Environmental Engineering Reference
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FIGURE 7.1 Hydrogen storage density in physisorbed materials, metal/complex, and chemical
hydrides. Source : Reproduced with permission from Niemann et al. [1]. (See color insert.)
density. The actual mechanism of hydrogen storage is still not well under-
stood, but is considered to involve either physisorption via van der Waals
attractive forces or chemisorption via strong interaction or bonding between
hydrogen and carbon atoms. Physisorption of hydrogen limits the hydrogen-
to-carbon ratio to less than one hydrogen atom per two carbon atoms (i.e.,
4.2 mass %), whereas chemisorption allows a higher hydrogen to carbon
ratio, with the ratio of two hydrogen atoms per one carbon atom reported in
the case of polyethylene [3, 4]. Physisorbed hydrogen has a binding energy
normally on the order of 0.1  eV, while chemisorbed hydrogen has C-H
covalent bonding, with a binding energy of more than 2-3 eV.
The first report of hydrogen storage using carbon nanotubes appeared in
1997 [5], which stimulated significant interest in this topic. Both single-
walled and multiwalled CNTs have been studied, with hydrogen storage
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