# Review of Flexible Supercapacitors Using Carbon Nanotube-Based Electrodes
## 1 Introduction  
Energy storage is a crucial aspect of modern society and is necessary to meet the growing demand for renewable and clean energy [1; 2; 3; 4; 5; 6; 7; 8]. Supercapacitors, also known as electrochemical capacitors, are energy storage devices that have gained significant attention in recent years due to their high energy density and fast charging/discharging capability [9; 10; 11; 12; 13; 14; 15]. The development of new electrode materials has been an area of active research in supercapacitor technology, as this leads to improvements in energy and power density (Figure 1) [16; 17; 18; 19; 20; 21; 22]. Supercapacitors are generally divided into two categories: pseudo capacitors and electronic double layer supercapacitors (EDLCs) [24,25]. Pseudo capacitors accompany redox reaction at the surface of electrode. Otherwise, having non-faradaic reaction, EDLCs accumulate charges between the electrode and electrolyte by forming a double layer, so carbon materials such as carbon nanotubes (CNTs), carbon black (CB), and graphene are beneficial for a large interfacial area due to their porosity. Among them, CNTs have high electrical conductivity, specific surface area, chemical/thermal stability, and good contact with the electrolyte [16,26,27,28,29,30,31,32,33,34,35,36,37,38,39,30,31,32,33]. Carbon materials can also be formed in various dimensions such as 1D carbon fiber, 2D carbon film, and 3D carbon composite. Therefore, CNTs with those unique properties are expected to serve as EDLCs electrode materials in various forms of supercapacitor. Synthesis and characterization of new materials are very important topics for several applications including supercapacitors [34,35,36,37,38]. One such promising material is carbon nanotube fiber fabrics (CNT-FF), which have emerged as a candidate for supercapacitor electrodes due to their combinatorial merits in high surface area, excellent conductivity, and mechanical strength [10,39]. CNT-FF is a type of fabric-like structure made by weaving together carbon nanotubes, which have high surface area and electrical conductivity. These unique properties of CNTs make them an excellent option for supercapacitor electrodes as they provide a large surface for charge storage and fast charging/discharging. Additionally, the high mechanical strength of CNT-FF makes it suitable for flexible devices, reducing the risk of mechanical degradation during cycling. The focus of the research into the use of CNT fiber fabrics as supercapacitor was on boosting energy density and rate performance. Researchers have employed various strategies such as functionalizing CNT fiber fabrics with conductive materials such as graphene and metal nanoparticles, and incorporating these materials into hybrid CNT-based fiber fabrics. These efforts have resulted in impressive energy storage performance, including specific capacitance values of up to 402 F/g and energy densities of up to 28 Wh/kg. However, it is important to note that these studies were limited in their comparison with other electrode materials and their lack of information on the long-term stability and durability of CNT-FF electrodes, which are critical factors for practical applications. Another promising approach in supercapacitor technology is the utilization of CNTs in combination with thin metal film electrodes. The combination of high surface area, electrical conductivity, and stability of CNTs with thin metal film electrodes has gained widespread attention in recent years. Research has been conducted to optimize the energy density and rate performance of supercapacitors utilizing this combination. One strategy for enhancing performance has involved functionalizing the CNTs with conductive materials, such as graphene and metal nanoparticles, while another method has involved controlling the  
Figure 1: Main issues in flexible supercapacitors (SCs). Reproduced with permission from ref. [23]. Copyright 2015 Elsevier. morphology of the CNTs on thin metal films.