Characterization and Optimization of Plasma Electrolytic Process for Conversion of Molecular Nitrogen to Ammonia in Aqueous Water at Atmospheric Pressure

Abstract

Ammonia (NH3) is widely used in the chemical and agricultural industries and is of growing interest as a potential energy carrier. Most of the current global NH3 production utilizes the Haber-Bosch process which has a large environmental and physical footprint. For this reason, there have been substantial efforts to develop an alternative process that can be sustainable and distributed. Recently, plasma-based electrolysis reactors have shown promise to convert molecular nitrogen to various forms of fixed nitrogen such as ammonia, nitrate, and nitrite in water at atmospheric pressure. However, selectivity and control over the products has not been adequately demonstrated. In this work, we studied a plasma formed in pure nitrogen gas or mixtures containing nitrogen gas at the surface of liquid water. Various products were characterized in the liquid phase including ammonium ions, nitrate ions, nitrite ions, and hydrogen peroxide as a function of pH, discharge current, and time. In addition, continuous direct current (DC) was compared to pulsed DC voltage plasma operation. We find that the selectivity to different nitrogen products depends on the solution conditions. These results could be used to inform optimization and scale up of similar plasma-liquid systems for sustainable nitrogen fixation.