[en] Highlights: • A new class of nitrogen-doped two-dimensional MXene (N-Ti₃C₂T_x) is synthesized. • A significant increase of the c-lattice parameter of MXene sheets is observed upon ammonia treatment. • A drastic increase of up to 460% in gravimetric capacitance is achieved in the N-Ti₃C₂T_x compared to its undoped counterpart. A new type of nitrogen-doped two-dimensional MXene (N-Ti₃C₂T_x) was synthesized by post-etch annealing Ti₃C₂T_x in ammonia as a promising electrode material for supercapacitors. The concentrations of nitrogen can be rationally controlled to produce N-Ti₃C₂T_x materials with 1.7–20.7 at% of nitrogen by simply tuning the annealing temperatures from 200 °C to 700 °C. The introduction of nitrogen as a heteroatom in the Ti₃C₂T_x structure leads to a remarkable increase of the c-lattice parameter of MXene sheets from 1.92 nm in Ti₃C₂T_x to 2.46 nm in N-doped ones upon ammonia treatment at 200 °C. More interestingly, the resultant doped MXene materials under optimized condition exhibited drastically improved electrochemical capacitances of 192 F g⁻¹ in 1 M H₂SO₄ and 82 F g⁻¹ in 1 M MgSO₄ electrolyte, which are remarkably higher than those of the un-doped Ti₃C₂T_x materials (34 F g⁻¹ in 1 M H₂SO₄ and 52 F g⁻¹ in 1 M MgSO₄).