Sustainable liquid nitrogen fertilizer production via air plasma bubbles: insights into plasma-enabled N₂ fixation chemistry

Liquid nitrogen fertilizers, such as potassium nitrate (KNO₃) have been commonly used in modern agriculture, playing a crucial role in agricultural production. However, its production involves energy-intensive and environmentally unfriendly processes such as the Haber-Bosch process. This study demonstrated a new strategy for the sustainable and distributed production of liquid KNO₃ fertilizer via air plasma bubbles. We investigated the effects of solution characteristics (initial liquid conductivity, pH) and discharge power on the nitrogen fixation performance of the air plasma bubble system. Using a strongly alkaline solution can induce the increase of vibrational temperature (T_vib) during air plasma discharges, thereby enhancing NO_x yield together with favoring the NO_x adsorption process. Moreover, through electrical characteristics and a simplified circuit diagram, we found a highly conductive liquid phase is not conducive to NO_x generation due to the significant energy dissipation in the liquid before discharge. By further adjusting discharge power parameters and coupling the introduction of O₃, the highest energy efficiency (58.5 mmol kWh⁻¹) of NO_x production, with an excellent production rate (1687.4 μmol h⁻¹) is achieved. These findings provide an overall understanding of the effects of solution characteristics on gas-liquid plasma chemistry and pave the way for the optimized production of liquid nitrogen fertilizers.