【作者】 饶德伟；

【导师】 邓开明； 陆瑞锋；

【作者基本信息】 南京理工大学 ， 材料科学与工程， 2013， 博士

【摘要】 氢能是一种理想的清洁能源,寻找合适的材料来存储足够的氢气满足实际需要是目前氢能利用的关键问题。本论文首先介绍了目前世界能源现状、氢能的优点及存储方法,回顾了多孔纳米材料的储氢研究进展,并总结了影响多孔材料氢气存储的因素。关于预测和模拟的理论基础,包括量子计算化学的发展历史、密度泛函理论和巨正则蒙特卡洛方法等,以及计算中用到的量子化学计算软件包,在论文中予以详细介绍。我们通过富勒烯嵌入和锂掺杂的组合方法改性IRMOF-10、-12和-14三种大孔径的金属有机骨架材料(MOF)。巨正则蒙特卡洛模拟预测了它们在近环境条件下,组合修饰的MOF结构可以获得良好的储氢质量密度和体积密度,从理论上实现了美国能源部制定的双重目标。第一性原理计算发现,锂原子在材料上失去部分电荷而引起的静电场对其周围的吸附的氢气具有极化效应,这将增强了氢气在材料中的结合能,最终引起氢气存储性能的提升。富勒烯掺杂的主要影响集中在两个方面——增加氢气体积存储量和提供锂原子掺杂的位置。材料的物理性质对氢气存储性能也有一定的影响,包括吸附焓、晶体密度、表面积、孔隙体积等。通过碳纳米管嵌入和锂掺杂相结合的组合方法,改性大孔径共价有机骨架材料COF-108,其氢气存储性能可以达到5.83wt%和32.4g/L,是一种非常良好的氢气存储材料。组合改性方法中,碳纳米管的主要作用是提供更多的锂掺杂位置,锂的主要作用是提高材料的氢气存储性能。我们嵌入不同直径的纳米管来探究氢气存储的最佳孔隙直径,分析孔隙与存储量的关系发现,最适合氢气存储的孔隙直径在4-5A之间,大约为氢气动力学直径的1.5倍。此外,我们还研究了二维多孔碳材料的改性及其储氢能力。密度泛函计算优化了氢气吸附在氮替代掺杂的多孔石墨烯(1Li-nN-PG)材料上的结构,发现单个锂原子周围能够稳定吸附至少3个氢气分子。差分电荷密度以及结合能确定了氢气的存储受到了锂原子和氮原子的双重影响；在硼掺杂的多孔石墨烯中,通过比较氢气吸附的几何结构、吸附能发现金属钙原子掺杂比锂掺杂对氢气储存更加有利,而GCMC模拟结果证实了这一点,4Li-2B-PG-H和4Ca-2B-PG-H中氢气的室温存储性能分别达到了6.4wt%和6.8wt%,它们均超过了U.S. DOE的目标；研究硼掺杂的石墨一炔发现,除了掺杂的硼、锂原子外,主体材料本身的基团——炔基对氢气的存储性能也有影响,而第一性原理计算、基于从头算的分子动力学模拟以及GCMC模拟均显示,锂修饰的BG是一种在室温下具有较高氢气存储性能的材料,在100bar下可高达7.41wt%。更多还原

【Abstract】 Hydrogen is an ideal clean energy resource, and the key issue for hydrogen application is to develop a suitable material which can store enough hydrogen to satisify the practical requiement. In this dissertation, we first introduce the current energy situation and the advantages of hydrogen energy as well as its storage methods, and review the capacities of hydrogen storage in porous materials materials. Besides, the factors affecting the capacity of hydrogen storage in porous materials are summarized. With regard to the fundamental theory and methodology for the simulations and prediction in our work, the development of quantum chemistry, the basic concept of density functional theory (DFT), grand canonical ensemble monte carlo (GCMC) method and some employed software packages are described in detail.Three materials, inculding IRMOF-10,-12and-14, were modified by fullerene impregnating and lithium doping, and their H2uptakes near ambient temperature were calculated by GCMC method. It is found that the gravimetric density and volumetric density for H2storage in the modified structures exceed the2017targets set by U.S. Department of Energy (U.S. DOE) in theory. First-principles results show that the electrostatic field caused by the charge transfer from lithium atoms to materials, polarizes the adsorbed H2, which could enhance the binding of the hydrogen, resulting in high performance of hydrogen storage in materials. The main effects of the fullerene impregnation are two folds:increasing the volumetric density of hydrogen storage and providing additional sites for lithium doping. Further, the hydrogen storage are also dependent on the physical properties of materials, including adsorption enthalpy, crystal density, surface area, pore volume, etc.The covalent organic framework, COF-108, a crystal material with large free volume, is modified by single-walled nanotube inserting (SWNT) and metal doping. Our calculation results shown that the modified COF-108have the capacities of5.83wt%and32.4g/L at298K and100bar, which demonstrated that the modified COF-108is suitable for hydrogen storage. Focused on modification methods, the SWNTs play an important role in supplying more places for Li doping, and Li atoms dominate the H2uptakes. We explored the best pore size for hydrogen storage through choosing inserted SWNTs with different diameters. By analysizing the relationship between the pore size and hydrogen uptake, we found that the favorable pore size for hydrogen storage should be in the range of4-5A, about1.5times of the kinetic diameter of hydrogen molecule.Furthermore, we studied the modified two-dimensional porous carbon materials (CMs) and their hydrogen storage properties. The H2adsorbed on nitrogen substituted porous graphene (1Li-nN-PG) were optimized by DFT calculations, and we found that at least three H2molecules can be adsorbed around each Li atom. The charge density difference plot and binding energy of H2verified that both the Li and N atoms are responsible for H2adsorption. In boron-substituted PG, by comparing the geometric structures and adsorption energies, we observed that the Ca doping is more effective than the Li doping to improve hydrogen storage, which is further confirmed by GCMC simulations. GCMC results show that in4Li-2B-PG-H and4ca-2B-PG-H, the hydrogen storage at room temperature are6.4wt%and6.8wt%, respectively, which are higher than the U.S. DOE target. The boron substituted graphyne (BG) was employed to store hydrogen at room temperature. The calculated results indicate that the boron, lithium, and alkynyl groups in graphyne all contribute to hydrogen uptake. In addition, multiscale simulations suggest that Li doped BG has a high capacity of hydrogen storage at298K,100bar which reaches7.41wt%.更多还原