Biomedical Engineering Reference
In-Depth Information
In particular, this means that the identification of a
chemisorption process III (the TPD peak III) in a carbon material
implies the experimental determination of the thermodynamic
characteristics ∆
and the sorption Eq. (2.5), as well as the
diffusion characteristics
H
(4)III
of the process (the appropriate method
is described in Section 2.2.3).
From Eq. (5a), it is possible to estimate that the equilibrium
concentration
Q
III
l
of the dissolved hydrogen atoms in GNF
and in nanostructured graphite at 300 K and at the hydrogen pressure
in the range 1-10 MPa, approaches 0.77, or 6 wt%. This value is
defined in Refs. [1-3] as the lower limit of the sorption capacity of
adsorbent materials suitable for storing hydrogen in vehicles. The
estimated value can be confirmed, as order of magnitude, by the
analysis of the sorption data in Refs. [12, 14, 54-56].
Unfortunately, the diffusion kinetics of chemisorbed hydrogen
discharge from carbon materials at room temperature (process III)
does not meet the technological requirements [1-3] for hydrogen-
driven vehicles, mainly because of the high value of the hydrogen-
diffusion activation energy
X
=
(H/C
)
III
III
.
There is one last aspect that should be mentioned. The indirect
experimental value of enthalpy of C-H bond formation, linking
hydrogen atoms to carbon centers in graphene layers (∆
Q
III
H
= −243
(3)III
−1
± 3 kJ mol
(H)), is determined independently from the experimental
values of ∆
H
and ∆
H
via Eq. (2.7) and from the experimental
(4)III
dis
l
values of
Q
and
Q
(in accordance with Eq. (2.9)). The value of
III
∆
H
thus obtained is close to the experimental value −255 ± 1 kJ
(3)III
-1
mol
(H) [59] of the enthalpy of C-H bond formation in the fullerene
hydride C
H
(i.e., the filling of quasigraphene spherical layer is
60
36
H/C = 36/60) and to the theoretical values −(220−260) ± 20 kJ mol
−1
(H) of the energy of C-H bond formation, linking hydrogen atoms to
the graphene (cylindrical) surface of various single-wall nanotubes
with a filling factor H/C = 0.5, obtained in Ref. [57] by the density
functional method.
At the same time, the experimental value of ∆
agrees only
as order of magnitude with the theoretical value of −194 kJ mol
H
(3)III
−1
(H) [36] (the calculations have been done by
MO method,
computing the energies of atomic hydrogen chemisorption on
graphite) corresponding to the assumed model (F*) in Fig. 2.8) for
the chemisorption process III. We have the same situation when
the experimental value of ∆
ab initio
H
is compared with the theoretical
(3)III
