Page 277 - Carbon Nanotube Fibres and Yarns

266 Carbon Nanotube Fibers and Yarns

active materials, electrolytes, designs of threadlike SCs, CNT-based SCs (sym-
metric and asymmetric SCs) and self-charging SCs. CNT yarn has been ex-
tensively used as electrode materials and/or current collectors for wearable
SCs due to their large surface area, high conductivity, excellent mechanical
flexibility, and electrochemical properties. Because of their structural diversity
and suitability for smart electronics, flexible, stretchable, and wearable CNT
yarn-based SCs have been under extensive research in recent years.
Despite the exciting progress on the design and fabrication of CNT-
yarn-based supercapacitors, there are still some major challenges. (1) The
unique 1D structure of CNT-based threadlike SCs shortens the distance
for ions to travel between the two electrodes, leading to high electro-
chemical performances. To further improve their performance, we need
a better understanding of the energy storage mechanisms, especially the
electrochemical reactions in the interface between the CNT yarn and
the electrolyte, and in situ characterization to provide direct experimen-
tal proof of the electrochemical activities. (2) CNT yarns have been used
extensively as active materials and current collectors in energy storage de-
vices to take advantage of their superior strength, flexibility, and electrical
conductivity. However, CNT yarns possess relatively low capacitance. To
significantly increase the capacitance of CNT yarn-based supercapacitors,
nanostructured pseudocapacitive materials, such as metal oxide/sulfides/
nitrides, conductive polymer, and other new high performance pseudo-
capacitive materials, are incorporated into carbon nanotube yarns. MOFs
and COFs are two types of fascinating materials with great potential for
supercapacitors. (3) Fabricating supercapacitors with high energy density
without sacrificing power density and cycle life is still a challenging topic.
Suitably designed asymmetric supercapacitors with balanced positive and
negative electrodes open an avenue for high energy density devices with
wide operating potential windows. New solid-state electrolytes, such as
organic electrolyte or ionic liquid, provide another route to high energy
density and power density supercapacitors with a working voltage higher
than 2.5 V. (4) Development of CNT yarn-based supercapacitors integrated
with other wearable functions, such as electrochromism, self-healing, actu-
ation, sensing, etc.

Acknowledgments

We gratefully acknowledge the financial support of the National Natural Science Foundation
of China (51702369, 51873233), Hubei Provincial Natural Science Foundation of China
(2018CFA023), and Hubei Major Projects of Technological Innovation (2017AAA131).