The synthesis and supercapacitor application of flexible free-standing Ti₃C₂T, Mo₂TiC₂T, and V₄C₃T MXene films

MXenes represent a fascinating category of two-dimensional materials made up of transition metal carbides and nitrides, currently attracting significant research attention, especially in energy storage. However, the electrochemical properties of MXene materials with varying elemental compositions may exhibit significant differences. In order to optimally select types of MXenes that are more suitable for energy storage and explore their energy storage mechanisms, three kinds of different elemental compositions of delaminated MXenes (d-Ti₃C₂T, d-Mo₂TiC₂T, and d-V₄C₃T) were prepared by solid-phase synthesis, liquid-phase etching, and mechanical exfoliation method, successively. The obtained single-layer or few-layer MXene nanosheets were self-assembled into flexible free-standing film electrodes via vacuum-assisted filtration, and the detailed material preparation and characterization can guide the synthesis of more MXenes. Furthermore, we conducted a comprehensive study on the effects of various aqueous electrolytes (3 M H₂SO₄, 3 M KOH, and 3 M Na₂SO₄) and temperatures (0 °C, 20 °C, and 40 °C) on their electrochemical performance. This work optimized the MXene types that are more suitable for electrochemical energy storage application (d-Ti₃C₂T and d-V₄C₃T), and also found that the V₄C₃T MXene has excellent rate performance and long cycling performance, and has guiding significance for the development of MXene materials in energy storage. More significantly, the d-V₄C₃T MXene exhibits exceptional specific capacitance in both acidic and alkaline electrolytes, reaching 292.0 F g^-1 in 3 M H₂SO₄, the highest among the three types of MXenes, and 184.3 F g^-1 in 3 M KOH, far surpassing the performance of the d-Mo₂TiC₂T and d-Ti₃C₂T MXenes (less than 100 F g^-1 at 2 mV s^-1). Furthermore, this reveals that H⁺ intercalation/deintercalation, showing pseudocapacitance characteristics, along with the large interlayer spacing play a vital role in energy storage for MXenes, and an asymmetric configuration is an effective means to improve the energy density of aqueous supercapacitors. The comparative analysis aims to enhance the understanding of MXene materials' potential in advanced energy storage systems.