Hydrogen Spillover-Bridged Volmer/Tafel Processes Enabling Ampere-Level Current Density Alkaline Hydrogen Evolution Reaction under Low Overpotential.

Water-alkaline electrolysis holds a great promise for industry-scale hydrogen production but is hindered by the lack of enabling hydrogen evolution reaction electrocatalysts to operate at ampere-level current densities under low overpotentials. Here, we report the use of hydrogen spillover-bridged water dissociation/hydrogen formation processes occurring at the synergistically hybridized NiS/CrS sites to incapacitate the inhibition effect of high-current-density-induced high hydrogen coverage at the water dissociation site and concurrently promote Volmer/Tafel processes. The mechanistic insights critically important to enable ampere-level current density operation are depicted from the experimental and theoretical studies. The Volmer process is drastically boosted by the strong HO adsorption at Cr sites of CrS, the efficient HO* dissociation a heterolytic cleavage process (Cr-HO* + S(#) → Cr-OH* + S-H) on the Cr/S sites in CrS, and the rapid desorption of OH* from Cr sites of CrS a new water-assisted desorption mechanism (Cr-OH* + HO(aq) → Cr-HO* + OH(aq)), while the efficient Tafel process is achieved through hydrogen spillover to rapidly transfer H from the synergistically located H-rich site (CrS) to the H-deficient site (NiS) with excellent hydrogen formation activity. As a result, the hybridized NiS/CrS electrocatalyst can readily achieve a current density of 3.5 A cm under an overpotential of 251 ± 3 mV in 1.0 M KOH electrolyte. The concept exemplified in this work provides a useful means to address the shortfalls of ampere-level current-density-tolerant Hydrogen evolution reaction (HER) electrocatalysts.