氨合金属硼氢化物储氢材料热分解机制和改性研究及二元复合材料设计

Ammine metal borohydrides are potential hydrogen storage materials due to their light weight and high hydrogen capacity. The direct use of these compounds as hydrogen energy carriers in on-board applications, however, is impeded by its sluggish dehydrogenation kinetics below 100 oC and by the concurrent release of a large amount of detrimental volatile by-products such as ammonia and borane. Therefore, a detailed investigation of electronic structure and decomposition pathway will be helpful for understanding the dehydrogenation mechanism of ammine metal borohydrides so that to further improve their hydrogen storage properties. In this project, density functional theory (DFT) calculations are conducted on the structure and decomposition mechanism of these materials. We will further apply an advanced computational screening to design novel ammine metal borohydrides, which can suppressed the releases of ammonia and improved the dehydrogenation properties.

氨合金属硼氢化物具有较高的理论含氢量、较低的放氢温度，是极具潜力的储氢材料。但这类材料仍存在100 oC以下放氢动力学缓慢、热分解放氢过程中伴随杂质气体的产生等问题，严重制约了其作为储氢材料的实际应用。深入地研究其的晶体结构和热分解微观机制，可为这类材料的改性及新型储氢材料的设计提供有价值的理论参考依据。本项目拟运用密度泛函理论计算研究氨合金属硼氢化物的微观热分解机制、提出完全抑制材料热分解过程中氨气释放、改善其放氢动力学性能的方案，在此基础上设计具有较好放氢性能的二元复合氨合金属硼氢化物。

氨合金属硼氢化物储氢材料兼具较高的理论含氢量、较低放氢温度等优点。但该体系储氢材料仍然存在放氢纯度较低、100oC以下放氢动力学缓慢等有待解决的科学问题。本项目运用密度泛函理论计算研究了单离子和双离子氨合金属硼氢化物的电子结构和热分解微观机制，揭示了材料放氢纯度与氨气扩散势垒的关系以及材料放氢微观机制。阐明了有Al、Li、Mg等轻质元素构成的氨合金属硼氢化物通过直接的(N)H和(B)H结合释放氢气，理论计算得到的放氢反应势垒与实验观测放氢温度较好吻合。理论计算结果表明，通过二元复合生成双离子氨合金属硼氢化物可提高氨气的扩散势垒、抑制氨气释放。而含3d轨道电子的过渡金属元素构成的氨合金属硼氢化物Zn(BH4)2•2NH3在放氢气前首先生成中间产物如NH3BH3、B2H7-，Zn2+起到转移BH4-中的H-，促进中间产物生成及氢气在较低温度下释放。因此，通过二元复合生成含有3d轨道过渡金属元素的双离子氨合金属硼氢化物有利于改善材料的放氢性能。