Two-dimensional (2D) antimonene is a promising anode material in sodium-ion batteries (SIBs) because of its high theoretical capacity of 660 mAh g and enlarged surface active sites. However, its Na storage properties and sodiation/desodiation mechanism have not been fully explored. Herein, we propose the sodiation/desodiation reaction mechanism of 2D few-layer antimonene (FLA) based on results acquired by in situ synchrotron X-ray diffraction, ex situ selected-area electron diffraction, and theoretical simulations. Our study shows that the FLA undergoes anisotropic volume expansion along the a/b plane and exhibits reversible crystalline phase evolution (Sb ⇋ NaSb ⇋ NaSb) during cycling. Density-functional theory calculations demonstrate that the FLA has a small Na-ion diffusion barrier of 0.14 eV. The FLA delivers a larger capacity of 642 mAh g at 0.1 C (1 C = 660 mA g) and a high rate capability of 429 mAh g at 5 C and maintains a stable capacity of 620 mA g at 0.5 C with 99.7% capacity retention from the 10th to the 150th cycle. Considering the 660 mAh g theoretical capacity of Sb, the electrochemical utilization of Sb atoms of FLA is as high as 93.9% at a rate of 0.5 C for over 150 cycles, which is the highest capacity and Sb utilization ratio reported so far. Our study discloses the Na storage mechanism of 2D FLA, boosting promising applications of 2D materials for advanced SIBs.

The adhesion force between modified AFM tips and a mica substrate has been studied using an atomic force microscope. The force−distance curves measured by the microscope show that the adhesion force is sensitive to the surface energies of the materials coated on the tips. The adhesion force between a gold-coated tip and a mica surface is much larger than that between a paraffin-coated tip and a mica surface. A simple calculation shows that this behavior of the adhesion forces can be accounted for by the van der Waals and capillary forces between the AFM tips and the substrate.

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The conductivity of a proton-conducting membrane ͑PEM͒ depends on the characteristics of the ionic clusters both within the polymeric structure and on its outer surfaces. This work explores the use of conductive atomic force microcopy to characterize the surface ionic activity of these membranes and investigates the effect of the surface ionic activity on the performance of PEM fuel cells. Results obtained show that only a fraction of the membrane surfaces is active and that this fraction increases with the relative humidity in the gas phase. Also, no correlation exists between the membrane surface ionic activity and its topography as expected.In a proton exchange membrane ͑PEM͒ fuel cell, a proton conducting membrane like Nafion is used as the electrolyte. The proton conductivity of this membrane, which strongly affects the performance of a PEM fuel cell, depends on the characteristics of the ionic clusters both within the polymeric structure and on its outer surfaces. The bulk conductivity of these membranes has been extensively studied. 1-12 However, no study has been conducted to determine the effect of the morphology and activity of the ionic clusters on the surface of these membranes on the performance of PEM fuel cells. Furthermore, the effects of membrane preparation and treatment processes ͑sulfonation, protonating, boiling, drying, hot pressing, etc.͒ on the topography and activity of the ionic clusters on the membrane surface are not known. It is known, however, from water absorption, contact angle, and MRI experiments that the bulk membrane and its outer surface behave differently. This difference in behavior is also known to depend on the membrane pretreatment processes. 1,4-6,13 It has been postulated that the ionic clusters that are normally found outside of the Teflon structure when the membrane is hydrated reorganize and migrate into the hydrophobic Teflon structure when the membrane becomes dehydrated, resulting in a membrane with greatly reduced surface ionic conductivity. Existence of a Fluorine-Rich SkinThe first study of the surface ionic activity of proton conducting membranes and its effect on the performance of a fuel cell using these membranes was conducted in 1999. 14,15 In this study X-ray photoelectron spectroscopy ͑XPS͒ was used to determine the surface elemental composition, specifically the carbon-to-sulfur ratio, of Nafion 1100 membranes subjected to various treatment and surface modification processes. 14-17 Next, these membranes were incorporated into membrane-electrode-assemblies ͑MEAs͒ and tested in a PEM fuel cell to determine whether there was a correlation between the surface elemental composition and the fuel cell performance. The results of the XPS study are summarized in Table I, and those from the fuel cell test are shown in Fig. 1.Note that the surface composition of a dry, as-received Nafion 1100 membrane has lower sulfur-to-carbon ratio than that calculated from the chemical formula for Nafion 1100, 0.024 vs 0.053. The results also show that pretreatment with sulfuri...