Page 271 - Carbon Nanotube Fibres and Yarns

CNT yarn-based supercapacitors 261

due to the spandex fiber was formed. The MnO 2 /CNT asymmetric
−2
−3
SC had a capacitance of 4.28 mF cm (11.4 F cm ), a power density of
−3
−2
−2
0.493 mWh cm (1.32 W cm ), and an energy density of 0.226 μWh cm
−3
(0.6 mWh cm ). The supercapacitor was able to undergo a large cyclic ten-
sile strain of 100%.
10.4.2 Asymmetric threadlike supercapacitors
Most threadlike SCs are based on a symmetric design with two identical
electrodes. The energy density (E) of the SC device is determined by the
capacitance (C) and the operating voltage (V) according to the equation
2
E = CV /2. Despite the increase in cell capacitance by using high perfor-
mance pseudocapacitive materials, the cell voltage limitation in symmetrical
devices limits the maximum energy density of supercapacitors. The max-
imum operating voltage for aqueous-based supercapacitors is about 1.2 V.
Other electrolytes, such as ionic liquid-based electrolyte, can widen the
operating window to as high as 2.7 V [89].
Another effective approach to widen the operating voltage window is to
use a hybrid or asymmetric supercapacitor architecture. In an asymmetric
supercapacitor, the negative electrode of a symmetrical pseudocapacitive
device is replaced by an EDLC electrode to reach a more negative potential.
Based on this approach, Su and Miao [90] constructed asymmetric superca-
pacitors from a CNT yarn EDLC electrode and a non-polarizable electrode
made from a CNT yarn coated with high performance pseudocapacitive
material MnO 2 . This increased the capacitance from 4 F/g to 12.5 F/g, the
operating voltage from 1 V to more than 2 V and the energy density from
−1
2.1 to 42 Wh kg . Zhang et al. developed a twisted threadlike asymmetric
SC using CNT@ZnO-NWS@MnO 2 fiber as the positive electrode and
CNT fiber as a negative electrode [91]. The voltage window of the assem-
bled SC was widened to 1.8 V, and the device displayed a maximum spe-
−2
−2
cific capacitance of 31.15 mF cm at a current density of 10 μA cm and a
−2
high energy density of 13.25 μWh cm . Fig. 10.9A shows optical photos of
the threadlike SCs integrated into a woven fabric. The five threadlike SCs
demonstrated high flexibility (Fig. 10.9B). Fig. 10.9C and D shows three
SCs connected in series and parallel. The electrochemical performances of
the SCs connected in series and parallel are shown in Fig. 10.9E and F.
Considering that the two-ply yarn configuration often leads to a low
contact area between the electrolyte and the electrodes, some efforts have
been directed toward the development of coaxial fiber SCs [92]. Chen et al.
fabricated an asymmetric coaxial threadlike SC from an aligned CNT fiber,