Phase‐Mixed Nickel Sulfide Nanoarrays for Electrochemical Water Oxidation at Low Overpotential

Abstract Transition metal sulfide materials have demonstrated exceptional electrocatalytic activities for the hydrogen evolution reaction (HER); however, their efficacy for the oxygen evolution reaction (OER) remains limited, particularly in alkaline environments. This study introduces a novel nanoarray structure composed of phase‐mixed nickel sulfides, specifically rhombohedral NiS and Ni 3 S 2 , directly grown on nickel foam (denoted as bi‐NiS x /NF) through a straightforward hydrothermal method. The optimized bi‐NiS x /NF catalyst achieves an impressive overpotential of only 43 mV to reach a current density of 10 mA cm −2 in 1 m KOH electrolyte. The stability test of bi‐NiS x /NF under alkaline conditions uncovers that the intrinsic sulfur‐rich surface plays an important role in its superior OER activity. Additionally, annealing treatments demonstrate that OER performance is closely linked to sulfur retention, further affirming the significance of this structural sulfur‐rich characteristic. These findings provide valuable insights into the design of phase‐mixed catalysts and the intrinsic OER activity of transition metal sulfides, paving the way for future advancements in electrocatalytic materials.