Page 294 - Polymer-based Nanocomposites for Energy and Environmental Applications

Polymer nanocomposite materials in energy storage: Properties and applications 263

porous structure favors close interaction of the electrode with the electrolyte. In
fact, GNS acted as nucleation site for the polypyrrole utilizing very high surface
area of GNS. Fig. 9.7B shows the CV curves for the polypyrrole, GNS, and poly-
1
pyrrole/GNS composite in 0.1 M LiClO 4 electrolyte at 10 mV s . It can be seen that
the polymer composite showed very good performance in comparison with the indi-
2
vidual components. The area capacitance of 151 mF cm , volume capacitance
3
151 F cm , and specific capacitance 1510 F g 1 were obtained for a scan rate of
10 mV s 1 for the sample polymerized for 1500 s. Moreover, the composite material
shows better stability than polypyrrole as shown in Fig. 9.7C and D.
Qian et al. synthesize a novel core-shell PPy/reduced graphene oxide polymer
nanocomposite for use as electrode material for supercapacitors using two-step syn-
thetic. Firstly, PPy microspheres were synthesized from pyrrole by the initiation of
FeCl 2 -H 2 O 2 mixture. These microspheres were then coated with the negatively
charged functional groups containing reduced graphene oxide sheets via electrostatic
and π-π interaction (Fig. 9.8A). Fig. 9.8B confirms the formation of composite where
the PPy microspheres are fully covered with the crumpled and graphene oxide sheets
of thickness 5–10 nm. Characteristic wrinkles associated with graphene sheets can be
seen in Fig. 9.8B. Fig. 9.8C shows the CV curves for the composite at different scan
rates. The shapes of all CV curves show rectangular and symmetrical current-potential
characteristics, indicating an ideal capacitive behavior. The specific capacitance of the
composite was found to be 430 F g 1 at 1 A g 1 discharge rate, which was very high
in comparison with either graphene or PPy. After 1000 cycles, the nanocomposite was
able to retain about 85% of its initial specific capacitance. The maximum energy den-
sity of the fabricated supercapacitor based on the mass of active electrodes is calcu-
1
lated to be 49.5 and 33.3 Wh kg 1 at a power density of 0.22 and 6.06 kW kg , which
exhibit higher energy and power densities than those of other types of commercially
available energy storage devices [207]. Chang et al. also reported high initial
capacitance for 424 A g 1 discharge rate for Ppy/graphene synthesized by the electro-
chemical polymerization [208]. On similar philosophy, Alvi et al. reported graphene-
PEFOT and graphene-PTh nanocomposite with very high specific capacitance
between 160 and 400 F g 1 [209].
Although the performance of EDLC-based supercapacitors has been gone through
tremendous amount of improvement, still they are far from being satisfactory. As
mentioned earlier, another mode of supercapacitor is called pseudocapacitors. In pseu-
docapacitance, charge is stored through a fast reversible reduction/oxidation reaction
taking place at the surface or near the boundary between the active electrode material
and the electrolyte. Although this charge storage mechanism involves redox reactions,
still it is considered as capacitance because of the linear charge vs potential relation-
ship that exists in the pseudocapacitance [177]. Different types of storage mechanism
in a pseudocapacitor electrode material can be distinctly identified: (1) physical
adsorption of the ions on the electrode from the electrolyte, (2) redox reaction of
the transition metal oxides, (3) insertion/extraction of active pseudocapacitance mate-
rial, and (4) reversible doping and undoping of conducting polymeric materials at the
electrode [182]. Common materials for pseudocapacitance include transition metal
oxides and conducting polymers and their derivatives [23,180]. As far as the