Transition metal carbides (TMCs) feature high catalytic activity and superior stability for the hydrogen evolution reaction (HER). However, their platinum‐like HER catalytic performance is heavily hindered, due to their strong interaction with hydrogen. Herein, Ni activation of TMCs (M = V, Fe, Cr, and Mo) is proposed through introducing adsorbed nickel atoms on the TMC surface (Ni/TMC). In both acidic and alkaline solutions, a sharp decrease of both overpotentials and Tafel slopes of the Ni/TMC catalysts for HER is achieved. At 10 mA cm−2, the overpotentials of the Ni/vanadium carbide (VC) and Ni/Fe3C catalysts are 128 and 93 mV in 1 m KOH, 111 and 112 mV in 0.5 m H2SO4, respectively. Even at 150 mA cm−2, they exhibit the overpotentials of as low as 270 and 291 mV, respectively. In the alkaline solutions, the performance of these Ni/TMC catalysts is even superior to a Pt/C catalyst. As confirmed from density functional theory calculations and X‐ray absorption fine structure analysis, such adsorbed Ni atoms effectively optimize the d‐electron structure and improve HER performance. As a versatile strategy, this work provides a universal route to activate TMCs for highly efficient HER in different media.

Although theoretical calculations predict vanadium carbide (V2C) MXene may possess superior performances as electrodes of supercapacitors and lithium-ion batteries, a milder synthesis process of high-purity V2C MXene undoubtedly becomes one of the greatest hindrances for extending its applications. Herein, we report a hydrofluoric acid (HF)-free synthesis of 2D V2C MXene by milder etching V2AlC powders in the mixture of lithium fluoride and hydrochloric acid. The etching time plays vital roles on the structure and morphology of V2C MXene. The as-synthesized V2C MXene etched for 120 h displays a uniform multilayered structure and higher purity (>90%). All that matter is it exhibits a maximum specific capacitance of 164 F g−1 at a scan rate of 2 mV s−1, good cycling stability and high specific capacitance retention (~90% after 10 000 cycles at 5 A g−1) in 1 M Na2SO4 electrolyte. Moreover, other fluorides, including potassium fluoride, sodium fluoride and ammonium hydrogen fluoride, are also testified to be effective to obtain highly pure V2C MXene. This work provides the certainty for synthesizing MXenes besides Ti3C2 via a HF-free etching process to beneficially expand their promising application.

Hydrate formation conditions of (H 2 + CH 4 ) binary gas mixtures in the presence of tetrahydrofuran (THF) in water were measured in a sapphire cell using the "pressure search" method. The experimental temperature ranged from 277.7 K to 288.4 K, and pressure ranged from 0.12 MPa to 8.86 MPa. The hydrogen composition in the gas mixture varied from 0 mol % to 97.85 mol %. The THF content in the aqueous solution was fixed at 6 mol %.

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