7.5 Electrical and Electrochemical Characterization …
129
The electrochemical behavior of the nanostructured carbon produced by the reactive molten salt treatment of PET was examined using a three-electrode system with
6 M KOH as the electrolyte [24]. The CV profiles were recorded at various scan rates
in the range of 5–200 mV s
−1 , and the results are shown in Fig. 7.13c. The CV curves
exhibit nearly rectangular shapes, indicating that the nanostructured carbon stores
the electric charge via the electrochemical double-layer capacitive mechanism, characterized by a reasonably high rate performance with no pseudo-capacitive effect.
The high purity of the nanostructured carbon and its high electrical conductivity can
also be realized. The potential-time profiles evaluated at various current densities
in the range of 0.2–20 A g
−1 are shown in Fig. 7.13d. The high reversibility of
the electric charge storage can be realized from the quasi-symmetric feature of the
charge—discharge profiles shown in this figure [24, 78–80].
7.6 Molten Salt Graphitization of Amorphous Carbons
As discussed in the previous sections, the molten salt treatment of waste plastic bottles
can be recognized as an effective strategy for large-scale preparation of nanostructured carbon materials with a high electrical conductivity and large specific surface
area. In this strategy, the mixture of plastic and NaCl is heated in air to a critical
temperature. During the heat treatment process, at about 260 °C, the plastic material
melts, and then the melt decomposes at about 470 °C to form carbon particles with
an amorphous structure, smooth surfaces, sharp edges and high resistance against
thermal oxidation in air. This amorphous carbon material remains stable, without
experiencing a considerable oxidation, until the melting point of the salt (~800 °C)
is reached. The molten salt formed protects the PET-derived carbon material from
oxidation at higher temperatures. Meanwhile, the molten salt causes the graphitization of the amorphous carbon. The signs of graphitization/exfoliation appear at
about 1100 °C, preferentially at the edge sites of amorphous carbon particles. The
promotion of this process at higher temperatures leads to the formation of exfoliated
graphene-like nanosheets with interesting properties such as high crystallinity, electrical conductivity, purity and surface area. This carbon material can be extracted
from its mixture with the solidified salt and the salt itself can be recycled, enhancing the economic and environmental sustainability of the process. The capability of
molten salts in protecting nanostructured carbon materials against thermal oxidation has been demonstrated [81]. It was reported that multiwalled carbon nanotubes
[82] and 3D graphene nanosheets [83, 84] produced by the electrochemical exfoliation of high-purity graphite in molten LiCl, and molten NaCl possess a greater
degree of crystallinity in comparison with their mother graphite. The high crystallinity observed was scribed to the healing of the structural defects, including the
removal of impurities from the graphite exposed to the molten salt and also the
improvement of the graphite lattice stacking order [81, 85]. Altogether, the findings
129
The electrochemical behavior of the nanostructured carbon produced by the reactive molten salt treatment of PET was examined using a three-electrode system with
6 M KOH as the electrolyte [24]. The CV profiles were recorded at various scan rates
in the range of 5–200 mV s
−1 , and the results are shown in Fig. 7.13c. The CV curves
exhibit nearly rectangular shapes, indicating that the nanostructured carbon stores
the electric charge via the electrochemical double-layer capacitive mechanism, characterized by a reasonably high rate performance with no pseudo-capacitive effect.
The high purity of the nanostructured carbon and its high electrical conductivity can
also be realized. The potential-time profiles evaluated at various current densities
in the range of 0.2–20 A g
−1 are shown in Fig. 7.13d. The high reversibility of
the electric charge storage can be realized from the quasi-symmetric feature of the
charge—discharge profiles shown in this figure [24, 78–80].
7.6 Molten Salt Graphitization of Amorphous Carbons
As discussed in the previous sections, the molten salt treatment of waste plastic bottles
can be recognized as an effective strategy for large-scale preparation of nanostructured carbon materials with a high electrical conductivity and large specific surface
area. In this strategy, the mixture of plastic and NaCl is heated in air to a critical
temperature. During the heat treatment process, at about 260 °C, the plastic material
melts, and then the melt decomposes at about 470 °C to form carbon particles with
an amorphous structure, smooth surfaces, sharp edges and high resistance against
thermal oxidation in air. This amorphous carbon material remains stable, without
experiencing a considerable oxidation, until the melting point of the salt (~800 °C)
is reached. The molten salt formed protects the PET-derived carbon material from
oxidation at higher temperatures. Meanwhile, the molten salt causes the graphitization of the amorphous carbon. The signs of graphitization/exfoliation appear at
about 1100 °C, preferentially at the edge sites of amorphous carbon particles. The
promotion of this process at higher temperatures leads to the formation of exfoliated
graphene-like nanosheets with interesting properties such as high crystallinity, electrical conductivity, purity and surface area. This carbon material can be extracted
from its mixture with the solidified salt and the salt itself can be recycled, enhancing the economic and environmental sustainability of the process. The capability of
molten salts in protecting nanostructured carbon materials against thermal oxidation has been demonstrated [81]. It was reported that multiwalled carbon nanotubes
[82] and 3D graphene nanosheets [83, 84] produced by the electrochemical exfoliation of high-purity graphite in molten LiCl, and molten NaCl possess a greater
degree of crystallinity in comparison with their mother graphite. The high crystallinity observed was scribed to the healing of the structural defects, including the
removal of impurities from the graphite exposed to the molten salt and also the
improvement of the graphite lattice stacking order [81, 85]. Altogether, the findings