态的变化。结合第一性原理计算，总结(NH4)xMFy对于络合硼氢化物储氢性能改进的规律和内在机制，为进一步寻找新型储氢体系提供指导。

金属硼氢化物因其储氢容量高，仍为当前研究的热点之一。其中，LiBH4高达18.3 wt%的质量储氢密度和121 kg H2/m3的体积储氢密度，是一种有潜力的储氢材料。但是LiBH4高的热力学稳定性和差的吸放氢动力学是它面临的两大主要挑战。.本项目针对LiBH4储氢材料存在的问题，利用正负氢 H−/H+偶合机理来实现LiBH4放氢温度的降低。众所周知，络合硼氢化物中的H带有负电性；而与氮结合的H具有一定的正电性；通过掺杂NH2-或NH4+来对LiBH4去稳定化。经过4年的努力，构建了 LiBH4-LiNH2、LiBH4-(NH4)3AlF6和LiBH4-(NH4)3TiF6三种储氢体系。分别对这三种储氢体系的储氢性能进行了研究；并考察了不同添加剂对它们储氢性能的影响；并分别探讨了这三个体系的吸放氢机理。如对于LiBH4-LiNH2储氢体系，分别探讨了自制的Ni的氧化物、Co和Ni的氟化物、CoB和NiB合金及Co@C对该体系放氢性能的影响。结果表明，它们的掺杂均能降低LiBH4-LiNH2的放氢温度、提高了LiBH4-LiNH2放氢动力学。针对LiBH4-(NH4)3TiF6释放的氢气中有杂质气体的问题，发现掺杂MgH2不但可以抑制杂质气体的产生，而且可以实现LiBH4-(NH4)3TiF6-MgH2体系可逆储放氢，且其可逆储氢量达3 wt%左右。其机理研究表明，由于掺杂了MgH2，该体系升温放氢时产生了Mg-B-N化合物，从而抑制了杂质气体的产生，达到纯化氢气的目的。.该项研究工作对于开发新的络合硼氢化物储氢材料具有一定的科学价值和理论指导意义。