162
Electrochemical Supercapacitors for Energy Storage and Delivery
Graphene is mechanically robust, exhibiting a quantum Hall effect at room
temperature and undergoing ballistic conduction of charge carriers along the
basal planes, resulting in good conductivity in the material [61]. Stankovich et
al. [62] illustrate in Figure 4.15b that the conductivity of chemically reduced
graphene along its basal plane, on the order of 200 S.m –1 , was close to that of
bulk graphite. The high conductivity helps reduce internal resistance in the
materials and provides power gains similar to those exhibited by CNTs.
The most important properties for the ES market include the ability of graphene to reach the high theoretical surface area of 2630 m 2 .g -1 of its parent
graphite and the highly regular pore spacing across the crystal lattice [63].
Due to the atomic thickness, negligible diffusion transport distances result
in low ionic resistance. The high theoretical surface area of the fully exposed
graphene sheets could provide a maximum theoretical capacitance of 550
F.g –1 [64].
The combination of good electrical conductivity, low internal resistance,
and high surface area makes graphene a competitive material for EDLCs.
However, as with CNTs, aggregation and poor macroscale controls still limit
the full potential of graphene materials. Restacking of graphene sheets can
occur in solution over time during annealing and during drying procedures,
leading to reduced surface area. Stoller et al. [63] were one of the first groups
to utilize chemically reduced graphene (CMG) for ES applications. They produced a CMG graphene that exhibited an area of 705 m 2 .g –1 and capacitances
reaching to 107 F.g –1 in KOH and 100 F.g –1 in acetonitrile using cyclic voltammetry with a 20 mV.s –1 scan rate.
Many graphene-based materials report 100 to 200 F.g –1 , outperforming CNT
devices in an aqueous electrolyte [65]. Most EDLC electrodes used in ESs use
thick material layers that are opaque and fairly brittle. For example, Yu et al.
[66] produced flexible, uniform, 25 nm thick layers of graphene that offered up
to 70% optical transparency (Figure 4.16) and capacitance of 135 F.g –1 .
Other techniques have been applied to help boost the performance of graphene in ESs by altering the graphene structure. Zhu et al. [67] investigated
the application of thermal KOH activation from AC materials to graphene
materials. Similar to its activity in AC devices, KOH was shown to boost the
surface area of TEGO and MEGO graphenes up to 3100 m 2 .g –1 by restructuring the carbon, exposing hidden graphene sheets and producing extra pores.
MEGO powder was weighed and mixed with KOH to undergo a secondary activation reaction (800°C, 400 torr, 1 hr) that increased the surface area
significantly and generated a well defined pore distribution (Figure 4.17) of
micropores (~0.8 nm) and mesopores (~4 nm). As a result, the device was able
to achieve 165 F.g –1 (stable up to 6 A.g –1 ) in organic electrolyte [67].
Jeong et al. [21] studied the effects of doping graphene planes with nitrogen to enhance local electronic interactions and improve binding with ions
in solution. Plasma treatment was used to impregnate nitrogen functionalities into defects along the basal planes of graphene sheets. The presence
of nitrogen functionalities could generate cross linking along the graphene
Electrochemical Supercapacitors for Energy Storage and Delivery
Graphene is mechanically robust, exhibiting a quantum Hall effect at room
temperature and undergoing ballistic conduction of charge carriers along the
basal planes, resulting in good conductivity in the material [61]. Stankovich et
al. [62] illustrate in Figure 4.15b that the conductivity of chemically reduced
graphene along its basal plane, on the order of 200 S.m –1 , was close to that of
bulk graphite. The high conductivity helps reduce internal resistance in the
materials and provides power gains similar to those exhibited by CNTs.
The most important properties for the ES market include the ability of graphene to reach the high theoretical surface area of 2630 m 2 .g -1 of its parent
graphite and the highly regular pore spacing across the crystal lattice [63].
Due to the atomic thickness, negligible diffusion transport distances result
in low ionic resistance. The high theoretical surface area of the fully exposed
graphene sheets could provide a maximum theoretical capacitance of 550
F.g –1 [64].
The combination of good electrical conductivity, low internal resistance,
and high surface area makes graphene a competitive material for EDLCs.
However, as with CNTs, aggregation and poor macroscale controls still limit
the full potential of graphene materials. Restacking of graphene sheets can
occur in solution over time during annealing and during drying procedures,
leading to reduced surface area. Stoller et al. [63] were one of the first groups
to utilize chemically reduced graphene (CMG) for ES applications. They produced a CMG graphene that exhibited an area of 705 m 2 .g –1 and capacitances
reaching to 107 F.g –1 in KOH and 100 F.g –1 in acetonitrile using cyclic voltammetry with a 20 mV.s –1 scan rate.
Many graphene-based materials report 100 to 200 F.g –1 , outperforming CNT
devices in an aqueous electrolyte [65]. Most EDLC electrodes used in ESs use
thick material layers that are opaque and fairly brittle. For example, Yu et al.
[66] produced flexible, uniform, 25 nm thick layers of graphene that offered up
to 70% optical transparency (Figure 4.16) and capacitance of 135 F.g –1 .
Other techniques have been applied to help boost the performance of graphene in ESs by altering the graphene structure. Zhu et al. [67] investigated
the application of thermal KOH activation from AC materials to graphene
materials. Similar to its activity in AC devices, KOH was shown to boost the
surface area of TEGO and MEGO graphenes up to 3100 m 2 .g –1 by restructuring the carbon, exposing hidden graphene sheets and producing extra pores.
MEGO powder was weighed and mixed with KOH to undergo a secondary activation reaction (800°C, 400 torr, 1 hr) that increased the surface area
significantly and generated a well defined pore distribution (Figure 4.17) of
micropores (~0.8 nm) and mesopores (~4 nm). As a result, the device was able
to achieve 165 F.g –1 (stable up to 6 A.g –1 ) in organic electrolyte [67].
Jeong et al. [21] studied the effects of doping graphene planes with nitrogen to enhance local electronic interactions and improve binding with ions
in solution. Plasma treatment was used to impregnate nitrogen functionalities into defects along the basal planes of graphene sheets. The presence
of nitrogen functionalities could generate cross linking along the graphene