Page 255 - Carbon Nanotube Fibres and Yarns

246 Carbon Nanotube Fibers and Yarns

10.1.2.3 MXenes
MXenes, a recently discovered family of 2D transition metal carbides
and carbonitrides, have received increasing attention since 2011 [33]. The
MXenes consist of M n + 1 X n layers, where M is an early transition metal, X
is carbon and/or nitrogen, and n = 1, 2, or 3. They exhibit various unique
properties (such as high conductivity, good mechanical properties, and hy-
drophilicity), making them attractive for energy storage applications. A hy-
drazine treated Ti 3 C 2 -based MXene demonstrated a high capacitance of
250 F/g in acidic electrolytes [34]. PPy/Ti 3 C 2 T x composite exhibited a
high gravimetric capacitance (416 F/g) with high cycling stability up to
25,000 cycles [35].

10.1.2.4 Metal oxides
The pseudocapacitance of metal oxides arises from both Faradaic redox re-
action and electrochemical adsorption/desorption of ions at the electrode/
electrolyte interface. Therefore, metal oxides generally have much higher
special capacitance than carbon materials based on EDLCs. Various metal
oxides (e.g., RuO 2 , IrO 2 , MnO 2 , V 2 O 5 , NiO, Co 3 O 4 , SnO 2 , and Fe 2 O 3 )
and some hydroxides (e.g., Co(OH) 2 , Ni(OH) 2 , and their composites) have
been widely investigated as pseudocapacitive electrode materials [36, 37].
Wang et al. fabricated an asymmetric SC based on Fe 2 O 3 /RuO 2 hollow
nanorods, which demonstrated excellent electrochemical performance,
3
3
including capacitance of 4.9 F/cm , energy density of 1.5 mWh/cm and
3
power density of 9.1 mW/cm [38]. The device also showed high cycling
stability of 97% capacitance retention after 5000 charge/discharge cycles.
MnO 2 has been considered to be a promising electrode material for SCs
also because of its low cost and high theoretical capacitance in the range
between 1100 and 1300 F/g [39]. Cheng et al. reported a MnO 2 nanowire/
graphene composite fabricated by solution-phase assembly of a graphene
sheet and α-MnO 2 nanowires [40]. The asymmetric SC based on MnO 2 /
graphene/graphene in a neutral Na 2 SO 4 solution, when being cycled re-
versibly in the high-voltage region of 0–2 V, exhibited an energy density
−1
of 30.4 Wh kg . Guo et al. synthesized hierarchical porous V 2 O 5 @C core-
2
shell nanowires that exhibited a remarkable areal capacitance of 128.5 F/cm
−1
at a scan rate of 10 mV s with a high rate capability [41].
Nickel- and cobalt-based binary and ternary materials have been used
as electrode materials for SCs because of their high capacitance through
Faradaic reactions. Wang et al. [42] designed flexible SCs using two NiCo 2 O 4
nanowire arrays supported on Ni foam as electrodes and PVA/KOH as