A wide variety of carbon materials has been used in the present work including: multi-wall carbon nanotubes (provided by Professor Hui-Ming Cheng, Institute of Metal Research Chinese Academy of Science, China); chemically activated multi-wall carbon nanotubes1; commercially available vapor grown carbon nanofibers (sample NF); sample NF after chemical activation with KOH; a series of activated carbons prepared from a Spanish anthracite and from Subituminous coal by chemical activation with KOH as described by D. Lozano-Castello et al.2'3; commercially pitch-based carbon fiber (sample CF) from Kureha Company; commercially available activated carbons AX-21 from Anderson Carbon Co., Maxsorb3000 from Kansai Coke & Chemicals and commercial activated carbon fibers A20 from Osaka Gas Co.

Porous texture characterization of all the samples was performed by physical adsorption of N2 at 77 K and CO2 at 273 K, using an automatic adsorption system (Autosorb-6, Quantachrome). The micropore volume was determined by application of Dubinin-Radushkevich equation to the N2 adsorption isotherm at 77 K up to P/P0 < 0.1 (Vjup (N2)). The volume of narrow micropores (mean pore size lower than 0.7 nm) was calculated from CO2 adsorption at 273 K (V^p (CO2)).

The packing density of the materials was determined by pressing a given amount of activated carbon (0.5 g approximately) in a mould at a pressure of 550 kg/cm2.4 The measurements were repeated several times. The densities obtained have an error smaller than 3%.

Hydrogen adsorption measurements were carried out at 298 and 77 K both at high pressures. Hydrogen isotherms at 298 K were carried out in an automatic volumetric apparatus designed and built up in our laboratory to perform hydrogen isotherms up to 20 MPa following the experimental process described previously.5 The bulk gas amounts have been calculated by the equation of state of Modified-Benedic-Webb-Rubin,6 and the cell volume has been calculated taken into account the procedure described in the literature.

Hydrogen isotherms at 77 K and up to 4 MPa were carried out in a DMT high-pressure microbalance following the experimental process described previously.5 The experimental results were corrected for buoyancy effects related to the displacement of gas by the sample, sample holder and pan.8

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