Constructing built-in electric field via CuO/NiO heterojunction for electrocatalytic reduction of nitrate at low concentrations to ammonia

Electrocatalytic reduction of nitrate (NO3−) at low concentrations to ammonia (NH4+) still faces challenges of low NO3− conversion and NH4+ selectivity due to the sluggish mass transfer and insufficient atomic hydrogen (H*) supply. Herein, we propose CuO/NiO heterojunction with the assistance of a built-in electric field to enhance mass transfer and H* provision. The built-in electric field in CuO/NiO is successfully formed as demonstrated by X-ray photoelectron spectroscopy and ultraviolet photoemission spectroscopy. The results reveal that CuO/NiO achieves high NO3− reduction activity (100%) and NH4+ selectivity (100%) under low NO3− concentration conditions (100 mg/L NO3−, ca. 22.6 mg/L NO3−–N), which is superior to that of many recently reported electrocatalysts. Density functional theory calculations further clarify that the built-in electric field triggers the enhanced adsorption of reactants on CuO/NiO heterojunction interface and strong d-p orbital hybridization between reactants and CuO/NiO. Besides, the free energy diagram of hydrogen evolution reaction of CuO/NiO confirms the realization of enhanced H* provision. Moreover, coupling experiments and consecutive cycle tests demonstrate the potential of CuO/NiO in practical applications. This work may open up a new path and guide the development of efficient electrocatalysts for electrocatalytic reduction of NO3− at low concentrations to NH4+.