Diffusion of atomic and molecular hydrogen in the interstitial space between graphite sheets was studied by molecular dynamics simulations. Interatomic interactions were modelled by a tight-binding potential fitted to density-functional calculations. Atomic hydrogen was found to be bounded to C atoms, and its diffusion consists in jumping from a C atom to a neighbouring one, with an activation energy of about 0.4eV. Molecular hydrogen was less attached to the host sheets and diffuses faster than isolated H. At temperatures lower than 500K, H2 diffused with an activation energy of 89meV, whereas at higher T its diffusion was enhanced by longer jumps of the molecule as well as by correlations between successive hops, yielding an effective activation energy of 190meV.

Diffusion of Hydrogen in Graphite: a Molecular Dynamics Simulation. Herrero, C.P., Ramírez, R.: Journal of Physics D, 2010, 43[25], 255402

Figure 12

Diffusion of Rb in graphite (C24Rb)