Electronic Effect and Key Intermediates in Electrochemical Ammonia Oxidation With Mn-Salen Complexes

Ammonia (NH₃) has emerged as a promising carbon-free fuel and hydrogen carrier, offering a potential solution to meet growing global energy demands. In this study, we investigate the role of ligand electronic effects in a series of Mn-salen electrocatalysts for the ammonia oxidation reaction (AOR) in nonaqueous media. Our results reveal that electron-withdrawing substituents promote an earlier onset potential and enhanced catalytic activity. A high-valent nitrido-manganese (V) salen species is identified as a key reaction intermediate through mass spectrometry during electrolysis, which undergoes nucleophilic attack by NH₃ under oxidative conditions to produce nitrogen (N₂). We also examined how solvent and electron-transfer redox mediators affect the reaction. While the solvent shows no significant impact on catalytic performance, the use of a mediator markedly enhances reaction kinetics. Furthermore, the salen catalyst was evaluated for AOR in aqueous conditions, demonstrating an onset potential of 1.65 V versus RHE along with appreciable Faradaic efficiencies for N₂ and H₂ production of 93.6% ± 4.5% and 98.9% ± 1.7%, respectively, making it one of the few examples of selective AOR in aqueous medium.