High-valent bimetal Ni3S2/Co3S4 induced by Cu doping for bifunctional electrocatalytic water splitting

Rational design of low-cost and efficient electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is imperative for renewable energy conversion. Herein, for the first time, a facile strategy is developed to synthesize M (M = Fe, Cu, Zn, Mo) doped bimetallic sulfide heterostructure Ni3S2/Co3S4 electrocatalysts. The as-prepared bifunctional Cu-Ni3S2/Co3S4 electrode exhibits excellent electrocatalytic activity for HER and OER in 1 M KOH electrolyte, and it requires only an overpotential of 79 mV (150 mV) to deliver 10 mA cm−2 (20 mA cm-2) current density for HER process. Moreover, it shows a considerable low cell voltage of 1.49 V at the current density of 10 mA cm-2 in a two-electrode configuration which is far surpassing most of the reported bifunctional metal sulfides. Meanwhile, besides increasing the specific surface area of the electrocatalyst by optimizing the microstructure, the introduction of Cu cation could also stimulate the formation of high-valent Ni/Co sites, which can be verified by XPS technique. Density function theory calculations demonstrate that the Cu-doping boosts the formation of high valent Co sites and enhances the charge transfer performance of Ni and Co species, thus promotes intrinsic catalytic activity through modulating the d-band center of Co and reducing the adsorption energy of H and O-containing intermediates (H*, OH*, OOH*) on the surface of the catalyst. This work suggests the importance of exploitation of transition metal ion-doping for promoting the electrocatalytic activity of bimetallic sulfides.