Biomedical Engineering Reference
In-Depth Information
cycle so reducing the hydrogen physisorption and storage. The same
authors have demonstrated also that Li
molecules can bind
up to 60 hydrogen molecules resulting in a gravimetric density of
13 wt.% with a nearly constant binding energy [97].
Using
C
12
60
B3LYP/3-21G(d,p) calculations, Chandrakumar
and Gosh have studied the C
ab initio
decoration with alkali metals of
the first group (Li, Na, and K) [22]. Due to their electron affinity,
alkali metals are positively charged with respect to fullerenes and
therefore can bind up to six (Li) or eight (Na and K) H
60
molecules.
Concerning alkali metals of the second group, Yoon and co-workers
have evidenced by DFT calculations that the binding of Ca and Sr
atoms to C
2
is accompanied by a strong electric field [118] that
can easily polarize H
60
molecules. Therefore the hydrogen uptake
enhancement (up to 8.4 wt.%) originates, in this case, from a totally
different mechanism with respect to the one occurring for TMs.
Carbon nanocones (CNCs) have been investigated as possible
alternatives to CNTs for hydrogen storage mainly because of their
peculiar topology [70]. Hydrogen adsorption isotherms at 77 K
in CNCs with different apex angles have been calculated by GCMC
simulations [39]; C-H
2
interactions are treated with second-order
Feynman-Hibbs LJ potentials showing that molecular hydrogen can
be confined in the apex region inside the cone, in agreement with
recent findings from neutron spectroscopy of H
2
interacting with
CNHs [30]. The theoretical data demonstrate that the hydrogen
uptake is larger in CNCs than in CNTs, which is attributed mainly to
the high-interaction region close to the apex.
2
8.4.2
Gas Physical Adsorption in Carbon Nanostructures
Noble gases are considered as important case studies for adsorption
in carbonaceous nanostructures; however, due to the specific interest
for low temperature physics, their condensation in nanostructured
carbon will be omitted. Moreover we will treat explicitly the theme
of methane adsorption in carbonaceous nanostructures that is
attracting a growing interest because of advantageous properties
for alternative automotive energy sources. Methane, indeed, can
be efficiently stored in nanostructured carbon because of its high
physisorption binding energy making it attractive for storage at RT
and moderate pressure.
