| Due to its high efficiency,harmlessness and renewable,hydrogen energy is widely considered as a very potential new energy.However,the lack of advanced materials with high capacity and efficient reversibility for hydro gen storage has severely hindered the promotion and application of hydrogen energy.Porous graphene?PG?is a collection of graphene-related materials with nanopores in the plane.It owns many good features including light weight,large surface area and high porosity.In recent years,the application of porous graphene in hydrogen storage has gained more and more attention from researchers.In this dissertation,we focus the attention on the hydrogen storage capacity of porous graphene decorated respectively with alkaline metal,rare earth metal and transition metal atoms.Base on the density functional theory and first-principle calculation,the adsorption abilities of H2 molecule on PG are investigated comprehensively and deeply.Ab initio molecular-dynamic simulation by using CASTEP code is also explored to demonstratethe stability of H2 adsorbed on metal-decorated PG at ambient temperature.Firstly,we study the adsorption ability of H 2 molecular on pure PG.It can be found that the adsorption of H2 molecules on PG is very weak,and the adsorption energy is higher than-0.1e V,which belong to physical adsorption.Then,adsorption abilities of H2 molecules on PG decorated with alkaline metal atoms?Li?Na?K?are investigated deeply.We find that Li,Na and K prefer to adsorb on the sites of hollow center of the C hexagon.The interaction between the PG and Li atom is the strongest.Four alkaline metal atoms can be adsorbed stably on both sides of PG unit cell,but there is no tendency of clustering among the m etal atoms.Three H2 molecules can be adsorbed around each metal atom.The adsorption of H 2 molecules on Li-PG system is the strongest.Twelve H2 molecules can be absorbed on both sides of Li-PG.The corresponding gravimetric hydrogen storage capacity is 1 2wt.%with the average adsorption energy of-0.246e V/H2.The adsorption of H2 molecules on Na-PG system is weaker,whose average adsorption energy is-0.129e V/H2.The adsorption of H2molecules on K-PG system is the weakest,and the corresponding average a dsorption energy is only-0.056e V/H2.So,K-PG system is not suitable for hydrogen storage materials.The adsorption process of hydrogen molecule on PG decorated with the alkaline metal atoms is achieved by means of a polarization mechanism.Secondly,the adsorption property of H2 molecules on PG decorated with the same family of rare earth atoms?Sc,Y and La?are studied deeply.We found that only one Sc atom can be stably adsorbed on the same side of PG unit cell.Although two Y atoms can be stably adsorbed on the same side of PG unit cell,they are prefer to occupy the hollow center of different C hexagon.When more than four H 2molecules are adsorbed around each Y atom,the Y atoms will deviate from the center of the C hexagon.We also found that a Y atom b onds with the nearest the Y atom from a 2×2 supercell of the PG.So,to avoid a tendency of clustering among the rare earth metal atoms,the single side of the PG unit cell should only contain one metal atom.Ten H2 molecules can be absorbed on both sides o f Sc-PG,whose corresponding gravimetric hydrogen storage capacity is 7.75wt.%with the average adsorption energy of-0.312e V/H2.Twelve H2 molecules can be absorbed on both sides of Y-PG,and the corresponding gravimetric hydrogen storage capacity is6.87wt.%with the average adsorption energy of-0.267e V/H2.The adsorption of H2molecules on La-PG system is the weakest.Therefore,it is not suitable for the hydrogen storage at ambient conditions.The PG decorated with Sc atoms is most suitable for the hydrogen storage.In this case,the adsorption process of hydrogen molecule on PG might be attributed to the polarization mechanism and the orbital hybridization among metal atoms,H2 molecules and C atoms.Finally,the adsorption properties of H2 molecules on PG decorated with transition metal atoms?Ti?V?Cr?Mn?Fe?Co?Ni?Cu?Zn?are studied comprehensively.The obtained results show that,if PG is decorated with a Zn atom,the adsorption intensity of H2 molecules on PG will decrease.Two H2 molecules can stably adsorb on PG decorated res pectively with a Cr,Mn,Fe,Co,Ni or Cu atom.The adsorption energy of the first H2 molecule adsorbed on PG decorated respectively with a Fe,Co or Ni atom is lower than-1.0 e V,which is close to chemisorption.PG decorated with Ti or V atoms is suitabl e for hydrogen storage.Under these circumstances,four H2 molecules can be adsorbed around each metal atom.By comparing the geometrical structures and the adsorption energies of hydrogen molecules,PG decorated with Ti atoms is more favorable to reversib le hydrogen storage than V atoms.Eight H2 molecules can be absorbed on both sides of Ti-PG.The corresponding gravimetric hydrogen storage capacity is 6.15wt.%with the average adsorption energy of-0.457e V/H2.In order to investigate the stability of H2 molecules adsorbed on the Li-PG,Sc-PG and Ti-PG system at ambient conditions,we also present the results from ab initio molecular-dynamic simulation in the last part of the chapters.For this purpose,NVT ensemble has been selected.The obtained result s hows that the configuration of the system is very stable at 300K and without external pressure.At 300k,nine H2molecules can be absorbed on both sides of Li-PG with the gravimetric hydrogen storage capacity of 9.25wt.%.Eight H2 molecules can be absorbed on both sides of Sc-PG with the gravimetric hydrogen storage capacity of 6.30wt.%.And,six H2molecules can be absorbed on both sides of Ti-PG with the gravimetric hydrogen storage capacity of 4.69wt.%. |
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