Chemistry Reference
In-Depth Information
capacitance was 768 F g
1
at 50 mV s
1
(10 A g
1
), and it could increase up
to
1100 F g
1
after 500 cycles.
In addition to the introduced cases, similar approaches are also applicable
with other combinations of nanomaterials. Furthermore, hierarchical
nanostructures consisting of more than two levels can be applied to super-
capacitors. Investigated ternary electrode systems are relatively rare, but
recently it has been reported that conductive polymer coating on binary
systems containing pseudocapacitive metal oxides can enhance the per-
formance of the supercapacitors by providing additional electron
conduction paths and protecting the metal oxides from corrosive electro-
lytes.
23,92
For instance, Yu et al.
23
demonstrated conductive wrapping of the
hybrid graphene-MnO
2
nanostructure system to enhance cyclic perform-
ance as well as specific capacitance of the electrode. They reported a
B
d
n
3
r
4
n
g
|
4
B
20%
and
45% increase of specific capacitance by a coating of poly(3,4-ethyle-
nedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) and single-walled
carbon nanotubes, respectively. The capacitance reached a maximum
(
B
380 F g
1
) at a scanning rate of 100 mV s
1
(current density: 0.1 mA cm
2
)
with 95% retention after 3000 cycles.
B
9.4 Summary and Outlook
In this chapter, we have discussed two kinds of supercapacitors: EDLCs and
pseudocapacitors. EDLCs have significantly higher power density compared
with lithium ion batteries, but their energy density is limited, due to the
restricted domain responsible for charge storage. The development of high
specific area electrodes is pursued to improve the capacitance of EDLCs.
Various carbon-based electrode materials of extremely high specific area
include activated carbon, carbon nanotubes, and graphene. Pseudocapaci-
tors based on electroactive metal oxides and conductive polymers can be
promising replacements for EDLCs because of their battery-like charge
storage mechanism. However, the increase of energy density is comprom-
ised by the decrease of power density.
Increasing demands for high energy and high power storage systems have
led to extraordinary interest in nanostructured electrodes for super-
capacitors. Capacitive nanomaterials are excellent candidates in themselves
for improving the performance of supercapacitors. They can have physical
and electrochemical properties that overcome the performance limit of
conventional bulk materials. Hierarchical hybrid nanomaterials are a most
advanced form of multi-functioning nanomaterial in that this novel material
system can synergistically combine the strengths of constituent nanoma-
terials. Therefore, capacitive hierarchical hybrid nanomaterials are promis-
ing electrode materials for next-generation supercapacitors.
Nevertheless, there are issues to consider before we fully adopt the hier-
archical material system for advanced supercapacitors. Firstly, the cost of the
sophisticated fabrication processes should not be underestimated. Secondly,
the resulting power performance (kW kg
1
) and energy storage (Wh kg
1
)
.
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