Reproducible and flexible Ti₂CT_x MXene films for high-rate proton-coupled pseudocapacitance in acidic aqueous electrolytes

Ti₂CT_x MXene presents significant fabrication challenges, including difficulties in producing freestanding films, high susceptibility to oxidation under ambient conditions, and inadequate mechanical toughness, which results in brittleness. Here, through optimization of post-etch solution selection and physical delamination, binder-free freestanding electrodes with a high electronic conductivity of 1098 S·cm⁻¹ are fabricated. The electrode exhibits high-rate specific capacitance and excellent rate capability, along with significant electrolyte-dependent behavior. In H₂SO₄, surface-dominated pseudocapacitance involving protons/hydronium ions prevails, delivering a specific capacitance of 617 F·g⁻¹ at 2 mV·s⁻¹ and 680 F·g⁻¹ at 1 A·g⁻¹, while retaining 298 F·g⁻¹ at 100 A·g⁻¹. This rate capability is comparable to that of previously reported Ti₂CT_x and many Ti₃C₂T_x-based electrodes. In contrast, electric double-layer capacitance dominates in NaCl or NaOH electrolytes. A rapid proton-coupled pseudocapacitance mechanism is confirmed using in situ X-ray diffraction, which reveals a reversible interlayer expansion–contraction of 0.22 nm during electrochemical cycling. First-principles analysis indicates strong coupling between the oxygen terminations and H₃O⁺, which lowers the ion migration barrier, enabling a high-rate response. These advancements in fabrication reliability, mechanistic insight, and rate capability offer a viable strategy for designing Ti₂CT_x-based flexible MXene electrodes with enhanced high-rate performance for high-power supercapacitors. Overall, this work provides a reproducible fabrication strategy, mechanistic insight, and competitive high-rate performance for Ti₂CT_x-based flexible MXene electrodes.