The Origin of High Activity of Amorphous MoS₂ in the Hydrogen Evolution Reaction

Molybdenum disulfide (MoS₂) and related transition metal chalcogenides can replace expensive precious metal catalysts such as Pt for the hydrogen evolution reaction (HER). The relations between the nanoscale properties and HER activity of well-controlled 2H and Li-promoted 1T phases of MoS₂, as well as an amorphous MoS₂ phase, have been investigated and a detailed comparison is made on Mo−S and Mo−Mo bond analysis under operando HER conditions, which reveals a similar bond structure in 1T and amorphous MoS₂ phases as a key feature in explaining their increased HER activity. Whereas the distinct bond structure in 1T phase MoS₂ is caused by Li⁺ intercalation and disappears under harsh HER conditions, amorphous MoS₂ maintains its intrinsic short Mo−Mo bond feature and, with that, its high HER activity. Quantum-chemical calculations indicate similar electronic structures of small MoS₂ clusters serving as models for amorphous MoS₂ and the 1T phase MoS₂, showing similar Gibbs free energies for hydrogen adsorption (ΔG_H*) and metallic character.