Elucidating the Charge Storage Mechanism on Ti₃C₂ MXene through In Situ Raman Spectroelectrochemistry

Abstract

Current climate issues can be partially remedied through the inclusion of renewable energy sources. However, these energy sources suffer from the need for highly efficient energy storage systems. To this end, studies have been conducted on developing energy storage materials that can provide high energy and power densities. Two-dimensional (2D) carbide and nitride MXenes have the potential to provide both if their mechanism of charge storage is understood. Here, we use in situ/operando Raman spectroelectrochemistry to investigate in real time the charge storage mechanism of the benchmark Ti 3 C 2 MXene in acidic and neutral media. We found that during charge/discharge cycling in acidic media, protons are pulled towards the surface, leading to a reversible reaction with the surface termination groups leading to a switch from À OÀ to À O(OH) to À OÀ chemistry, indicative of a pseudocapacitive mechanism. In neutral media, no pseudocapacitive behavior is observed and it is found that the material exhibits an electrostatic double layer charge storage mechanism through attracting sodium ions towards the surface for transient adsorption processes. Taking into consideration the mechanism we propose, the Ti 3 C 2 MXene can exhibit a pseudocapacitance of 358 F/g within a voltage window of 1.35 V. Ultimately, these fundamental insights can be used to design electrode materials with both high energy and power densities.

Elucidation of the Charging Mechanisms and the Coupled Structural–Mechanical Behavior of Ti3C2Tx (MXenes) Electrodes by In Situ Techniques

“…which showed slightly larger changes of 0.134$$\bar{e}$$ in the Ti oxidation state when a larger potential window was applied (0.9 V). [ 31 ] These findings are in line with in situ Raman studies conducted for acidic electrolyte solutions (0.1 m HCl) [ 32,33 ] As can be seen in Figure 2b significant voltage‐dependent changes of the Raman spectra, expressed by a continuous red shifting of the 730 cm −1 bands toward a lower wavenumber of 722 cm −1 were detected upon scanning of the potential from 0.4 to −0.6 V (vs Ag/AgCl). These variations in the observed spectra are attributed to the transition of Ti 3 C 2 O 2 to Ti 3 C 2 O(OH).…”

“…Based on this observation one can assume the occurrence of two charging mechanisms – double layer adsorption of protons which takes place at a positive potential range, and the subsequent protons‐induced surface redox reaction at more negative potentials. [ 32 ] The morphological changes of MXene films during proton insertion/extraction were further evaluated by in situ XRD measurements. [ 34 ] It was found that the changes of the interlayer spacing of the Ti 3 C 2 T x films are strongly affected by the applied voltage – upon charging to −0.6 V a lattice shrinking of 0.1 Å was probed, while further polarization to more negative potentials resulted in a significant expansion of the interlayer space by 0.1 Å (see Figure 2c).…”

“…Reproduced with permission. [ 32 ] Copyright 2022, Wiley c) in situ XRD analysis of MXene electrodes in 1 m H 2 SO 4 : i)CV response, ii) XRD patterns collected during charging/discharging, iii) the corresponding variation in the d spacing with the applied potential. Reproduced with permission.…”

Subsurface oxygen reduction reaction activity on Ti2N MXene revealed byin situRaman spectroelectrochemistry

“…In general, an increase in the intensity of a certain peak is typically indicative of an increase in the electron density, and consequently, the decrease in the peak intensity corresponds to a decrease in the electron density. 22,35 With this idea, the increase in the intensity of the OH peak when probing the surface corresponds to an increase in the density of adsorbed OH (Fig. 6c).…”

“…The Ti 3 C 2 MXene was delaminated in a similar manner as previously reported. 22,35 In short, the as-purchased Ti 3 C 2 MXene (NanoChemAZone 99 wt%) was intercalated using DMSO by stirring for 18 hours. The mixture was then centrifuged at 3500 RPM for 30 minutes (decanting the supernatant and replacing it with water between each centrifuge) until all DMSO was removed, then, the precipitate was membrane ltered and dried in a vacuum oven.…”