One-step rapid synthesis Ti-O-rich Ti3C2 MXene via NH4HF2 hydrothermal etching for high charge storage

MXene has emerged as a promising alternative to carbon-based capacitor materials since its inception, however, the current practical implementation still faces dual challenges: environmentally hazardous, time-consuming synthesis procedures and intrinsically low-concentration pseudocapacitive active sites. Herein, a short-duration, eco-friendly and high-efficiency double fluoride salt hydrothermal strategy was proposed to fabricate Ti 3 C 2 MXene with unparalleled electrochemical properties. A comprehensive and good balance, including rapid MAX phase etching, just-right titanium terminal water oxidation, and ammonia reduction activation, is achieved via a one-step NH 4 HF 2 hydrothermal reaction at 160 °C for merely 1 h (TOR-MXene). It drastically outperforms conventional HF acid etching at 60 °C for 48 h (P-MXene). Systematic experimental characterizations (HRTEM, SAXS, XPS, EXAFS, EPR and in-situ XRD, in-situ Raman) and DFT calculations consistently reveal that instead of the generally believed pore expansion, the abundant highly-reduced titanium dioxide nanoclusters anchored on the surface of MXene exhibit substantial pseudocapacitive activation, accompanied by the threshold Bader charge of Ti atoms exceeding 10.5 |e|. Consequently, the prepared TOR-MXene delivers a remarkable specific capacitance of 446.1 F g −1 even at the ultrahigh current density of 30 A g −1 , a sixfold improvement over the P-MXene of 74.8 F g −1 . Moreover, the TOR-MXene electrode displays around 91.2% capacitance retention over 50,000 cycles, showing an extremely high cycling stability. This strategy establishes atomic-level design principles for MXene-based energy storage, advancing efficient synthesis and performance optimization.