Two-dimensional Ti2CT x MXene nanosheets were prepared by the selective etching of Al layer from Ti2AlC MAX phase using HF treatment. The MXene sheets retained the hexagonal symmetry of the parent Ti2AlC MAX phase. Effect of the postetch annealing ambient (Ar, N2, N2/H2, and air) on the structure and electrochemical properties of the MXene nanosheets was investigated in detail. After annealing in air, the MXene sheets exhibited variations in structure, morphology, and electrochemical properties as compared to HF treated MAX phase. In contrast, samples annealed in Ar, N2, and N2/H2 ambient retained their original morphology. However, a significant improvement in the supercapacitor performance is observed upon heat treatment in Ar, N2, and N2/H2 ambients. When used in symmetric two-electrode configuration, the MXene sample annealed in N2/H2 atmosphere exhibited the best capacitive performance with specific capacitance value (51 F/g at 1A/g) and high rate performance (86%). This improvement in the electrochemical performance of annealed samples is attributed to highest carbon content, and lowest fluorine content on the surface of the sample upon annealing, while retaining the original two-dimensional layered morphology and providing maximum access of aqueous electrolyte to the electrodes.

Herein we demonstrate that a prominent member of the MXene family, Ti2C, undergoes surface oxidation at room temperature when treated with hydrogen peroxide (H2O2). The H2O2 treatment results in opening up of MXene sheets and formation of TiO2 nanocrystals on their surface, which is evidenced by the high surface area of H2O2 treated MXene and X-ray diffraction (XRD) analysis. We show that the reaction time and the amount of hydrogen peroxide used are the limiting factors, which determine the morphology and composition of the final product. Furthermore, it is shown that the performance of H2O2 treated MXene as an anode material in Li ion batteries (LIBs) was significantly improved as compared to as-prepared MXenes. For instance, after 50 charge/discharge cycles, specific discharge capacities of 389 mA h g(-1), 337 mA h g(-1) and 297 mA h g(-1) were obtained for H2O2 treated MXene at current densities of 100 mA g(-1), 500 mA g(-1) and 1000 mA g(-1), respectively. In addition, when tested at a very high current density, such as 5000 mA g(-1), the H2O2 treated MXene showed a specific capacity of 150 mA h g(-1) and excellent rate capability. These results clearly demonstrate that H2O2 treatment of Ti2C MXene improves MXene properties in energy storage applications, such as Li ion batteries or capacitors.

In this report, we show that oxide battery anodes can be grown on two-dimensional titanium carbide sheets (MXenes) by atomic layer deposition. Using this approach, we have fabricated a composite SnO 2 /MXene anode for Li-ion battery applications. The SnO 2 /MXene anode exploits the high Li-ion capacity offered by SnO 2 , while maintaining the structural and mechanical integrity by the conductive MXene platform. The atomic layer deposition (ALD) conditions used to deposit SnO 2 on MXene terminated with oxygen, fluorine, and hydroxylgroups were found to be critical for preventing MXene degradation during ALD. We demonstrate that SnO 2 /MXene electrodes exhibit excellent electrochemical performance as Liion battery anodes, where conductive MXene sheets act to buffer the volume changes associated with lithiation and delithiation of SnO 2. The cyclic performance of the anodes is further improved by depositing a very thin passivation layer of HfO 2 , in the same ALD reactor, on the SnO 2 /MXene anode. This is shown by high-resolution transmission electron microscopy to also improve the structural integrity of SnO 2 anode during cycling. The HfO 2 coated SnO 2 /MXene electrodes demonstrate a stable specific capacity of 843 mAh/g when used as Li-ion battery anodes.

coplanar interdigitated electrode architecture and shorter paths for in-plane diffusion of electrolyte ions are expected to exhibit improved power and rate capabilities over the conventional electrode designs. [4,5] Such architecture can also be simply integrated on the same substrate as other electronic components such as sensors or rectifiers. However, most of the efforts have been focused on fabricating microsupercapacitors on rigid and flexible substrates employing conventional microfabrication techniques. [1,4] For example, various carbonaceous materials including activated carbon, [6] onionlike carbon [7] carbide-derived carbons, [8] carbon nanotubes, [9] and graphene [10] were employed to demonstrate state-of-theart electrical double layer type capacitors. Furthermore, pseudocapacitive materials which store charge through fast surface redox reactions such as metal oxides, [11] hydroxides, [12] sulfides, [13] and conducting polymers [14] were employed to fabricate micropseudocapacitors. While aiming at the optimal electrochemical performance within a given foot-print area, fabricating thick coplanar electrodes on unconventional substrates such as paper and textiles could increase the amount of energy stored, however this proves to be a difficult task.Paper and textiles are among the most widely used materials, but building devices on their surfaces remains a challenge. [14][15][16] Attempts have been made to use graphite in pencils and inks for writing electrodes on paper. [17,18] Paper has a hierarchical arrangement of cellulose fibers (typical diameter of 20 μm), resulting in a porous and rough surface texture that is helpful for good adhesion of ink without any additional treatments. [17] Moreover, the capillary nature of cellulose fibers, inherent surface charge, and functional groups make the paper surface a universal platform for obtaining thick coatings up to 100 μm of various functional materials by solution processing. [17][18][19][20] For example, Hu et al. demonstrated a solution processable approach to make highly conductive paper for energy storage by integrating single-walled carbon nanotubes and metal nanowires. [19] Similarly, paper-based supercapacitors employing commonly used materials such as graphene, metal oxides, and conducting polymers have been demonstrated. [20][21][22] However, there have been no reports on direct fabrication of microsupercapacitors based on a new class of layered materials, MXenes, coated on an inexpensive paper substrate.MXenes are a new family of 2D layered transition metal carbides and nitrides, [23] which have shown great promise as potential electrode materials for electrochemical energy storage devices. [24,25] Ti 3 C 2 is the most studied member of the A simple and scalable direct laser machining process to fabricate MXeneon-paper coplanar microsupercapacitors is reported. Commercially available printing paper is employed as a platform in order to coat either hydrofluoric acid-etched or clay-like 2D Ti 3 C 2 MXene sheets, followed by laser machining to fabr...

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