Theoretical investigation of the effect of graphite interlayer spacing on hydrogen absorption

We investigate the absorption of hydrogen molecules between graphite layers using both first-principles calculations and classical grand-canonical Monte Carlo simulations. While a recent theoretical study showed that graphite layers have high storage capacity at room temperature, previous simulation results on hydrogen-graphite systems showed otherwise. Our first-principles calculations suggest that it is possible to store hydrogen molecules between the graphite layers if the energetically unfavorable initial absorption stage could be overcome. The barrier to the initial absorption originates from the large lattice strain required for H(2) absorption: small amounts of initial absorption cause an interlayer expansion of more than 60%. To determine if significant storage is indeed possible at finite temperature (and pressure), we performed grand-canonical Monte Carlo H(2)-absorption simulations with variable graphite interlayer spacing. Using two different potentials for the H(2)-C interaction, we found low-H(2)-mass uptake at room temperature and moderate pressures (e.g., close to 2 wt % at 298 K and 5 MPa). Our results suggest that a pore width or interlayer spacing around 6 A in the graphite layers has the optimum absorption capacity.