Methanol and Diethanolamine Assisted Synthesis of Flexible Nitrogen-Doped Ti₃C₂ (MXene) Film for Ultrahigh Volumetric Performance Supercapacitor Electrodes

To acquire better nitrogen doping results of titanium carbides (MXenes), methanol (MT) solvent with high fluidity and smaller micromolecule size is selected to assist the introduction of nitrogen of diethanolamine (DEA) in the Ti₃C₂ flakes, and thus, the flexible, free-standing, and compact nitrogen-doped Ti₃C₂ (abbreviated as MD-Ti₃C₂) film can be obtained by the facile ex situ solvothermal method as high-performance supercapacitor electrode. Results indicate that due to an integrated effect of the mixed solvents with liquid N-doping source DEA and high fluidity MT, the MD-Ti₃C₂ film electrode shows an outstanding capacitive performance of 3123 F cm^-3 at a scan rate of 5 mV s^-1 and excellent stability (∼100% retention of initial capacitance after 10 000 charging-discharging cycles), which is more advantageous than MXenes and nitrogen-doped carbon-based materials. The possible reasons should be related to the layered structure, suitable nitrogen element, relatively high surface area, wettability, and conductivity, as well as rich and conductive pores channels, which are conducive to the intercalation of more cations in the MD-Ti₃C₂ film. Finally, MT as an effective auxiliary solvent with nitrogen source DEA can be suitable for improving the electrochemical performance of binary metal carbides (MXenes) through optimization of the ex situ nitrogen doping process.