Surface Functional Groups and Interlayer Water Determine the Electrochemical Capacitance of Ti₃C₂ T _x MXene

MXenes, an emerging class of conductive two-dimensional materials, have been regarded as promising candidates in the field of electrochemical energy storage. The electrochemical performance of their representative Ti₃C₂ T _x, where T represents the surface termination group of F, O, or OH, strongly relies on termination-mediated surface functionalization, but an in-depth understanding of the relationship between them remains unresolved. Here, we studied comprehensively the structural feature and electrochemical performance of two kinds of Ti₃C₂ T _x MXenes obtained by etching the Ti₃AlC₂ precursor in aqueous HF solution at low concentration (6 mol/L) and high concentration of (15 mol/L). A significantly higher capacitance was recognized in a low-concentration HF-etched MXene (Ti₃C₂ T _x-6M) electrode. In situ Raman spectroscopy and X-ray photoelectron spectroscopy demonstrate that Ti₃C₂ T _x-6M has more components of the -O functional group. In combination with X-ray diffraction analysis, low-field ¹H nuclear magnetic resonance spectroscopy in terms of relaxation time unambiguously underlines that Ti₃C₂ T _x-6M is capable of accommodating more high-mobility H₂O molecules between the Ti₃C₂ T _x interlayers, enabling more hydrogen ions to be more readily accessible to the active sites of Ti₃C₂ T _x-6M. The two main key factors ( i.e., high content of -O functional groups that are involved bonding/debonding-induced pseudocapacitance and more high-mobility water intercalated between the MXene interlayers) simultaneously account for the superior capacitance of the Ti₃C₂ T _x-6M electrode. This study provides a guideline for the rational design and construction of high-capacitance MXene and MXene-based hybrid electrodes in aqueous electrolytes.