Plasma-Driven Decomposition of HAN-Based Ionic Liquids

A nanosecond pulse transient plasma is employed to initiate and control the exothermic decomposition of ionic liquids, namely, a mixture of hydroxylammonium nitrate (HAN) and 1-ethyl-3-methylimidazolium ethyl sulfate [EMIM]/[EtSO₄], as well as some noncombustible ionic liquids. Here, the plasma is discharged in a cylindrical geometry with a coaxial center wire electrode. High voltage (20 kV) nanosecond pulses (20 ns) at various frequencies up to 10 kHz produce a plasma discharge in the ionic liquid that initiates its nonthermal decomposition. Realtime imaging was used to observe and characterize electrically driven bubble formation (i.e., electrostriction), plasma initiation, and the evolution of ignition and extinguishing processes. This high-speed imaging shows that the flame can be toggled on and off within 66 ms. We hypothesize that the plasma-driven decomposition mechanism proceeds as follows: (1) bubble formation due to the high voltage-induced electric fields, (2) plasma initiation in the bubble regions where dielectric breakdown occurs, (3) the plasma then generates highly energetic electrons that, in turn, form highly reactive atomic and diatomic species, and (4) these radical species serve as short-lived reaction intermediates that drive nonthermal chemical reaction pathways when interacting with the ionic liquid, resulting in decomposition and ignition. Using in situ plasma emission spectroscopy, we have identified the following reaction intermediates: H, O, S, NO, and CO, which accelerate the conventional decomposition reaction pathways. Using FTIR emission spectroscopy of the flame plume, we have identified CO₂ and H₂O as products, verifying the combustion of the HAN-EtSO₄ mixture. These plasma-driven reaction intermediates deviate from the available chemical mechanisms of conventional thermal catalysis and thermal decomposition chemical pathways. As a control, we repeated the experiment with a DC voltage, which led to no decomposition or combustion up to 500 V, above which decomposition occurred. Here, the DC voltage provides Joule heating, which evaporates off water, destabilizing the HAN and causing ignition.