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

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 (E-b) 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 H-2 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 H-2 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 H-2 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. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.