7.2 Double-Layer Capacitor
197
7.2 Double-Layer Capacitor
Nanostructured carbon materials are prospective electrode materials for supercapacitors due to their abundance, non-toxicity, high specific surface area, high electronic
conductivity, and good chemical stability. Various carbon materials, including activated carbon, mesoporous carbon, carbon nanotubes (CNTs), and graphene, have
been widely reported as electrode materials for EDLCs over the past years. Numerous
studies have shown that specific surface area, pore size distribution, pore structure,
pore volume, pore size, pore surface curvature, electrical conductivity, and surface
functional groups of carbon materials have decisive influence on the performance of
EDLCs. Herein, we briefly summarize the relevant advances in the development of
carbon materials for EDLCs.
7.2.1 Activated Carbon
Activated carbons have been the earliest carbon materials for supercapacitor because
of their good electrical conductivity, high specific surface area, abundant resources,
and low cost. In recent years, the studies about the preparation of activated carbons
mainly focused on raw materials and activation technologies. The precursors for the
preparation of activated carbon mainly include natural renewable resources (such as
wood, coconut shell), fossil fuel and its derivatives (such as asphalt, coal, coke), and
synthetic polymers. The activation method includes chemical activation and physical
activation. For the chemical activation, the carbon materials, which were obtained via
the carbonization for precursors, were mixed with activating agents (such as KOH,
K 2 CO 3 , H 3 PO 4 , ZnCl 2 ) and a subsequent heat treatment under inert atmosphere. For
the physical activation, the carbon material was treated by oxidizing gas (such as
CO 2 and H 2 O) under high temperature. The activated conditions directly influenced
the pore structure parameters of the products. The activated carbons obtained by the
activation method have a dispersive pore distribution. Numerous studies have proved
that there is no linear relationship between the specific capacitance and the specific
surface area. The pore size and pore distribution also have an important influence on
the specific capacitance.
Gogotsi et al. (Chmiola 2006) overturns the erroneous view that the pore smaller
than solvent ionic diameter is invalid. They found an abnormal phenomenon by
testing the specific capacitance of carbon materials with different pore sizes. When
the pore size was more than 2 times of the solvent ionic diameter, the area specific
capacitance was basically stable and did not change with the pore size. When the
pore size was larger than the solvent ionic diameter and less than 2 times of the
solvent ionic diameter, the area specific capacitance decreased with the decreasing
pore size, owing to not forming effectively the dense layer and the dispersed double
layer. When the pore size is less than the solvent ionic diameter, due to the existence
197
7.2 Double-Layer Capacitor
Nanostructured carbon materials are prospective electrode materials for supercapacitors due to their abundance, non-toxicity, high specific surface area, high electronic
conductivity, and good chemical stability. Various carbon materials, including activated carbon, mesoporous carbon, carbon nanotubes (CNTs), and graphene, have
been widely reported as electrode materials for EDLCs over the past years. Numerous
studies have shown that specific surface area, pore size distribution, pore structure,
pore volume, pore size, pore surface curvature, electrical conductivity, and surface
functional groups of carbon materials have decisive influence on the performance of
EDLCs. Herein, we briefly summarize the relevant advances in the development of
carbon materials for EDLCs.
7.2.1 Activated Carbon
Activated carbons have been the earliest carbon materials for supercapacitor because
of their good electrical conductivity, high specific surface area, abundant resources,
and low cost. In recent years, the studies about the preparation of activated carbons
mainly focused on raw materials and activation technologies. The precursors for the
preparation of activated carbon mainly include natural renewable resources (such as
wood, coconut shell), fossil fuel and its derivatives (such as asphalt, coal, coke), and
synthetic polymers. The activation method includes chemical activation and physical
activation. For the chemical activation, the carbon materials, which were obtained via
the carbonization for precursors, were mixed with activating agents (such as KOH,
K 2 CO 3 , H 3 PO 4 , ZnCl 2 ) and a subsequent heat treatment under inert atmosphere. For
the physical activation, the carbon material was treated by oxidizing gas (such as
CO 2 and H 2 O) under high temperature. The activated conditions directly influenced
the pore structure parameters of the products. The activated carbons obtained by the
activation method have a dispersive pore distribution. Numerous studies have proved
that there is no linear relationship between the specific capacitance and the specific
surface area. The pore size and pore distribution also have an important influence on
the specific capacitance.
Gogotsi et al. (Chmiola 2006) overturns the erroneous view that the pore smaller
than solvent ionic diameter is invalid. They found an abnormal phenomenon by
testing the specific capacitance of carbon materials with different pore sizes. When
the pore size was more than 2 times of the solvent ionic diameter, the area specific
capacitance was basically stable and did not change with the pore size. When the
pore size was larger than the solvent ionic diameter and less than 2 times of the
solvent ionic diameter, the area specific capacitance decreased with the decreasing
pore size, owing to not forming effectively the dense layer and the dispersed double
layer. When the pore size is less than the solvent ionic diameter, due to the existence