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

Abstract

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 Ni₃S₂/Cr₂S₃ 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 H₂O adsorption at Cr_5c sites of Cr₂S₃, the efficient H₂O* dissociation <i>via</i> a heterolytic cleavage process (Cr_5c-H₂O* + S_3c(#) → Cr_5c-OH* + S_3c-H^#) on the Cr_5c/S_3c sites in Cr₂S₃, and the rapid desorption of OH* from Cr_5c sites of Cr₂S₃ <i>via</i> a new water-assisted desorption mechanism (Cr_5c-OH* + H₂O(aq) → Cr_5c-H₂O* + OH^-(aq)), while the efficient Tafel process is achieved through hydrogen spillover to rapidly transfer H^# from the synergistically located H-rich site (Cr₂S₃) to the H-deficient site (Ni₃S₂) with excellent hydrogen formation activity. As a result, the hybridized Ni₃S₂/Cr₂S₃ 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.