MoOxSy/Ni3S2 Microspheres on Ni Foam as Highly Efficient, Durable Electrocatalysts for Hydrogen Evolution Reaction

Hydrogen energy derived from water splitting is the cleanest renewable energy source, but it is also very challenging to achieve because the hydrogen evolution reaction (HER) requires highly efficient and low-cost electrocatalysts. Here, we have fabricated a novel hierarchical system of amorphous molybdenum oxy/sulfide microspheres with crystalline Ni₃S₂ intergrown in situ on Ni foam (MoO_xS_y/Ni₃S₂/NF) as an outstanding electrocatalyst for HER. The MoO_xS_y/Ni₃S₂/NF demonstrates an ultra-low overpotential of 58 mV at a current density of 10 mA cm^-2 and extremely durable stability (>200 h), suggesting superior performance comparable to that of Pt-C/NF under acidic conditions. The X-ray absorption fine structure (XAFS) determines the average valence state of Mo to be +(5 + δ), with a coordination motif by O and S. To explain such high HER activity, a [Mo₂O₂(S,O)₄] dimer-based periodic model structure with the average composition of [Mo₄O₈S₄] interfaced with the Ni₃S₂(101) surface is proposed. The interactions between the Ni of Ni₃S₂ and bridging S/O of [Mo₄O₈S₄] result in an average formal Mo charge state between +5 and +6, and significant charge transfer from Ni₃S₂ to [Mo₄O₈S₄] activates the Mo = O bonds. The calculated |ΔG_H*| of less than 50 meV suggests that the double-bonded O is the most active site. This work points to the importance of oxy/sulfides with Mon+ (+5 < n < +6) as exceptional electrochemical catalysts for HER.