| The hydrogen storage properties of some carbon materials were studied. Graphite nanofibers (GNF) were synthesized by catalytic decomposition of ethylene and hydrogen. Catalyst supported carbon materials were prepared by impregnation process. Hydrogen desorption and adsorption properties of graphite nanofibers, single-walled carbon nanotubes (SWNT), fullerene materials and catalysts supported carbon materials were measured volumetrically using a Sievert's apparatus. The hydrogen desorption capacity of GNF was typically less than 0.2 wt%. A phase transition between crystal SWNT and a new hydride phase was found at high pressures at 80K. The phase transition was of first order, and involved the separation of the individual tubes within a rope, exposing a high surface area for hydrogen adsorption. From the change in chemical potential of the hydrogen gas upon adsorption, we were able to calculate the cohesive van der Waals energy between the tubes as 5 meV/C atom. This is much smaller than expected from previous theoretical work, and shows that defects in the crystal structure cause large suppressions of the cohesive energy. We were able to alter this cohesive energy by changing the state of the material. Over several cycles of isotherm measurements at 77 K, the hydrogen storage capacities of one of the fullerite samples increased from an initial value of 0.4 wt% for the first cycle to a capacity of 4.2 wt% for the fourth cycle. Correspondingly, the surface area increased from 0.9 m2/gm to 11 m2/gm, and showed a phase transformation, characterized by X-ray powder diffraction. By adding Ni particles onto the sample, the hydrogen storage capacity of fullerite and activated carbon sample was increased. The adsorption of hydrogen on Ni particle can not account for the total increased capacity even by assuming complete coverage of hydrogen molecules on the Ni particle surface. |
