# Effect of MXene Nanosheet Sticking on Supercapacitor Device Performance
## 2 MXenes as Supercapacitor Electrodes
### Advantages of MXenes in Supercapacitors
#### 2.2.2 MXene-Carbon Allotropes: Pioneering Energy Storage Innovation  
In the realm of supercapacitors, MXenes have not limited themselves to collaborating solely with CNTs but have also ventured into partnerships with various carbon allotropes. [PERSON] et al. [31] demonstrated that MXene combined with acid-activated carbon (AAC) could prevent the aggregation of 2D MXene sheets while increasing the interlamellar spacing. This breakthrough resulted in significantly improved electrochemical performance. An asymmetric supercapacitor was formed with MXene/AAC (2:1) as the positive electrode and AAC as the negative electrode, achieving a specific capacitance of 177 F g\({}^{-1}\) at 0.5 Ag\({}^{-1}\), with an impressive 97.4% retention over 10,000 cycles. This achievement stands as a testament to the potential of MXene-carbon allotrope composites in extending the life and performance of supercapacitors [31]. This progress has extended to micro supercapacitors, which are gaining prominence in various miniaturized energy storage applications. A micro supercapacitor incorporating MXene as the cathode and activated carbon as the anode, immersed in a neutral PVA/Na\({}_{2}\)SO\({}_{4}\) electrolyte, showed an expanded potential window of 1.6 V. Even after 10,000 cycles, this supercapacitor maintained 91.4% of its capacitance, yielding an areal capacitance of 7.8 mF cm\({}^{-2}\), an energy density of 3.5 mWh cm\({}^{-3}\), and a power density of 100 mW cm\({}^{-3}\)[32]. A significant breakthrough occurred with the development of the Ti\({}_{3}\)C\({}_{2}\)T\({}_{\rm x}\) MXene-carbon quantum dots (MXene-CQD) composite electrode, which demonstrated an outstanding capacitance of 441.3 F g\({}^{-1}\) at 1 Ag\({}^{-1}\). Impressively, it retained 100% of its capacitance after 10,000 cycles at a current density of 10 Ag\({}^{-1}\)[33]. This notable increase in capacitance can be ascribed to the excellent conductivity and increased surface area of Ti\({}_{3}\)C\({}_{2}\)T\({}_{\rm x}\) MXene sheets, leading to a higher double-layer capacitance. By preventing layer restacking and activating surface functional groups with CQDs, the performance of the MXene-CQD composite electrode was further enhanced. An innovative approach was used to create a hybrid supercapacitor by spray-injecting Ti\({}_{3}\)C\({}_{2}\)T\({}_{\rm x}\) MXene onto a nickel-cobalt sulfide (NiCO\({}_{2}\)S\({}_{4}\)) material previously deposited on a carbon cloth (CC) substrate. This process resulted in the formation of a composite material known as Ti\({}_{3}\)C\({}_{2}\)T\({}_{\rm x}\)/NiCO\({}_{2}\)S\({}_{4}\)@CC, which is abbreviated as TNSC. TNSC showed a gravimetric capacitance of 2326 F g\({}^{-1}\) at 1 Ag\({}^{-1}\), with superior cyclic stability of 93.8% at a current density of 10 Ag\({}^{-1}\). This enhancement in the electrochemical behavior of the complex electrode, achieved by modulating the mass-loading of Ti\({}_{3}\)C\({}_{2}\)T\({}_{\rm x}\) MXene, contributes to electrical conductivity and manages ion permeability to the nickel-cobalt sulfide. The outcome was a semi-solid flexible supercapacitor capable of a power density of 800 W kg\({}^{-1}\) at an energy density of 57.5 Wh kg\({}^{-1}\), accompanied by a coulombic efficiency of 99.2% and 90.2% of its capacitance retained beyond 5000 cycles [34]. The advantages of hydrogel electrolytes has also come to the forefront. A zinc ion hybrid micro supercapacitor was constructed using battery-type vanadium pentoxide as the cathode and capacitor-type Ti\({}_{3}\)C\({}_{2}\)T\({}_{\rm x}\) MXene as the anode, set in a ZnSO\({}_{4}\) dissolved PAM hydrogel electrolyte. This micro supercapacitor displayed an areal capacitance of 129 mF cm\({}^{-2}\) at a current density of 0.34 mA cm\({}^{-2}\) and an energy density of 48.9 mWh cm\({}^{-2}\) at 673 mW cm\({}^{-2}\). Impressively, it retained 77% of its capacitance over 10,000 cycles.