Flexible Nitrogen‐Doped 2D Titanium Carbides (MXene) Films Constructed by an Ex Situ Solvothermal Method with Extraordinary Volumetric Capacitance

Abstract

A facile and controllable strategy, which combines solvothermal treatment with ex situ nitrogen doping by using urea saturated alcohol solution or monoethanolamine as nitrogen source, is used to prepare flexible, freestanding, and compact nitrogen‐doped delaminated Ti3C2 (abbreviated N‐Ti3C2) film electrodes for symmetric electrochemical capacitors (ECs). Compared with the N sites from in situ N solid solution doping, those of ex situ N solvothermal doping enable larger contributions to the capacitance through regulating nitrogen species and content. As a result, the urea‐assisted N‐Ti3C2 (UN‐Ti3C2) film exhibits an ultrahigh volumetric capacitance of 2836 F cm−3 (927 F g−1) at 5 mV s−1 in 3 m H2SO4 solution. This value surpasses the all previously reported volumetric performance of MXenes. A large capacitance of 2643 F cm−3 (786 F g−1) is also obtained for the monoethanolamine‐assisted N‐Ti3C2 film. In addition, the symmetric electrochemical capacitor fabricated from the binder‐free UN‐Ti3C2 film exhibits a high volumetric energy density of 76 Wh L−1, which is the highest value achieved compared to those of MXenes so far. This work presents the effects of nitrogen species and solvothermal treatment on the electrochemical performance of MXene, and opens up an exciting opportunity for fabricating highly flexible and integrated ECs.

Hydrous RuO2‐Decorated MXene Coordinating with Silver Nanowire Inks Enabling Fully Printed Micro‐Supercapacitors with Extraordinary Volumetric Performance

“…An intense (002) diffraction peak at around 6.5° is observed in the XRD pattern of the pure MXene in Figure 2f, corresponding to an interlayer spacing of ≈1.4 nm. [39] The low intensity and broad diffraction peak at 28.3° is attributed to the low crystallinity, hydrous RuO 2 . In contrast, the XRD pattern of the RuO 2 @MXene nanocomposite exhibited a clear shift downward in the (002) peak from ≈6.5° to ≈6.1°, corresponding to an increase of 0.09 nm in the spacing between MXene layers (inset in Figure 2f).…”

Two‐Dimensional Transition Metal Carbides and Nitrides (MXenes): Synthesis, Properties, and Electrochemical Energy Storage Applications

“…By modifying the surface chemistry and/or doping heteroatoms, the theoretical capacitance of MXene can be pushed further. For example, Yang et al successfully fabricated flexible and freestanding N‐doped Ti 3 C 2 T x films through solvothermal treatment. The resultant nitrogen‐doped Ti 3 C 2 T x films exhibited an ultrahigh capacitance of 2836 F cm −3 (927 F g −1 ) at 5 mV s −1 in the 3 m H 2 SO 4 electrolyte, being the record for all known MXene‐based materials …”

“…We note that specifying the scan rate when reporting the capacitance is necessary, since the scan rate heavily affect the ion diffusion kinetics and thus the utilization of active sites. On the other hand, the specific capacitance of Ti 3 C 2 T x can be greatly improved by increasing the redox active sites such as nitrogen doping, enlarging the interlayer spacing, and/or mixing with pseudocapacitive materials (i.e., metal oxide) …”

“…b) Schematic illustration of film preparation and electrode fabrication of the urea‐assisted nitrogen‐doped delaminated Ti 3 C 2 . Reproduced with permission from ref . Copyright 2018, John Wiley & Sons.…”

“…Discovered in 2011 by Gogotsi, Barsoum and other researchers, MXenes have quickly drawn research attention due to their unique properties and excellent performance in many applications. MXenes are synthesized by selectively etching the A element (typically Al and Ga) from the precursor layered ternary carbides or nitrides (MAX phase) (where M represents a transition metal and X is carbon and/or nitrogen) using hydrofluoric acid‐containing solutions or other etchants . Consequently, the surfaces of MXenes are terminated with abundant functional groups such as –OH, –O, and/or –F (hereafter called T), with the type and amount depending on the etching route .…”