Vapor-phase hydrothermal transformation of a nanosheet array structure Ni(OH)2 into ultrathin Ni3S2 nanosheets on nickel foam for high-efficiency overall water splitting

Electrocatalytic water splitting has been widely accepted as an environmentally friendly approach to generate clean H2. However, the sluggish oxidation half reaction, namely, the oxygen evolution reaction (OER), usually requires a high overpotential, which is the obstacle to high-efficiency overall water splitting to generate H2. Herein, we report the preparation of ultrathin nanosheet array Ni3S2 with a 9–14 nm nanosheet thickness grown directly on a commercial Ni foam substrate (Ni3S2/NF) by in situ vapor-phase hydrothermal (VPH) transformation of the nanosheet array structure Ni(OH)2/NF precursor. As a multifunctional electrocatalyst, the as-fabricated Ni3S2/NF-2 (VPH time of 2 h) displays excellent electrocatalytic activities toward the hydrazine oxidation reaction (HzOR) with a potential of 0.415 V (vs. RHE) to deliver a current density of 100 mA cm−2, an OER with an overpotential of 425 mV obtaining the same current density, and a hydrogen evolution reaction (HER) with an onset potential of −0.05 V (vs. RHE) in 1.0 M KOH media. A two-electrode system is therefore constructed using Ni3S2/NF-2 as both the anode and cathode, capable of achieving 100 mA cm−2 at 0.867 V in 1.0 M KOH with 0.2 M hydrazine. Density functional theory (DFT) calculations reveal that the adsorption of N2H4 molecules on the Ni3S2 (110) is more thermodynamically favourable than H2O, thus contributing to the high HzOR activity.