Spillover of hydrogen on nanostructured carbons is a phenomenon that is critical to understand in order to produce efficient hydrogen storage adsorbents for fuel cell applications.The spillover and interaction of atomic hydrogen with single-walled carbon nanotubes (SWNTs) is the focus of this combined theoretical and experimental work.To understand the spillover mechanism,very low occupancies (i.e.,1 and 2 H atoms adsorbed) on (5,0),(7,0),(9,0) zigzag (semiconducting) SWNTs and a (5,5) armchair (metallic) SWNT,with corresponding diameters of 3.9,5.5,7.0,and 6.8 A,were investigated.The adsorption binding energy of H atoms depends on H occupancy,tube diameter,and helicity (or chirality),as well as endohedral (interior) vs exohedral (exterior) binding.Exohedral binding energies are substantially higher than endohedral binding energies due to easier sp~2-sp~3 transition in hybridization of carbon on exterior walls upon binding.A binding energy as low as -8.9 kcal/mol is obtained for 2H atoms on the exterior wall of a (5,0) SWNT.The binding energies of H atoms on the metallic SWNT are significantly weaker (about 23 kcal/mol weaker) than that on the semiconductor SWNT,for both endohedral and exohedral adsorption.The binding energy is generally higher on SWNTs of larger diameters,while its dependence on H occupancy is relatively weak except at very low occupancies.Experimental results at 298 K and for pressures up to 10 MPa with a carbon-bridged composite material containing SWNTs demonstrate the presence of multiple adsorption sites based on desorption hysteresis for the spillover H on SWNTs,and the experimental results were in qualitative agreement with the molecular orbital calculation results.
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