48
4 Cathodic Exfoliation of Graphite in Molten Salt Electrolytes
Table 4.1 Characteristics of the molten salt cathodic exfoliation of graphite for the preparation
of graphene, in comparison with typical state–of-the-art alternative electrochemical methods,
reproduced from Ref. [1], copyright 2019, with permission from Elsevier
Approach
Carbon
source
Electrolyte
Product/specifications Current density
(A cm −2 )
Room
temperature
anodic exfoliation
Graphite
[61]
Acetamide, urea
and ammonium
nitrate
1–5-layer graphene, σ
= 7.6 × 10 3 S m −1
0.010
CO 2 capture and
conversion
CO 2
[31]
CaCl 2 –CaO
(850 °C)
Carbon sheets and
nanotubes
0.2–0.6
Molten salt
cathodic
exfoliation
Graphite
[4, 5]
LiCl under
Ar–4% H 2
(800 °C)
1–5-layer graphene,
450 g graphene per
liter of LiCl per day,
σ = 5.8 × 10 5 S m −1
1 A cm −2
(40 A)
Graphite
[26]
NaCl under
Ar–4%H 2
(900 °C)
1–5-layer graphene,
200 g graphene per
liter of NaCl per day,
σ = 2.1 × 10 5 S m −1
1 A cm −2
(35 A)
4.4 Molten Salt Preparation of Metal-Filled Carbon
Nanostructures
In previous sections, we discussed the molten salt preparation of carbon nanostructures by the electrochemical exfoliation/erosion of graphite electrodes in molten salts.
These molten salt-based technologies open up the opportunity for low-cost production of high-quality carbon nanostructures. One attractive feature of the molten salt
approach is its ability of producing filled carbon nanomaterials; in which an inorganic
second phase is encapsulated within graphitic shells.
Earlier investigations [32–34] demonstrated the possibility of filling molten salt—
produced carbon nanostructures with metals such as Sn. In a pioneering study in the
field, Terrones et al. [33] produced tin-filled CNTs by the electrolysis of molten
LiCl–SnCl 2 at 600 °C using a graphite cathode immersed in the melt (Fig. 4.10a).
The same group also reported on the preparation of CNTs filled with Sn–Pb alloy by
the electrolysis of molten LiCl containing 0.5% Pb and 0.5% Sn [32] (Fig. 4.10b).
However, it was realized that the electrolysis of molten LiCl containing only small
amounts of SnCl 2 using graphite electrodes leads to the formation of a small quantity of Sn-filled carbon nanostructures. On the other hand, any SnCl 2 concentration
greater than 2 wt% was found to be preventive against the formation of carbon nanostructures [24, 32–35]. For example, Fig. 4.11 shows photographs of a graphite rod
before and after being used as the cathode material during the electrolysis of molten
LiCl containing an initial SnCl 2 concentration of 30 wt%. It can be observed that only
a minor electrode erosion took place during the process. The cathodic polarization
of the graphite rod immersed in molten LiCl–SnCl 2 under a potential of around 7 V
leads to the electrodeposition of Sn from the molten salt on the graphite cathode.
4 Cathodic Exfoliation of Graphite in Molten Salt Electrolytes
Table 4.1 Characteristics of the molten salt cathodic exfoliation of graphite for the preparation
of graphene, in comparison with typical state–of-the-art alternative electrochemical methods,
reproduced from Ref. [1], copyright 2019, with permission from Elsevier
Approach
Carbon
source
Electrolyte
Product/specifications Current density
(A cm −2 )
Room
temperature
anodic exfoliation
Graphite
[61]
Acetamide, urea
and ammonium
nitrate
1–5-layer graphene, σ
= 7.6 × 10 3 S m −1
0.010
CO 2 capture and
conversion
CO 2
[31]
CaCl 2 –CaO
(850 °C)
Carbon sheets and
nanotubes
0.2–0.6
Molten salt
cathodic
exfoliation
Graphite
[4, 5]
LiCl under
Ar–4% H 2
(800 °C)
1–5-layer graphene,
450 g graphene per
liter of LiCl per day,
σ = 5.8 × 10 5 S m −1
1 A cm −2
(40 A)
Graphite
[26]
NaCl under
Ar–4%H 2
(900 °C)
1–5-layer graphene,
200 g graphene per
liter of NaCl per day,
σ = 2.1 × 10 5 S m −1
1 A cm −2
(35 A)
4.4 Molten Salt Preparation of Metal-Filled Carbon
Nanostructures
In previous sections, we discussed the molten salt preparation of carbon nanostructures by the electrochemical exfoliation/erosion of graphite electrodes in molten salts.
These molten salt-based technologies open up the opportunity for low-cost production of high-quality carbon nanostructures. One attractive feature of the molten salt
approach is its ability of producing filled carbon nanomaterials; in which an inorganic
second phase is encapsulated within graphitic shells.
Earlier investigations [32–34] demonstrated the possibility of filling molten salt—
produced carbon nanostructures with metals such as Sn. In a pioneering study in the
field, Terrones et al. [33] produced tin-filled CNTs by the electrolysis of molten
LiCl–SnCl 2 at 600 °C using a graphite cathode immersed in the melt (Fig. 4.10a).
The same group also reported on the preparation of CNTs filled with Sn–Pb alloy by
the electrolysis of molten LiCl containing 0.5% Pb and 0.5% Sn [32] (Fig. 4.10b).
However, it was realized that the electrolysis of molten LiCl containing only small
amounts of SnCl 2 using graphite electrodes leads to the formation of a small quantity of Sn-filled carbon nanostructures. On the other hand, any SnCl 2 concentration
greater than 2 wt% was found to be preventive against the formation of carbon nanostructures [24, 32–35]. For example, Fig. 4.11 shows photographs of a graphite rod
before and after being used as the cathode material during the electrolysis of molten
LiCl containing an initial SnCl 2 concentration of 30 wt%. It can be observed that only
a minor electrode erosion took place during the process. The cathodic polarization
of the graphite rod immersed in molten LiCl–SnCl 2 under a potential of around 7 V
leads to the electrodeposition of Sn from the molten salt on the graphite cathode.