Electrocatalytic Oxidation of Ammonia to Nitrate Occurs on NiOOH with OH/O Vacancies

The ammonia electrooxidation reaction (AOR) has attracted significant attention for both wastewater treatment and energy storage applications. However, AOR pathways generating oxygenated products such as nitrite or nitrate remain unresolved. Attenuated total reflection-surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) and density functional theory (DFT) have been combined to determine the potential-dependent intermediates, active catalytic sites, and AOR pathways on Ni electrodes under alkaline conditions. The OH/O vacancy sites on NiOOH, revealed by XPS, are found by DFT calculations to be the active sites for the AOR, catalyzing the complete (eight-electron) oxidation of ammonia into nitrate. The formation of isolated OH/O vacancies on NiOOH is thermodynamically more favorable than that of paired vacancies in the potential range conducive to NiOOH formation and the AOR, with an increasing formation barrier at higher potentials. This synergistic ATR-SEIRAS and DFT study reveals that the selectivity of nitrite and nitrate production is potential-dependent, with nitrite and hydrazine formation initiated at moderate anodic potentials, whereas larger potentials promote conversion of nitrite into nitrate species. The release of adsorbed nitrite and nitrate, which is crucial for liberating catalytic sites for the continuous AOR, is spectroscopically and computationally shown to be facilitated at relatively low potentials. Comprehending AOR mechanisms on Ni-based catalysts as achieved in this study can pave the way for future research on catalyst design and optimization of AOR performance.