Nitrogen-doped Ti3C2 MXene films with low -F terminal groups achieving an ultrahigh volumetric capacitance

Two-dimensional Ti3C2 MXene with high bulk density can act as prospective electrode material to construct high volumetric energy density supercapacitors. Herein, a series of nitrogen-doped Ti3C2 (N-Ti3C2) films are successfully synthesized by adjusting the quantity of urea using a facile solvothermal method. The incorporation of nitrogen atoms can enlarge interlayer spacing and diminish -F terminal groups of Ti3C2 to promote the diffusion and intercalation of electrolyte ions, and bring about defect sites to provide more electrochemical reactive sites, thus improving the specific capacitance of N-Ti3C2. As a result, the 2.0 N-Ti3C2 film shows the highest volumetric capacitance of 2898.5 F cm-3 at scan rate of 2 mV s-1 in 3 M H2SO4 solution, which is mainly attributed to its high pyridinic N content and low -F terminal groups. Density functional theory (DFT) calculations indicate that the doped N atoms have a higher adsorption energy for protons to enhance the pseudocapacitance of 2.0 N-Ti3C2 electrode, and the increased density of states demonstrate the improvement of its electric conductivity after N doping, which promotes the rate performance of 2.0 N-Ti3C2 electrode. The assembled symmetric supercapacitor based on 2.0 N-Ti3C2 film delivers a high volumetric energy density of 40.8 Wh L-1 and good cycling stability. Our work provides an effective and alternative strategy to improve capacitance of other MXenes for highperformance supercapacitors.