130
7 Molten Salt Conversion of Plastics into Highly Conductive …
demonstrate that molten salts can cause graphitization in amorphous carbon materials produced by the thermal treatment of PET. This observation can be explained
based on a combination of effects mentioned above.
Investigations have been shown that the presence of oxygen atoms in amorphous
carbon materials can be a considerable barrier against their graphitization. Also,
it is known that the effective removal of oxygen from carbon materials requires a
prolonged heat treatment at very high temperatures [86]. By comparison of the C:O
ratio in the amorphous carbon material produced at 620 °C (5.2) with that of the
nanostructured carbon material obtained at 1300 °C (28.4), a substantial reduction
in the oxygen content of the carbon material upon the molten salt treatment can be
realized. This observation can be corresponded to the dissolution of oxygen from
the carbon material into the molten salt. This facilitates the graphitization of the
amorphous carbon at such low temperatures.
It is worth mentioning that generally amorphous carbons obtained by the pyrolysis of polymers are classified as “non-graphitizable” carbons, since it is extremely
difficult to convert the disordered structure of such carbon materials into ordered
graphitic structures even at temperatures around 3000 °C. This characteristic of
non-graphitizable carbons is attributed to the existence of a complicated network of
oxygen-containing cross-links between randomly oriented crystallites [87, 88].
The potential of molten salts to eliminate impurities from carbon materials [81]
can be employed to reduce the oxygen content of non-graphitizable carbons, facilitating the graphitization of such carbons. Kim et al. [89] heat treated a non-graphitizable
carbon derived from phenol resin in molten NaOH for 1 h at 900 °C under N 2 . Then,
the NaOH content of the resulting mixture was removed by washing the mixture by
HCl solution at room temperature. Finally, the graphitization process was performed
at 2800 °C. The possible function of the molten NaOH in this process was suggested
to be the removal of gases and also reducing alkyl group content of the carbon material. The toxicity of NaOH as well as the acid leaching requirement, however, can
limit the application of this approach. Moreover, a very high-temperature treatment
was still required in order to achieve the graphitization. To reduce the graphitization temperature, Jin et al. [86, 90] die pressed an amorphous carbon black powder
into cylindrical pellets and then consolidated the pellets using porous nickel foams
or graphite containers. The consolidated pellets were immersed in molten CaCl 2 at
820 °C, and negatively polarized at the potential of −1.7 V, based on an electrochemical deoxidation method known as the FFC-Cambridge process [91]. This process
led to the graphitization of the amorphous carbon black, and this was attributed to
the removal of oxygen from the amorphous carbon material under the influence of
the cathodic polarization applied, facilitating the graphitization process. As noted in
pervious sections of this chapter, the graphitization of the PET-derived amorphous
carbons can occur by a simple molten salt heat treatment, without the involvement
of high-temperature electrochemical equipment.
The increase of the electrical conductivity occurred upon the heat treatment process can be an indication of the occurrence of the graphitization, caused due to the
development of the hexagonal stacking structures, which would facilitate the electron transfer [92]. Considering the high diffusion rate of oxygen ions in molten salts
7 Molten Salt Conversion of Plastics into Highly Conductive …
demonstrate that molten salts can cause graphitization in amorphous carbon materials produced by the thermal treatment of PET. This observation can be explained
based on a combination of effects mentioned above.
Investigations have been shown that the presence of oxygen atoms in amorphous
carbon materials can be a considerable barrier against their graphitization. Also,
it is known that the effective removal of oxygen from carbon materials requires a
prolonged heat treatment at very high temperatures [86]. By comparison of the C:O
ratio in the amorphous carbon material produced at 620 °C (5.2) with that of the
nanostructured carbon material obtained at 1300 °C (28.4), a substantial reduction
in the oxygen content of the carbon material upon the molten salt treatment can be
realized. This observation can be corresponded to the dissolution of oxygen from
the carbon material into the molten salt. This facilitates the graphitization of the
amorphous carbon at such low temperatures.
It is worth mentioning that generally amorphous carbons obtained by the pyrolysis of polymers are classified as “non-graphitizable” carbons, since it is extremely
difficult to convert the disordered structure of such carbon materials into ordered
graphitic structures even at temperatures around 3000 °C. This characteristic of
non-graphitizable carbons is attributed to the existence of a complicated network of
oxygen-containing cross-links between randomly oriented crystallites [87, 88].
The potential of molten salts to eliminate impurities from carbon materials [81]
can be employed to reduce the oxygen content of non-graphitizable carbons, facilitating the graphitization of such carbons. Kim et al. [89] heat treated a non-graphitizable
carbon derived from phenol resin in molten NaOH for 1 h at 900 °C under N 2 . Then,
the NaOH content of the resulting mixture was removed by washing the mixture by
HCl solution at room temperature. Finally, the graphitization process was performed
at 2800 °C. The possible function of the molten NaOH in this process was suggested
to be the removal of gases and also reducing alkyl group content of the carbon material. The toxicity of NaOH as well as the acid leaching requirement, however, can
limit the application of this approach. Moreover, a very high-temperature treatment
was still required in order to achieve the graphitization. To reduce the graphitization temperature, Jin et al. [86, 90] die pressed an amorphous carbon black powder
into cylindrical pellets and then consolidated the pellets using porous nickel foams
or graphite containers. The consolidated pellets were immersed in molten CaCl 2 at
820 °C, and negatively polarized at the potential of −1.7 V, based on an electrochemical deoxidation method known as the FFC-Cambridge process [91]. This process
led to the graphitization of the amorphous carbon black, and this was attributed to
the removal of oxygen from the amorphous carbon material under the influence of
the cathodic polarization applied, facilitating the graphitization process. As noted in
pervious sections of this chapter, the graphitization of the PET-derived amorphous
carbons can occur by a simple molten salt heat treatment, without the involvement
of high-temperature electrochemical equipment.
The increase of the electrical conductivity occurred upon the heat treatment process can be an indication of the occurrence of the graphitization, caused due to the
development of the hexagonal stacking structures, which would facilitate the electron transfer [92]. Considering the high diffusion rate of oxygen ions in molten salts