Page 269 - Carbon Nanotube Fibres and Yarns

CNT yarn-based supercapacitors 259

Transitional metal oxide pseudocapacitive materials, such as Co 3 O 4 ,
NiO, and MnO x , can also be deposited on CNT yarns using a simple elec-
trodeposition process [75, 85]. The metal oxide nanoparticles are deposited
not only on the CNTs on the yarn surface but also to CNTs below the yarn
surface, increasing the loading of the transitional metal oxide nanomaterials
in the electrodes. Wang et al. reported a parallel threadlike SC integrating
two Cu/CNT yarn electrodes decorated with NiCo 2 O 4 nanowires [86].
The NiCo 2 O 4 nanowires were grown in situ on the surface of the CNT
yarn intertwined with the Cu filament using a hydrothermal method. Due
to the high electrochemical performance of both NiCo 2 O 4 nanowires and
CNT yarn and the high charge transport efficiency of the Cu filament as a
current collector, the SC demonstrated a specific areal capacitance as high
−2
−2
as 277.3 mF cm and an energy density of 35.76 μWh cm coupled with a
−2
power density of 0.154 mW cm .
Stretchable electronics, including transistors, polymer light-emitting di-
odes, polymer solar cells, and active matrix displays, have been developed
to maintain their electronic performance under high level of deformation
(up to 40% strain). Yang et al. proposed a threadlike SC design with CNT
sheets as active materials and an elastic fiber as supporting substrate [87].
Aligned CNT sheets were sequentially wrapped on an elastic rubber fiber
with polymer gel electrolyte (Fig. 10.7A). The use of aligned CNT sheets
renders high flexibility, high electrical conductivity, good tensile strength,
and mechanical stability. The CV curve of the SC (Fig. 10.7B) remained
unchanged even under 75% strain while its specific capacitance was main-
tained by more than 95% over 100 stretching cycles with a strain of up to
75% without any obvious structural damage (Fig. 10.7C). The obtained SC
−1
provided a capacitance of 20 F/g, an energy density of 0.515 Wh kg , and
−1
a power density of 421 W kg . Because of the stretchable nature of both
elastic fiber substrates and the gel electrolyte, the threadlike SC was flexible
and easily stretched without an obvious deterioration in structural integrity.
Xu et al. reported a twisted ply asymmetric supercapacitor using a
MWNT fiber attached on a spandex fiber (Fig. 10.8A and B) [88]. A pris-
tine MWNT fiber spun from a vertically aligned CNT array was used as
one electrode and another MWNT fiber precipitated with nanostructured
MnO 2 particles was used as a second electrode. The two electrodes were
twined together with the PVA/H 2 SO 4 electrolyte to produce an asymmet-
ric SC. The straight fiber SC was then “glued” on the pre-strained spandex
fiber using additional gel electrolyte as adhesive. After the prestrain on the
spandex fiber was released, a sinusoidal buckling fiber SC with stretchability