Porosity, surface area, surface energy, and hydrogen adsorption in nanostructured carbons

Hydrogen adsorption isotherms at 77, 87, and 298 K have been measured on three samples of single-wall carbon nanotubes. The highest adsorption capacity (1.58 wt % at 77 K, 1.15 wt % at 87 K, and 0.02 wt % at 298 K) at atmospheric pressure has been observed in a chemically activated sample (activated with KOH), which has hybrid porosity between a carbon nanotube material and a microporous activated carbon. According to CO2 adsorption at 273 K and density functional theory pore size distributions from N2 adsorption, it is deduced that pores up to approximately 0.5−0.7 nm can adsorb hydrogen at ambient conditions. Isosteric heat of hydrogen adsorption has been calculated for the three samples, having initial values around 7−7.5 kJ mol-1. It is concluded that the hydrogen adsorption capacity of carbon nanotubes depends both on the extent of their surface area and on the adsorption energy of the surface sites.This work was supported by the CSIC Fuel Cell Network and the Spanish MCYT Project NANOENER MCYT; MAT2002-04630-C02-01.Peer reviewe