200
7 Nanomaterials for Supercapacitors
temperature was necessary to increase the specific capacitance and reduce the CNTelectrode resistance. Furthermore, a maximum specific capacitance of 180 F g
−1
with a large power density of 20 kW kg
−1 at an energy density of 6.5 Wh kg
−1 was
realized.
Compared with randomly entangled CNT, vertically aligned CNT (VA-CNT)
array possesses obvious advantages (Dai et al. 2012). The vertically aligned nanostructures and the well-defined tube spacing in the array are beneficial to the contact of
the electrolyte and solid surface. For VA-CNT array, CNTs were directly grown on the
substrate, which is beneficial to the rate performance. Moreover, the top end of CNTs
can be opened via appropriate approaches, which results in more active surface area.
Chen et al. (2004) fabricated VA-CNT array via CVD method using porous alumina
as the template. A specific capacitance of 365 F g
−1 was achieved at 210 mA g
−1
in 1 mol L
−1 H 2 SO 4 electrolyte. Dai et al. (Lu et al. 2009) synthesized VA-CNT
array on SiO 2 /Si wafers that were pre-deposited with Fe nanocatalysts by vacuum
CVD method. The end tips of nanotube were opened via Plasma etching method.
The array electrode showed a remarkable capacitance of 440 F g
−1 in the ionic
liquid electrolyte ([EMIM][Tf2N]). Furthermore, the supercapacitor device exhibited
a high cell voltage of 4 V and superior energy and power densities (148 Wh kg
−1 and
315 kW kg
−1 ).
7.2.4 Graphene-Based Materials for Supercapacitor
Graphene possesses a honeycombed 2D crystal structure formed by the arrangement
of sp2 hybrid carbon atoms. It has been widely studied as electrode materials for
supercapacitors due to its good electric conductivity and large specific surface area.
The theoretical capacity of graphene is up to 550 F g
−1 , which is the largest value
among all carbon-based materials. However, due to the characteristics of its twodimensional plane structure and strong π-π interaction, graphene is easy to stack,
which results in the decrease of specific surface area and the low utilization rate
of electrolyte. Therefore, the actual specific capacitance value is far less than the
theoretical value. Preventing the accumulation of graphene nanosheets is the focus
of current research in this field.
Kaner et al. (El-Kady et al. 2012) presented a simple strategy to fabricate the
graphene-based EDLCs by an all-solid-state laser-reducing approach (Fig. 7.2a).
Graphene oxide (GO) was reduced directly to laserscribed graphene (LSG) on DVD,
which avoided the restacking of graphene sheets. Figure 7.2b, c shows the SEM
images of the well-exfoliated LSG sample and the initially stacked GO precursor.
Moreover, the produced robust films possessed a high electrical conductivity of
1738 S m
−1 and a large specific surface area of 1520 m
2 g
−1 . In an aqueous electrolyte,
the LSG electrode showed a nearly rectangular CV shape at 1000 mV s
−1 (Fig. 7.2d).
When LSG was designed and fabricated to a flexible and all-solid-state electrochemical capacitor, its performance was completely independent of the bending angle
7 Nanomaterials for Supercapacitors
temperature was necessary to increase the specific capacitance and reduce the CNTelectrode resistance. Furthermore, a maximum specific capacitance of 180 F g
−1
with a large power density of 20 kW kg
−1 at an energy density of 6.5 Wh kg
−1 was
realized.
Compared with randomly entangled CNT, vertically aligned CNT (VA-CNT)
array possesses obvious advantages (Dai et al. 2012). The vertically aligned nanostructures and the well-defined tube spacing in the array are beneficial to the contact of
the electrolyte and solid surface. For VA-CNT array, CNTs were directly grown on the
substrate, which is beneficial to the rate performance. Moreover, the top end of CNTs
can be opened via appropriate approaches, which results in more active surface area.
Chen et al. (2004) fabricated VA-CNT array via CVD method using porous alumina
as the template. A specific capacitance of 365 F g
−1 was achieved at 210 mA g
−1
in 1 mol L
−1 H 2 SO 4 electrolyte. Dai et al. (Lu et al. 2009) synthesized VA-CNT
array on SiO 2 /Si wafers that were pre-deposited with Fe nanocatalysts by vacuum
CVD method. The end tips of nanotube were opened via Plasma etching method.
The array electrode showed a remarkable capacitance of 440 F g
−1 in the ionic
liquid electrolyte ([EMIM][Tf2N]). Furthermore, the supercapacitor device exhibited
a high cell voltage of 4 V and superior energy and power densities (148 Wh kg
−1 and
315 kW kg
−1 ).
7.2.4 Graphene-Based Materials for Supercapacitor
Graphene possesses a honeycombed 2D crystal structure formed by the arrangement
of sp2 hybrid carbon atoms. It has been widely studied as electrode materials for
supercapacitors due to its good electric conductivity and large specific surface area.
The theoretical capacity of graphene is up to 550 F g
−1 , which is the largest value
among all carbon-based materials. However, due to the characteristics of its twodimensional plane structure and strong π-π interaction, graphene is easy to stack,
which results in the decrease of specific surface area and the low utilization rate
of electrolyte. Therefore, the actual specific capacitance value is far less than the
theoretical value. Preventing the accumulation of graphene nanosheets is the focus
of current research in this field.
Kaner et al. (El-Kady et al. 2012) presented a simple strategy to fabricate the
graphene-based EDLCs by an all-solid-state laser-reducing approach (Fig. 7.2a).
Graphene oxide (GO) was reduced directly to laserscribed graphene (LSG) on DVD,
which avoided the restacking of graphene sheets. Figure 7.2b, c shows the SEM
images of the well-exfoliated LSG sample and the initially stacked GO precursor.
Moreover, the produced robust films possessed a high electrical conductivity of
1738 S m
−1 and a large specific surface area of 1520 m
2 g
−1 . In an aqueous electrolyte,
the LSG electrode showed a nearly rectangular CV shape at 1000 mV s
−1 (Fig. 7.2d).
When LSG was designed and fabricated to a flexible and all-solid-state electrochemical capacitor, its performance was completely independent of the bending angle