Page 260 - Carbon Nanotube Fibres and Yarns

CNT yarn-based supercapacitors 251

were sandwiched together, forming a thin film SC, which gave a specific
capacitance of ~36 F/g. Apart from plastic films, other low-cost lightweight
substrates deposited with CNTs have also been used as electrodes in flex-
ible SCs. For instance, Kang et al. [61] deposited CNTs onto a bacterial
nanocellulose substrate using a vacuum filtering process. The resultant pa-
pers showed high flexibility, large specific surface area, and good chemical
stability. The assembled all-solid-state flexible SC exhibited a high specific
−1
capacitance of 46.9 F/g at a scan rate of 0.1 V s , and excellent stability with
a less than 0.5% loss of capacitance after 5000 charge-discharge cycles at a
high current density of 10 A/g. A freestanding SWNT film electrode was
studied in aqueous gel and organic liquid electrolytes [63]. The aqueous
gel electrolyte was composed of PVA/H 3 PO 4 electrolyte and the organic
liquid electrolyte was composed of 1 M LiPF 6 in ethylene carbonate/diethyl
carbonate (in 1/1 weight ratio). Their capacitances were comparable in all
aqueous electrolytes ranging from ~90 to 120 F/g. However, the capaci-
tance was relatively low for the LiPF 6 /EC:DEC electrolyte, especially when
measured at a higher current density.
CNT can also serve as fiber scaffold, current collector, and active ma-
terial for electrodes. Studies on CNT-based SCs have been on the increase
in recent years. Ren et al. [64] were one of the first groups to utilize CNTs
in fiber-shaped SC with a twisted structure and achieved a relatively low
−1
specific capacitance (0.006 mF cm ). The specific capacitance for one sin-
−1
gle electrode was then improved to as high as 294 F/g or 282 mF cm by a
pseudocapacitive effect generated from conductive PANI hybridized with
the CNT fiber [65]. Jiang et al. fabricated a double-layered symmetric SC
utilizing vertically aligned 80 μm high CNT forests grown on silicon wafers,
−2
showing a capacitance of 428 μF cm [66]. Lee et al. [67] used a biscrolling
method to produce fast-ion-transport yarn electrodes (Fig. 10.2A and B),
in which hundreds of MWNT layers infiltrated with conductive polymer
were scrolled into a yarn of about 20 μm thickness. The discharge current
of the plied yarn SC increased linearly with the voltage scan rate up to
−1
80 and 20 V s for liquid and solid electrolytes, respectively. The resultant
fiber-shaped SC exhibited high stability. The capacitance changed little when
the capacitor was bent (Fig. 10.2D) or woven onto a glove (Fig. 10.2E).
To increase the specific capacitances of CNT-based SCs, the CNTs were
functionalized to introduce functional groups and defects to offer pseudo-
capacitance; they were incorporated with other carbon materials to form a
hierarchical three-dimensional structure, and they were incorporated with
pseudocapacitive materials such as metal oxides and conducting polymers.