Electronic modulation of nickel sites via Cu-doping and sulfur oxyanions coordination for highly efficient ammonia electrooxidation to nitrite production

Abstract

Electrocatalytic ammonia oxidation (AOR) offers an energy-efficient alternative to electrochemical water splitting for hydrogen (H₂) production and the sustainable synthesis of high-value nitrogen-containing species, such as nitrites. To optimize this process, designing catalysts for selective ammonia-to-nitrite electrooxidation with enhanced electron transfer and high current density is practically important. Nickel-based materials are effective AOR catalysts, particularly through the self-oxidation reconstruction into NiOOH. However, pristine NiOOH typically yields multiple products and catalyzes the competing oxygen evolution reaction (OER) within a similar potential range, hindering the high selectivity required for valuable AOR products. Herein, we report a Cu-doped nickel sulfide catalyst (Cu-NiS₂) with an optimized electronic configuration, which undergoes surface reconstruction to form sulfur oxyanion-coordinated Cu-NiOOH-SO_x at ∼1.60 V vs. RHE. The incorporation of Cu facilitates electron transfer in Ni, while the SO_x anions refines the coordination environment of Ni, modulating its electronic structure and enabling balanced adsorption of nitrogen-containing intermediates. This synergistic effect enhances both AOR activity and selectivity. As a result, the catalyst achieves an exceptional AOR current density of 104.8 mA cm⁻² at 1.65 V, along with an ammonia conversion efficiency of 90.3 % over 8-h and a NO₂⁻ selectivity of 95.4 %. Notably, the Cu-NiS₂ catalyst demonstrates remarkable stability, maintaining consistent performance for 56 h. This work provides new insights into the design and regulation of active sites in transition metal compound catalysts for efficient AOR, paving the way for selective nitrite production.