Non-thermal air bubble discharge in water for atmospheric nitrogen fixation: mechanistic insights from experiments and simulations

Abstract A bubble discharge reactor, termed CHIEF (concentrated high-intensity electric field), was investigated to elucidate how reactor configuration, liquid conductivity, and bubble properties (size, deformation, polarization, and water vapor content) govern electric field distribution, streamer dynamics, and plasma chemistry. Experiments revealed that increasing liquid conductivity enhances conduction current and Joule heating within the orifice, leading to thermal instability at the bubble boundary, a decrease in neutral gas density, and an increase in the reduced electric field. Collectively, these effects lower the applied voltage required for streamer initiation, providing a mechanistic basis for leveraging conduction current to facilitate plasma formation. COMSOL simulations showed that the electric field around and inside a deformed air bubble is highly non-uniform, whereas nanobubbles and microbubbles exhibit minimal electric-field enhancement. Experimental analysis, optical emission spectroscopy (OES), and BOLSIG + calculations were used to investigate the reaction pathways of nitrogen fixation in air bubble discharges. The highest NO x production rate of 10.09 µ mol min −1 was achieved using a 6.0 mm orifice length with a solution conductivity of 140 µ S cm −1 . The effects of water molecules on streamer initiation, plasma chemistry, and discharge characteristics were analyzed using OES, BOLSIG + calculations, and simulations. This study provides mechanistic insights into plasma–liquid interactions for atmospheric nitrogen fixation and proposes a sustainable electrosynthesis approach using only air, water, and electricity to produce liquid nitrogen fertilizer.