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

Abstract

Fabrication\nof high-performance noble-metal-free bifunctional electrocatalysts\nfor both hydrogen evolution reaction (HER) and oxygen evolution reaction\n(OER) in water is a promising strategy toward future carbon-neutral\neconomy. Herein, a one-pot hydrothermal synthesis of cobalt sulfide/nickel\nsulfide heterostructure supported by nickel foam (CoS_<i>x</i>/Ni₃S₂@NF) was performed. The\nNi foam acted as the three-dimensional conducting substrate as well\nas the source of nickel for Ni₃S₂. The formation\nof CoS_<i>x</i>/Ni₃S₂@NF\nwas confirmed by X-ray diffraction and X-ray photoelectron spectroscopy.\nThe formation of CoS_<i>x</i>/Ni₃S₂@NF facilitated easy charge transport and showed synergistic\nelectrocatalytic effect toward HER, OER, and overall water splitting\nin alkaline medium. Remarkably, CoS_<i>x</i>/Ni₃S₂@NF showed catalytic activity comparable with\nthat of benchmarking electrocatalysts Pt/C and RuO₂. For\nCoS_<i>x</i>/Ni₃S₂@NF, overpotentials\nof 204 and 280 mV were required to achieve current densities of 10\nand 20 mA cm^–2 for HER and OER, respectively, in\n1.0 M KOH solution. A two-electrode system was formulated for overall\nwater splitting reaction, which showed current densities of 10 and\n50 mA cm^–2 at 1.572 and 1.684 V, respectively. The\nprepared catalyst exhibited excellent durability in HER and OER catalyzing\nconditions and also in overall water splitting operation. Therefore,\nCoS_<i>x</i>/Ni₃S₂@NF could\nbe a promising noble-metal-free electrocatalyst for overall water\nsplitting application.