134
7 Molten Salt Conversion of Plastics into Highly Conductive …
On the other hand, graphene-like materials have been synthesized by multistep
processes using non-graphitic carbonaceous materials, such as polymeric molecular
frameworks [105], resorcinol [106, 107] and pomelo peel [108] as the carbon precursor. The carbon products, however, exhibit a low electrical conductivity in the
range of 10–300 S m
−1 . As discussed in the previous sections of this chapter, the
reactive molten salt treatment of PET can offer an efficient method for the preparation of graphene-based nanostructures with an electrical conductivity of greater than
1100 S m
−1 , which is remarkable. The upscaling of the molten salt process is easy
and does not require expensive equipment. Furthermore, the process is green, since
it only uses waste plastic bottles and electricity in order to produce the graphene
material. The carbon product has a high quality, which can be compared with those
of high-quality commercial conductive carbon materials [109]. The preparation of
high-quality conductive carbons from PET wastes can provide economical motivation for converting the waste plastic bottles into carbon nanostructures with a wide
use in energy storage devices.
It should also be mentioned that PET bottles is fabricated from derivatives of
oil and natural-gas such as naphtha and ethane. The conversion of PET into useful
carbon materials applicable in energy storage devices can also provide a perspective
in sustainable utilization of fossil fuels. In the laboratory-scale, for the preparation
of 1 kg nanostructure carbon, around 8.5 kg waste plastic and 30 kWh energy are
required. By considering the current average price of electricity to be around 20 US
cents per kWh, the cost of producing nanostructured carbon from waste PET can be
estimated to be about US $6 kg
−1 . In larger scales, the utilization of more efficient
equipment for a continuous-type production can further reduce the overall cost. The
carbon product should be attractive for many applications, including energy storage
devices [24].
For example, the PET-derived nanostructured carbon produced in molten salt can
be compared with the activated carbon produced from coconut shells. The latter is
currently used by supercapacitor manufacturers as the electrode material, due to its
low cost (around) $15 per kilogram, moderate conductivity (around 250 S m
−1 ), and
relatively high availability. However, this electrical conductivity level is known to
be a main barrier toward the enhanced performance of supercapacitors [110–112].
In contrast, the nanostructured carbon produced by molten salt treatment of PET
exhibits a conductivity of around four times greater than that of coconut shell, and
also an attractive cost [24].
As a conclusion, the heat treatment of PET in air leads to the formation of large
irregular shaped amorphous carbon particles with sharp edges and sizes of up to
several hundred micrometers. This carbon material has an exceptionally high resistance against oxidation in air up to around 900 °C, and this is assigned to its large
particle sizes, high purity, low porosity and low density of surface defects. On the
other hand, the reactive molten salt heat treatment of the plastic material in air at a
temperature above the melting point of the salt can lead to the formation of nanostructured carbon-containing graphene-based nanosheets, with interesting properties
such as high conductivity and surface area. The graphitization was found to initiate
from the edge sites of amorphous carbon particles at around 1100 °C, and progressed
7 Molten Salt Conversion of Plastics into Highly Conductive …
On the other hand, graphene-like materials have been synthesized by multistep
processes using non-graphitic carbonaceous materials, such as polymeric molecular
frameworks [105], resorcinol [106, 107] and pomelo peel [108] as the carbon precursor. The carbon products, however, exhibit a low electrical conductivity in the
range of 10–300 S m
−1 . As discussed in the previous sections of this chapter, the
reactive molten salt treatment of PET can offer an efficient method for the preparation of graphene-based nanostructures with an electrical conductivity of greater than
1100 S m
−1 , which is remarkable. The upscaling of the molten salt process is easy
and does not require expensive equipment. Furthermore, the process is green, since
it only uses waste plastic bottles and electricity in order to produce the graphene
material. The carbon product has a high quality, which can be compared with those
of high-quality commercial conductive carbon materials [109]. The preparation of
high-quality conductive carbons from PET wastes can provide economical motivation for converting the waste plastic bottles into carbon nanostructures with a wide
use in energy storage devices.
It should also be mentioned that PET bottles is fabricated from derivatives of
oil and natural-gas such as naphtha and ethane. The conversion of PET into useful
carbon materials applicable in energy storage devices can also provide a perspective
in sustainable utilization of fossil fuels. In the laboratory-scale, for the preparation
of 1 kg nanostructure carbon, around 8.5 kg waste plastic and 30 kWh energy are
required. By considering the current average price of electricity to be around 20 US
cents per kWh, the cost of producing nanostructured carbon from waste PET can be
estimated to be about US $6 kg
−1 . In larger scales, the utilization of more efficient
equipment for a continuous-type production can further reduce the overall cost. The
carbon product should be attractive for many applications, including energy storage
devices [24].
For example, the PET-derived nanostructured carbon produced in molten salt can
be compared with the activated carbon produced from coconut shells. The latter is
currently used by supercapacitor manufacturers as the electrode material, due to its
low cost (around) $15 per kilogram, moderate conductivity (around 250 S m
−1 ), and
relatively high availability. However, this electrical conductivity level is known to
be a main barrier toward the enhanced performance of supercapacitors [110–112].
In contrast, the nanostructured carbon produced by molten salt treatment of PET
exhibits a conductivity of around four times greater than that of coconut shell, and
also an attractive cost [24].
As a conclusion, the heat treatment of PET in air leads to the formation of large
irregular shaped amorphous carbon particles with sharp edges and sizes of up to
several hundred micrometers. This carbon material has an exceptionally high resistance against oxidation in air up to around 900 °C, and this is assigned to its large
particle sizes, high purity, low porosity and low density of surface defects. On the
other hand, the reactive molten salt heat treatment of the plastic material in air at a
temperature above the melting point of the salt can lead to the formation of nanostructured carbon-containing graphene-based nanosheets, with interesting properties
such as high conductivity and surface area. The graphitization was found to initiate
from the edge sites of amorphous carbon particles at around 1100 °C, and progressed