Atomic Sulfur Filling Oxygen Vacancies Optimizes H Absorption, Boosts Hydrogen Evolution Reaction in Alkaline Media.

Hydrogen evolution reaction (HER) usually has sluggish kinetics in alkaline solutions due to the difficulty in the formation of  binding protons. Herein we report an electrocatalyst by sulfur atoms doping into the oxygen vacancies (V O ) of inverse spinel NiFe 2 O 4 (S-NiFe 2 O 4 ) to create active sites with enhanced electron transfer capability. Our results indicated this electrocatalyst has an ultralow overpotential of 61 mV at the current density of 10 mA cm -2 and long-term stability of 60 h at 1.0 A cm -2 in 1.0 M KOH media . After coupled with V O , the high electroactive S atoms facilitate binding with protons. In-situ Raman spectroscopy revealed that S sites absorb hydrogen adatom (H*) and in-situ form S-H*, that favor the production of hydrogen and boosts HER in the alkaline solution. DFT calculations further verified lowered energy barrier of the H 2 O dissociation induced by the S introduction, which is the key factor for this remarkable HER performance. Both experimental and theoretical investigations confirmed S atoms are active sites of the S-NiFe 2 O 4 . This work provides an facile and powerful solution in designing novel electrocatalysts to accelerate water splitting via vacancies filling strategy.