Porous Graphite Sheets Are Used As Theoretical Studies For Hydrogen Purification And Storage Materials

In recent years, hydrogen have attracted great attention in the field of renewable en-ergy and is expected to be a major power source in the future because of its abundance,cleanness, and high energy density. At the same time, it has important industrial appli-cations, especially in the petrochemical, metallurgy, food processing, precision electronics industry synthesis and other fields. However, the biggest challenge in making industrial use of hydrogen practical is to find materials which are able to separate and store H2 efficiently.The successfully synthesis of graphene nanosheets C₂₁₆ in the laboratory attracts our lots of attention. As is known to all, the existence of defined defects can alter the electrical and optical properties of graphene nanosheets. Therefore, in this work, three kinds of defects on C₂₂₂ nanographene molecule are studied theoretically with density functional theory, and the influence of defects to different properties of C₂₂₂ are explored.It is demonstrated that C₂₁₆, which has a prefect hole with 0.6 nm diameter, increases the HOMO-LUMO gap of C₂₂₂, showing a semi-conducting character. While C221 and C₂₂₂0,with a single vacancy and a double vacancy respectively, decrease the HOMO-LUMO gap of C₂₂₂,showing a conducting character. The peaks of UV-vis absorption spectrum for these nanosheets have blue shift and red shift relative to C₂₂₂ caused by the gap change.Furthermore, if the density of the defect is increased, the HOMO-LUMO gap will keep increasing up to 4.28 eV and the band of absorption spectrum moves from 400-800 nm to 300-500 nm.The energy barrier and the selectivity for H2, O2, N2, NO, N02, H20, CO and CO2 on C₂₁₆ surface are studied. The results show that the energy barrier of the diffusion reaction for H2 penetrating the C₂₁₆ is the lowest, about 0.76 eV. At the room temperature,the selectivity for H2 to other gas molecules is high up to 1048, indicating that C₂₁₆ is a good candidate material as a hydrogen purification membrane, which is superior to porous graphene, polyphenylene, and other traditional membranes.Based on the study above, we designed a Li decorated porous nanosheet C180 as a hydrogen storage material. Our calculation shows the Li atoms are easily adsorbed to the surface of C180 without forming the clusters. The average adsorption energy of H2 in the surface of Li decorated C180 is about -0.17 eV, lower than that in Li-doped graphene as reported by the previous studies. This energy is an appropriate value for adsorption and desorption of H2. Furthermore, if the concentration of Li on the surface of C180 is increased, the weight density of hydrogen storage can be significantly enhanced to 4.76 wt%, higher than Li doped graphene and carbon nanotubes. Therefore, C180 is predicted to be a promising hydrogen storage material.