Understanding the MXene Pseudocapacitance

Abstract

MXenes have attracted great attention as next-generation capacitive energy-storage materials, but the mechanisms underlying their pseudocapacitive behavior are not well understood. Here we provide a theoretical description of the surface redox process of TiCT (T = O, OH), a prototypical MXene, in 1 M HSO electrolyte, based on joint density functional theory with an implicit solvation model and the analysis of Gibbs free energy under a constant-electrode potential. From the dependence of the O/OH ratio (or the surface H coverage) and the surface charge on the applied potential, we obtain a clear picture of the capacitive energy-storage mechanism of TiCT that shows good agreement with previous experimental findings in terms of the integral capacitance and Ti valence change. We find a voltage-dependent redox/double-layer co-charging behavior: the capacitive mechanism is dominated by the redox process, but the electric double-layer charge works against the redox process. This new insight may be useful in improving the capacitance of MXenes.

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

“…[47,48] When potentials were applied, a shift of the (002) peak was only observed for anodic potentials (−0.1 to 0.2 V/Ag), where the expansion is diminished (Δc = −0.6 Å) ( Figure 4b). [34,35,37] Demonstrated by Hu et al, the change of surface terminations of Ti 3 C 2 T x from O to OH when a cathodic potential is applied can be followed using in situ Raman spectroscopy. Therefore, we turned our attention to the relationship between the pseudocapacitive nature of Ti 3 C 2 T x and the electrochromic properties observed.…”

“…[3] This material property, electrochromism, is associated with change in the oxidation state or ion insertion and extraction upon reduction and oxidation of the active general formula of M n+1 X n T x , where M is an early transition metal, X is C and/or N, and T x represents surfaces termination such as O, OH, and/or F. [19,20] Titanium carbide (Ti 3 C 2 T x ) was the first discovered and is the most studied MXene [17] and it can be synthesized at a large scale with high quality. [35] This mechanism allows for the storage of more energy than in electric double layer capacitors (EDLCs) due to the fast and reversible redox process involved, even at high rates (>100 mV s −1 ). [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.…”

Automated Scalpel Patterning of Solution Processed Thin Films for Fabrication of Transparent MXene Microsupercapacitors

“…Recently, a new family of 2D materials—termed MXenes—composed of transition metal carbides and nitrides became the front runner in 2D materials research . Their metallic conductivity combined with the hydrophilic nature of their oxide‐like‐surfaces make MXenes special, in terms of processing routes available for a variety of energy storage applications, especially for high‐rate pseudocapacitive energy storage . They are typically synthesized by etching “A” layers of corresponding layered, hexagonal carbide, popularly known as MAX phase (where M is early transition metal, A is group 13 or 14, X is C, N, or C/N) or other layered ceramics using aqueous fluoride etchants.…”

Delaminating Vanadium Carbides for Zinc‐Ion Storage: Hydrate Precipitation and H+/Zn2+ Co‐Action Mechanism

“…After being discharged to 0 V the intensity reversibly returns back which is opposite with the changes of O peak at 528 eV, accompanied by protonation of the oxygen functional groups. These changes clearly suggest the redox of vanadium and (de)intercalation of protons during cycling process, which can largely contribute to the pseudocapacitance of V 2 C MXene electrodes …”