Modulating the Electron Temperature of Spark Discharge for Highly Efficient Production of HNO₃

Low-temperature plasma offers a promising pathway for sustainable production of HNO₃ directly from air and H₂O under ambient conditions. However, most plasma electrodes suffer from a low yield rate and selectivity of NO₂, which limits the efficiency of HNO₃ production. In this study, we propose a strategy to improve the NO₂ yield by regulating the electron temperature (T_e) in spark discharge. An array electrode with multiple vertically oriented hollow Cu rods is designed to reduce localized energy deposition and optimize T_e. At an airflow rate of 300 SCCM, the array electrode achieves a yield rate for NO₂ of 17.85 mmol h^-1, which is 10.8 times higher than that (1.65 mmol h^-1) of the conventional single electrode. Notably, after absorption in 300 mL of H₂O, the array electrode produces a pure HNO₃ solution with a concentration as high as 680.5 mM within a 12 h discharge time.