Ammonia (NH3) has emerged as a promising carbon-free energy carrier owing to its high energy density, moderate storage conditions, and established global infrastructure. Within the framework of the ammonia economy, the electrochemical ammonia oxidation reaction (eAOR) plays a critical role in enabling sustainable energy conversion, wastewater treatment, and green production of nitrogen-containing chemicals. Despite its favorable thermodynamics and low equilibrium potential relative to water oxidation, the practical implementation of AOR remains limited by sluggish kinetics, high overpotentials, catalyst deactivation, and poor control over product selectivity.

In alkaline media, eAOR proceeds through multiple reaction pathways, yielding dinitrogen (N2), nitrite (NO2⁻), or nitrate (NO3⁻). Two classical mechanisms, the Oswin–Salomon and Gerischer–Mauerer pathways, have been proposed to describe N2 formation based on Pt catalysts. At higher anodic potentials, further oxidation of nitrogen intermediates leads to the formation of N–O species and overoxidized products, highlighting the strong dependence of AOR selectivity on electrode potential and surface chemistry.

Platinum-based catalysts exhibit high intrinsic activity and excellent selectivity toward N2 formation due to their optimal nitrogen adsorption energies. However, their high cost and susceptibility to poisoning by strongly adsorbed nitrogen species significantly hinder large-scale application. Consequently, non-noble metal catalysts have attracted increasing attention. Among them, Ni-based catalysts, particularly NiOOH formed in situ under alkaline conditions, have shown considerable promise as redox mediators for AOR. Their activity and selectivity are strongly influenced by pH, ammonia concentration, applied potential, and surface oxygen coverage. Cu-based catalysts also demonstrate notable activity, especially toward nitrite and nitrate formation, though stability remains a key challenge.

Herein, we investigated the effects of oxygen species and pH on the activity and selectivity of the eAOR using in situ analysis. 1) The oxygen species, especially dissolved O2, are significantly modulated the reaction activity and selectivity. 2) Increasing NH3 concentration suppresses OER by competing for surface sites, particularly at low OH− concentrations (i.e., low pH), while promoting AOR pathways.