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. [ABSTRACT FROM AUTHOR]