Figure 1: Schematic for the working principles of LIBs [Sandeep et al., 2018]

Supercapacitors

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

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.