5.3 Electrochemical Erosion of Graphite Under Humid Ar
69
hydrogen atoms to create hydrogen molecules, reaction (5.7), is probable to happen in the porosity of graphite. Hence, the molecules created have a lower chance
of escaping from the graphite electrode, and consequently, progressively dissolve
into graphite material. Theoretical calculations explain that H 2 with a size of 2.5 Å
is able to diffuse in the interlayer space of graphite quicker than atomic hydrogen
[14–20]. The activation energy required for the diffusion of atomic hydrogen in the
interlayer space between graphene sheets in the hexagonal structure of graphite can
be calculated to be extremely high (5 eV). This is because hydrogen atoms are able
to bind to carbon atoms. Consequently, the probability of hydrogen intercalation in
graphite is low [15]. It is known that no direct bonding can be formed between the
molecular hydrogen and graphite. Moreover, the diffusion coefficient of molecular
hydrogen in graphite at room temperature does not follow a single straight Arrhenius line, since the diffusion of molecular hydrogen proceeds via jumps between the
nearest-neighbor adsorption sites in a random walk of the H 2 molecule in the graphite
interlayer space. However, at temperatures higher than around 200 °C, the hydrogen molecules jump mostly one-directional and about twice as much longer. This
behavior enhances the effective diffusion length of hydrogen molecules in graphite.
The diffusion coefficient of H 2 in the interlayer space of graphite (D H2 ) at 800 °C
(3.5 × 10
−4 cm
2 s
−1 ) is much greater than that at room temperature (6.9 × 10
−6 cm
2
s
−1 ). Furthermore, the activation energy required for the diffusion of H 2 in graphite
at 800 °C is relatively small at 0.19 eV [14, 15].
The thermal behavior of hydrogen in graphite lattice has been the subject of
few studies [21]. Molecular dynamics modeling of the hydrogen movement into the
graphite structure has delivered constructive information that aids in explaining the
exfoliation of graphite under the effect of hydrogen intercalation. It was presented
that the kinetic energy of H 2 intercalated into the graphite interlayer space increases
from about 23 kJ mol
−1 to 33 kJ mol
−1 as the temperature increases from 25 °C to
800 °C [15]. Since the molten salt process takes place at high temperatures, either the
diffusion rate and the kinetic energy of H 2 molecules intercalated into the graphite
cathode material are substantially high which advance the exfoliation process.
In highly oriented graphite, the binding energy between the individual graphene
layers can be estimated to be around 19 J m
−2 . Therefore, the energy needed to isolate
one graphene layer from its graphite mother can be found to be in the range of 0.26–
0.32 J m
−2 [21, 22]. The values of the exfoliation energy, the density and the interlayer
space of typical graphite materials used in the molten salt exfoliation process can be
assumed to be 0.31 J m
−2 [23], 2.7 g cm
−3 and 0.34 nm, respectively. Based on this
information, the presence of H 2 in graphite at local concentrations of greater than 2
wt% can provide adequate kinetic energy to overcome the van der Waals adhesion
force which occurs between the individual layers of the graphite, leading to the
exfoliation of the material into graphene nanosheets. One important thing to mention
is that the solubility of hydrogen molecules in graphite can theoretically be as high
as 6 wt% [24]. The cathodic polarization of the graphite electrodes in the presence
of protons in the molten salt can thus offer the likelihood of surpassing this critical
concentration, thus bringing about the exfoliation of graphite. Moreover, it should
be noted that in the presence of a humid Ar atmosphere, the as-synthesized graphene
69
hydrogen atoms to create hydrogen molecules, reaction (5.7), is probable to happen in the porosity of graphite. Hence, the molecules created have a lower chance
of escaping from the graphite electrode, and consequently, progressively dissolve
into graphite material. Theoretical calculations explain that H 2 with a size of 2.5 Å
is able to diffuse in the interlayer space of graphite quicker than atomic hydrogen
[14–20]. The activation energy required for the diffusion of atomic hydrogen in the
interlayer space between graphene sheets in the hexagonal structure of graphite can
be calculated to be extremely high (5 eV). This is because hydrogen atoms are able
to bind to carbon atoms. Consequently, the probability of hydrogen intercalation in
graphite is low [15]. It is known that no direct bonding can be formed between the
molecular hydrogen and graphite. Moreover, the diffusion coefficient of molecular
hydrogen in graphite at room temperature does not follow a single straight Arrhenius line, since the diffusion of molecular hydrogen proceeds via jumps between the
nearest-neighbor adsorption sites in a random walk of the H 2 molecule in the graphite
interlayer space. However, at temperatures higher than around 200 °C, the hydrogen molecules jump mostly one-directional and about twice as much longer. This
behavior enhances the effective diffusion length of hydrogen molecules in graphite.
The diffusion coefficient of H 2 in the interlayer space of graphite (D H2 ) at 800 °C
(3.5 × 10
−4 cm
2 s
−1 ) is much greater than that at room temperature (6.9 × 10
−6 cm
2
s
−1 ). Furthermore, the activation energy required for the diffusion of H 2 in graphite
at 800 °C is relatively small at 0.19 eV [14, 15].
The thermal behavior of hydrogen in graphite lattice has been the subject of
few studies [21]. Molecular dynamics modeling of the hydrogen movement into the
graphite structure has delivered constructive information that aids in explaining the
exfoliation of graphite under the effect of hydrogen intercalation. It was presented
that the kinetic energy of H 2 intercalated into the graphite interlayer space increases
from about 23 kJ mol
−1 to 33 kJ mol
−1 as the temperature increases from 25 °C to
800 °C [15]. Since the molten salt process takes place at high temperatures, either the
diffusion rate and the kinetic energy of H 2 molecules intercalated into the graphite
cathode material are substantially high which advance the exfoliation process.
In highly oriented graphite, the binding energy between the individual graphene
layers can be estimated to be around 19 J m
−2 . Therefore, the energy needed to isolate
one graphene layer from its graphite mother can be found to be in the range of 0.26–
0.32 J m
−2 [21, 22]. The values of the exfoliation energy, the density and the interlayer
space of typical graphite materials used in the molten salt exfoliation process can be
assumed to be 0.31 J m
−2 [23], 2.7 g cm
−3 and 0.34 nm, respectively. Based on this
information, the presence of H 2 in graphite at local concentrations of greater than 2
wt% can provide adequate kinetic energy to overcome the van der Waals adhesion
force which occurs between the individual layers of the graphite, leading to the
exfoliation of the material into graphene nanosheets. One important thing to mention
is that the solubility of hydrogen molecules in graphite can theoretically be as high
as 6 wt% [24]. The cathodic polarization of the graphite electrodes in the presence
of protons in the molten salt can thus offer the likelihood of surpassing this critical
concentration, thus bringing about the exfoliation of graphite. Moreover, it should
be noted that in the presence of a humid Ar atmosphere, the as-synthesized graphene