Biomedical Engineering Reference
In-Depth Information
,
Table 1 (I)] of desorption of two hydrogen atoms from the carbon
atom of the sorption center [model H in Fig. 2.8 (I) is much higher
than the energy (-∆
The explanation of this phenomenon is that the energy [-∆
H
(12)II
≈ 485 kJ/mol)] of detachment of this same
carbon atom from its two nearest carbon neighbors. This can be
related to the vibration contribution at 331 meV due to a dimmer of
hydrogen atoms bonding to graphite [9]. Such type of contributions
may be also attributed to the process I (Table 2.1 (I), model F in
Fig. 2.8).
As mentioned in Ref. [5], these results lead to the assumption
that atomic hydrogen could be stored in closed graphite nanotubes,
through graphene sheet walls of the nanotubes, as molecular H
H
C-C
gas,
2
and that this type of storage would be stable.
In this context, it is relevant to consider data regarding
interactions of low-kinetic-energy hydrogen atoms (0.5-30 eV) with
SWNT based on molecular dynamics and, in particular, on
ab initio
calculations [20-23]. According to these references, hydrogen atoms
with an energy of 16-25 eV are characterized by a high probability
of penetration through the side faces of closed SWNTs and of
accumulation inside these capsules in form of molecular hydrogen
(Fig. 2.24). Owing to the high mechanical strength of nanotubes,
hydrogen molecules can be concentrated therein up to volume
densities considerably exceeding the one observed for capillary
condensation (Fig. 2.24).
According to the values estimated in Ref. [21], the pressure of
molecular hydrogen embedded into SWNT can reach a value as high
as 60 GPa. This hydrogen, depending on hydrogen volume density
and pressure, condensates inside individual SWNT, and can undergo
several phase transitions, giving rise to the building of different
crystal lattices made by hydrogen molecules [22]. If the hydrogen
pressure inside a (5,5) SWNT is 37.4 GPA, the bulk sorption capacity
can reach 63 kg/m
3
, which value meets the DOE requirements [1] for
the automotive industry for the year 2015. In Refs.[23, 24], potential
ways for practical achievement of such high hydrogen sorption levels
by bundles of closed SWNTs have been suggested, together with
methods for hydrogen pull out through the walls of the nanotubes.
As a matter of fact, using core-level photo electron spectroscopy and
X-ray absorption spectroscopy, recently it has been confirmed that
5.1 wt% of hydrogen storage can be achieved by hydrogenation of
single-wall carbon nanotubes with atomic hydrogen [25, 26]. It is
