Surface Functional Groups and Interlayer Water Determine the Electrochemical Capacitance of Ti₃C₂T_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 Ti3C2Tx, 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 Ti3C2Tx 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 (Ti3C2Tx–6M) electrode. In situ Raman spectroscopy and X-ray photoelectron spectroscopy demonstrate that Ti3C2Tx–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 Ti3C2Tx–6M is capable of accommodating more high-mobility H2O molecules between the Ti3C2Tx interlayers, enabling more hydrogen ions to be more readily accessible to the active sites of Ti3C2Tx–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 Ti3C2Tx–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.