| Hydrogen, unlike the fossil fuels, is an abundant, clean, and renewable "green" energy carrier. However, storing hydrogen efficiently and safely is still a major challenge to realize hydrogen economy. Recent studies show that carbon nanostructures are capable of storing hydrogen in molecular form. However, the binding energy of hydrogen to these host materials is too small to be used at room temperature. In this paper, the hydrogen storage properties of metal-decorated nanomaterials and the Li2O clusters are studied by using first-principles method based on density functional theory. The results show that the binding energy of the molecular hydrogen meets the requirement of the reversible hydrogen adsorption/desporption near ambient temperature. The findings also show that the storage capacity of these materials also meets the DOE system target for hydrogen gravimetric density, presenting a good potential as hydrogen storage materials.In Chapter1, the current situation of store materials of hydrogen energy has been introduced, which include carbon materials, boron materials and boron-nitrogen materials and so on.In Chapter2, density functional theory method is introduced.In Chapter3, we have studied the hydrogen storage properties of metal-decorated nanomaterials including Li and Ca co-decorated carbon nitride nanostructures, Li-decorated planar boron sheets and metal decorated monolayer BC2N. It is found that these materials are feasible for hydrogen storage without metal clustering owing to the strong interaction between metal atoms and the host materials. The hydrogen binding energies and storage capacities can be markedly increased by doping metal atoms onto these host materials. Both the polarization mechanism and the orbital hybridizations contribute to the adsorption of hydrogen molecules. In the fully loaded cases, the above metal-decorated nanomaterials can contain up7.69-9.81wt%of molecular hydrogen with moderate binding energy.In Chapter4, we have studied the hydrogen storage properties of Ca-decorated graphyne nanotubes. The results show that Ca atoms can be adsorbed stably on the acetylenic ring of the graphyne nanotube without Ca atom clustering. Both the polar interactions and the orbital hybridizations contribute to the adsorption of H2molecules. The average adsorption energy is in the range of0.13～0.33eV/H2which is almost independent of the tube diameter. Each Ca atom can adsorb up to four H2molecules due to the steric hindrance of the H2molecules. With a hydrogen uptake of7.44～8.96wt%, the Ca-decorated GNT is an optimal choice for hydrogen recycling at near ambient conditions.In Chapter5, we have investigated Li-decorated boron nitride atomic chains for their applications as hydrogen storage materials. We have shown that the interaction between hydrogen molecules and the pristine chains is too weak to be used at room temperature, and the hydrogen adsorption energies and storage capacities can be obviously increased by decorating Li atoms on the ends of the BNACs. Moreover, the bonding energy between Li and boron nitride atomic chains is much greater than the cohesive energy of bulk Li so that the clustering of Li atoms will not occur once Li is bonded with BNAC. Our studies also revealed that the van der Waals interaction plays an important role in the adsorption of the H2molecules. Therefore, with two Li atoms binding to each end of the BNACs, the hydrogen storage capacity can reach up to29.2wt%theoretically.In Chapter6, we have studied the geometries, electronic structures, and hydrogen storage properties of (Li20)n clusters. The results show that for a given n its ground-state structure usually has the maximum Li-O bond. Due to transferring of charges from Li to O, the Li-O bond is polar. The (Li20)n (n=1-6) clusters each can bind12-24hydrogen molecules with average adsorption energy of0.19-0.22eV/H2, desirable for reversible hydrogen storage. |
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