Nanosecond-Pulsed Plasma Oxidation with Electrocatalytic Reduction for Sustainable Ammonia Production

Ammonia is primarily used as a fertilizer, and its potential as a hydrogen carrier is critical for the emerging global hydrogen economy.¹ Current industrial ammonia synthesis process (Haber-Bosch) has adverse environmental effects due to its reliance on fossil fuels. To achieve sustainable ammonia production, electricity-driven processes have stimulated intensive research such as plasma and electrochemical technologies. However, they have been significantly limited either by the low activation of stable nitrogen or by the high energy consumption respectively, and both result in the low production rates and selectivity towards ammonia. We recently demonstrated integrating non-thermal plasma with electrochemistry is the most practical approach to addressing above issues.² The non-thermal plasma provides reactivity and solubility for N2 gas in water, while the electrocatalytic conversion of plasma-produced species synthesize ammonia selectively. However, this hybrid process is far too young to compete with Haber-Bosch's productivity, especially due to the relatively high energy consumption and unclear underlying mechanism for the plasma ignition. In this study, a nanosecond pulse-driven plasma bubble reactor with multiple discharge schemes is developed to couple with electrochemistry for sustainable ammonia production. DBD and Spark discharge modes and their evolution processes are studied by comparing discharge images and electrical characterizations via an intensified charge-coupled device and a digital oscilloscope. Optical emission spectra and Fourier Transform infrared spectroscopy are employed to determine the composition of the gaseous plasma. Moreover, the effects of discharge modes on plasma-liquid interactions are discussed in terms of the physio-chemical properties of the treated aqueous solution, which can provide fundamental insights into the physics and chemistry of plasma bubbles as well as their potential a ppl ications.