An Efficient Way To Suppress the Competition between Adsorption of H₂ and Desorption of nH₂–Nb Complex from Graphene Sheet: A Promising Approach to H₂ Storage

We performed first-principles calculations to investigate the electronic structure and hydrogen storage capacity of bare niobium (Nb) and niobium-decorated graphene (GR@Nb). The results predict that bare Nb can bind 6 H₂ molecules in the quasi-molecular form to reach saturation with the binding energy of hydrogen in the range 0.2–0.7 eV. In addition, the maximum temperature of desorption was 466 K. We demonstrated that the most favorable site for Nb atoms is the hollow site of graphene with a binding energy of 1.783 eV. Moreover, we analyzed the stability of Nb dopant on the graphene surface by means of reaction barriers calculations and an ab initio molecular dynamics simulation. Our calculations reveal that an energy barrier of 0.435 eV is required for a Nb atom to move from one hollow site to the adjacent hollow site, which is far greater than the energy of thermal vibration of Nb at 300 K. We show that Nb-decorated graphene doped with nitrogen atoms at 7.25% can absorb 12 H₂ molecules in the quasi-molecular form with an average binding energy of 0.410 eV at an average desorption temperature of 520 K. Our results predict that desorption of nH₂–Nb complex from a graphene sheet can be suppressed by increasing the concentration of nitrogen atoms to 7.25%. Finally, the storage capacity of 2NGR@2Nb is about 8 wt %. These results clearly demonstrate that Nb-decorated graphene with a 7.25% concentration of nitrogen atoms is a promising candidate for H₂ storage for mobile applications.