Hydrogen Storage in Graphite Nanofibers

Abstract

Graphite nanofibers are a novel material that is produced from the dissociation of carbon-containing gases over selected metal surfaces. The solid consists of very small graphite platelets, 30−500 Å in width, which are stacked in a perfectly arranged conformation. We have discovered that the material is capable of sorbing and retaining in excess of 20 L (STP) of hydrogen per gram of carbon when the nanofibers are exposed to the gas at pressures of 120 atm at 25 °C, a value that is over an order of magnitude higher than that found with conventional hydrogen storage systems. This behavior is rationalized in terms of the unique crystalline arrangement existing within the graphite nanofiber structure, where the platelets generate a system comprised entirely of slit-shaped nanopores, in which only edge sites are exposed. Since the interplanar distance within the material is 3.37 Å, sorption of molecular hydrogen, which possesses a kinetic diameter of 2.89 Å, is a facile process owing to the short diffusion path. In addition, owing to the weak (van der Waals) bonding of the platelets, these nonrigid wall nanopores can expand to accommodate hydrogen in a multilayer configuration. Subsequent lowering of the pressure to nearly atmospheric conditions results in the release of a major fraction of the stored hydrogen at room temperature.