62
5 Mechanisms Involved in the Electrolytic Fabrication …
Experimental results revealed that the molten salt exfoliation of graphite electrodes immersed in the LiCl melt leads to the increase of the temperature of molten
salt, for example, from 770 to 800 °C. This observation can be attributed to the
occurrence of exothermic reactions such as (5.1). The electrochemical formation of
lithium carbide during the electrolysis of LiCl melt by graphite cathodes may be
explained as reaction (5.2) [1].
2LiCl + 2C = Li 2 C 2 + Cl 2 (g) E
◦
800 ◦ C = −3.14V H
◦
800 ◦ C = 607kJ
(5.2)
The use of standard electrochemical potential (E°), which is an intensive property
of the reaction, provides a tool to evaluate the possibility of occurring the reaction
(5.2). During the production of carbon nanostructures in LiCl melt under a dry argon
atmosphere, the typical cell potential was measured to be about 8 V, which is sufficient
for the formation of Li 2 C 2 . Lithium carbide formed during the electrolysis can be
washed away during the washing step according to the reaction (5.3) [1]:
Li 2 C 2 + 2H 2 O = 2LiOH(aq) + 2C + H 2 (g) )G
◦
25 ◦ C
= −347 kJ H
◦
25 ◦ C = −339 kJ
(5.3)
The formation of carbon nanoparticles, therefore, can be attributed to the formation of lithium carbides on the surface of the flakes, leading to the electrochemical
etching of graphite flakes, followed by removing the carbides formed either by dissolving into the LiCl melt, or by reacting with water during the washing process.
This subsequently leads to the disintegration of the graphite fakes into graphitic carbon nanoparticles containing a proportion of carbon atoms with amorphous bonding
structure [1].
At higher current densities (higher than 0.6 A cm
−2 ), however, the intercalation of
lithium electrodeposited on the graphite’s surface from the molten lithium chloride
seems to play a significant role in the cathodic exfoliation. The mechanism of this
process can be explained by the discharge of lithium cations on the cathode and
their subsequent intercalation into the graphite between the graphene layers under
the effect of the cathodic potential applied [2–4]. It is worthy to mention that the
atomic size of lithium is similar to the interlamellar spacing in graphite. Therefore,
the high-temperature diffusion of lithium in graphite might generate sufficient stress
to extrude carbon sheets from the graphite material into the molten salt where the
sheets have the chance to roll up in order to minimize their surface area exposed to
the melt. The intercalation of Li into graphite cathodes can also take place in molten
LiOH at 600 °C leading the exfoliation of graphite. The exfoliation rate, however,
was reported to be much lower than that observed in molten LiCl [5].
Xu et al. measured the cyclic voltammograms related to the reduction of lithium
from molten LiCl at 625 °C on molybdenum and graphite working electrodes, and the
results are shown in Fig. 5.1a, b, respectively [6]. Table 5.1 summarizes the various
cathodic and anodic events that can be observed in the cyclic voltammograms. The
results suggest that lithium ions can deposit on sufficiently cathodically polarized
5 Mechanisms Involved in the Electrolytic Fabrication …
Experimental results revealed that the molten salt exfoliation of graphite electrodes immersed in the LiCl melt leads to the increase of the temperature of molten
salt, for example, from 770 to 800 °C. This observation can be attributed to the
occurrence of exothermic reactions such as (5.1). The electrochemical formation of
lithium carbide during the electrolysis of LiCl melt by graphite cathodes may be
explained as reaction (5.2) [1].
2LiCl + 2C = Li 2 C 2 + Cl 2 (g) E
◦
800 ◦ C = −3.14V H
◦
800 ◦ C = 607kJ
(5.2)
The use of standard electrochemical potential (E°), which is an intensive property
of the reaction, provides a tool to evaluate the possibility of occurring the reaction
(5.2). During the production of carbon nanostructures in LiCl melt under a dry argon
atmosphere, the typical cell potential was measured to be about 8 V, which is sufficient
for the formation of Li 2 C 2 . Lithium carbide formed during the electrolysis can be
washed away during the washing step according to the reaction (5.3) [1]:
Li 2 C 2 + 2H 2 O = 2LiOH(aq) + 2C + H 2 (g) )G
◦
25 ◦ C
= −347 kJ H
◦
25 ◦ C = −339 kJ
(5.3)
The formation of carbon nanoparticles, therefore, can be attributed to the formation of lithium carbides on the surface of the flakes, leading to the electrochemical
etching of graphite flakes, followed by removing the carbides formed either by dissolving into the LiCl melt, or by reacting with water during the washing process.
This subsequently leads to the disintegration of the graphite fakes into graphitic carbon nanoparticles containing a proportion of carbon atoms with amorphous bonding
structure [1].
At higher current densities (higher than 0.6 A cm
−2 ), however, the intercalation of
lithium electrodeposited on the graphite’s surface from the molten lithium chloride
seems to play a significant role in the cathodic exfoliation. The mechanism of this
process can be explained by the discharge of lithium cations on the cathode and
their subsequent intercalation into the graphite between the graphene layers under
the effect of the cathodic potential applied [2–4]. It is worthy to mention that the
atomic size of lithium is similar to the interlamellar spacing in graphite. Therefore,
the high-temperature diffusion of lithium in graphite might generate sufficient stress
to extrude carbon sheets from the graphite material into the molten salt where the
sheets have the chance to roll up in order to minimize their surface area exposed to
the melt. The intercalation of Li into graphite cathodes can also take place in molten
LiOH at 600 °C leading the exfoliation of graphite. The exfoliation rate, however,
was reported to be much lower than that observed in molten LiCl [5].
Xu et al. measured the cyclic voltammograms related to the reduction of lithium
from molten LiCl at 625 °C on molybdenum and graphite working electrodes, and the
results are shown in Fig. 5.1a, b, respectively [6]. Table 5.1 summarizes the various
cathodic and anodic events that can be observed in the cyclic voltammograms. The
results suggest that lithium ions can deposit on sufficiently cathodically polarized