Unraveling the Ionic Storage Mechanism of Flexible Nitrogen-Doped MXene Films for High-Performance Aqueous Hybrid Supercapacitors

2D MXene nanomaterials have excellent potential for application in novel electrochemical energy storage technologies such as supercapacitors and batteries, but the existing pure MXene is difficult to meet the practical needs. Although the electrochemical properties of modified MXene have been improved, the unclear ion storage mechanism still hinders the development of MXene-based electrode materials. Herein, the study develops flexible self-supported nitrogen-doped Ti₃C₂ (Py-Ti₃C₂) films by the highly mobile, high nitrogen content, oxygen-free pyridine-assisted solvothermal method, and then deeply investigates the energy storage mechanism of hybrid supercapacitors in four aqueous electrolytes (H₂SO₄, Li₂SO₄, Na₂SO₄, and MgSO₄). The experimental results suggest that the Py-Ti₃C₂ film electrode exhibits a pseudocapacitance-dominated energy storage mechanism. Particularly, the specific capacity of the Py-Ti₃C₂ in 1 M H₂SO₄ (506 F g⁻¹ at 0.1 A g⁻¹) is 4–5 times higher than other electrolytes (≈110 F g⁻¹), which could be attributed to the substantially higher ionic diffusion coefficient of H⁺ than those of Li⁺, Na⁺, Mg²⁺ with small ionic size, high ionic conductivity, and fast pseudocapacitance response. Theoretical analysis further confirms that Py-Ti₃C₂ has strengthened conductivity and electrical double-layer capacitance performance. Meanwhile, it has lower free energy for protonation and deprotonation of functional groups, which gives excellent pseudocapacitance performance.