Theoretical Studies On Hydrogen Storage Propetries Of Graphene-based Nanomateirals

Making full use of the characteristics of dispersion and discontinuous of hydrogen is to solve theproblem of hydrogen storage. The key is the hydrogen storage material, which must be of low cost, highsafety, high efficiency, high stability of reversible use. The emergence of new nanomaterials for hydrogenstorage provides an expansion of research. By using the first-principles density functional theory, thispaper investigates the hydrogen storage properties and adsorption mechanism of graphene-basednanomaterials. Following are the main contents:（1） Due to its pore structure and high surface to volume ration, graphene is considered as promisinghydrogen storage materials. The mass preparation of graphene makes its wide application in hydrogenstorage. The binding energy of Li-doped graphene is smaller than the cohesive energy of bulk Li, leadingto the preferable clustering of Li atoms, which reduces the weight percentage of hydrogen storage. Ourstudy shows that Li-doped fluorinated graphene enhances the binding strength of Li, preventing fromforming an Li cluster. And in fluorinated graphene, the Ï€ bands of graphene rigidly shift downward withthe level broadening, and the other levels below the Fermi level are also strongly dispersive. It causes amarked weakening of the Ï€ bonds that exist among the carbon atoms of the graphene layer, which allowsfor the formation of new chemical bonds with the F atom, namely, a covalent bond. Therefore, thesp²-hybridized C atoms turn to sp³, leading to the distortion of the graphene. After doping Li on fluorinatedgraphene, It is found that a mixture between sp³and a higher degree of sp²of the carbon orbitals wouldrestore the distorted fluorinated graphene. Considering this material for hydrogen storage, Li adsorbed onsingle or double-sides could store hydrogen up to9or16.2wt%. The enhanced electrostatic field aroundthe Li atom originates from the increased charge transfer from Li to graphene and F atoms with moreelectronegativity. Hybridization interaction between Li and graphene is also responsible for the adsorptionof H₂molecules. Therefore tuning the bonding mechanism between the metal atom and substrate throughappropriate doping element is a possible solution for developing reversible hydrogen storage materials.Compared with monolayer graphene, there exits van der waals force in few-layer graphene. Theexisting experiments have been able to prepare single-, bi-, and few-layer graphene and expand theinterlayer distance. Because BN and C₂are isoelectronic, C in carbon-based nanomaterials can be substituted by B and N, forming new BN-based nanomaterials, considering as hydrogen storage materialswould show great different properties from carbon-based nanomaterials. We study the effect of variableinterlayer distance on the adsorption of hydrogen molecules between bilayer solid matrix layers （bilayerboron nitride sheets and graphene/boron nitride heterobilayers）. The results show that the H₂adsorptionenergy has a minimum by expanding the interlayer spacing, along with further interlayer expansion, arisingfrom many H₂binding states and electrostatic interaction induced by the polar nature of B-N bonds. Todetermine if successive addition of H₂molecules is indeed possible using the minimal H₂adsorption energyas the reference state, we then simulate the hydrogen storage capacity of BBN and GBN with differentstacking types, and find that the GBN with Bernal stacking is superior for reversible hydrogen storage. Upto eight H₂molecules can be adsorbed with the average adsorption energy of-0.20eV/H₂, corresponding to7.69wt%hydrogen uptake. We also use ab initio molecular-dynamics simulations on the GBN with ABstacking decorated with eight H₂molecules with two different methods to check the stability of thehydrogen storage system. We find that the system is stable in normal temperature-pressure. This stability isof vital importance to the hydrogen storage, which not only reduces the cost of storing hydrogen but alsoensures the safety in the transportation section. The theoretical results here will provide a useful referencefor searching reversible hydrogen storage materials in the laboratory.