Biomedical Engineering Reference
In-Depth Information
8.4.1
Hydrogen Physical Adsorption in Carbon
Nanostructures
In order to evaluate the hydrogen storage performance of carbon
nanostructures, one should always refer to the target indicated
by the U.S. DOE of 6.5 wt.% hydrogen as the minimum extractable
loading for addressing the commercial storage needs.
A typical parameter used to measure the storage is the gravimetric
excess (excess hydrogen adsorption):
m
-
m
0
H
H
E
(8.11)
2
2
a
0
m
-
m
m
H
H
SWCNTs
2
2
Hydrogen physisorption is expected to exhibit a faster
adsorption-desorption thermodynamic cycles with respect to
chemisorption due to the lower adsorption energy; however, severe
limitation emerged from the theoretical and experimental values of
gravimetric and volumetric excess available.
The theoretical studies on physical adsorption in CNTs or
fullerenes could benefit of the well known atomistic arrangement in
these nanostructures, so that, in this case, the observed discrepancies
with respect to experimental data are attributed, apart from the limits
inherent the model and the simulation techniques, to impurities or
samples inhomogeneity.
Moreover, complex carbon nanostructures, such as AC or
microporous carbon (MPC), are particularly challenging due to
the lack of satisfactory and reliable modeling of their atomistic
structure.
8.4.1.1
CNTs
CNTs have been considered as ideal candidates to fulfill the DOE
requirements in the light of some experimental results supporting
this idea [28]. Since then, many theoretical and experimental articles
appeared with controversial results providing a large spread of
experimental data.
Recent review papers [48, 67, 68] have evidenced this dispersion,
particularly for early adsorption measurements as reported in
Fig. 8.1. It is evidenced that the improvement of CNT technology
and new purification strategies have reduced the data dispersion
evidencing low storage capacity values of CNTs.
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