Page 256 - Carbon Nanotube Fibres and Yarns

CNT yarn-based supercapacitors 247

electrolyte. The as-fabricated SCs displayed a high cell areal capacitance of
2
2
161 mF/cm at 1 mA/cm . A porous vanadium-doped zinc-nickel-cobalt
ternary oxide (VZnNiO) was grown on nickel foil for flexible SC [43].
It exhibited a capacitance of 590 mF/g, which was fourfold higher than
NiCo 2 O 4 /NiCo 2 O 4 -based SC and twofold higher ZnNiCo/ZnNiCo-
based SC. This significant increase in capacitive response was attributed to
the synergistic redox reactions of all the ions and the direct growth of a
unique porous nanostructure on the flexible current collector to provide
excellent ion diffusion efficiency for a high electrochemically active surface
2
area. The device delivered an areal capacitance of 0.463 mF/cm with an
2
2
energy density of 0.93 mWh/cm and a power density of 75 mW/cm .
10.1.2.5 Metal nitrides
Metal nitrides, such as titanium nitride (TiN) and vanadium nitride (VN),
are an emerging class of electrode materials for high performance SCs ow-
ing to their excellent electrical conductivity [44]. Xiao et al. [45] prepared
flexible freestanding VN nanowires on CNTs. The SC device was con-
structed based on the freestanding VN/CNT hybrid electrodes in an H 3 PO 4 -
3
PVA electrolyte and exhibited a high volume capacitance of 7.9 F/cm
3
at a current density of 0.025 A/cm , together with energy and power den-
3
3
sity values of 0.54 mWh/cm and 0.4 W/cm , respectively. The device dis-
played 82% capacity retention over 10,000 charging-discharging cycles and
demonstrated excellent flexibility with almost no change in the cyclic vol-
tammogram (CV) shapes under different bending conditions. Ma et al. [46]
developed a freestanding electrode based on Mo 2 N nanobelts and rGO
sheets. An SC was then fabricated by assembling the Mo 2 N/rGO hybrid
electrodes with PVA/H 3 PO 4 -silicotungstic acid (SiWA) gel electrolyte. The
3
device exhibited a high volumetric capacitance of 15.4 F/cm , and energy
3
3
and power densities of 1.05 mWh/cm and 0.035 W/cm , respectively. Lu
et al. [47] demonstrated the electrochemical cycling stability of TiN nanow-
ires in PVA/KOH gel electrolyte. As the polymer gel electrolyte suppressed
the oxidation reaction on the electrode surface, the TiN nanowire-based
SC displayed extraordinary stability up to 15,000 cycles and a volumetric
3
energy density of 0.05 mWh/cm .

10.1.2.6 Conducting polymers
Conducting polymers, including polyacetylene (PA), polypyrrole (PPy), poly-
aniline (PANI), and poly(3,4-ethylenedioxythiophene) (PEDOT), have been
considered as promising pseudocapacitive electrode materials. They offer