Synergistic Active Phases of Transition Metal Oxide Heterostructures for Highly Efficient Ammonia Electrosynthesis

Electrochemically converting waste nitrate (NO₃⁻) into ammonia (NH₃) is a green route for both wastewater treatment and high-value-added ammonia generation. However, the NO₃⁻-to-NH₃ reaction involves multistep electron transfer and complex intermediates, making it a grand challenge to drive efficient NO₃⁻ electroreduction with high NH₃ selectivity. Herein, an in-operando electrochemically synthesized Cu₂O/NiO heterostructure electrocatalyst is proven for efficient NH₃ electrosynthesis. In situ Raman spectroscopy reveals that the obtained Cu₂O/NiO, induced by the electrochemistry-driven phase conversion, is the real active phase. This electronically coupled phase can modulate the interfacial charge distribution, dramatically lower the overpotential in the rate-determining step and thus requiring lower energy input to proceed with the NH₃ electrosynthesis. The orbital hybridization calculations further identify that Cu₂O is beneficial for NO₃⁻ adsorption, and NiO could promote the desorption of NH₃, forming an excellent tandem electrocatalyst. Such a tandem system leads to NH₃ Faradaic efficiency of 95.6%, a super-high NH₃ selectivity of 88.5% at −0.2 V versus RHE, surpassing most of the NH₃ electrosynthesis catalysts at an ultralow reaction voltage. © 2023 Wiley-VCH GmbH.