Engineering additional edge sites on molybdenum dichalcogenides toward accelerated alkaline hydrogen evolution kinetics

Abstract

The sluggish reaction kinetics of the hydrogen evolution reaction (HER) in alkaline media is a great obstacle to alkaline water electrolysis, and it remains a great challenge to develop precious metal-free efficient catalysts for the alkaline HER. Transition metal dichalcogenides (TMDs), in particular MoS₂ and MoSe₂, are promising catalysts for the HER in acidic media, but they exhibit much inferior catalytic activity for the alkaline HER owing to the slow water dissociation process. In this work, we, for the first time, demonstrate that TMD heterostructures with abundant edge sites deliver substantially accelerated alkaline HER kinetics, which is in great part due to the enhanced water adsorption/dissociation capability. As a proof of concept, MoS₂/MoSe₂ heterostructures with ultrasmall MoS₂ nanoclusters anchored on MoSe₂ nanosheets are synthesized via a solution-phase process and are investigated as alkaline HER catalysts in detail. MoSe₂ nanosheets serve as excellent substrates to hinder the agglomeration of MoS₂ nanoclusters, resulting in abundant edge sites. Benefiting from the decent water adsorption/dissociation capability of the edge sites, the optimal MoS₂/MoSe₂ heterostructure shows exceptional catalytic activity in 1 M KOH with an overpotential of 235 mV at 10 mA cm⁻² and a Tafel slope of 96 mV dec⁻¹, which is substantially improved as compared with the individual MoSe₂ (330 mV, 135 mV dec⁻¹) and MoS₂ (400 mV, 157 mV dec⁻¹). The success of this catalyst design strategy for enhancing alkaline HER kinetics is also demonstrated in MoSe₂/MoSe₂ and MoS₂/MoS₂ heterostructures. The results suggest that engineering additional edge sites that have a strong affinity for H₂O is critical for TMDs towards enhanced alkaline HER activity, and also open new avenues in the design of precious metal-free efficient catalysts for the alkaline HER.