Effect of the porous structure of graphite on atomic hydrogen diffusion and inventory

A multi-scale model for particle diffusion in porous structures is used to study the effect of the internal structure of graphite on atomic hydrogen transport and inventory in graphite. The diffusion of trace amounts of atomic hydrogen are modelled as a trapping-detrapping mechanism within the porous network typical in graphites. Activation energies for the different traps are taken from experiments and from molecular dynamics simulations. Different diffusion mechanisms dominate at different graphite temperatures. It is seen that the trace diffusion coefficient of hydrogen scales as the square of the jump lengths of the dominant mechanism. Depending on the detrapping mechanism and on the internal structure of graphite, the jump lengths after each detrapping event can vary over a few orders of magnitude. This gives rise to the possibility of anomalous diffusion in graphite. The effect of closing the pores of graphite on atomic hydrogen diffusion is also presented.