First-principles Study Of Influence Of Dopants Ca And B On The Hydrogen Storage Performance Of Graphene And Carbon Nanotubes

In the environment human depended,more than 80% of the energy we rely on is from the burning of fossil fuels still.At present,environmental problems are becoming more and more serious and threatening the humanity’s survival.We have to develop a new energy which has a little of pollution to environment.So the hydrogen energy emerges as the times require.But the hydrogen storage and uses are restricted because of it mainly in the form of gas,flammable and explosive.Thus how to storage the hydrogen become a major focus in academia.The storage materials of hydrogen energy is mainly include metal materials, carbon materials,metal organic frameworks,and so on.Because of the light quality,large specific surface area and pore structures,carbon materials has become a hot topic in the research of hydrogen storage since it was discovered.In this paper,we use CASTEP program in the MS(Material Studio) software to study the hydrogen storage properties of Ca and B doped graphene and carbon nanotubes.This is a method of density functional theory based on first-principles calcution.Some physical quantities related to hydrogen storage,for example,the combination heat,the net charge of atoms,Mulliken population,overlap population,density of states,band structure and the electron density are calculated and analyzed.The studies shows that there is no interaction between C and H atoms on graphene before doped and the adsorption of hydrogen is physical adsorption.The combination heat of graphene system are raised after B doped and the stability of the structure is enhanced.Moreover electrons move to C atoms and the band gap range from insulator to metal near the Fermi energy.These changes all can make electrons moving freely and improve the hydrogen storage ability of system.Here,the hydrogen storage capacity of the system can reach 8.0wt% theoretically.Graphene system becomes semiconductor after Ca decorated.There is the maximum electron density around C atoms because of the electrons moving.So the strong covalent interaction come into being between C and H atoms,but not as good as B doped.Compared with the undoped CNTs system,the electrons density increase of C andH atoms and chemical bond interaction enhances after Ca doped.At the same time,the overlap of the conduction band and valence band is more strong and the hydrogen storage capacity gets enhanced.CNTs system changes metal into semiconductor by B doped.Thus free moving of electrons are inhibited.But part of the electrons still can skip over the energy gap and move to C and H atoms from B atoms.The electrons density about C-H is enlarged to enhance the chemical bond between C and H atoms,but not as good as Ca doped.In summary,B and Ca doped all can improve the hydrogen storage properties of graphene and carbon nanotube system.B doping is helpful to improve the hydrogen storage properties of graphene system,while adding the Ca atom is more applicable to improve the hydrogen storage properties of carbon nanotube system.