# Review of Flexible Supercapacitors Using Carbon Nanotube-Based Electrodes
## 2 Flexible Supercapacitor with Various Types of CNT-Based Electrodes  
### CNT-FF-Based Electrodes  
Supercapacitors, also known as electrochemical capacitors, have gained significant attention as energy storage devices in recent years due to their high energy density and fast charging/discharging capability. One of the key research areas in supercapacitors is the development of new electrode materials that can improve their energy and power density. CNT-FFs have emerged as promising materials for supercapacitor electrodes due to their unique combination of merits in high surface area, excellent conductivity, and mechanical strength. CNT-FF is a type of fabric-like structure made by weaving carbon nanotubes together. The high surface area of the CNTs makes them excellent supercapacitor electrodes due to their large surface for charge storage. Their electrical conductivity is also much higher compared with other carbon-based materials, making fast charging and discharging reactions possible. Additionally, CNT-FF have a high mechanical strength, making them suitable for flexible devices and reducing the risk of mechanical degradation during cycling. Research into using CNT-FF as supercapacitor electrodes has been largely focused on boosting the energy density and rate performance. For example, in the work by [PERSON] et al., researchers fabricated CNT-FF using a solution-blowing method as shown in Figure 2a, which resulted in a remarkable specific surface area of 15.995 m\({}^{2}\)/g [12]. When used as supercapacitor electrodes, the CNT-FF displayed impressive energy storage performance, including a specific areal capacitance of up to 353 mF/cm\({}^{2}\) and an energy density of up to 247 uW/cm\({}^{2}\). Strengths of this study include the high surface area of CNT-FF and the impressive energy storage performance. However, one potential weakness could be the scalability of the solution-blowing method used in creating the CNT-FF. [PERSON], Y., et al. knitted CNT yams using a weaving machine followed by heating at 400 \({}^{\circ}\)C for 60 min under air condition [39]. After oxidation, the CNT-FF is immersed into HNO\({}_{3}\) and H\({}_{2}\)SO\({}_{4}\) solution and magnetically stirred at 45 \({}^{\circ}\)C for 60 min. Then, after cleaning with deionized water, vacuum drying is performed at 60 \({}^{\circ}\)C. This method exhibited improved cyclic voltammetry (CV) curves and longer discharge time compared with bare, oxidized, and acidized CHT-FF. High surface area and an additional oxygen-containing functional group by oxidation and acid treatment caused the increased capacitance of 1033 mF cm\({}^{-2}\) at a current density of 2 mA cm\({}^{-2}\). In addition, the activated CNT-FF had excellent cyclic capacitance retention for 10,000 cycles. This weaving process makes it easy to fabricate a large-scale electrode for flexible supercapacitor. Another work by [PERSON] et al. involved functionalizing CNT-FF with MnO\({}_{2}\) nanoparticles to form hybrid CNT-MnO\({}_{2}\) fiber fabrics (Figure 2b) [14]. The results showed improved electrochemical performance, with a specific capacitance reaching up to 231.3 F/g and an energy density of up to 86 nWh/cm. Strengths of this study include the significant improvement in electrochemical performance achieved through functionalization with MnO\({}_{2}\) nanoparticles. However, one potential weakness could be the cost and scalability of synthesizing the MnO\({}_{2}\) nanoparticles. Researchers have also sought to enhance the mechanical and electrical conductivity of CNT-FF. This has been achieved through various strategies, such as incorporation of conductive materials such as graphene and metal nanoparticles or utilization of hybrid CNT-graphene fiber fabrics. For instance, [PERSON] et al. coated CNT-FF with graphene to create hybrid CNT-graphene fiber fabrics (Figure 3a-f) [10]. These hybrid fiber fabrics showed improved conductivity and mechanical strength, with a specific capacitance of up to 350 mF/cm\({}^{2}\).