Transition-Metal Decoration Enhanced Room-Temperature Hydrogen Storage in a Defect-Modulated Graphene Sheet

Using first-principles density functional calculations, we show that a transition-metal (TM)-doped defected graphene sheet with periodic repetition of a C atom vacancy (V-c) can be used as a promising system for hydrogen storage. The TM atoms adsorbed above and below the defected site are found to have a strong bonding to the graphene sheet, thereby circumventing the problem of TM clustering, which is the main impediment for efficient hydrogen storage in nanostructure systems. The results reveal that, when the vacancy-modulated graphene sheet is decorated on both sides by a combination of less than half-filled (TM1) and more than half-filled (TM2) elements, it results in the adsorption of molecular hydrogen with a binding energy lying in the desirable energy window. Among all the different TM1-TM2 combinations at a C vacancy site, Fe-Ti turns out to be the best choice where five H-2 molecules get attached on each pair. To underscore the stability of these hydrogenated systems, we have performed an ab initio molecular dynamics simulation for a fully decorated defected graphene structure. The results show that, at room temperature, the system is stable with a gravimetric efficiency of 5.1 wt % of hydrogen, whereas desorption starts only at similar to 400 K.