However, limited by their high contact resistance, insufficient charge storage, and low electrode density, the activated carbons only show an inadequate capacitance range of 60-200 F cm −3 . [7,8] In comparison, the pseudocapacitive materials, such as conducting polymers and metal oxides, exhibit higher capacitance based on their fast non-diffusion limited surface Faraday redox reactions, [9] whereas poor conductivity and unstable cycle performance restrict their wide-spectrum application. [10,11] Several efforts have been made to develop the intercalation pseudocapacitive electrode materials, which store charge via fast ion intercalation and thus combine the high energy density of batteries and the high power density of supercapacitors. [12,13] 2D transitional metal carbides/nitrides, also known as MXenes, have attracted much attention since firstly reported in 2011. [14] MXenes have a general formula of M n + 1 X n T x , where M is a transition metal, X is carbon or/and nitrogen, n can be an integer between 1 and 4, and T x stands for surface-terminating functional groups (-OH, -F, -O, etc.). [15,16] MXenes could be obtained by selectively etching the A atomic layer from their layered MAX phase precursors (M n + 1 AX n ) with different etchants. [14,17,18] Currently, more than 100 kinds of MXenes have been predicted, among which about 40 compositions have been experimentally synthesized and Ti 3 C 2 T x is the most studied member. Various compositions and ratios of M and X elements, together with different surface terminations, endow MXene materials with special and tunable structures and properties. As a result, they have rapidly aroused tremendous interest for various applications, including energy storage, [19][20][21][22][23] catalysis, [24] electromagnetic interference shielding, [25] water purification, [26] reinforcement for composites, [27] etc.More specifically, MXene materials are promising electrode materials for supercapacitors, as they have 2D lamellar structure, high electrochemically active surface, metallic-like electrical conductivity, tunable surface-terminating functional groups, good dispersibility in various solvents, and outstanding mechanical flexibility. [28][29][30] Due to their 2D structure and polar terminations, MXenes allow ions (e.g., H + , Li + , Na + ) to intercalate/de-intercalate fast between the layers. [20,31] In basic and neutral aqueous electrolytes, the intercalated ions are surrounded by water molecules, leading to an inner EDLC mechanism with common capacitances of 60-150 F g -1 , [32] while in H 2 SO 4 electrolyte, the intercalated H + protons can bond/ debond with the oxygen-containing terminations on the MXene Supercapacitors have the characteristics of high power density and long cycle life, but the low energy density limits their further development. The 2D transitional metal carbides/nitrides (MXenes) show great application prospects in the field of supercapacitors due to their superior volumetric capacitance, metallic-like conductivity, tunable surface termination...
