# Review of Flexible Supercapacitors Using Carbon Nanotube-Based Electrodes
## 2 Flexible Supercapacitor with Various Types of CNT-Based Electrodes
### Self-Assembled CNTs with NiCoO\({}_{2}\) Electrode  
Self-assembled CNTs with NiCoO\({}_{2}\) electrodes have been the focus of several studies to improve the performance of supercapacitors. The combination of CNTs and NiCoO\({}_{2}\) provides a high-performing material system with enhanced energy storage and charge-discharge capability. In the work by [PERSON], [PERSON], et al., CNT was grown on the Ni foam (NF) by immersing in CNTs solution [3]. Then, it was immersed in a solution that contains precursor of NiCoO\({}_{2}\) in deionized water and ethanol. After hydrothermal treatment and calcination under N\({}_{2}\) atmosphere at 300 \({}^{\circ}\)C, the obtained NiCoO\({}_{2}\)/CNT@NF acts as an electrode (Figure 7a). The electrode has a certain structure where CNT covered with NiCoO\({}_{2}\) nanosheets is intertwined on the surface of NF due to the two-step grown approach of CNTs and NiCoO\({}_{2}\). At a current density of 5 mA cm\({}^{-2}\), it showed a specific capacitance of 929 F g\({}^{-1}\) (3350 mF cm\({}^{-2}\)), and it is almost 3-8 times higher than that of NiCoO\(\underline{\text{\text{\text{\text{\text{\text{\text  
5000 cycles. The results of this study indicate that the self-assembled CNTs/NiCoO\({}_{2}\) system is a promising candidate for high-performance supercapacitors. [PERSON] et al. also explored the potential of self-assembled CNTs/NiCoO\({}_{2}\) system for supercapacitors [15]. The study modified CNTs' surface with metal oxides such as NiCoO\({}_{2}\) (Figure 7f), which showed a high specific capacitance of 1360 F/g, and an excellent rate capability, showing only a 1.4% loss of capacitance after 3000 cycles. The authors concluded that the CNTs/NiCoO\({}_{2}\) system had a great potential for use as high-performance supercapacitor electrodes. Figure 6: (**A**) General scheme of hydrothermal synthesis procedure employed to prepare VS\({}_{2}\), VS\({}_{2}\)-MX, and VS\({}_{2}\)-MX-CNT. (**B**) Comparison plot showing the specific capacitance of VS\({}_{2}\)-MX, VS\({}_{2}\)-MX-CNT-30, VS\({}_{2}\)-MX-CNT-50, and VS\({}_{2}\)-MX-CNT-70 samples at a current density from 0.2 A/g to 1 A/g. (**C**) Nyquist plot showing the EIS fit graphs in the circuit (see Inset). (**D**) Contribution of capacitive and diffusion-controlled areas of VS\({}_{2}\)-MX-CNT-50 at a scan rate of 20 mV/s. (**E**) Contribution of capacitive and diffusion-controlled reactions at scan rates ranging from 10 to 100 mV/s for VS\({}_{2}\)-MX-CNT-50 ternary hybrid electrode. (**F**) Comparison of MXene and VS\({}_{2}\)-MX-CNT-50 CV profiles at a scan rate of 80 mV/s to estimate the potential window. (**G**) The stability of potential window of VS\({}_{2}\)-MX-CNT-50 // MWCNT asymmetric devices from 1.6 to 1.8 V at 100 mV/s, as shown by CV curves. Reproduced with permission from ref. [42]. Copyright 2022 Wiley. However, the practical implementation of the self-assembled CNTs/NiCoO\({}_{2}\) system is still facing several challenges. One major challenge is the consistency and stability of the CNTs self-assembling process, as this greatly impacts the performance of the supercapacitor. Additionally, the cost-effective and large-scale production of the CNTs/NiCoO\({}_{2}\) system remains a technical challenge for commercialization. Self-assembled CNTs with NiCoO\({}_{2}\) electrodes have shown great potential as high-performance supercapacitor electrodes. Despite the challenges, the results of the studies indicate that the self-assembled CNTs/NiCoO\({}_{2}\) system is a promising candidate for future energy storage systems. Further research is needed to address the challenges and optimize the electrochemical performance of self-assembled CNTs/NiCoO\({}_{2}\) supercapacitors. Figure 7: (**a**) Schematic illustration of the fabrication process of the NiCoO\({}_{2}\)@CNTs@NF integrated electrode. Reproduced with permission from ref. [3]. Copyright 2021 Elsevier. (**b**-**e**) SEM images of (**a**) pure CNT, (**b**) pure NiCoO\({}_{2}\), and (**c**,**d**) NiCoO\({}_{2}\)@CNT. Reproduced with permission from ref. [9].