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 Ni 3 S 2 /Cr 2 S 3 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 H 2 O adsorption at Cr 5c sites of Cr 2 S 3 , the efficient H 2 O* dissociation via a heterolytic cleavage process (Cr 5c -H 2 O* + S 3c (#) → Cr 5c -OH* + S 3c -H # ) on the Cr 5c /S 3c sites in Cr 2 S 3 , and the rapid desorption of OH* from Cr 5c sites of Cr 2 S 3 via a new water-assisted desorption mechanism (Cr 5c -OH* + H 2 O(aq) → Cr 5c -H 2 O* + OH - (aq)), while the efficient Tafel process is achieved through hydrogen spillover to rapidly transfer H # from the synergistically located H-rich site (Cr 2 S 3 ) to the H-deficient site (Ni 3 S 2 ) with excellent hydrogen formation activity. As a result, the hybridized Ni 3 S 2 /Cr 2 S 3 electrocatalyst can readily achieve a current density of 3.5 A cm -2 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.