2D transition metal carbides and nitrides, named MXenes, are attracting increasing attentions and showing competitive performance in energy storage devices including electrochemical capacitors, lithium- and sodium-ion batteries, and lithium-sulfur batteries. However, similar to other 2D materials, MXene nanosheets are inclined to stack together, limiting the device performance. In order to fully utilize MXenes' electrochemical energy storage capability, here, processing of 2D MXene flakes into hollow spheres and 3D architectures via a template method is reported. The MXene hollow spheres are stable and can be easily dispersed in solvents such as water and ethanol, demonstrating their potential applications in environmental and biomedical fields as well. The 3D macroporous MXene films are free-standing, flexible, and highly conductive due to good contacts between spheres and metallic conductivity of MXenes. When used as anodes for sodium-ion storage, these 3D MXene films exhibit much improved performances compared to multilayer MXenes and MXene/carbon nanotube hybrid architectures in terms of capacity, rate capability, and cycling stability. This work demonstrates the importance of MXene electrode architecture on the electrochemical performance and can guide future work on designing high-performance MXene-based materials for energy storage, catalysis, environmental, and biomedical applications.

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1393wileyonlinelibrary.com the practical applications of SIBs have been hamstrung by the lack of suitable anode materials to host Na + , which has a larger radius than that of Li + . Graphite with a highly ordered structure is considered to be not suitable to accommodate Na + because Na hardly forms staged intercalation compounds with graphite. [ 2 ] Twodimensional layered metal sulfi des (LMSs) with analogous structures to graphite, such as MoS 2 , [ 3 ] WS 2 , [ 4 ] SnS, [ 5 ] and SnS 2 , [ 6 ] have been reported as potential electrode materials for SIBs. The open framework of these types of materials allows Na + to insert reversibly with acceptable mobilities. However, the further application of 2D LMSs is impeded by their inherent limitations. First, these semiconductor metal sulfi des have inherently low electronic conductivity, which affects their electrochemical performances for Na + storage. Second, owing to the high surface energy and interlayer van der Waals attractions, [ 7 ] these thermally unstable 2D nanomaterials have a tendency to restack to minimize the surface energy. Furthermore, the signifi cant volume change and mechanical stress as a concomitant of sodium-ion insertion and extraction can induce the failure of the electrode and the loss of contact between active materials and the current collector, resulting in poor cycling stability.Graphene has established itself as a promising candidate to circumvent these challenges. For example, WS 2 /graphene, [ 4 ] SnS/graphene, [ 5b ] and SnS 2 /graphene [ 6 ] nanocomposites have already been successfully applied as anode materials for SIBs, showing a synergistic effect for sodium-ion storage, including improved capacity, rate capability, and cycling stability. In these reports, it is generally recognized that the enhanced electrochemical performances are attributed to the good electronic conductivity and mechanical resilience of graphene as 2D conformal building blocks for these layered sulfi des. However, a fundamental understanding of the exact interaction mechanism between LMSs and graphene for improving Na + storage performance is still not clear. The heterointerface between LMSs and graphene has been proven to contribute to novel properties and new functionalities that cannot be achieved by individual constituting materials. [ 8 ] Therefore, investigations Graphene has been widely used as conformal nanobuilding blocks to improve the electrochemical performance of layered metal sulfi des (MoS 2 , WS 2 , SnS, and SnS 2 ) as anode materials for sodium-ion batteries. However, it still lacks in-depth understanding of the synergistic effect between these layered sulfi des and graphene, which contributes to the enhanced electroactivity for sodium-ion batteries. Here, MoS 2 /reduced graphene oxide (RGO) nanocomposites with intimate two-dimensional heterointerfaces are prepared by a facile one-pot hydrothermal method. The heterointerfacial area can be effectively tuned by changing the ratio of MoS 2 to RGO. When used as anode materials for sodiu...

Two-dimensional (2D) heterostructured materials, combining the collective advantages of individual building blocks and synergistic properties, have spurred great interest as a new paradigm in materials science. The family of 2D transition-metal carbides and nitrides, MXenes, has emerged as an attractive platform to construct functional materials with enhanced performance for diverse applications. Here, we synthesized 2D MoS -on-MXene heterostructures through in situ sulfidation of Mo TiC T MXene. The computational results show that MoS -on-MXene heterostructures have metallic properties. Moreover, the presence of MXene leads to enhanced Li and Li S adsorption during the intercalation and conversion reactions. These characteristics render the as-prepared MoS -on-MXene heterostructures stable Li-ion storage performance. This work paves the way to use MXene to construct 2D heterostructures for energy storage applications.

materials to host sodium ions. Nanostructuring and hierarchical electrode architectures are required to facilitate transport of Na-ions within electrodes. [ 3 ] Recent studies have provided a leap forward in developing promising anode materials, such as carbonaceous materials, [ 4 ] phosphorus, [ 5 ] metallic alloys, [ 6 ] titanates, [ 7 ] and 2D metal carbides (MXenes). [ 8 ] MoS 2 , a layered material with S-Mo-S motifs stacked together by van der Waals forces, received extraordinary attention in the last few years. Many inroads have been made recently in developing MoS 2 based electrode materials for SIBs. For example, MoS 2 nanofl owers with expanded interlayers have been prepared as intercalationtype electrode materials in the voltage window of 0.4-3.0 V. [ 9 ] When the voltage window is expanded to 0.01-3.0 V, MoS 2 follows an intercalation-conversion mechanism for Na + storage. Due to the low conductivity and the huge volume variations of MoS 2 during charge/discharge processes, bare MoS 2 electrodes exhibited poor rate capability and fast capacity decay upon cycling. [ 10 ] To overcome this limitation, dispersing MoS 2 in carbon matrices with high electronic conductivity has been proved effective for improving the electrochemical properties in SIBs. Different MoS 2 -carbon hybrid materials have been tested as anodes for SIBs, such as MoS 2 nanodots embedded in carbon nanowires, [ 11 ] MoS 2 /graphene composites, [ 10,12 ] MoS 2 /CNT composites, [ 13 ] and MoS 2 /carbon nanospheres. [ 14 ] Usually, current collectors, conductive agents and binders are needed to fabricate fi lm electrodes, which inherently increases the total weight and cost of SIBs. Furthermore, they suffer from low initial Coulombic effi ciency (ICE <60%). The low ICE originates from: (i) the formation of solid electrolyte interfaces (SEI) caused by electrolyte decomposition; (ii) adverse side reactions between inactive components (conductive agent and binder) and sodium metal; (iii) electrical contact failure of electrode; and (iv) an excessive interface between carbon and electrolyte, which leads to considerable side reactions. [ 15 ] This low ICE requires a larger mass of the corresponding cathode material in full cells, thereby increasing the total weight and cost of SIBs. Rational geometrical design to give electrode materials a high ICE is a key research topic in SIBs. Moreover, anodeThe development of sodium-ion batteries for large-scale applications requires the synthesis of electrode materials with high capacity, high initial Coulombic effi ciency (ICE), high rate performance, long cycle life, and low cost. A rational design of freestanding anode materials is reported for sodium-ion batteries, consisting of molybdenum disulfi de (MoS 2 ) nanosheets aligned vertically on carbon paper derived from paper towel. The hierarchical structure enables suffi cient electrode/electrolyte interaction and fast electron transportation. Meanwhile, the unique architecture can minimize the excessive interface between carbon and electrolyte,...

This work demonstrates the molecular engineering of active sites on a graphene scaffold. It was found that the N-doped graphene nanosheets prepared by a hightemperature nitridation procedure represent a novel chemical function of efficiently catalyzing aerobic alcohol oxidation. Among three types of nitrogen species doped into the graphene latticepyridinic N, pyrrolic N, and graphitic N the graphitic sp 2 N species were established to be catalytically active centers for the aerobic oxidation reaction based on good linear correlation with the activity results. Kinetic analysis showed that the N-doped graphene-catalyzed aerobic alcohol oxidation proceeds via a Langmuir−Hinshelwood pathway and has moderate activation energy (56.1 ± 3.5 kJ•mol −1 for the benzyl alcohol oxidation) close to that (51.4 kJ•mol −1 ) proceeding on the catalyst Ru/Al 2 O 3 reported in literature. An adduct mechanism was proposed to be different remarkably from that occurring on the noble metal catalyst. The possible formation of a sp 2 N−O 2 adduct transition state, which can oxidize alcohols directly to aldehydes without any byproduct, including H 2 O 2 and carboxylic acids, may be a key element step. Our results advance graphene chemistry and open a window to study the graphitic sp 2 nitrogen catalysis.

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Lithium-sulfur batteries have attracted increasing attention as one of the most promising candidates for next-generation energy storage systems. However, the poor cycling performance and the low utilization of sulfur greatly hinder its practical applications. Here we report the improved performance of lithium-sulfur batteries by coating TiCT MXene nanosheets (where T stands for the surface termination, such as -O, -OH, and/or -F) on commercial "Celgard" membrane. In favor of the ultrathin two-dimensional structure, the TiCT MXene can form a uniform coating layer with a minimum mass loading of 0.1 mg cm and a thickness of only 522 nm. Owing to the improved electric conductivity and the effective trapping of polysulfides, the lithium-sulfur batteries with MXene-functionalized separators exhibit superior performance including high specific capacities and cycling stability.