Quantitative Assessment on the Water Change of 2D Ti ₃ C ₂ T _X MXene Under Alkali Treatment

Alkali treatment is commonly applied to remove -F termination of Ti₃C₂T_x MXene, tailoring its structural and chemical characteristics for specialized applications; yet no guarantee of basal spacing expansion brings uncertainty in improving MXene's electrochemical performance, no mention the deteriorated electrical conductivity. Herein, through treating MXene with alkalis, the initial basal spacing has been noticed to have a major impact on the alkalization effects that proper spacing (≈1.35-1.40 nm) favors not only-F reduction but also spacing expansion. Moreover, distinct alkalization results between NaOH and KOH are demonstrated and explained by varied cation hydrophilicity and water diffusion dynamics, inferring different water changes related to a triple-threaded water motion process. Based on this, a diagnosis scheme is proposed that can experimentally quantify the water change of each process, further clarifying the water motion within the layered MXene structures. Interestingly, the electrochemical performance of Ti₃C₂T_x MXene after varied alkali treatments displays a shift of capacitance dependence from upon basal spacing under slow scan rate to upon F content under fast scan rate. The findings further deepen the understanding of water dynamics and structural changes of the 2D layered MXenes during alkali treatment, and should shed light in fine-designing the dehalogenation strategies to achieve desired properties.