As discussed previously, CNTs have excellent electron conductivity,
large surface to volume ratio and outstanding electrolyte accessible
capability. For these mentioned reasons, CNT based electrodes exhibit
good electrochemical performance and has proven to be critical in
enhancing the performance of electrochemical capacitors or
supercapacitors. They are found in diverse fields of applications such
as consumer electronics (e.g., mobile phones, camera, and toys), medical
appliances, transportation, electrical utilities, and defense systems.
In comparison to other energy storage systems, supercapacitors offer
rapid charging, higher power density (10 kW/kg), excellent reversibility
(90%- 95%), longer cycle life, higher columbic efficiencies, lack of
maintenance, and operational safety. There are two different types of
supercapacitors, namely, electrical double layer capacitors (EDLCs) and
pseudo-capacitors. EDLCs contain two electrodes and the charges get
accumulated on these electrodes. The charge separation between the
electrodes results in energy storage. Thus, materials with large surface
area and good conductivity are required for high performance EDLCs.
Pseudo-
capacitors transfer charges by redox reaction that takes place between
electrode and redox material on the surface of electrode.
Both SWCNTs and MWCNTs have been widely investigated as effective
supercapacitor electrodes with high surface areas to support aqueous and
non-aqueous electrolytes. CNT/graphene hybrid 3D structures are also
used to increase the performance of supercapacitors. These hybrids can
be used in two ways in a supercapacitor. They can be applied directly as
electrodes for electrical double layer capacitors (EDLCs). CNT/graphene
hybrids are also used as highly conductive skeleton for loading
pseudocapacitive metal oxides and sulphides[Yi Li et al.]. The
requirement of high energy density without sacrificing the power density
is difficult to be obtained using pure CNT electrodes as they store
charges based on a physical process. Thus pseudocapacitive active
materials like conductive polymers and MeOx, in combination with CNTs
are used to modify its properties.
Transparent conducting films or TCFs are optically transparent
conducting films. They can be used in thin film photovoltaic cells, in
organic LEDs and in liquid crystal displays (LCDs). The challenge of
making TCF flexible led to the utilization as a potential material of
carbon-based nanostructures. The conventional TCFs consist of tin oxide
doped with indium [ITO] and tin oxide doped with fluorine [FTO].
A TCF material should be selected carefully to balance the optical
transmittance and electrical conductivity. CNTs, because of its
excellent flexibility and high optical transmittance are used as
alternatives to conventional ITO and FTO conducting films [Sandeep et
al, 2018]. CNT based TCFs are fabricated in both dry and wet
conditions.
Dry process of CNT based TCF fabrication include the famous arc
discharge, laser ablation and CVD techniques while wet processes include
spray pyrolysis, spray coating, spin coating, screen printing etc.