The density functional theory (DFT) is used to estimate hydrogen adsorption in single-walled carbon nanotube (SWNT) arrays. Gravimetric storage density, excess gravimetric storage density, and volumetric storage capacity of hydrogen inside the tubes and in the interstices formed by external surfaces of the neighboring carbon tubes in square- packed SWNT arrays have been investigated for the tube diameters of 1. 225 nm, 2.04 nm and 2.719 nm at 77 K and 300 K. The results of DFT calculations indicate that adsorption in the interstices of large pores is not negligible even when the interstitial spacing is set to 0. 334 nm. The calculations indicate that the storage capacity of square arrays of SWNT for a range of configurations exceed 6.5 wt%, the target set by the Department of Energy (DOE) of US, at 77 K. The gravimetric hydrogen storage for 2.719 nm SWNT arrays at 77 K and 6 MPa can reach 13.2 wt%, about two times of the target, while the volumetric capacity is below but near to the equivalent volumetric density of the target. However, the gravimetric hydrogen storage of 1. 5 wt% is found at 300 K, far below the DOE standard. Our results suggest that SWNT arrays are promising hydrogen storage material at low temperature, in particular. In addition, the storage capacities from the DFT calculations for SWNT arrays at 77 K and 300 K are in reasonable agreement with experimentally measured capacities.

Key words: HYDROGEN, ADSORPTION, CARBON, NANOPHASE MATERIALS