Page 257 - Carbon Nanotube Fibres and Yarns

248 Carbon Nanotube Fibers and Yarns

capacitive behavior through redox reactions that occur not only on the surface
but also throughout the entire bulk. PANI and PPy are the most promising
conductive polymers due to their low costs and facile synthesis processes. Guo
et al. synthesized PANI hydrogel electrode, exhibiting a high capacitance of
750 F/g at 1 A/g [48a], Wang et al. deposited highly ordered PANI nanow-
ires on carbon nanotube yarns by an in situ polymerization process, resulting
2
in SCs with areal capacitance up 38 mF/cm [48b]. Huang et al. deposited
PPy on stretchable stainless steel mesh, which exhibited a capacitance of up
to 170 F/g at 0.5 A/g [49]. Wang fabricated threadlike asymmetric SC based
on NiCo 2 O 4 @PPy negative electrode [50]. The device expanded the stability
voltage window from 0–1.0 to 0–1.7 V and demonstrated a high volume energy
3
density of 5.18 mWh/cm and high flexibility. To improve the cycling stability
of conductive polymer, Wang et al. [51] synthesized cobalt-based MOF onto
carbon cloth and deposited with PANI. The electrode exhibited an areal capaci-
2
2
−1
tance of 2.1 F/cm at 10 mV s , higher than the PANI electrode (727 mF/cm ).
10.1.3 Electrolyte
Electrolyte plays a critical role in supercapacitor performance. A good
electrolyte offers a wide voltage window, high electrochemical stability,
high ionic concentration and conductivity, low viscosity, and low toxicity.
Common electrolytes can be classified into three types: aqueous, organic
liquid, and ionic liquid. Aqueous electrolytes, such as H 2 SO 4 and KOH,
dissolve in water, have high ionic conductivity, and low internal resistance.
The maximum working voltage of aqueous electrolyte is limited to 1.23 V.
Organic electrolyte and ionic electrolyte can provide broader working volt-
age windows (for example, higher than 2 V) but they are often accompanied
with higher internal resistance. Small-size lightweight flexible energy stor-
age devices for wearable electronics require solid-state polymer electrolytes
that do not leak without the need of bulky packaging. In practice, solid-state
polymer electrolytes are usually fabricated by mixing an electrolyte solution
with a polymer matrix, such as polyvinyl alcohol (PVA), polyethylene ox-
ide (PEO) and polyvinylidene difluoride (PVDF). The maximum working
voltage of solid-state polymer electrolyte is determined by the electrolyte
solution used, which is below1.23 V for aqueous solution-based electrolyte
or over 2 V for organic solution and ionic liquid-based electrolyte.

10.1.4 Performance evaluation of SCs

The common way to carry out material capacitance measurements is to
coat the chosen material onto an inert electrode surface, then measure this