Biomedical Engineering Reference
In-Depth Information
GCMC simulations have been used also to study how hydrogen
physisorption onto CNTs is affected by oxidation or by hydrogen
chemisorptions [106] showing that oxidation should favor the
endohedral physical adsorption thus increasing both the volumetric
and the gravimetric densities.
The theoretical limits of the hydrogen physical adsorption in
SWCNTs have been discussed by Bahtia and Myers [12] who recast
the problem as a “delivery” one involving storage and release. By
combining GCMC simulations with thermodynamic predictions,
it has been demonstrated that, for the measured adsorption heat
(∆
≈ 6 kJ/mol), the theoretical maximum values of the delivery
is lower than 4.6 wt.%. In this context, the authors evidenced with
persuasive arguments that the adsorption heat of H
H
0
physically
adsorbed on SWCNTs (that is related to the LJ energy parameter
e
2
≈ 0.31 K) makes pure CNTs unfit to satisfy the DOE re-
quirements; thus it is recognized that the hydrogen storage in CNTs is
still a challenging problem with unambiguous physical limitations.
Even though the above results should be corrected to account
for the tube curvature [40], they are not expected to change, at least
qualitatively, the conclusions drawn.
The binding energy of physisorbed H
/
k
C-H
B
2
has been calculated by
accurate DFT showing that both zig-zag and armchair CNTs bind
weakly the hydrogen molecules (0.049-0.113 eV) via the van der
Waals interactions [121]; the H
2
adsorption is slightly improved
on nanotube bundles because the adsorption energy and charge
transfer for the interstitial and groove sites are larger than those on
the surface site since the adsorbate interacts with many more carbon
atoms. Dag and co-workers [26] have tried to clarify the nature and
the strength of H
2
adsorption on an armchair CNT by using a hybrid
model accounting for accurate short-range interactions and Van der
Waals long-range interactions. The equilibrium configuration was
found at a distance
2
≈ 0.31 nm with a binding energy of 0.057 eV
that is almost independent on the tube curvature.
Theoretical calculations have shown that, in order to have good
delivery properties with efficient charging/discharging cycles, a
system with an adsorption heat of about 15 kJ/mol is needed [60].
Therefore some authors have tried to improve the CNT uptake
properties by doping with different species, mostly alkali and
TMs [26]. Generally speaking, if the interaction potential and the
d
0
