Selective Synthesis of Either Nitric Acid or Ammonia from Air by Electrolyte Regulation in a Plasma Electrolytic System

Abstract

The sustainable production of nitric acid (HNO3) and ammonia (NH3) using renewable energy sources and readily available feedstock under mild conditions is highly desirable as it can bypass the energy-intensive Ostwald and Haber–Bosch processes. Here, we report a plasma electrolytic system to selectively synthesize HNO3 or NH3 from air by an electrolyte-regulation strategy. The plasma is generated by air-pulsed positive direct current (DC) corona discharge, which occurs in a 2 mm gap between a tip electrode and the aqueous solution surface under atmospheric pressure. HNO3 was generated in 0.1 M HCl solution with a selectivity of 91.75 ± 6.34%. In contrast, nitrite (NO2–) as the main product (92.38 ± 1.17%) is obtained in 0.1 M KOH solution, which is utilized as the electrolyte in the subsequent electrocatalytic reduction step to further convert NOx– (NO3– and NO2–) into NH3 by employing a Cu2O nanowire electrocatalyst. The lowest total energy consumption of NH3 synthesis is 3.91 kWh mol–1NH3, lower than that of most previous reports. This electrolyte-regulation strategy opens avenues for adjustable production of HNO3 and NH3 in an energy-efficient and sustainable way by a plasma electrolytic system.