High-Capacitance Mechanism for Ti₃C₂T_x MXene by in Situ Electrochemical Raman Spectroscopy Investigation

Abstract

MXenes represent an emerging family of conductive two-dimensional materials. Their representative, TiCT, has been recognized as an outstanding member in the field of electrochemical energy storage. However, an in-depth understanding of fundamental processes responsible for the superior capacitance of TiCT MXene in acidic electrolytes is lacking. Here, to understand the mechanism of capacitance in TiCT MXene, we studied electrochemically the charge/discharge processes of TiCT electrodes in sulfate ion-containing aqueous electrolytes with three different cations, coupled with in situ Raman spectroscopy. It is demonstrated that hydronium in the HSO electrolyte bonds with the terminal O in the negative electrode upon discharging while debonding occurs upon charging. Correspondingly, the reversible bonding/debonding changes the valence state of Ti element in the MXene, giving rise to the pseudocapacitance in the acidic electrolyte. In stark contrast, only electric double layer capacitance is recognized in the other electrolytes of (NH)SO or MgSO. The charge storage ways also differ: ion exchange dominates in HSO, while counterion adsorption in the rest. Hydronium that is characterized by smaller hydration radius and less charge is the most mobile among the three cations, facilitating it more kinetically accommodated on the deep adsorption sites between the MXene layers. The two key factors, i.e., surface functional group-involved bonding/debonding-induced pseudocapacitance, and ion exchange-featured charge storage, simultaneously contribute to the superior capacitance of TiCT MXene in acidic electrolytes.

Electrochromic Effect in Titanium Carbide MXene Thin Films Produced by Dip‐Coating

“…However, the expansion upon intercalation for cathodic potentials (−0.1 to −0.8 V/Ag) is not significant compared to the cycling of Ti 3 C 2 T x with other electrolytes, [46] and suggests the origin of the optical peak shift is not because of the intercalation/deintercalation of H + ions alone. [33] The scattering peak at 723 cm −1 is assigned to the out-of-plane vibration of a CTi bond surrounded by an O-termination, such as in Ti 3 C 2 O 2 , whereas the peak at 708 cm −1 corresponds to that of C-Ti in a Ti 3 C 2 O(OH) environment. The pseudocapacitive mechanism relies on the reduction and oxidation of Ti-O/Ti-OH terminations, and the variation of the oxidation state of Ti in Ti 3 C 2 T x .…”

“…The pseudocapacitive mechanism relies on the reduction and oxidation of Ti-O/Ti-OH terminations, and the variation of the oxidation state of Ti in Ti 3 C 2 T x . [33] Adv. [33] The scattering peak at 723 cm −1 is assigned to the out-of-plane vibration of a CTi bond surrounded by an O-termination, such as in Ti 3 C 2 O 2 , whereas the peak at 708 cm −1 corresponds to that of C-Ti in a Ti 3 C 2 O(OH) environment.…”

“…When materials are capable of changing their optical properties under an electrical potential, electrochromic devices are obtained. [9,[26][27][28][29][30][31][32] Additionally, Ti 3 C 2 T x has shown pseudocapacitive behavior when used as electrode material with acidic electrolytes based on the protonation of the oxidelike surface terminations (from Ti-O to Ti-OH), [33] changing the oxidation state of Ti, [34] and the electron density of the material. [21,22] Ti 3 C 2 T x shows high metallic conductivity (8-10 kS cm −1 ), [23] hydrophilicity, sufficient chemical and environmental stability, [24,25] and promise as a transparent conductor due to the absorption of only ≈3% of visible light per nanometer thickness.…”

Antimonene Engineered Highly Deformable Freestanding Electrode with Extraordinarily Improved Energy Storage Performance

“…[9][10][11] However, as for supercapacitor applications, the explorations are extremely scarce. [15,16] However, although breakthroughs were made by pristine MXene films, development of the energy storage performances of MXene-based flexible supercapacitors is still in its infancy.Advanced 2D materials have spurred great interest as a new paradigm in pursuing improved energy storage performance. [14] As a result, the multistacked MXene "paper" freestanding electrodes demonstrate excellent energy storage performances, which are on the topmost level among conventional freestanding electrodes based on graphene and carbon nanotube.…”

“…[14] As a result, the multistacked MXene "paper" freestanding electrodes demonstrate excellent energy storage performances, which are on the topmost level among conventional freestanding electrodes based on graphene and carbon nanotube. [15,16] However, although breakthroughs were made by pristine MXene films, development of the energy storage performances of MXene-based flexible supercapacitors is still in its infancy.…”