MoS₂ and WS₂ Nanosheets Decorated on Metal–Organic Framework-Derived Cobalt/Carbon Nanostructures as Electrocatalysts for Hydrogen Evolution

Abstract

The replacement of platinum with nonprecious metal electrocatalysts for hydrogen evolution reaction (HER) remains an important challenge. We report facile synthesis of precious-metal-free HER electrocatalysts that are made up of metal–organic framework-derived cobalt/carbon nanostructures and semicrystalline ultrathin MoS2 or WS2 nanosheets. The as-synthesized catalysts MoS2/Co@NC and WS2/Co@NC delivered an electrochemical HER current density of 25 mA cm–2 at overpotentials of 0.23 and 0.28 V, respectively. Both the catalysts were found to be highly stable in 0.5 M H2SO4 with small Tafel slope values. The high-performance HER activity can be related to (i) covalent cobalt doping into MoS2 and WS2 layers confirmed by X-ray photoelectron spectroscopy, (ii) the presence of cobalt nanoparticles in close vicinity of MoS2 and WS2 layers, (iii) the presence of bridging disulfide S22– into MoS2 and WS2 layers, and (iv) synergistic cooperation among multicomponents present in the catalyst structure. Density functional theory calculations suggested that Co doping at Mo sites in MoS2 has a favorable Gibb's free energy (ΔG) value for HER. Interestingly, the interface between the Co nanocluster and MoS2 is found to be a favorable HER active site with localization of electrons. To the best of our knowledge, the simultaneous effect of single metal substitution and metal clusters in/on MoS2 and WS2 layers has not been studied for HER. Moreover, we have also demonstrated a durable acid–base water electrolyzer using MoS2/Co@NC and WS2/Co@NC as cathodes, generating 10 mA cm–2 current density at a cell voltage of ∼0.89 V.