Page 357 - Polymer-based Nanocomposites for Energy and Environmental Applications

324 Polymer-based Nanocomposites for Energy and Environmental Applications

11.8.3.2 Graphene based electrodes for supercapacitors

Graphene can be assembled into several different structures, that is, the freestanding
particles or dots, one-dimensional fibers or yarns, two-dimensional films, and three-
dimensional foams and composites. Graphene-based materials in different forms of
0-D, 1-D, 2-D, to 3-D have proved to be excellent candidates of electrode materials
in electrochemical power storage space system, such as supercapacitors. The function
of graphene-like electrode for supercapacitors is striking so its high surface region (in
2
order of 2630 m /g) [44] inherent mechanical strength, superior mechanical immov-
ability, and thermal conductivity and high electric conductivity [45] that bring high
energy density. On the other hand, the packing of graphene nanosheets shows a restric-
tion for growth of supercapacitors [44]. In order to avoid the gathering of grapheme
sheets, latest labors have been intended for the progress of mix carbon conductive
structure such as carbon composite of metal oxide/graphene and nanotube/graphene
films N-doped graphene [46]. In the hybrid electrodes, graphene will function as a
conductive channel for charge transmit and thus advance the general conductivity,
while pseudocapacitance from the metal oxides/hydroxides or conductive polymers
[47]. Graphene-based fibers and yarns have a great application potential due to the
combined merits of tiny volume, high flexibility, and weave ability that promise some
applications in the next-generation supercapacitors for wearable and portable devices
and electric vehicles [48].

11.9 Electrolytes

Electrolytes used in supercapacitor cells have great importance like electrode mate-
rials, as they describe the routine of supercapacitors, mainly the energy density. An
appropriate electrolyte must have a low toxicity, high electrochemical immovability,
high-voltage window, higher ionic concentration, low viscosity, low volatility, low
solvated ionic radius, low resistivity, small cost, and accessibility at high transparency
[3]. The most common electrolyte that is generally used for commercial super-
capacitors is Et 4 NBF 4 or TEABF 4 (tetraethylammonium tetrafluoroborate) salt in
molecular liquid [49]. However, the use of ammonium salt in supercapacitors results
in poor performance of the device. Therefore, ionic liquids are introduced for super-
capacitors to solve these issues.

11.9.1 Aqueous electrolyte
These electrolytes are collected of aqueous solution of acids. The most common aque-
ous electrolytes are 6 mol L 1 KOH and 1 mol L 1 H 2 SO 4 [50]. On the other hand,
they have a tendency to show extremely good kinetic conduct of the electrolyte ions
lead to incredibly proficient discharge-charge rates. The performance of these aque-
ous electrolytes is owing to a comparatively low viscosity and high conductivity of the
concerted solutions. The inner support of this type of supercapacitor is extremely
small because of its high conductivity (0.8 S/cm for H 2 SO 4 ) of aqueous electrolyte.