Dynamic saturation of active sites in Ti3CNTx MXene enabled by coupled proton transport and surface redox kinetics

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Understanding how electrochemically active sites are utilized under high-rate conditions remains a fundamental challenge for MXene-based energy storage systems. In particular, the dynamic interplay between proton transport and surface redox reactions, which governs the accessibility and effective utilization of active sites, is still poorly understood. Herein, we investigate the charge-storage mechanism of Ti 3 CNT x MXene in acidic electrolytes by systematically tuning surface terminations and structural features via thermal annealing. The optimized sample (annealing at 300 °C) delivers a high specific capacitance of 470 F g −1 at 2 A g −1 and maintains excellent rate capability. Comprehensive characterization reveals that annealing induces coupled modifications in surface chemistry (reduction of F terminations and increase in O groups), electronic structure (increased Ti valence state), and porosity (formation of mesoporous channels), collectively facilitating proton transport and surface redox kinetics. More importantly, we introduce the concept of dynamic saturation of active sites, defined as a rate-dependent steady-state condition in which the occupation of active sites is governed by a dynamic balance between proton insertion and extraction processes. In situ EQCM measurements, together with electrochemical analysis, demonstrate that the progressive activation of active sites during initial cycling evolves into a stable dynamic equilibrium, enabling efficient utilization of active sites even at high rates. This work provides a mechanistic framework linking structural evolution, proton transport, and redox kinetics in MXenes, offering new insights for the rational design of high-rate pseudocapacitive materials.