Improve Hydrogen Storage Capacities Of Several Porous Materials

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%.