Page 267 - Carbon Nanotube Fibres and Yarns

CNT yarn-based supercapacitors 257

SCs did not follow the order of the conductivity of the metal filaments, due
to the extra contributions from surface oxidation by acidic electrolyte, which
brought in considerable pseudocapacitance. The capacitance of SCs derived
from Cu + CNT and PtCu+CNT electrodes was more than 400% higher
than those of others, though the conductivity of the Pt and Au filaments were
higher than that of Cu and its alloys. Further experiments showed that the
electrochemical potential window of the symmetric SC Cu + CNT could be
extended from 1 to 1.4 V. Fig. 10.5C and D shows the CV and charge-discharge
curves of the SC at 1, 1.2 and 1.4 V. The gravimetric capacitance showed a 20%
increase as the potential window was extended from 1 to 1.4 V, as shown in
Fig. 10.5E. The Ragone plot in Fig. 10.5F shows significant increase of energy
density and power density due to this extension of the potential window, in-
cluding a 134% increase in energy density and a 729% increase in power density.
Another strategy to improve the capacitance of threadlike SC based on
CNT yarn is incorporating electrochemically active guest components, es-
pecially psuedocapacitive materials. Extending the work of in situ polym-
erized ordered PANI nanowires on carbon nanotube yarns [48b], Wang
et al. produced a metal filament reinforced carbon nanotube composite yarn
which was further in situ polymerized with polyaniline nanowires for use
as electrodes (Fig. 10.6A) [84]. The combination of the Pt filament and the
CNT yarn provided high electrolyte accessibility and high efficiency charge
transport as well as a strong structural backbone for the two-ply composite
yarn SC. The PANI nanowires polymerized in situ on the surface of the
twisted Pt/CNT yarn served as the main electrochemically active mate-
rial of the working electrode. Fig. 10.6B shows an SEM image of the as-
spun pure CNT yarn with a diameter of approximately 25 μm. Fig. 10.6C
shows the PANI nanowires in situ deposited on the surface of the twisted
Pt/CNT yarn substrate. The PANI nanowires were uniform in diameter
and formed a porous layer on the CNT yarn surface. The as-fabricated SC
−2
possessed an areal capacitance of 91.67 mF cm and an energy density of
−2
12.68 μWh cm (Fig. 10.6D). The high energy density was ascribed to the
PANI nanowire network on the CNT yarn surface that offers a large effi-
cient specific area for ion absorption in the electrolyte and the high charge
transport efficiency of the metal filament incorporated in the electrode.
Solutions of separately synthesized PANI nanowires can also be coated on
carbon nanotube yarns incorporated with metal filament to improve capac-
itor performance. For example, supercapacitors made from Pt/CNT core/
sheath yarn coated with PANI nanowires demonstrated a capacitance of
−1
86.2 F/g and a power density of up to 10.69 W kg [74].