Microscopic-Level Insights into the Mechanism of Enhanced NH3 Synthesis in Plasma-Enabled Cascade N2 Oxidation–Electroreduction System

Integrated/cascade plasma-enabled N₂ oxidation and electrocatalytic NO_x^- (where x = 2, 3) reduction reaction (pNOR-eNO_x^-RR) holds great promise for the renewable synthesis of ammonia (NH₃). However, the corresponding activated effects and process of plasma toward N₂ and O₂ molecules and the mechanism of eNO_x^-RR to NH₃ are unclear and need to be further uncovered, which largely limits the large-scale deployment of this process integration technology. Herein, we systematically investigate the plasma-enabled activation and recombination processes of N₂ and O₂ molecules, and more meaningfully, the mechanism of eNO_x^-RR at a microscopic level is also decoupled using copper (Cu) nanoparticles as a representative electrocatalyst. The concentration of produced NO_x in the pNOR system is confirmed as a function of the length for spark discharge as well as the volumetric ratio for N₂ and O₂ feeding gas. The successive protonation process of NO_x^- and the key N-containing intermediates (e.g., -NH₂) of eNO_x^-RR are detected with in situ infrared spectroscopy. Besides, in situ Raman spectroscopy further reveals the dynamic reconstruction process of Cu nanoparticles during the eNO_x^-RR process. The Cu nanoparticle-driven pNOR-eNO_x^-RR system can finally achieve a high NH₃ yield rate of ∼40 nmol s^-1 cm^-2 and Faradaic efficiency of nearly 90%, overperforming the benchmarks reported in the literature. It is anticipated that this work will stimulate the practical development of the pNOR-eNO_x^-RR system for the green electrosynthesis of NH₃ directly from air and water under ambient conditions.