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Slow hydrogen diffusion in potassium-intercalated graphite studied by quasielastic neutron scattering

Justin Purewal (Cal Tech)

Hydrogen is adsorbed by stage-two, potassium-intercalated graphite (KC24) at low temperatures. The potassium and adsorbed hydrogen form a quasi-two-dimensional solid solution between the galleries of the host graphitic layers. Due to the presence of both the graphite corrugation and the metal sites, the hydrogen molecules experience a strong anisotropic potential. This is manifested in the large splitting of the hydrogen ortho-para rotational transition. Quasielastic neutron scattering measurements were performed on the KC24(H2)1 system at temperatures between 40 K and 80 K. We find that hydrogen diffusion in KC24 is almost an order of magnitude slower than diffusion in metal-organic-frameworks and nanostructured carbons. The slow diffusion is caused by both strong binding interactions and steric diffusion barriers. The latter are associated with the quasi-two-dimensional confinement of the hydrogen and with the volume blocked by potassiums. Molecular dynamics simulations yield self-diffusion coefficients in reasonable agreement with experimental ones. Additional measurements on the high flux backscattering spectrometer reveal an even slower, secondary hydrogen diffusion process in KC24 over the same temperature range. Together, these results provide direct information on the interaction of hydrogen with a carbon slit-pore and an alkali-metal dopant, both topics of interest to the hydrogen storage community.

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