Electrostatic Confinement of Plasma Electrons by Water Microdroplets Enables Dinitrogen Oxidation

Nonthermal plasma (NTP) offers exceptional theoretical energy efficiency for nitrogen fixation, yet realizing this potential in practice remains a formidable challenge due to rapid energy dissipation, particularly through low-energy electrons that typically lose their energy as heat. Conventional plasma modulation largely relies on mass-transfer-dominated frameworks, lacking mechanisms to selectively channel these electrons into productive chemical pathways. Here we demonstrate that positively charged water microdroplets constitute active electrostatic confinement interfaces to harvest and re-energize low-energy plasma electrons. Under minimized microdroplet charge relaxation, plasma energy becomes spatially and energetically confined at the droplet-air interface, promoting interfacial water splitting and triggering a distinctive •OH-mediated dinitrogen (N₂) oxidation pathway at reduced discharge voltage. Such energetic regulation boosts dinitrogen oxidation rates 60-fold while cutting power consumption by two-thirds, achieving a nitrate energy efficiency of 1.65 μmol J^-1 that rivals electrolysis-based industrial benchmarks. These results reveal an unrecognized chemical role of ubiquitous microdroplets as plasma energy concentrators and establish an interfacial electrostatic confinement mechanism for directing NTP reactivity, with implications for nitrogen fixation chemistry, atmospheric chemistry, and plasma catalysis.