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

Bi-functional electrocatalysts are desired for both hydrogen and oxygen evolution reactions (HER and OER). Herein, facile O-plasma activation is introduced to improve the bi-functionality via constructing nickel sulfide-oxide heterostructures. NiS-NiO supported by nickel foam delivers obviously elevated HER and OER activity in comparison with pristine NiS and recently reported NiS-based electrocatalysts, featured by the low overpotentials for HER (104 mV) and OER (241 mV) at ±10 mA cm 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) generated from NiS 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 NiS and thereby declining activity. These changes are interpreted by the distinct thermodynamic relationship under specific electrochemical conditions via density functional theory calculations.