56
4 Cathodic Exfoliation of Graphite in Molten Salt Electrolytes
niobates [47] and titanates [48]. The fabrication of Li 2 CO 3 in LiCl melt by the occurrence of the reaction (4.1) is generally accepted in the literature [49–51]. The Li 2 CO 3
nanocrystals formed, however, are very difficult to be evidenced, since the Li 2 CO 3
formed has a high solubility in molten LiCl at temperatures greater than 730 °C [52].
It should also be mentioned that the solubility of Li 2 CO 3 in LiCl decreases with the
temperature and eventually becomes negligible at room temperature [52].
4.7 Encapsulation of Li 2 CO 3 Nanocrystals in Carbon
Layers
Lithium carbonate nanocrystals formed in the process explained in 4.6 can be encapsulated in graphitic layers. For this, a graphite rod was used as the carbon source.
The rod had a well-defined graphitic structure with an average crystalline size of
36 nm and contained graphite flakes of several micrometers in size. After 10 min
of bubbling the moist CO 2 gas into the melt (Fig. 4.15a), the graphite rod and a
Pt tube, which were both already immersed in the molten salt, were connected to
the negative and positive terminals of a power supply, respectively, and a potential
difference of 5 V was applied between the electrodes, corresponding to an electric
current of 35 A. Under this condition, H
+ cations, arisen from the ionization of HCl
in the LiCl melt, were neutralized on the graphite cathode to form hydrogen atoms.
The atomic hydrogen formed could easily intercalate into the interlayer spaces of the
graphite cathode. Such an intercalation can occur, considering that the size of hydrogen atoms (0.5 A) is considerably smaller than the average interlayer space of the
graphite material (3.35 A). Furthermore, the high diffusion coefficient of hydrogen in
graphite at 800 °C (3.3 × 10
−5 cm
2 s
−1 ) is five-order of magnitude greater than that
at room temperature [53]. The subsequent combination of the already intercalated
hydrogen atoms within the graphite lattice could lead to the formation of H 2 gas. The
gas generated will have enough kinetic energy to exfoliate the graphite lattice into
graphene nanosheets. The details of this process will be discussed in Chap. 5. The
graphene nanosheets formed are released in the bulk of LiCl melt, which already
contains dissolved Li 2 CO 3 formed by the occurrence of the reaction (4.1). The subsequent cooling of the molten salt after the electrolysis leads to the reduction of
the Li 2 CO 3 solubility and hence the formation of Li 2 CO 3 nanocrystals. Graphene
nanosheets available in the melt could then easily wrap the Li 2 CO 3 nanoparticles
in order to minimize their surface energy. This process results in the formation of
carbon-encapsulated Li 2 CO 3 nanostructures, as shown in Fig. 4.15. SEM micrograph
of this material (Fig. 4.15d ) exhibits a hierarchical-like morphology comprising 3D
assemblies of nanoparticles, containing Li 2 CO 3 nanocrystals with a size of 5–30 nm
tightly sealed by 2–5-nm-thick graphitic layers. The (002) planes of the crystalline
carbon, wrapping the Li 2 CO 3 nanoparticles, can clearly be observed in the HRTEM
micrograph of Fig. 4.15e [43]. This core–shell hybrid nanostructured material can be
employed as a precursor to produce nanodiamonds, as will be discussed in Chap. 8.
4 Cathodic Exfoliation of Graphite in Molten Salt Electrolytes
niobates [47] and titanates [48]. The fabrication of Li 2 CO 3 in LiCl melt by the occurrence of the reaction (4.1) is generally accepted in the literature [49–51]. The Li 2 CO 3
nanocrystals formed, however, are very difficult to be evidenced, since the Li 2 CO 3
formed has a high solubility in molten LiCl at temperatures greater than 730 °C [52].
It should also be mentioned that the solubility of Li 2 CO 3 in LiCl decreases with the
temperature and eventually becomes negligible at room temperature [52].
4.7 Encapsulation of Li 2 CO 3 Nanocrystals in Carbon
Layers
Lithium carbonate nanocrystals formed in the process explained in 4.6 can be encapsulated in graphitic layers. For this, a graphite rod was used as the carbon source.
The rod had a well-defined graphitic structure with an average crystalline size of
36 nm and contained graphite flakes of several micrometers in size. After 10 min
of bubbling the moist CO 2 gas into the melt (Fig. 4.15a), the graphite rod and a
Pt tube, which were both already immersed in the molten salt, were connected to
the negative and positive terminals of a power supply, respectively, and a potential
difference of 5 V was applied between the electrodes, corresponding to an electric
current of 35 A. Under this condition, H
+ cations, arisen from the ionization of HCl
in the LiCl melt, were neutralized on the graphite cathode to form hydrogen atoms.
The atomic hydrogen formed could easily intercalate into the interlayer spaces of the
graphite cathode. Such an intercalation can occur, considering that the size of hydrogen atoms (0.5 A) is considerably smaller than the average interlayer space of the
graphite material (3.35 A). Furthermore, the high diffusion coefficient of hydrogen in
graphite at 800 °C (3.3 × 10
−5 cm
2 s
−1 ) is five-order of magnitude greater than that
at room temperature [53]. The subsequent combination of the already intercalated
hydrogen atoms within the graphite lattice could lead to the formation of H 2 gas. The
gas generated will have enough kinetic energy to exfoliate the graphite lattice into
graphene nanosheets. The details of this process will be discussed in Chap. 5. The
graphene nanosheets formed are released in the bulk of LiCl melt, which already
contains dissolved Li 2 CO 3 formed by the occurrence of the reaction (4.1). The subsequent cooling of the molten salt after the electrolysis leads to the reduction of
the Li 2 CO 3 solubility and hence the formation of Li 2 CO 3 nanocrystals. Graphene
nanosheets available in the melt could then easily wrap the Li 2 CO 3 nanoparticles
in order to minimize their surface energy. This process results in the formation of
carbon-encapsulated Li 2 CO 3 nanostructures, as shown in Fig. 4.15. SEM micrograph
of this material (Fig. 4.15d ) exhibits a hierarchical-like morphology comprising 3D
assemblies of nanoparticles, containing Li 2 CO 3 nanocrystals with a size of 5–30 nm
tightly sealed by 2–5-nm-thick graphitic layers. The (002) planes of the crystalline
carbon, wrapping the Li 2 CO 3 nanoparticles, can clearly be observed in the HRTEM
micrograph of Fig. 4.15e [43]. This core–shell hybrid nanostructured material can be
employed as a precursor to produce nanodiamonds, as will be discussed in Chap. 8.