Page 264 - Carbon Nanotube Fibres and Yarns

254 Carbon Nanotube Fibers and Yarns

infiltrated with an electrolyte, or a gel polymer electrolyte serves as both
separator and electrolyte (Fig. 10.3B). Coaxial threadlike SCs are assembled
by wrapping a gel-electrolyte coated core electrode with an outer electrode
layer (Fig. 10.3C). In serial threadlike SCs, two non-contacting electrodes
are placed on a common linear substrate in an end-to-end configuration
and a continuous coat of gel-polymer electrolyte provides the electrical
connection between the two electrodes (Fig. 10.3D).

10.4 Symmetrical and asymmetrical (or hybrid) threadlike
supercapacitors
10.4.1 Symmetric threadlike supercapacitors
A symmetric device is fabricated from two electrodes of the same mate-
rial with the same mass and same configuration. Carbon materials exhibit
excellent properties, including electrical conductivity, flexibility, low cost,
which make them ideal candidates for symmetric SC application. CNTs
[76, 77] and graphene [78] are two types of new carbon electrode mate-
rials under extensive research in recent years. CNTs have a unique one-
dimensional structure with good electrical conductivity that facilitates rapid
charge transport as well as high mechanical strength and lightweight [79].
Threadlike SCs based on CNT yarns are valued for their great potential in
future portable and wearable electronics. Dalton et al. [80] were probably
the first to report a threadlike SC composed of two CNT/PVA composite
fibers, which were separately coated with electrolyte by dipping in aqueous
PVA/phosphoric acid (19% phosphoric acid and 4% PVA by weight), twisted
together and then recoated with electrolyte. This threadlike SC (100 μm in
diameter) provided a capacitance of 5 F/g and an energy storage density of
−1
0.6 Wh kg at 1 V, values that are comparable to those of large commercial
SC. The performance was unchanged over 1200 charge- discharge cycles.
The CNT threadlike SC was woven into a fabric for possible electronic
textile application. Gao et al. [81] adopted a coaxial wet-spinning assembly
approach to continuously spin polyelectrolyte-wrapped graphene/CNT
core-sheath fibers, which could be used directly as safe electrodes in two-
ply threadlike SCs. The threadlike SC using liquid and solid electrolytes
−2
showed capacitance between 117 and 269 mF cm and energy densities
−2
between 3.84 and 5.91 μWh cm , respectively.
Ren, et al. assembled a threadlike SC by twisting two MWNT yarns
coated with PVA/H 3 PO 4 gel electrolyte [82]. The device showed a low
−1
capacitance of 6 μF cm because of the low specific surface area of the