4.3 Production of Graphene in Molten NaCl
47
Recently, the exfoliation of graphite nanosheets in molten NaCl under the influence of H 2 containing inert atmospheres was evaluated. From this, it was discovered
that graphite is able to be exfoliated in molten NaCl to create highly crystalline 3D
graphene nanosheets with a uniform mesoporous nanostructure, high electrical conductivity of 2.1 × 10
−5 S m
−1 and thermal stability in air at temperatures below
500 °C. The SEM and TEM micrographs of graphene nanosheets created in molten
NaCl are shown in Fig. 4.9 [26]. The comparison of these images with the SEM
micrographs of Fig. 4.8 reveals the overall morphological similarity of the graphene
material produced in molten NaCl to that of produced in molten LiCl. It should
be noticed that the mechanisms suggested for the formation of graphene in molten
NaCl are appealed to be nearly the same as that in molten LiCl. In fact, the main
mechanism is based on the reduction of hydrogen cations (protons) from the melt
on the graphite surface, followed by the intercalation of hydrogen into the graphite
lattice structure. Later in Chap. 5, this will be discussed. On the other hand, the yield
of producing graphene in molten NaCl was seen to be considerably lower than that
in molten LiCl. This was blamed on the lower solubility of protons in molten NaCl
in comparison with molten LiCl. Nonetheless, it was supposed that a molten salt
electrochemical cell with a capacity of 1000 L molten NaCl (or 20 kg if the density
of molten NaCl at 900 °C is considered to be 1.94 g cm
−3 [27]) is still able to make
200 kg graphene nanosheets per day. The specific energy consumption could also be
estimated to be around 50 kW h kg
−1 [26]. Assuming the average price of electricity
in China is around 10 US cent per kW h, and that of graphite electrodes and NaCl
to be about US$2000 and US$50 per ton, respectively, we can see that the cost of
the production of one kilogram of graphene in NaCl melt in China would be around
US$12. What’s more, it should be said that salt can be retrieved after the washing
process and reused in the process. This reduces the total cost to only about US$8 per
kilogram of the graphene.
Table 4.1 provides a comparison between the electrochemical exfoliations of
graphite in molten LiCl and NaCl with typical state–of-the-art alternative electrochemical processes. We should take into account that the cathodic exfoliation of
graphite in molten salts is much faster if compared to the low-temperature electrochemical methods, indicated by a much higher value of the current density which can
be achieved. Moreover, since the graphene is produced at the cathode, a high-quality
graphene is able to be produced in a short amount of time, without occurrence of
oxidation. These attributes are compelling, particularly when looking at large-scale
production. On the other hand, like any other molten salt-based methods, the molten
salt exfoliation of graphene has its own challenges. One of these challenges is the
corrosion of the equipment and the structural materials used for operating molten
salt reactors [28–30]. Additionally, the level of expertise needed to conduct molten
salt electrochemical experiments and initial preparations could be much higher than
those in low-temperature operations. This creates a barrier for researchers to adequately employ this approach. These restrictions can be markedly expelled in largescale molten salt operations, such as the case of Hall–Héroult process for aluminum
smelting.
47
Recently, the exfoliation of graphite nanosheets in molten NaCl under the influence of H 2 containing inert atmospheres was evaluated. From this, it was discovered
that graphite is able to be exfoliated in molten NaCl to create highly crystalline 3D
graphene nanosheets with a uniform mesoporous nanostructure, high electrical conductivity of 2.1 × 10
−5 S m
−1 and thermal stability in air at temperatures below
500 °C. The SEM and TEM micrographs of graphene nanosheets created in molten
NaCl are shown in Fig. 4.9 [26]. The comparison of these images with the SEM
micrographs of Fig. 4.8 reveals the overall morphological similarity of the graphene
material produced in molten NaCl to that of produced in molten LiCl. It should
be noticed that the mechanisms suggested for the formation of graphene in molten
NaCl are appealed to be nearly the same as that in molten LiCl. In fact, the main
mechanism is based on the reduction of hydrogen cations (protons) from the melt
on the graphite surface, followed by the intercalation of hydrogen into the graphite
lattice structure. Later in Chap. 5, this will be discussed. On the other hand, the yield
of producing graphene in molten NaCl was seen to be considerably lower than that
in molten LiCl. This was blamed on the lower solubility of protons in molten NaCl
in comparison with molten LiCl. Nonetheless, it was supposed that a molten salt
electrochemical cell with a capacity of 1000 L molten NaCl (or 20 kg if the density
of molten NaCl at 900 °C is considered to be 1.94 g cm
−3 [27]) is still able to make
200 kg graphene nanosheets per day. The specific energy consumption could also be
estimated to be around 50 kW h kg
−1 [26]. Assuming the average price of electricity
in China is around 10 US cent per kW h, and that of graphite electrodes and NaCl
to be about US$2000 and US$50 per ton, respectively, we can see that the cost of
the production of one kilogram of graphene in NaCl melt in China would be around
US$12. What’s more, it should be said that salt can be retrieved after the washing
process and reused in the process. This reduces the total cost to only about US$8 per
kilogram of the graphene.
Table 4.1 provides a comparison between the electrochemical exfoliations of
graphite in molten LiCl and NaCl with typical state–of-the-art alternative electrochemical processes. We should take into account that the cathodic exfoliation of
graphite in molten salts is much faster if compared to the low-temperature electrochemical methods, indicated by a much higher value of the current density which can
be achieved. Moreover, since the graphene is produced at the cathode, a high-quality
graphene is able to be produced in a short amount of time, without occurrence of
oxidation. These attributes are compelling, particularly when looking at large-scale
production. On the other hand, like any other molten salt-based methods, the molten
salt exfoliation of graphene has its own challenges. One of these challenges is the
corrosion of the equipment and the structural materials used for operating molten
salt reactors [28–30]. Additionally, the level of expertise needed to conduct molten
salt electrochemical experiments and initial preparations could be much higher than
those in low-temperature operations. This creates a barrier for researchers to adequately employ this approach. These restrictions can be markedly expelled in largescale molten salt operations, such as the case of Hall–Héroult process for aluminum
smelting.