Flexible Nitrogen-Doped 2D Titanium Carbides (MXene) Films Constructed by an Ex Situ Solvothermal Method with Extraordinary Volumetric Capacitance

A facile and controllable strategy, which combines solvothermal treatment with ex situ nitrogen doping by using urea saturated alcohol solution or monoethanolamine as nitrogen source, is used to prepare flexible, freestanding, and compact nitrogen-doped delaminated Ti₃C₂ (abbreviated N-Ti₃C₂) film electrodes for symmetric electrochemical capacitors (ECs). Compared with the N sites from in situ N solid solution doping, those of ex situ N solvothermal doping enable larger contributions to the capacitance through regulating nitrogen species and content. As a result, the urea-assisted N-Ti₃C₂ (UN-Ti₃C₂) film exhibits an ultrahigh volumetric capacitance of 2836 F cm⁻³ (927 F g⁻¹) at 5 mV s⁻¹ in 3 m H₂SO₄ solution. This value surpasses the all previously reported volumetric performance of MXenes. A large capacitance of 2643 F cm⁻³ (786 F g⁻¹) is also obtained for the monoethanolamine-assisted N-Ti₃C₂ film. In addition, the symmetric electrochemical capacitor fabricated from the binder-free UN-Ti₃C₂ film exhibits a high volumetric energy density of 76 Wh L⁻¹, which is the highest value achieved compared to those of MXenes so far. This work presents the effects of nitrogen species and solvothermal treatment on the electrochemical performance of MXene, and opens up an exciting opportunity for fabricating highly flexible and integrated ECs.