159
Components and Materials for Electrochemical Supercapacitors
surface area, CNT quality, and type [31]. This shows that even SWNTs that
exhibit moderate surface area underperform AC materials when gravimetric
capacitance is considered.
However, the low gravimetric capacitance of CNTs is offset by their higher
packing density that creates volumetric capacitances exceeding those of
many AC materials [52]. ACs also suffer degradation of capacitance more
rapidly than CNTs at high rates of operation because they exhibit less ionic
and electrical conductivity. As a result, CNTs offer superior performance in
high power electrodes. The higher conductivity along with the large aspect
ratio enables a lower percolation threshold (0 to 4% for CNTs) compared to
conductive carbon black additives (3 to 15%) [56].
An example of this is shown by Liu et al. [57] through the addition of a
small percentage of CNTs to AC, resulting in a lower resistance and a higher
capacitance of 180 F.g –1 , compared to 130 F.g –1 when carbon black binder was
used. The low percolation means that no carbon additives are needed in CNT
electrodes to lower internal electrode resistance. Further, sheet resistances
were observed to be as low as 1 to 10 Ω.cm 2 for CNT-based films [47,49] and
this suggested that heavy metal current collectors are not needed to provide
conduction along the length of an electrode.
As a result, complete devices can be designed with less dead weight and
improved gravimetric performance. The strong ionic conduction throughout
entangled CNT electrodes could create conductive, lightweight current collectors for pseudocapacitive electrodes [36]. Zhou et al. [50] created a super
aligned CNT film with low surface area (100 m 2 .g –1 ) and baseline EDLC
capacitance of only 5 F.g –1 . However, the strong binding of MnO x nanoparticles to the CNTs and conductive structures enabled performance of 245
F.g –1 (including CNT collector mass) at a very high current density (155 A.g –1 )
and reliable stability over 2500 cycles [50].
Vertically aligned CNT “forests” created by CVD can have highly ordered
structures, resulting in even higher ionic conductivity compared to filmbased CNT electrodes. The increased packing density improves volumetric capacitance. Recently, an improved (“super growth”) CVD process was
developed by Hate et al. [58]. This process involved the addition of water
into the CVD chamber during deposition, which could increase the activity
and lifetime of the metal catalysts for growth. The super growth generated
ordered SWNT forests with heights of millimeters. The method is highly
efficient, exhibiting short growth times of only 10 min and yielding SWNT
with 99.9% purity without further purification steps [58].
Similar to standard CVD, vertical alignment of the CNTs is performed
through the application of a high electric field during growth. When CNT
growth is complete, further alignment occurs through a “zipping effect”
caused by immersing a CNT electrode in solvent. The effect is the by-product
of the solvent surface tension and the Van der Waals interactions of the CNTs
as the electrode dries. The super growth process enables a surface area of
Components and Materials for Electrochemical Supercapacitors
surface area, CNT quality, and type [31]. This shows that even SWNTs that
exhibit moderate surface area underperform AC materials when gravimetric
capacitance is considered.
However, the low gravimetric capacitance of CNTs is offset by their higher
packing density that creates volumetric capacitances exceeding those of
many AC materials [52]. ACs also suffer degradation of capacitance more
rapidly than CNTs at high rates of operation because they exhibit less ionic
and electrical conductivity. As a result, CNTs offer superior performance in
high power electrodes. The higher conductivity along with the large aspect
ratio enables a lower percolation threshold (0 to 4% for CNTs) compared to
conductive carbon black additives (3 to 15%) [56].
An example of this is shown by Liu et al. [57] through the addition of a
small percentage of CNTs to AC, resulting in a lower resistance and a higher
capacitance of 180 F.g –1 , compared to 130 F.g –1 when carbon black binder was
used. The low percolation means that no carbon additives are needed in CNT
electrodes to lower internal electrode resistance. Further, sheet resistances
were observed to be as low as 1 to 10 Ω.cm 2 for CNT-based films [47,49] and
this suggested that heavy metal current collectors are not needed to provide
conduction along the length of an electrode.
As a result, complete devices can be designed with less dead weight and
improved gravimetric performance. The strong ionic conduction throughout
entangled CNT electrodes could create conductive, lightweight current collectors for pseudocapacitive electrodes [36]. Zhou et al. [50] created a super
aligned CNT film with low surface area (100 m 2 .g –1 ) and baseline EDLC
capacitance of only 5 F.g –1 . However, the strong binding of MnO x nanoparticles to the CNTs and conductive structures enabled performance of 245
F.g –1 (including CNT collector mass) at a very high current density (155 A.g –1 )
and reliable stability over 2500 cycles [50].
Vertically aligned CNT “forests” created by CVD can have highly ordered
structures, resulting in even higher ionic conductivity compared to filmbased CNT electrodes. The increased packing density improves volumetric capacitance. Recently, an improved (“super growth”) CVD process was
developed by Hate et al. [58]. This process involved the addition of water
into the CVD chamber during deposition, which could increase the activity
and lifetime of the metal catalysts for growth. The super growth generated
ordered SWNT forests with heights of millimeters. The method is highly
efficient, exhibiting short growth times of only 10 min and yielding SWNT
with 99.9% purity without further purification steps [58].
Similar to standard CVD, vertical alignment of the CNTs is performed
through the application of a high electric field during growth. When CNT
growth is complete, further alignment occurs through a “zipping effect”
caused by immersing a CNT electrode in solvent. The effect is the by-product
of the solvent surface tension and the Van der Waals interactions of the CNTs
as the electrode dries. The super growth process enables a surface area of