Electrocatalysts for hydrogen evolution reaction (HER) from water splitting have been widely studied as one of the efficient energy storage methods by producing hydrogen (H₂), the important clean energy resource. The best-known electrocatalyst for HER is platinum (Pt) that is expensive and scarce on earth. Molybdenum disulfide (MoS₂) has attracted great attentions as one of promising catalysts for replacing Pt not only because of the relatively low cost and earth abundancy but also great HER activity, selectivity and stability as a great catalyst. Since the edge site of MoS₂ was proved as an active site for HER, a great number of efforts on maximally exposing edge site have contributed to the studies about MoS₂ based catalysts for HER.

The basal plane of MoS₂ is catalytically inert for HER since the free energy of adsorbed hydrogen (delG_H*) known to a great descriptor for activation energy is thermodynamically uphill (~1.92eV) compared with that of active edge site (~0.06eV). However, sulfur (S) vacancy in the basal plane was very recently investigated as a new active site for HER demonstrating the basal plane that composes the majority part of MoS₂ can be utilized for HER. Diverse methods to generate S-vacancy to make a use of the basal plane of MoS₂ for efficient HER catalysts have been investigated such as Ar plasma treatment, hydrogen annealing and electrochemical desulfurization. However, HER activity of desulfurized MoS_2-x still needs to be improved to be competitive with the HER activities of platinum.

Cobalt is one of the non-precious metal promotors for MoS₂ based catalysts for hydrogen evolution reaction, water gas shift reaction and oxygen reduction reaction. In very recently, Co was doped in the basal plane of S-vacancy rich MoS₂ and Co-^SMoS₂ achieved a great catalytic activity for the hydrodeoxygenation (HDO) reaction. Since S-vacancy site is coordinatively unsaturated and has a high surface free energy like edge site, it is expected to provide a great nucleation site for doping or anchoring transition metal atoms or clusters. Thus, Co promotor can be employed for enhancing the catalytic activity of the basal plane of MoS₂ based catalysts.

Herein we report the great HER activity enhancement of the catalytically inert basal plane of MoS₂ by combining the catalytic effects of S-vacancy and Co cluster addition as a non-precious metal oxide promotor. We use both density functional theory (DFT) calculations and experiments to support the claim. The polycrystalline 2H phase MoS₂ multilayer whose basal plane is dominantly exposed was synthesized on fluorine-doped tin oxide (FTO) and carbon foam electrodes through thermolysis in the tube furnace. S-vacancy was generated on the basal plane of MoS₂ through the same electrochemical desulfurization method. On top of that, Co cluster was electrodeposited to further promote the HER activity of the basal plane under different desulfurization conditions. Consequently, we demonstrate S-vacancy plays a critical role for Co nucleation during electrodeposition and thus both electrochemically active surface area and the intrinsic catalytic activity increased. Especially, the overpotential at -10 mA/cm² was reduced around 300mV making the inert basal plane of MoS₂ have comparable overpotential about -0.21 V vs. RHE at -10 mA/cm² with the state of the art non-precious MoS₂ based catalysts.