Hierarchical Nanostructures: Application to Supercapacitors - Hierarchical Nanostructures for Energy Devices - page 216

nanomaterials. (In section 9.3.3, the performance of this hybrid structure
will be discussed.)
Capacitance of CNT electrodes ranges from 50-200 F g 1 in general
aqueous electrolytes, and the energy density and the power density span
8-20 kW kg 1 and 1-10 Wh kg 1 , 26 respectively. As reported by Lu et al., 35
additional surface treatment and pore opening can enhance the capacitance.
Indeed, they obtained an extraordinarily high capacitance of VACNTs
(440 F g 1 ) with an ionic liquid. The energy and power densities
(148 Wh kg 1 , 315 kW kg 1 ) were also high, compared with pristine VACNTs.
To apply CNTs to flexible supercapacitors, the nanotubes should be in-
tegrated on flexible materials such as polymers. However, direct growth of
CNTs on polymeric substrates is highly challenging, due to the high tem-
perature that CVD growth of CNTs requires. Instead of direct growth of CNTs
on flexible substrates, CNTs can be transferred and assembled on flexible
receptor substrates. This transfer process can incorporate CNTs into various
flexible materials such as carbon micro-fibers, 36 cotton paper, 37 metal
foils, 38 and plastic sheets. 39 Wet transfer techniques with solubilized CNTs
are versatile and easy to apply for this purpose, but the CNTs become en-
tangled and bundled. Performance of the entangled CNTs is significantly
undermined, due to their high inter-CNT and CNT-current collector contact
resistances.
In this regard, dry transfer of VACNTs to polymer sheets that does not
affect the arrangement of the aligned nanotubes can be a plausible approach
to realize high-performance CNT-based flexible capacitors. However,
VACNTs have relatively poor in-plane conductivity in spite of good axial
conductivity of the individual CNTs. Common flexible polymers are insu-
lating and therefore the lateral electrical resistance of VACNTs can become
even higher after the transfer. Recently, Marschewski et al. 40 reported a
significant decrease in lateral resistance of VACNT electrodes by placing a
nickel layer coat on top of the VACNTs. They used a laser welding technique
to simultaneously transfer and pattern the nickel-coated VACNTs into
interdigitated electrodes on polycarbonate substrates 41 (Figure 9.7). The fast
conduction path provided by the nickel film enabled significant decrease of
ESR, and the micro-supercapacitor that used the laser-welded VACNT
electrodes showed excellent bendability. 42
Despite the superior performance compared with the conventional acti-
vated carbon, use of CNT electrodes is limited by the following reasons.
Firstly, commercialized CNTs still have a much higher price than activated
carbon. Secondly, CNT growth requires catalyst particles, so these metal
elements can act as impurities unless the CNTs are properly purified. As a
result, unexpected pseudo-capacitance can appear. Purifying treatments
with acidic solutions can readily remove the metal catalysts, but the wet post-
processing can induce defects and surface groups on CNTs that can cause
unwanted electrochemical behavior. Thirdly, despite the aforementioned
effort, the in-plane (lateral) resistance of the CNT forest is still high. Metallic
growth substrates can enhance the in-plane conductivity. However, direct

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