Coil-structured
carbon nanotube (CNT) yarns have recently attracted
considerable attention. However, structural instability due to heavy
twist insertion, and inherent hydrophobicity restrict its wider application.
We report a twist-stable and hydrophilic coiled CNT yarn produced
by the facile electrochemical oxidation (ECO) method. The ECO-treated
coiled CNT yarn is prepared by applying low potentiostatic voltages
(3.0–4.5 V vs Ag/AgCl) between the coiled CNT yarn and a counter
electrode immersed in an electrolyte for 10–30 s. Notably,
a large volume expansion of the coiled CNT yarns prepared by electrochemical
charge injection produces morphological changes, such as surface microbuckling
and large reductions in the yarn bias angle and diameter, resulting
in the twist-stability of the dried ECO-treated coiled CNT yarns with
increased yarn density. The resulting yarns are well functionalized
with oxygen-containing groups; they exhibit extrinsic hydrophilicity
and significantly improved capacitance (approximately 17-fold). We
quantitatively explain the origin of the capacitance improvement using
theoretical simulations and experimental observations. Stretchable
supercapacitors fabricated with the ECO-treated coiled CNT yarns show
high capacitance (12.48 mF/cm and 172.93 mF/cm2, respectively)
and great stretchability (80%). Moreover, the ECO-treated coiled CNT
yarns are strong enough to be woven into a mask as wearable supercapacitors.
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