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Density functional theory calculations are carried out to study the hydrogen evolution reaction (HER) at the electrochemical double-layer interface of monolayer 2H phase VS2 and water. Under typical conditions of HER, the catalyst surface is predicted to have a low hydrogen coverage of about 12%, whereas the aqueous solution side features a high hydronium concentration of about 8.3%. As a result, the HER takes place through the Volmer–Heyrovsky route, with an overall reaction barrier of about 1.0 eV, much larger than that of 1T phase VS2. This result demonstrates that 2H phase VS2 is much less reactive than its 1T phase counterpart, and the 1T-to-2H phase transformation induced by thickness reduction may deteriorate the HER activity of VS2.

Transition metal oxide nanostructures are drawing much attention as promising electrodes for advanced rechargeable batteries. However, due to the intrinsic low electronic conductivity and substantial volume change during cycling, the electrodes usually show low rate capability and poor cycling life. Herein, we report a route combining surface/interface engineering and defect chemistry to tune the lithium storage properties in nanoscale cubic Co 3 O 4 crystallites. The Co 3 O 4 crystallites were annealed in an inert atmosphere by carefully controlling the temperature, which induces the conformal formation of CoO layers with a tunable thickness on the surface of initial cubes. Microstructural characterizations and electrochemical measurements indicate that the optimized sample possesses a CoO thickness of ∼1.1 nm and shows a reversible lithium storage capacity of 1069.4 mAh•g −1 after 100 cycles of a current density of 0.1 A•g −1 . A capacity of 807.9 mAh•g −1 can be obtained at a rate of 5 A•g −1 . The improved lithium storage performance is attributed to the unique CoO−Co 3 O 4 interface structure and defect chemistry, which induces a strong electric field at the sharp Co 3 O 4 −CoO interface according to density functional theory calculations. The optimized sample also shows it improved sodium storage properties. This work thus provides an effective strategy for the design and synthesis of advanced electrode materials for energy applications.

Nickel hydroxide [Ni(OH) 2 ] thin films were synthesized by a solution process and characterized by x-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and time-of-flight secondary-ion mass spectrometry. Unlike nickel oxide (NiO) memristors, which exhibit only nonvolatile memory properties, the Ag/Ni(OH) 2 /Pt memristor can be operated in volatilethreshold resistive-switching mode with the compliance current (I cc ) varying from 10 nAto 100 μA. When I cc exceeds 1 mA, the nonvolatile switching mode is triggered. The resistiveswitching mechanism of Ag ion diffusion in Ni(OH) 2 is discussed by combining experimental analysis and density functional theory calculations. The results provide a useful guideline for designing volatile-threshold memristor.