Mechanism of Hydrogen Storage in the Graphene Nanoflake–Lithium–H2 System

Carbon materials, such as graphene nanoflakes, carbon nanotubes, and fullerenes, can be used for hydrogen storage. Alkali doping of these materials generally increases their H 2 -storage density. In this study, the interaction of hydrogen molecules with Li-doped graphene nanoflakes was systematically investigated using density functional theory (DFT). A large polycyclic aromatic hydrocarbon composed of 37 benzene rings (referred to as GR) was used as a model of a graphene nanoflake, and GR–Li–(H 2 ) n and GR–Li + –(H 2 ) n ( n = 0–13) clusters were used as hydrogen storage systems. The first and second coordination shells of H 2 around GR–Li were saturated at n = 3 and 7, respectively. The binding energy of H 2 to GR–Li decreased with increasing n, reaching a limiting value at n = 10. For GR–Li + –(H 2 ) n, similar results were obtained and the Li atom on GR was shown to be activated by electron transfer from Li to GR. The diffusion barrier of Li + on GR decreased upon addition of H 2 to Li + from 6.35 ( n = 0) to 3.62 kcal/mol ( n = 4). The mechanism of H 2 addition is discussed herein based on the calculated results.