Surface Functional Groups and Interlayer Water Determine the Electrochemical Capacitance of Ti₃C₂T_x MXene

Abstract

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 TiC T , 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 TiC T MXenes obtained by etching the TiAlC 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 (TiC T -6M) electrode. In situ Raman spectroscopy and X-ray photoelectron spectroscopy demonstrate that TiC T -6M has more components of the -O functional group. In combination with X-ray diffraction analysis, low-fieldH nuclear magnetic resonance spectroscopy in terms of relaxation time unambiguously underlines that TiC T -6M is capable of accommodating more high-mobility HO molecules between the TiC T interlayers, enabling more hydrogen ions to be more readily accessible to the active sites of TiC T -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 TiC T -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.

Scalable Synthesis of Ti3C2Tx MXene

“…The sharp peaks, at 126, 209, and 723 cm −1 are distinct features of delaminated Ti 3 C 2 T x . They are attributed to a Ti 3 C 2 T x resonant peak, A 1g (Ti, O, C) and A 1g (C) vibrations, respectively . The position of the peaks, as well as full width at half maximum are the same in samples prepared from large‐ and small‐batch synthesis, which suggests no significant differences in atomic vibrations within both films.…”

Electromagnetic Shielding of Monolayer MXene Assemblies

“…Thermal treatment increases the carrier mobility from 0.6 to 1.25 cm 2 V −1 s −1 and carrier density from 2.6 × 10 16 to 3.2 × 10 16 cm −2 . These enhancements are also attributed to the removal of surface terminations, generally known as electron scattering sites in 2D materials . The removal of hydrophilic terminal groups can also be visualized by water contact angle measurement (Figure F).…”

2 D MXene‐based Energy Storage Materials: Interfacial Structure Design and Functionalization

“…Hu et al. synthesized two types of Ti 3 C 2 T x MXenes at different HF concentrations . Compared to the sample etched at a high HF concentration (15 mol L −1 ), the MXene obtained at a low HF concentration (6 mol L −1 ) delivered a higher capacitance.…”

“…It is well knownt hat MXenes preparedu sing HF or fluoridebased salt etchingh ave abundant termination groups (i.e., ÀOH, ÀF, ÀO), which directly influence their energy-storage capacity.M any pioneering works have already verified thata high content of ÀOt ermination groups and/or al ow content of ÀFt ermination groupsa re favorable to improve the energystoragec apacity. [95,96] Hu et al synthesized two types of Ti 3 C 2 T x MXenesa td ifferent HF concentrations. [95] Compared to the sample etched at ah igh HF concentration (15 mol L À1 ), the MXene obtained at al ow HF concentration( 6mol L À1 )d elivered ah igher capacitance.…”