Cobalt Sulfide/Nickel Sulfide Heterostructure Directly Grown on Nickel Foam: An Efficient and Durable Electrocatalyst for Overall Water Splitting Application

Fabrication of high-performance noble-metal-free bifunctional electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in water is a promising strategy toward future carbon-neutral economy. Herein, a one-pot hydrothermal synthesis of cobalt sulfide/nickel sulfide heterostructure supported by nickel foam (CoS _x/Ni₃S₂@NF) was performed. The Ni foam acted as the three-dimensional conducting substrate as well as the source of nickel for Ni₃S₂. The formation of CoS _x/Ni₃S₂@NF was confirmed by X-ray diffraction and X-ray photoelectron spectroscopy. The formation of CoS _x/Ni₃S₂@NF facilitated easy charge transport and showed synergistic electrocatalytic effect toward HER, OER, and overall water splitting in alkaline medium. Remarkably, CoS _x/Ni₃S₂@NF showed catalytic activity comparable with that of benchmarking electrocatalysts Pt/C and RuO₂. For CoS _x/Ni₃S₂@NF, overpotentials of 204 and 280 mV were required to achieve current densities of 10 and 20 mA cm^-2 for HER and OER, respectively, in 1.0 M KOH solution. A two-electrode system was formulated for overall water splitting reaction, which showed current densities of 10 and 50 mA cm^-2 at 1.572 and 1.684 V, respectively. The prepared catalyst exhibited excellent durability in HER and OER catalyzing conditions and also in overall water splitting operation. Therefore, CoS _x/Ni₃S₂@NF could be a promising noble-metal-free electrocatalyst for overall water splitting application.