Electric field induced enhancement of hydrogen storage capacity for Li atom decorated graphene with Stone-Wales defects

• The Li atom is binding strongest to the hollow site of heptatomic ring of the graphene with Stone-Wales defects and do not suffer from clustering.
• The upward electric field can enhance the binding energy of the Li atom to the graphene and the average adsorption energy of hydrogen molecules.
• Graphene-(Li-nH2) can release H2 molecules at the room temperature and the downward electric field help to desorb hydrogen molecules.

The density functional theory calculations are carried out to investigate the enhancement of the hydrogen storage ability induced by the electric field (E-field) vertical to the surface of the Li atom decorated graphene with Stone-Wales defects. When without E-field, the binding energy (Eb) of the Li atoms to the hollow site of the heptatomic ring is 2.89 eV, which is larger than that at the hollow sites of the hexatomic ring and the pentatomic ring, indicating the Stone-Wales defect on the graphene surface can effectively enhance the binding strength of the Li atom. When the upward +0.01 au E-field is imposed, the Eb of the Li atom on the hollow site of the heptatomic ring increases to 3.41 eV, which is larger than the double of the experimental cohesive energy of bulk Li (1.63 eV), consequently allowing the dispersion of Li atoms without clustering, which is the basis for large amount hydrogen storage. The maximum number of H2 molecules adsorbed by each Li on the surface of the SW defective graphene under the upward 0.01 au E-field is 5, one more than that without E-field. The average adsorption energies of molecular hydrogen around Li in the presence of upward 0.01 au E-field are in the range of 0.21–0.46 eV, which are larger than that in the field-free case and intermediate between physisorbed and chemisorbed states (0.1–0.6 eV). The SW defective graphene adsorbs hydrogen molecules mainly through the polarization interaction. The calculated desorption temperature and molecular dynamic simulation indicate that the H2 molecules are easier to be desorbed under the downward E-field. Therefore, the Li decorated graphene with SW defects is appropriate for hydrogen storage under near-ambient conditions with the application of an E-field. Keeping five H2 molecules adsorbed per Li and stabilizing the dispersion of individual Li atoms under the upward E-field, the structure can serve as better building blocks of polymers. These findings suggest an effective route to control the hydrogen storage abilities of nanomaterials.

The binding energy (Eb) for the Li atom to the hollow site of the heptatomic ring for the Stone-Wales defective graphene is 2.89 eV without the electric field (E-field) and increases to 3.41 eV when the upward +0.01 au E-field is imposed, which is larger than the double of the corresponding cohesive energy of bulk Li (1.63 eV), consequently allowing the dispersion of Li atoms without clustering, which is the basis for large amount hydrogen storage. With the assistance of an upward +0.01 au E-field, the maximum H2 molecules adsorbed on the SW defective graphene-Li can up to 5 with the average adsorption energy per H2 (Ead) of 0.21 eV, one more than that without E-field. Moreover, the calculated desorption temperatures and molecular dynamic simulations indicate that the SW defective graphene-Li is easier to desorb H2 molecules under the downward −0.01 au E-field. Therefore, the SW defective graphene-Li is appropriate for the hydrogen storage under near-ambient conditions with the application of an external E-field. Under the upward positive +0.01 au E-field, stabilizing the dispersion of individual Li atoms with the average Eb of Li atoms to the Li-graphene polymer of 3.19 eV and keeping five H2 molecules adsorbed per Li with the Ead of 0.18 eV, the polymerization do not affect the binding strength of Li atoms to the surface of the SW defective graphene and the graphene-Li-5H2 structure can serve as better building blocks of polymers. These findings suggest an effective route to control the hydrogen storage abilities of nanomaterials.Figure optionsDownload as PowerPoint slide