MXene, an important and increasingly popular category of postgraphene 2D nanomaterials, has been rigorously investigated since early 2011 because of advantages including flexible tunability in element composition, hydrophobicity, metallic nature, unique in-plane anisotropic structure, high charge-carrier mobility, tunable band gap, and favorable optical and mechanical properties. To fully exploit these potentials and further expand beyond the existing boundaries, novel functional nanostructures spanning monolayer, multilayer, nanoparticles, and composites have been developed by means of intercalation, delamination, functionalization, hybridization, among others. Undeniably, the cutting-edge developments and applications of clay-inspired 2D MXene platform as electrochemical electrode or photo-electrocatalyst have conferred superior performance and have made significant impact in the field of energy and advanced catalysis. This review provides an overview of the fundamental properties and synthesis routes of pure MXene, functionalized MXene and their hybrids, highlights the state-of-the-art progresses of MXene-based applications with respect to supercapacitors, batteries, electrocatalysis and photocatalysis, and presents the challenges and prospects in the burgeoning field.

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Microporous carbon materials with extremely small pore size are prepared by employing polyaniline as a carbon precursor and KOH as an activating agent. CO(2) sorption performance of the materials is systematically investigated at the temperatures of 0, 25 and 75 °C. The prepared carbons show very high CO(2) uptake of up to 1.86 and 1.39 mmol g(-1) under 1 bar, 75 °C and 0.15 bar, 25 °C, respectively. These values are amongst the highest CO(2) capture amounts of the known carbon materials. The relation between CO(2) uptake and pore size at different temperatures is studied. An interesting and innovative point that the micropores with pore size smaller than a critical value play a crucial role in CO(2) adsorption at different temperatures is demonstrated. It is found that the higher the sorption temperature is, the smaller this critical value of pore size is. Pores smaller than 0.54 nm are manifested to determine CO(2) capture capacity at high sorption temperature, e.g. 75 °C. This research proposes a basic principle for designing highly efficient CO(2) carbon adsorbents; that is, the adsorbents should be primarily rich in extremely small micropores.

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Graphene aerogel (GA) is successfully prepared through hydrogen reduction of graphene oxide aerogel (GOA) which is self-assembled from graphene oxide solution and subsequently dried by a supercritical CO 2 method. The morphology, structure and surface property evolution in the preparation of GA are investigated intensively by a variety of means such as atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), N 2 adsorption, X-ray diffraction (XRD), Raman spectroscopy, ultraviolet-visible absorption spectroscopy (UV-Vis), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR). A self-assembly mechanism based on the hydrogen-bonding interactions between hydroxyl groups and carbonyl groups is proposed for the first time to explain the formation of GA. As evidenced by elemental analysis (EA) and electrochemical measurements, this three dimensional GA has an unprecedented high C/O molar ratio of 69.9, which contributes to the excellent high-rate performance of this material for supercapacitor applications.

Open carbon nanotube materials with hierarchical porosity and N-doping are prepared from polyaniline nanotubes via a combination method of pre-carbonization and post-KOH activation. The morphology, pore texture and surface properties of the carbon materials are investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), N 2 adsorption, X-ray photoelectron spectroscopy (XPS), energy dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. The prepared carbon materials have a typical hierarchical pore texture and very high specific surface area up to 3253 m 2 g À1 . The electrochemical capacitive performance of the prepared carbons was systematically investigated in the 6 M KOH electrolyte. HPCT-4 exhibits high charge storage capacity with a specific capacitance of 365.9 F g À1 at a current density of 0.1 A g À1 , good rate capability of 60% in the range of 0.1-10 A g À1 , and excellent stability over 10 000 cycles. The high capacitive performance could be due to the hierarchical porosity combined with high effective surface area and heteroatom doping effects, resulting in both electrochemical double layer and Faradaic capacitance contributions.

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