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Hydrogen in

Nanostructured, Carbon-Related, and Metallic Materials Andreas Züttel and Shin-ichi Orimo

Abstract Recent developments in hydrogen interaction with carbonaceous materials are reviewed in this article. The interaction is based on van der Waals attractive forces (physisorption), or the overlap of the highest occupied molecular orbitals of carbon with the hydrogen electron, overcoming the activation-energy barrier for hydrogen dissociation (chemisorption). While the physisorption of hydrogen limits the hydrogen-to-carbon ratio to less than one hydrogen atom per two carbon atoms (i.e., 4.2 mass%), in chemisorption, a ratio of two hydrogen atoms per one carbon atom is realized (e.g., in polyethylene). However, the materials with large hydrogen-to-carbon ratios only liberate the hydrogen at elevated temperature. No evidence, apart from theoretical calculations, was found for a new hydrogen-adsorption phenomenon on carbon nanotubes (CNTs), as compared with high-surface-area graphite. The curvature of CNTs and fullerenes increases the reactivity of these materials with hydrogen and leads more easily to the formation of hydrocarbons, as compared with graphite. Nanocrystalline or amorphous carbon exhibits an intermediate state for hydrogen between physisorption and chemisorption and absorbs up to one hydrogen atom per carbon atom. Nanostructured carbonaceous and metallic materials offer a large potential for hydrogen storage and must therefore be investigated in more detail. Keywords: adsorption, carbon nanotubes, chemical reactivity, hydrogen storage, nanofibers.

Preparation of Carbon Nanostructures Synthesis of Nanotubes and Nanofibers In 1991, Iijima1 described for the first time the new form of carbon called carbon nanotubes (CNTs). CNTs are rolled graphite sheets with an inner diameter ranging from 0.7 nm up to several nanometers and a length of 10–100
m. They are usually closed on both ends by a hemisphere, that is, half of a fullerene. Nanotubes formed from a single graphite layer are called single-walled nanotubes (SWNTs). Nanotubes consisting of multiple concentric graphite layers are called multiwalled nanotubes (MWNTs). The interlayer distance in a MWNT is close to the interlayer distance in graphite, which is equal to half of the unit-cell parameter c (0.5c
0.3355 nm).

MRS BULLETIN/SEPTEMBER 2002

The diameters of SWNTs vary from 0.671 nm to 3 nm, whereas MWNTs show typical diameters of 30–50 nm. The helicity of the nanotubes is usually described by the Hamada vector,2 which indicates how the graphene sheet is rolled up along a lattice vector with components (n, m). The values of the integers n and m identify the general geometry of a SWNT. Nanotubes with n
m are called “armchair”; nanotubes with either n
0 or m
0 are called “zigzag”; all others have chiral symmetry.3 SWNTs tend to agglomerate and form bundles of several tens of nanotubes. The nanotubes in the bundles are in two dimensions and close-packed, and the intertube distance is 0.334 nm.4 Carbon fibers are similar to carbon nanot