MoS₂ and\nWS₂ Nanosheets Decorated\non Metal–Organic Framework-Derived Cobalt/Carbon Nanostructures\nas Electrocatalysts for Hydrogen Evolution

Abstract

The replacement of platinum with nonprecious metal electrocatalysts\nfor hydrogen evolution reaction (HER) remains an important challenge.\nWe report facile synthesis of precious-metal-free HER electrocatalysts\nthat are made up of metal–organic framework-derived cobalt/carbon\nnanostructures and semicrystalline ultrathin MoS₂ or WS₂ nanosheets. The as-synthesized catalysts MoS₂/Co@NC\nand WS₂/Co@NC delivered an electrochemical HER current\ndensity of 25 mA cm^–2 at overpotentials of 0.23\nand 0.28 V, respectively. Both the catalysts were found to be highly\nstable in 0.5 M H₂SO₄ with small Tafel slope\nvalues. The high-performance HER activity can be related to (i) covalent\ncobalt doping into MoS₂ and WS₂ layers confirmed\nby X-ray photoelectron spectroscopy, (ii) the presence of cobalt nanoparticles\nin close vicinity of MoS₂ and WS₂ layers, (iii)\nthe presence of bridging disulfide S₂^2– into MoS₂ and WS₂ layers, and (iv) synergistic\ncooperation among multicomponents present in the catalyst structure.\nDensity functional theory calculations suggested that Co doping at\nMo sites in MoS₂ has a favorable Gibb’s free energy\n(Δ<i>G</i>) value for HER. Interestingly, the interface\nbetween the Co nanocluster and MoS₂ is found to be a favorable\nHER active site with localization of electrons. To the best of our\nknowledge, the simultaneous effect of single metal substitution and\nmetal clusters in/on MoS₂ and WS₂ layers has\nnot been studied for HER. Moreover, we have also demonstrated a durable\nacid–base water electrolyzer using MoS₂/Co@NC and\nWS₂/Co@NC as cathodes, generating 10 mA cm^–2 current density at a cell voltage of ∼0.89 V.