Hydrogen diffusion in potassium intercalated graphite studied by quasielastic neutron scattering

The graphite intercalation compound KC24 adsorbs hydrogen gas at low temperatures up to a maximum stoichiometry of KC_(24)(H_2)_2, with a differential enthalpy of adsorption of approximately −9 kJ mol^(−1). The hydrogen molecules and potassium atoms form a two-dimensional condensed phase between the graphite layers. Steric barriers and strong adsorption potentials are expected to strongly hinder hydrogen diffusion within the host KC_24 structure. In this study, self-diffusion in a KC_(24)(H_2)_0.5 sample is measured experimentally by quasielastic neutron scattering and compared to values from molecular dynamics simulations. Self-diffusion coefficients are determined by fits of the experimental spectra to a honeycomb net diffusion model and found to agree well with the simulated values. The experimental H2 diffusion coefficients in KC_24 vary from 3.6 × 10^(−9) m^2 s^(−1) at 80 K to 8.5 × 10^(−9) m^2 s^(−1) at 110 K. The measured diffusivities are roughly an order of magnitude lower that those observed on carbon adsorbents, but compare well with the rate of hydrogen self-diffusion in molecular sieve zeolites