Abstract
AbstractMonometallic Mo and W carbides as well as highly mixed (Mo,W) carbides with various Mo/W ratios were synthesized directly on oxygen‐functionalized carbon nanotubes (OCNTs), and used as noble‐metal‐free electrocatalysts in the hydrogen evolution reaction (HER) under both acidic and alkaline conditions. A purely orthorhombic structure was found in both monometallic and mixed carbide samples by X‐ray diffraction. Transmission electron microscopy images showed that the carbide particles were highly dispersed on the OCNTs with well‐controlled particle size. The homogeneous distribution of Mo and W in the carbides was confirmed by elemental mapping. (Mo,W)2C/OCNT with a Mo/W ratio of 3 : 1 showed the lowest overpotential to reach a current density of 10 mA/cm2 (87 mV in 0.1 M KOH and 92 mV in 0.5 M H2SO4), and the smallest Tafel slope of 34 mV/dec. Long‐term stability under both alkaline and acidic conditions was demonstrated for 24 h. Our results revealed that an optimal amount of W in the mixed carbide can significantly improve its performance in the HER following the Tafel reaction pathway, most likely due to the weakened Mo−Hads bond.
Highlights
Monometallic Mo and W carbides as well as highly mixed (Mo,W) carbides with various Mo/W ratios were synthesized directly on oxygen-functionalized carbon nanotubes (OCNTs), and used as noble-metal-free electrocatalysts in the hydrogen evolution reaction (HER) under both acidic and alkaline conditions
Our results revealed that an optimal amount of W in the mixed carbide can significantly improve its performance in the HER following the
Metal chlorides MoCl5 and WCl6 were used as metal precursors instead of the commonly used ammonium molybdate to obtain mixed carbides with high dispersion of Mo and W and finely controlled nanoparticle size
Summary
Monometallic Mo and W carbides as well as highly mixed (Mo,W) carbides with various Mo/W ratios were synthesized directly on oxygen-functionalized carbon nanotubes (OCNTs), and used as noble-metal-free electrocatalysts in the hydrogen evolution reaction (HER) under both acidic and alkaline conditions. Molybdenum carbides can be found in four phases including α-MoC1-x, β-Mo2C, η-MoC, and γ-MoC, which share a very similar hexagonal structure with different stacking sequences.[7] Previous studies demonstrated that β-Mo2C exhibits the highest activity in the HER.[7,10] The strong bonding between Mo and Hads, which is mainly due to the high density of empty d-orbitals in Mo, is the main obstruction that slows down the reaction kinetics of the HER.[11] to intrinsically improve the performance of Mo carbide in catalyzing the HER, the most straight-forward solution is to weaken the strong.
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