# Review of Flexible Supercapacitors Using Carbon Nanotube-Based Electrodes
## 2 Flexible Supercapacitor with Various Types of CNT-Based Electrodes
### Hybrid Electrodes with Carbon Nanotube and MXene  
Recently, the use of hybrid electrodes composed of CNTs and MXene materials has gained much attention for supercapacitor applications. Both CNTs and MXenes possess significant advantages, including high surface area, good electrical conductivity, and durability, making them ideal components for hybrid electrodes.  
Studies have indicated that incorporating CNTs and MXenes into hybrid electrodes can lead to improved supercapacitor performance. For example, [PERSON], [PERSON] et al. fabricated MXene (especially Ti\({}_{3}\)C\({}_{2}\)T\({}_{\rm x}\))-MWCNT electrodes and polypyrrole (PPy)-coated MWCNT electrodes as anode and cathode, respectively [4]. Ni foam substrate was immersed in the isopropanol solution containing Ti\({}_{3}\)C\({}_{2}\)T\({}_{\rm x}\) and MWCNT at various ratios. Ni foam is effective due to its lightweight characteristics, porous structure, and high load mass of active material. PPy-coated MWCNT was prepared by impregnation of Ni foam with PPy-MWCNT composite in ethanol. The asymmetric supercapacitor which comprises of a Ti\({}_{3}\)C\({}_{2}\)T\({}_{\rm x}\)-MWCNT cathode and PPy-coated MWCNT anode showed a specific capacitance of 0.94 F cm\({}^{-2}\) at scan rate of 2 mV s\({}^{-1}\). The capacitance is higher than that of pure Ti\({}_{3}\)C\({}_{2}\)T\({}_{\rm x}\) by optimizing the ratio of MWCNT with increased voltage range. The asymmetric supercapacitor is also electrochemically stable at a voltage window of 0-1.6 V. Another work by [PERSON] et al. reported that a hybrid electrode combining CNTs, Fe\({}_{3}\)O\({}_{4}\), and Ti\({}_{3}\)C\({}_{2}\)T\({}_{\rm x}\) MXene (Figure 5a-d) exhibited a specific capacitance of 5.52 F/cm\({}^{2}\)[5]. [PERSON] et al. utilized a hybrid electrode made from CNTs and Ti\({}_{3}\)C\({}_{2}\) MXene (Figure 5e-j), achieving a specific capacitance of 134 F/g [13].  
A way to enhance the performance of hybrid electrodes even further is through the functionalization of CNTs and MXenes. Researchers have tried to modify CNTs with metal nanoparticles and MXenes with conductive materials. For instance, [PERSON] et al. uses functionalized CNTs and 1T metallic vanadium disulfide (VS\({}_{2}\)) hybridized with MXene as an electrode for supercapacitor (Figure 6A). The VS\({}_{2}\)-MX-CNT-50 // MWCNT-based asymmetric supercapacitor showed excellent electrochemical performances in supercapacitor applications, showing a specific capacitance of 505 F/g and an energy density of 7303 Wh/Kg (Figure 6B-G) [42].  
Despite these promising findings, challenges persist in the development of hybrid electrodes. One major challenge is maintaining a stable and consistent mixture of CNTs and MXenes, as this can greatly impact the performance of the hybrid electrode. Moreover, scaling up the production of MXenes remains a technical challenge. Hybrid electrodes made from CNTs and MXenes show great promise for supercapacitor applications. Further research is necessary to overcome the challenges in the synthesis and functionalization of these materials to fully unlock their potential in supercapacitor technology.