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

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 assynthesized 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 S-2(2-) into MoS(2 )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 (Delta G) 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 current density at a cell voltage of similar to 0.89 V.