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 NOx– (where x = 2, 3) reduction reaction (pNOR-eNOx–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 eNOx–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 eNOx–RR at a microscopic level is also decoupled using copper (Cu) nanoparticles as a representative electrocatalyst. The concentration of produced NOx 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 NOx– and the key N-containing intermediates (e.g., −NH2) of eNOx–RR are detected with in situ infrared spectroscopy. Besides, in situ Raman spectroscopy further reveals the dynamic reconstruction process of Cu nanoparticles during the eNOx–RR process. The Cu nanoparticle-driven pNOR-eNOx–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-eNOx–RR system for the green electrosynthesis of NH3 directly from air and water under ambient conditions.