Nickel sulfide-oxide heterostructured electrocatalysts: Bi-functionality for overall water splitting and in-situ...

Bi-functional electrocatalysts are desired for both hydrogen and oxygen evolution reactions (HER and OER). Herein, facile O₂-plasma activation is introduced to improve the bi-functionality via constructing nickel sulfide-oxide heterostructures. Ni₃S₂-NiO_x supported by nickel foam delivers obviously elevated HER and OER activity in comparison with pristine Ni₃S₂ and recently reported NiS_x-based electrocatalysts, featured by the low overpotentials for HER (104 mV) and OER (241 mV) at ±10 mA cm^-2 in 1.0 M KOH, as well as a voltage of 1.52 V for overall water splitting. As revealed by in-situ Raman spectroscopy, on the one hand, Ni(OH)₂ generated from Ni₃S₂ during alkaline HER accelerates water dissociation toward the gradually improved performance; on the other hand, this heterostructure undergoes extensive oxidation during OER, leading to excessive NiOOH covering on Ni₃S₂ and thereby declining activity. These changes are interpreted by the distinct thermodynamic relationship under specific electrochemical conditions via density functional theory calculations.