氨硼烷放氢过程的热力学、缺陷性质及微观结构演变的理论研究

Hydrogen energy is a potential ideal energy due to the advantage of high caloric value, non-polluting, abound, etc. The lack of hydrogen storage materials to fully meet the application requirements is the bottleneck of hydrogen economy. Ammonia borane (AB) is a promising hydrogen storage material because of its gavimetric density as high as 19.6 wt%. However, its decomposition temperature is relatively high and the kinetics is slow, making it not suitable for on-board application. In order to modify thermodynamic and kinetic properties of AB from experimental point of view, it is particularly important to understand the detailed mechanism of thermal decomposition of AB as well as establish thermodynaic database related to decomposition of AB. The present project will investigate all kinds of intrinsic defects in AB by first-principles calculations.We will calculate defect fomation energy and migaration barrier, and find out the predominate defect in AB. Ab initio molecular dynamics simulation will be used to investigate the structural, vibrational and diffusion characteristics of AB across a range of temperature above its melting point, and figure out microscopic process for the decomposition of AB. Thermodynamic functions in the decoposition process of AB are obtained by a combination of first-principles calculation and CALPHAD (CALculation of PHAse Diagrams) method, and eventurally establish the thermodynamic database for the decomposition of AB. The completion of the research work is expected to reveal the microscopic process and mechanism of thermal decomposition of AB, and provide theorectic guidance for the design of ammonia borane based hydrogen storage materials.

氢能因热值高、无污染以及资源丰富等优势而被视为一种潜在的理想能源，完全满足应用要求的储氢材料的缺乏是制约氢能规模利用的瓶颈。氨硼烷的储氢含量高达19.6wt%，是一种具有广泛应用前景的新型储氢材料，但其放氢温度偏高、放氢速率缓慢制约了它在车载氢源方面的应用。阐明氨硼烷放氢的详细过程并建立其相关的热力学数据库对氨硼烷的改性尤为重要。项目拟研究氨硼烷热解放氢过程中缺陷性质、微观结构演变以及建立与其热分解过程相关的热力学数据库。通过第一原理计算氨硼烷中各类型点缺陷的形成和扩散过程，找出主要缺陷及其扩散路径和能垒；采用第一原理分子动力学模拟不同温度下其微观结构演变过程和原子迁移规律，分析放氢的微观机理；通过耦合第一原理计算和CALPHAD方法来获得氨硼烷放氢过程的热力学参数，并建立与放氢过程相关的热力学数据库。该项目的完成可望揭示氨硼烷热解放氢的微观过程和机制，并为氨硼烷系列储氢材料提供设计准则。

氢能的规模应用需要解决氢的制取、储存、运输和转化应用四大相关技术，而高效的储氢材料的研发是目前氢能利用的主要瓶颈。氨硼烷(ammonia borane,NH3BH3)具有非常高的质量储氢密度和体积储氢密度(分别为19.6 wt%H 和145 gH2/L)，使其成为极具应用前景的新型储氢材料。但氨硼烷存在脱氢动力学阻力高、热分解放氢过程缓慢和热分解产生有毒副产物等问题，这阻碍了氨硼烷的实际应用，也要求对氨硼烷放氢过程进行深入研究。课题采用第一原理计算方法对氨硼烷等储氢材料的性质进行了研究。研究了范德华(van der Waals)作用对氨硼烷的结构、弹性及能量的影响，发现若不考虑范德华作用，第一原理计算的晶格常数和弹性常数不合理。研究了与氨硼烷放氢过程有关的H缺陷、NH3相关及BH3相关缺陷的形成能及缺陷导致的晶格原子弛豫，发现氨硼烷中主要的H缺陷是BH3单元中的正电H空位和NH3单元中的负电H空位，对于BH3和NH3相关的分子类型缺陷，某些中性缺陷的形成能较低，说明氨硼烷放氢时最先断裂N-B键；带电缺陷对晶格影响较大，或使晶格中自发生成H2分子，或使晶格中生成复杂的基团以进一步生成氨硼烷放氢时的中间产物或副产物。研究了氨硼烷在放氢过程中的结构演变规律,发现氨硼烷在600K以上温度时其体相的原子才能有效扩散。研究了Ca修饰的单层硼片的吸氢性能，发现每个Ca原子可以吸附住6个H2分子，最高储氢容量可高达12.68 wt%H，而且在常温和温和的氢压下所有吸附状态均稳定。此外，还研究了Mg的氢化过程，分析了Mg表面附近的空位对氢化的影响。受项目资助，在J Phys Chem C 等国际主流期刊上发表科研论文5篇，合作培养博士研究生2名。