# Review of Flexible Supercapacitors Using Carbon Nanotube-Based Electrodes
## 2 Flexible Supercapacitor with Various Types of CNT-Based Electrodes
### Carbon Nanotube on Thin Metal Film Electrodes  
The utilization of CNTs in combination with thin metal film electrodes has gained widespread attention in recent years due to the high surface area, electrical conductivity, and stability of the CNTs. Research has been conducted to optimize the energy density and rate performance of supercapacitors utilizing CNTs on thin metal film electrodes.  
One strategy for enhancing performance has involved functionalizing the CNTs with conductive materials such as graphene and metal nanoparticles. [PERSON] et al. improved electrical conductivity by functionalizing CNTs on a Ti thin film with graphene (Figure 4a), resulting in a specific capacitance of 535.7 F/g and an energy density up to 26.1 Wh/Kg [40]. Meanwhile, another study by [PERSON] et al. utilized Ni nanoparticles on CNTs on Cu thin films (Figure 4b,c), which resulted in a specific capacitance of 12.2 F/cm\({}^{2}\) and improved rate performance [41]. Another method for improving the performance of CNTs on thin metal film electrodes is by controlling their morphology. [PERSON] et al. used a two-step growth process to synthesize CNTs on Ni thin films (Figure 4d,e), leading to a significant increase in surface area and a specific capacitance of 1200 F/g [2].  
Figure 3: A schematic showing the fabrication process and structure of the wearable supercapacitor (WSC). (**a**) The carbon cloth was used as a current collector. (**b**) The CNTs were loaded on the carbon cloth through filtration, where sodium dodecylbenzene sulfonate (SDBS) was used as a dispersant. (**c**) The graphene was mounted on the CNTs layer through filtration, where boron nitride nanosheets (BNNS) were used to prevent graphene from collapsing. (**d**) The as-fabricated WSC was assembled with hydrogel used as a solid electrolyte. (**e**) The schematic structure of the WSC. (**f**) The WSCs could be integrated into clothing to power wearable devices. Reproduced from ref. [10] under the Creative Commons Attribution 4.0 International (CC BY 4.0) License). (**g**) Schematic illustration for the formation of composites. Reproduced from ref. [1] under the Creative Commons Attribution 4.0 International (CC BY 4.0) License.  
Figure 4: (**a**) Schematics of the synthesis procedure for the PANI/Gr/Ti wire electrode and symmetrical supercapacitor. Reproduced with permission from ref. [40]. Copyright 2018 Elsevier. (**b**) Schematic illustration of the preparation of Ni(OH)\({}_{2}\)/Ni–Cu/CW. (**c**) Typical digital images of the Cu-wire, Ni–Cu/CW and Ni(OH)\({}_{2}\)/Ni–Cu/CW. Reproduced with permission from ref. [41]. Copyright 2018 American Chemical Society. (**d**) Schematic illustration of the continuous fabrication of CNT films and the synthesis of the Ni(OH)\({}_{2}\)/CNTs composite and (**e**) digital photo of the obtained CNT films with length \(\sim\)1.5 m. Reproduced with permission from ref. [2]. Copyright 2015 Elsevier.  
Despite the impressive results, there are some challenges that need to be addressed. For example, the volumetric energy density is relatively low in comparison with other types of electrodes, such as activated carbon. The synthetic process of conductive materials and morphologies of CNTs can be difficult to scale up. CNTs on thin metal film electrodes show immense potential for supercapacitor applications, with numerous studies demonstrating improved energy density and rate performance. However, further work is necessary to overcome the limitations and fully harness the potential of this material.