Reproducible flexible Ti ₂ CT <em> _x </em> MXene films for high-rate proton-coupled pseudocapacitance in acidic aqueous electrolytes

Abstract Ti2CTx MXene presents significant fabrication challenges, such as difficulties in producing freestanding films, a high susceptibility to oxidation under ambient conditions, and inadequate mechanical toughness that results in brittleness. Here, through optimization of post-etch solution selection and physical delamination, the binder-free freestanding electrodes are fabricated with high electronic conductivity of 1098 S·cm-1. 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 compares favorably with previously reported Ti2CTx and many Ti3C2Tx-based electrodes. In contrast, electric double-layer capacitance dominates in NaCl or NaOH electrolytes. A rapid proton-coupled pseudocapacitance mechanism is confirmed by in situ XRD characterization, which reveals a reversible interlayer expansion-contraction of 0.22 nm during electrochemical cycling. First-principles analysis indicates that strong coupling between oxygen terminations and H3O+ lowers ion migration barriers, enabling high-rate response. These advancements in fabrication reliability, mechanistic insight, and rate capability offer a viable strategy for designing high-power flexible MXene supercapacitors. Overall, this work provides a reproducible fabrication strategy, mechanistic insight, and competitive high-rate performance for Ti2CTx-based flexible MXene electrodes.