Synergistic Coupling of Multiphase Transport and Chemical Capture Enables High-Rate Plasma Nitrogen Fixation

Nitrogen fixation using air cold plasma offers a decentralized, electricity-driven process to support sustainable agriculture and global food security under climate volatility. Maximizing the uptake of plasma-generated reactive nitrogen species (RNS) in aqueous NO _x ^- is essential to achieve high nitrogen-fixation rates. Here, we report a synergistic coupling of gas transport, microbubble dynamics, and chemical capture to achieve a high NO _x ^- molar production rate of 162.5 μmol/min. In this continuous-flow system, the dielectric-barrier discharge channel simultaneously serves as the plasma-discharge region and the gas inlet, through which the activated gas is delivered into the liquid flow via the microbubble cloud. The RNS production rate in the afterglow region increases with the gas-flow Reynolds number and plateaus in the turbulent regime, suggesting that plasma activation transits to a plasma-activation-limited stage. A pressure-regulated operating mode produces a stable microbubble dispersion and suppresses the formation of large gas slugs during gas flow-rate regulation. In parallel, alkaline chemical capture synergizes with microbubble hydrodynamics to promote NO _x ^- formation. Relative to water, the production rate of NO _x ^- is 1.2-fold higher at pH 13.7, increases by an additional 3.4-fold in the pressure-regulated mode of gas flow, and is 3.8-fold higher at 50 L liquid volume than at 0.5 L, multiplicatively yielding an overall ~ 15-fold enhancement. Furthermore, subsequent electrocatalysis converted 51 mM NO _x ^- in plasma-activated water into ammonia with a Faradaic efficiency of 63.6%. This work deepens the mechanistic understanding of multiphase transport in cold plasma activation and establishes a high-rate, electricity-driven platform for green nitrogen fixation.