54
4 Cathodic Exfoliation of Graphite in Molten Salt Electrolytes
is slightly greater than that of the graphite raw material (1.14 wt%). However, the
calculated C/O ratio of the nanostructured interconnected carbon (72.5) is considerably high, in comparison with other reported graphene-based hierarchical carbon
nanostructures [13, 37–39].
The morphology of the nanostructured interconnected carbon produced is exhibited in TEM images of Fig. 4.14d–f. As it can be observed, the presence of rolled-up
graphene species incorporated between the graphene nanosheets promotes the formation of the nanostructured interconnected porous carbon. The network structure
may involve the formation of rolled-up graphene bridges between the interconnected
carbon nanosheets. This nanostructured carbon material has a large specific surface
area of around 470 m
2 g
−1 , with a high electrical conductivity of about 2 × 10
4
S m
−1 . The nanostructured carbon material produced in the molten salt system has
a much higher conductivity than that of other known hierarchical nanostructured
carbon materials [13, 40–42] and also that of the graphite raw material (1.8 × 10
4
S m
−1 ). These results indicate that the high-temperature molten salt process is capable
of fabricating a high-quality well-interconnected carbon nanostructure. Since both
the carbon nanoscrolls and graphene nanosheets have intrinsically high electrical
conductivity and large electrochemically active sites, the observed network nanostructure with well-defined pores can serve as fast ionic and electronic conducting
channels, making the nanostructured carbon material ideal to be used as electrodes
for high-performance supercapacitors. It will be discussed in Chap. 6.
4.6 Molten Salt Conversion of CO 2 into Li 2 CO 3
Nanocrystals
Molten salt process can be used for the preparation of carbon nanostructures
encapsulated with inorganic crystals such as Li 2 CO 3 . Li 2 CO 3 -encapsulated carbon
nanostructures can be used as a precursor for the preparation of nanodiamond. The
transformation of this nanostructured hybrid material into nanodiamond will be discussed in Chap. 8. This section concerns the formation of Li 2 CO 3 -encapsulated
carbon. Figure 4.15a shows the setup used for the conversion of CO 2 into Li 2 CO 3
nanocrystals and also the encapsulated carbon nanostructures.
In order to demonstrate the transformation of CO 2 to Li 2 CO 3 , a moist CO 2 flow
of 200 cm
3 min
−1 was bubbled into molten LiCl–2 wt% Li 2 O as shown in Fig. 4.15a
for 60 min, without the involvement of the graphite electrodes. Then, the furnace
was allowed to be cooled down to room temperature. In order to study the possible
conversion of carbon dioxide, the content of the crucible was exposed to boiling
deionized distilled water. It should be considered that LiCl has a considerably high
solubility of 1200 g L
−1 in water at 100 °C, while the water solubility of Li 2 CO 3
is relatively low (7 g L
−1 ) under the same condition. This contrast in water solubility allows the selective dissolution of LiCl. In order to analyze Li 2 CO 3 crystals,
gold TEM grids were immersed in the solution and then allowed to be dried. TEM
4 Cathodic Exfoliation of Graphite in Molten Salt Electrolytes
is slightly greater than that of the graphite raw material (1.14 wt%). However, the
calculated C/O ratio of the nanostructured interconnected carbon (72.5) is considerably high, in comparison with other reported graphene-based hierarchical carbon
nanostructures [13, 37–39].
The morphology of the nanostructured interconnected carbon produced is exhibited in TEM images of Fig. 4.14d–f. As it can be observed, the presence of rolled-up
graphene species incorporated between the graphene nanosheets promotes the formation of the nanostructured interconnected porous carbon. The network structure
may involve the formation of rolled-up graphene bridges between the interconnected
carbon nanosheets. This nanostructured carbon material has a large specific surface
area of around 470 m
2 g
−1 , with a high electrical conductivity of about 2 × 10
4
S m
−1 . The nanostructured carbon material produced in the molten salt system has
a much higher conductivity than that of other known hierarchical nanostructured
carbon materials [13, 40–42] and also that of the graphite raw material (1.8 × 10
4
S m
−1 ). These results indicate that the high-temperature molten salt process is capable
of fabricating a high-quality well-interconnected carbon nanostructure. Since both
the carbon nanoscrolls and graphene nanosheets have intrinsically high electrical
conductivity and large electrochemically active sites, the observed network nanostructure with well-defined pores can serve as fast ionic and electronic conducting
channels, making the nanostructured carbon material ideal to be used as electrodes
for high-performance supercapacitors. It will be discussed in Chap. 6.
4.6 Molten Salt Conversion of CO 2 into Li 2 CO 3
Nanocrystals
Molten salt process can be used for the preparation of carbon nanostructures
encapsulated with inorganic crystals such as Li 2 CO 3 . Li 2 CO 3 -encapsulated carbon
nanostructures can be used as a precursor for the preparation of nanodiamond. The
transformation of this nanostructured hybrid material into nanodiamond will be discussed in Chap. 8. This section concerns the formation of Li 2 CO 3 -encapsulated
carbon. Figure 4.15a shows the setup used for the conversion of CO 2 into Li 2 CO 3
nanocrystals and also the encapsulated carbon nanostructures.
In order to demonstrate the transformation of CO 2 to Li 2 CO 3 , a moist CO 2 flow
of 200 cm
3 min
−1 was bubbled into molten LiCl–2 wt% Li 2 O as shown in Fig. 4.15a
for 60 min, without the involvement of the graphite electrodes. Then, the furnace
was allowed to be cooled down to room temperature. In order to study the possible
conversion of carbon dioxide, the content of the crucible was exposed to boiling
deionized distilled water. It should be considered that LiCl has a considerably high
solubility of 1200 g L
−1 in water at 100 °C, while the water solubility of Li 2 CO 3
is relatively low (7 g L
−1 ) under the same condition. This contrast in water solubility allows the selective dissolution of LiCl. In order to analyze Li 2 CO 3 crystals,
gold TEM grids were immersed in the solution and then allowed to be dried. TEM