Biomedical Engineering Reference
In-Depth Information
Several potential functions [94, 102] have been employed to
model the graphene-H
interactions through the “slit pore” model
[12]. Most of these studies predict similar values for the heat of
adsorption and for the excess gravimetric percentage that is well
below the DOE requirements at operating pressures and temperature.
Recently, reliable atomistic models of ACs and MPCs have been
obtained [77] using the Hybrid Reverse Monte Carlo (HRMC) scheme
[79]. In particular, the first guess of the atomistic configuration has
been defined on the basis of the experimental pore size distribution
(PSD) and pore wall thickness distribution; then, the HRMC algorithm,
based on acceptance criteria involving the total energy and the radial
distribution function (RDF), has been used in a simulated annealing
experiment involving multiple canonical ensembles. In this way, a
reliable atomistic model for AC has been obtained by minimizing
contextually both the total energy and the difference between the
experimental and the theoretical RDFs (see Fig. 8.3) [77].
On this basis, GCMC simulations with the Feynman-Hibbs (FH)
correction for the quantum dispersion effect [31, 88] have been
performed at cryogenic temperatures [76]. We remind that the
effective FH potential for interaction of H
2
with an immobile planar
2
carbon wall is
2
W H
2
U
( )
z
2
LJ
W H
W H
U
( )
z
U
( )
z
(8.13)
2
2
FH
LJ
2
24
z
where
is the LJ potential between a smooth graphitic
wall and the hydrogen molecule and has different functional forms
depending on the number of graphitic sheets included in the wall.
For the H
W H
U
( )
z
2
LJ
LJ interactions, the authors have chosen the Levesque
parameters [65], while the C-C interactions have been treated
appropriately [32, 109]. On this basis, reliable RT isotherms for
ACs and MPCs have been obtained using new LJ parameters with
an enhanced well depth (about
-H
2
2
flat
C C
e
= 37.26 K) to correct for the
increased surface polarizability occurring when H
molecules
2
approach the carbon surface.
8.4.1.3
Other Carbonaceous Structures
Other carbonaceous nanostructures, such as nanostructured
graphite, GNFs, fullerenes, nanohorns, etc. are frequently found in
the literature as potential materials for hydrogen adsorption.
