# Effect of MXene Nanosheet Sticking on Supercapacitor Device Performance
## 2 MXenes as Supercapacitor Electrodes  
### Overview of MXenes  
The exploration of graphene, a groundbreaking two-dimensional (2D) material, marked the beginning of a new chapter in materials science. This significant achievement paved the way for the investigation of additional 2D materials with properties and potential applications similar to graphene's. Graphene, with its single-layered structure, is renowned for its remarkable conductivity at room temperature and its ability to transmit 97.7% of light in the visible spectrum, positioning it as one of the most extensively researched 2D materials nowadays. The distinct properties and broad applications of 2D materials can be attributed to their larger specific surface area and quantum confinement effects. Following the success of graphene, a wide variety of 2D materials have been explored, including transition metal dichalcogenides (TMDs), metal-organic frameworks (MOFs), metal-oxide compounds, hydroxides, hexagonal boron nitrides, germanene, phosphorene, silicene, and monoelemental 2D Xenes like graphdiyne, arsenene, borophene, bismuthene, antimonene, and tellurene [11, 12, 13]. The attractiveness of these materials stems from their atomically thin layers, distinct electronic structures, generous surface areas, high sur face/volume ratios, adaptability, and resilience to mechanical stress, distinguishing them from their bulk equivalents [14]. However, the most groundbreaking development in the context of 2D materials lies in the emergence of MXenes. These materials, termed the \"wonder materials\" of the 2D world, have introduced a new paradigm in material science research [15]. MXenes are part of the 2D material family, hailing from layered transition metal carbides, nitrides, or carbonitrides. They are obtained by selective wet chemical etching processes, exfoliating them from their parent MAX phases [16]. The name \"MXene\" is reminiscent of graphene, a nod to their analogous properties [17]. The precursor of MXenes, known as the MAX phase, possesses a chemical formula of M\({}_{(n+1)}\)AX\({}_{n}\) (\(n\) = 1 to 4), where M stands for an early transition metal (e.g., Ti, V, Cr), A represents an element from groups 13 to 15 of the periodic table (e.g., Al, Ga, Si), and X includes carbon and/or nitrogen. MAX phases are characterized by a hexagonal layered structure within a space group of D6h4-P63/mmc. This structure consists of alternating M and A layers, with X atoms occupying octahedral sites. Notably, the M-X bond displays metallic, covalent, or ionic characteristics, while the M-A bond is primarily metallic, yet slightly weaker than the M-X bond. This subtle weakening facilitates the chemical etching process for obtaining MXenes [16]. The selective chemical etching process leads to the removal of covalently bonded A layers from MAX phases, culminating in the formation of MXenes as M\({}_{(n+1)}\)X\({}_{n}\)T\({}_{X}\). The \"T\" in the formula signifies surface termination groups, such as -F, -Cl, -OH, or -O, and \"x\" represents the number of terminations. Generally, higher \"_n\"_ values correspond to increased MXene stability. The surface termination groups, originating from the chemical etching process, impart hydrophilicity to MXene surfaces and influence their electronic and ion transport properties, ultimately impacting their conductivity [17]. The discovery of the first MXene material, Ti\({}_{3}\)C\({}_{2}\)T\({}_{x}\), in 2011 marked a watershed moment. It was etched from its parent MAX phase, Ti\({}_{3}\)AlC\({}_{2}\), using hydrofluoric acid (HF) as the etchant [15].