Dependence of hydrogen storage characteristics of mechanically milled carbon materials on their host structures

We investigated whether the hydrogen storage characteristics of carbon materials prepared by mechanical milling in an H-2 atmosphere were dependent on their host structures. We used natural graphite (NG) and activated carbon fibers (ACF) and compared them with activated carbon (AC) powders. The XRD patterns of NG and ACF milled for over 20h and SEM images of these samples milled for 80h were almost the same as those of AC. The hydrogen storage capacities of NG and ACF estimated by the inert gas fusion-thermal conductivity method increased with the mechanical milling time up to 10h and showed little milling time dependence thereafter. The capacities of NG and ACF reached about 3.0wt.% and were similar to that of AC. However, it should be noted that the hydrogen storage mechanism of NG and ACF mechanically milled in an H-2 atmosphere might be different because the changes in their specific surface areas with milling time were opposite. Thermal desorption mass spectroscopy (TDS) revealed that the desorption spectra of the hydrogen molecules (mass number = 2) of NG and ACF milled for 10h in the same way as AC contained two peaks at about 500 and 800degreesC. The desorption activation energies of hydrogenated NG and ACF at these peaks calculated from a Kissinger plot were almost with the same as those of hydrogenated AC. This suggests that the state of the hydrogen trapped in the structural defects in NG introduced by the mechanical milling may be almost the same as that of AC. In addition, we assumed the possibility that the state of the hydrogen in ACF hydrogenated by mechanical milling could be almost the same as that in hydrogenated AC. We considered that the nanocarbon materials hydrogenated under our milling conditions had very similar physical shapes and hydrogen storage capacities, independent of their host structures. (C) 2003 Elsevier B.V. All rights reserved.