# Effect of MXene Nanosheet Sticking on Supercapacitor Device Performance
## 3 Factors Influencing Sticking in MXene Electrodes
### Surface Chemistry of MXene Flakes
#### 3.2.1 Surface Chemistry  
The surface chemistry of MXene flakes refers to the composition and functional groups present on their surface. MXenes typically exhibit functional groups such as -OH, -O, -E, or -Cl, resulting from the etching process used to synthesize them. The presence of these functional groups affects the electrostatic and van der Waals forces between adjacent flakes, thus influencing sticking behavior.  
Surface functional groups can alter the surface charge of the MXene flakes, influencing inter-particle interactions. For instance, hydrophilic functional groups like -OH promote stronger inter-particle adhesion due to increased hydrogen bonding and dipole-dipole interactions. These interactions can facilitate the agglomeration of MXene flakes, resulting in sticking. On the other hand, hydrophobic functional groups can weaken attractive forces between the flakes, reducing the tendency to stick.  
MXenes display a broad spectrum of surface terminations except -OH, including -O, -Cl, or -F (as depicted in Figure 6), attributable to the etching process during their synthesis. These terminations affect the surface chemistry, stability, and interactions of MXenes with other materials, emphasizing their versatility across various applications. Diverse techniques are employed for the synthesis and delamination of MXenes, resulting in varied outcomes related to surface chemistry, interlayer spacing, flake dimensions, and defect density. The choice of etching method significantly influences the surface chemistry (Figure 6a,b). The synthesis of MXenes in acidic solutions containing F-ions generally results in limited control over terminations. The dominant surface terminations consist of -F and -O/-OH, with the modulation of -F terminations achievable by adjusting acid concentrations. Additionally, the complete replacement of -F terminations with -O/-OH terminations can be accomplished through treatment with a base such as NaOH or TBAOH [50]. In molten salt environments, the cations undergo a Lewis acid-base reaction with the A layer, initially replaced by Zn. Subsequent removal of Zn results in MXenes with exclusive -Cl terminations (Figure 6c). Manipulation of the molten salt composition for MAX phase etching allows for the attachment of various halogen terminations in desired stoichiometries to the MXene surface. Beyond halogens (-Cl, -Br, and -I), a diverse range of terminations such as -S, -Se, -Te, -P, and -Sb can be uniformly applied to MXenes.  
Figure 6: MXenes with surface terminations. (**a**) The terminal groups attached to MXene surfaces; (**b**) variety and adaptability of MXene terminations on MXene surfaces resulting from the etching of MAX phases; (**c**) surface terminations located on the surface Ti, HCP sites on the surface, and bridge sites between surface Ti atoms [51].  
The attachment of elements to these terminations is an effective approach to prevent the self-stacking of the MXene layers and thus to maximize ion conduction and improve energy storage capabilities of MXene-based supercapacitors. The predisposition of certain atomic-thickness MXene nanosheets to undergo progressive layering is affected by the robust van der Waals interaction between the sheets. This layering process leads to the creation of a relatively large inactive volume, resulting in a reduced active surface area for ion storage and impeding optimal ionic transport pathways [52; 53]. In addition to the incorporation of elements in the terminated sites between the sheets, enhanced ion transport can be achieved through the vertical orientation of the MXene sheets as opposed to the traditional horizontal alignment, as illustrated in Figure 7, offering a more accessible transport mechanism.