Enhanced electrocatalytic ammonia oxidation using nickel-copper sulfide catalysts: surface reconstruction and performance evaluation

• CV polarization offers more in situ oxidation compared to CA polarization. • An efficient strategy to accelerate the generation of NiOOH on the surface of NiS. • The presence of S-O species on the surface enhances the intrinsic activity of Ni. • Sulfur enhances charge transfer and electron flow between the metal sites. Transition metal sulfides are promising catalysts for the ammonia oxidation reaction (AOR), though their active species and sulfur’s role remain unclear. This study presents a high-performance bimetallic nickel–copper sulfide catalyst synthesized on nickel foam via hydrothermal deposition, sulfurization, and electrochemical surface reconstruction. Cyclic voltammetry (CV) treatment induced more effective in situ oxidation than chronoamperometry (CA), producing a needle-like structure and partially converting sulfides into (oxy)hydroxides, thereby enhancing active site area and charge transfer. The Ni 0.5 Cu 1.5 -S/NF_CV catalyst showed the highest performance, achieving a current density of 152.52 mA·cm −2 at 1.55 V vs . RHE, an electrochemical surface area (ECSA) of 192.12 cm 2 , Tafel slope of 90.72 mV·dec −1 , and turnover frequency of 0.114 s −1 . Sulfur plays a dual role, facilitating hydroxide formation through partial dissolution and modifying NiCu electronic structure to promote electron transfer from Cu to Ni. The catalyst demonstrated long-term stability, retaining over 95 % of its activity over 24 h, along with 89.28 % ammonia removal efficiency and 93.34 % nitrogen selectivity after 5 h. These findings highlight the catalyst’s potential for efficient AOR and offer insights into how surface reconstruction and sulfur species enhance activity and selectivity.