Structure Design And Properties Prediction Of Several Low-dimension Hydrogen Storage Materials

Hydrogen is considered to be an ideal clean and harmless energy source that could one day replace fossil fuels in the21century. One of the biggest challenges that hinder the widespread application of hydrogen economy is to find a safe, effective and economical method of storage. Therefore, it is extremely necessary to find suitable hydrogen storage media which can absorb hydrogen molecules with high gravimetric and volumetric density, and release the stored hydrogen easily when used. A lot of work has focused on the carbon-based nanomaterials due to their light weight and high surface-volume. Some recent studies have shown that their storage abilities can be largely improved by doping the nanostructure with metal atoms including transition metals (TMs) and alkali or alkaline-earth metals (AMs). In this research, we presented several theoretical models for hydrogen material, and investigated their storage capacity, adsorption structure and the mechanism of hydrogen adsorption through the first-principle calculations. Moreover, we investigated the effect of the external electric field on the hydrogen storage capability of the Ti-decorated graphene complex and the adsorption of Ti atoms on graphene. Our results may provide a useful reference for experimental scientists in searching for effective hydrogen storage materials in the future, and the main conclusions are summarized as follows:1. The hydrogen storage capacity of Ti coated B2C sheet:the behavior of hydrogen molecules adsorbed on Ti@B2C complex has been investigated for the first time by using the first-principle calculations. It is revealed that Ti atoms can be bounded to the B2C sheet tightly due to a strong hybridization between Ti and B2C sheet, and a single Ti atom can strongly bind up to four hydrogen molecules. The adsorption energy is in the range of-0.36～-0.82eV/H2, which is suitable for ambient temperature hydrogen storage. Considering the fact that Ti can be loaded on both sides of B2C sheet, corresponding gravimetric storage capacity of Ti@B2C system was also calculatedand it can reach to about7.0wt%, exceeding the minimum requirement of6.0wt%for applications. Through the further analysis of the projected densities of states (PDOS) and charge differences for Ti@B2C complex, we found that the hydrogen binding to Ti is originated from its polarization under the electric field introduced by Ti atom. Hence the main interaction mechanism is the electrostatic Coulomb attraction between Ti and hydrogen. The PDOS shows the interaction between the hydrogen molecules when more than two H2are adsorbed on Ti@B2C complex, indicating the formation of a super-hydrogen molecule. It is the formation of the super hydrogen molecule that further lowers the hydrogen adsorption energy.2. The hydrogen storage capacity of metal decorated graphyne:by the first principle calculations we studied Li, Ca, Sc, Ti decorated graphyne as a medium for hydrogen storage due to the larger pores of graphyne. It is found that all these metals prefer to pin at the hollow of the triangle rings rather than to form clusters on the graphyne surface. The results showed that the graphyne with both sides adsorbed Li, Ca, Sc, Ti can exhibit the hydrogen storage capacity as high as18.6wt%,10.5wt%,9.9wt%and9.5wt%, respectively. All the adsorbed H2remain the molecule form and their average adsorption energies are in the range of-0.26～-0.54eV/H2, promising for hydrogen adsorption and release at ambient conditions. Through the analyses of charge transfer, the adsorption mechanism originates from the following three facts:(ⅰ) the polarization of hydrogen under the electric field induced by the metal atoms;(ⅱ) the weak hybridization between hydrogen molecules and metal atoms modulated by the electrostatic potential induced by the metals; and (ⅲ) the formation of hydrogen super-molecule due to the interaction between the adsorbed H2.3. The hydrogen storage capacity of Ti coated graphene under electic field:we introduce an external electric field which is perpendicular to graphene to improve the hydrogen storage of Ti@graphene complex for the first time. The effect of the external electric field on the adsorption of Ti atoms on graphene and the hydrogen storage capability of the Ti-decorated graphene complex are investigated by using the first-principle calculations. It was found that the dispersive Ti atoms adsorbed on graphene are against aggregation under a negative electric field larger than0.006au. The external electric field can remarkably manipulate the charge transfer between Ti atom and C rings, enhancing the interaction between Ti and C atoms and preventing Ti atoms from clustering. Furthermore, the external electric field could effectively tune the hydrogen adsorption energies on Ti@graphene complex and provide a way to adjust hydrogen adsorption and desorption during hydrogen storage applications. The variations of the hydrogen adsorption energy originate from the redistribution of the charge density and the change of the adsorption geometry under the external electric field. Moreover, we found that the external electric field further enhances the hydrogen storage capacity with additional hydrogen molecules adsorbed on the top of Ti atom with F>-0.01au.