Supercapacitors are considered as the next generation of energy storage devices due to their higher power density than batteries and higher energy density than traditional flat capacitors. Among the numerous electrode materials, two-dimensional solid materials have been the research focus of industry and academia because of their large electrochemical active surface. Although graphene is a common two-dimensional layered material, the main capacity of intrinsic defect free graphene comes from electrical double layer capacitance (EDLC). Without the introduction of redox reaction, namely pseudocapacitance, the electrochemical properties of graphene are greatly limited. However, in the pseudocapacitor materials such as MnO2, Co3O4, etc., the cycle life is short although the energy density is relatively high.
In recent years, as a new member of two-dimensional solid materials, mxene has attracted extensive attention due to its excellent physical and chemical properties. As an electrode material, mxene also shows excellent electrochemical energy storage performance and electronic conductivity. Mxene is a kind of Mn + 1xntx composite materials, in which m is the pre transition metal, X can be carbon or nitrogen, n = 1,2,3, TX represents the surface functional groups (- O, - Oh, or - F). It is prepared by etching a in Max phase materials, forming a stable MX layer and adsorbing F - / OH -. The results show that the electrochemical energy storage performance of mxene is greatly affected by the surface functional groups. On the one hand, F - / OH - and other groups adsorbed on the surface are not conducive to the rapid migration of electrolyte ions between layers, on the other hand, it affects the redox reaction of transition metal M in mxene layer. Therefore, the adsorption of F - / OH - groups on the surface is one of the main reasons that affect the performance of mxene as electrode materials for supercapacitors.
